apple2_hgr_font_tutorial/README.md

#Apple ]\[ //e HGR Font 6502 Assembly Language Tutorial

Revision: 61, Jan 27, 2016.

# Table of Contents

* Introduction
* The Problem
* A Solution
 * Hard-Coded: A
* Functions we want & will write
* Quirks of the Apple HGR screen
 * Non-Linear Memory
 * No FONT data in ROM
 * HGR bytes are reversed
 * Half-pixel shift
* Font Data
 * Raw Font Data
 * Image to Font Data (JavaScript)
* DrawChar()
 * Font -> Screen Memory Trace
 * DrawChar() version 1
 * X Cursor Position
 * CursorCol()
 * Introduction to Optimization
 * Recap - Listing 5
 * DrawChar() version 2
 * DrawChar() version 3
* Character Inspector
 * Character Inspector version 2
 * Character Inspector version 3
* Y Cursor Position
* Natural Params SetCursorColRow()
* DrawString()
* Copy text screen to HGR
* Exercises
 * Exercise 1: ScrollHgrUpLine()
 * Exercise 2: ScrollHgrUpPixel()
* Recap - Font Draw
* Other Fonts
 * Fat Stroke Fonts
  * Beautiful Boot
* What's Next?
* Conclusion
* Solutions
 * Solution 1: ScrollHgrUpLine()
 * Solution 2: ScrollHgrUpPixel()
* References
* Misc. Utilities and Files
* TODO:

# Introduction

A lot of people in comp.sys.apple2.programmer and other places on the internet have wondered how to "print" text onto the Apple's High Resolution Graphics (HGR) screen. Here's a tutorial on "6502 Font Blitting."

**Note**: We will prefix hex numbers with `$` (or C's notation of `0x`). We will prefix binary numbers with `%`.

Fire up your favorite Apple emulator (*cough* AppleWin) or real hardware.

If you use:

* [AppleWin](https://github.com/AppleWin/AppleWin) press `F2` (to reboot), `Ctrl-F2` to Ctrl-Reset, and then press `F9` until you get a Monochrome screen.

* [Jace](https://github.com/Michaelangel007/jace/blob/master/target/jace-2.0-SNAPSHOT-jar-with-dependencies.jar) press `Ctrl-Delete` to reset.

 * You will also need the Java JRE.

 * ~~On OSX, Jace has an copy-paste bug and won't paste in the first line of the clipboard. :-/~~ (**Update**: Fixed! Thanks Blurry!) 

* [Virtual II](http://www.virtualii.com/) press `Ctrl-F12` to reset.

There are other emulators written in JavaScript but they are poor due to 2 reasons:

* Don't support paste -- you'll be forced to manually enter in the hex code. :-/  (Yeah, right!
* Don't emulate the half-pixel shift of real hardware at all -- not an issue, but you won't see the full effect for one section.

Some emulators that run in the browser:

* [Apple 2 js](https://www.scullinsteel.com/apple2/) make sure you select:

 * Options, [x] Green Screen

* [David's Caldwall's Apple 2 JavaScript + WebGL](http://porkrind.org/a2/)


**Note**: If you are using an emulator -- I've added "comments" in the lines of machine code you would paste  by having a semi-colon and a description at the end of the line.  

 * **You may want to mute your sound** since the Apple will beep at the semi-colon "comments" as that part of the input is not tehcnically valid input.  (The rest of the line WILL be processed, though.)


# The Problem

When you are at the Applesoft `]` prompt type in (or paste) the following:

    CALL-151
    FC58G
    400:41

We used the ASCII character `A` which has a hex value 0x41.  Hmm, OK, so we see an `A` but why is it flashing??

<hr>
**Note**: If you use AppleWin, select the lines, copy, switch back to the emulator, and press Shift-Insert to paste.
<hr>

The 40x24 text screen of the Apple is _"memory-mapped"_ -- that is by directly setting memory the Video Controller circuitry will read those contents and display that.  In later computers this `video ram` or `VRAM` is not directly accessible by the CPU; all the CPU can do is set `registers` of the GPU.

 One of the _quirks_ of the Apple is that it has support for Inverse, Flashing, and Normal characters.  It technically uses High-Bit ASCII to show normal characters.  This doesn't really concern us but just so you understand:

    400:01 41 C1

The control characters show up in `inverse`, ASCII characters show up `flashing`, and our normal character requires the high bit to be set. 0x41 + 080 = 0xC1.

<hr>
A slight fun diversion: If you are on an enhanced Apple //e or //c we can activate a 2nd character set called `Mouse Text`.  This replaces all the flashing text with special drawing charactes:

    C00F:1

You should see the flashing `A` has been replaced with an open apple symbol.

To turn `Mouse Text` off:

    C00E:1

Anyways, back to drawing text.
<hr>

If we switch to the HGR screen and tried to enter in 0x41 what would happen? (Ignore the beeping the emulator will make.)

    F399G    ; `TEXT`
    F3E2G    ; `HGR`
    2000:41  ; A

Hmm, that doesn't look like an `A` at all, only gibberish -- 2 dots. :-/  (If you see 2 magenta dots ignore the color for now.)

# A Solution:

## Hard-Coded: A

Enter in:

    2000:4
    2400:A
    2800:11
    2C00:11
    3000:1F
    3400:11
    3800:11

Voila!

You should see an uppercase A appear in the top left of the HGR screen.

![Screenshot 1](pics/hgrfont_01.png?raw=true)

Magic? :-)

Nah, just Computer Science. :-)

The first question you probably have is _"How did I know what bytes to use?"_  We'll get to that in a second.

## Functions we want & will write

When we are done we will have 6502 assembly code that implements the equivalent of these C functions names:

```c
    void DrawChar();
    void DrawCharCol( char c, int col );
    void DrawCharColRow( char c, int col, int row );
    void SetCursorRow( int row );
    void SetCursorColRow3( int col, int row );
    void SetCursorCol( int col );
    void IncCursorCol();
    void DrawHexByte( char c );
    void DrawString( char *text );
    void CopyTextToHGR();
    void ScrollHgrUpPixel();
```


## Quirks of the Apple HGR screen

There are couple of things we need to discuss first. The preceding example showed that the Apple's TEXT and HGR screen behaves a little "funky." The Apple's, shall we say, esoteric use of hardware, is one of the reasons us fans love (or hate) it.  "There are 4 lights!" Er, There are 4 things that stand out:


### Non-Linear Memory

First, we should notice that video memory is non-linear. :-( You'll want to get familiar with the HGR address for the various Y scanlines:

"Understanding the Apple II", page 5-14 has this table `HGR Memory-mapped IO`:

* https://archive.org/stream/understanding_the_apple_ii#page/n105/mode/2up

With all the decimal cruft removed:

<hr>

    Table 1: HGR Y Address for every scanline

| Y    |Address|   | Y   |Address|   | Y   |Address|   | Screen Hole  |
|-----:|:-----:|---|----:|-------|---|----:|:-----:|---|:------------:|
| **0**| $2000 | | **64**| $2028 | |**128**| $2050 | | $2078..$207F |
|   1  | $2400 | |   65  | $2428 | |  129  | $2450 | | $2478..$247F |
|   2  | $2800 | |   66  | $2828 | |  130  | $2850 | | $2878..$287F |
|   3  | $2C00 | |   67  | $2C28 | |  131  | $2C50 | | $2C78..$2C7F |
|   4  | $3000 | |   68  | $3028 | |  132  | $3050 | | $3078..$307F |
|   5  | $3400 | |   69  | $3428 | |  133  | $3450 | | $3478..$347F |
|   6  | $3800 | |   70  | $3828 | |  134  | $3850 | | $3878..$387F |
|   7  | $3C00 | |   71  | $3C28 | |  135  | $3C50 | | $3C78..$3C7F |
| **8**| $2080 | | **72**| $20A8 | |**136**| $20D0 | | $20F8..$20FF |
|   9  | $2480 | |   73  | $24A8 | |  137  | $24D0 | | $24F8..$24FF |
|  10  | $2880 | |   74  | $28A8 | |  138  | $28D0 | | $28F8..$28FF |
|  11  | $2C80 | |   75  | $2CA8 | |  139  | $2CD0 | | $2CF8..$2CFF |
|  12  | $3080 | |   76  | $30A8 | |  140  | $30D0 | | $30F8..$30FF |
|  13  | $3480 | |   77  | $34A8 | |  141  | $34D0 | | $34F8..$34FF |
|  14  | $3880 | |   78  | $38A8 | |  142  | $38D0 | | $38F8..$38FF |
|  15  | $3C80 | |   79  | $3CA8 | |  143  | $3CD0 | | $3CF8..$3CFF |
|**16**| $2100 | | **80**| $2128 | |**144**| $2150 | | $2178..$217F |
|  17  | $2500 | |   81  | $2528 | |  145  | $2550 | | $2578..$257F |
|  18  | $2900 | |   82  | $2928 | |  146  | $2950 | | $2978..$297F |
|  19  | $2D00 | |   83  | $2D28 | |  147  | $2D50 | | $2D78..$2D7F |
|  20  | $3100 | |   84  | $3128 | |  148  | $3150 | | $3178..$317F |
|  21  | $3500 | |   85  | $3528 | |  149  | $3550 | | $3578..$357F |
|  22  | $3900 | |   86  | $3928 | |  150  | $3950 | | $3978..$397F |
|  23  | $3D00 | |   87  | $3D28 | |  151  | $3D50 | | $3D78..$3D7F |
|**24**| $2180 | | **88**| $21A8 | |**152**| $21D0 | | $21F8..$21FF |
|  25  | $2580 | |   89  | $25A8 | |  153  | $25D0 | | $25F8..$25FF |
|  26  | $2980 | |   90  | $29A8 | |  154  | $29D0 | | $29F8..$29FF |
|  27  | $2D80 | |   91  | $2DA8 | |  155  | $2DD0 | | $2DF8..$2DFF |
|  28  | $3180 | |   92  | $31A8 | |  156  | $31D0 | | $31F8..$31FF |
|  29  | $3580 | |   93  | $35A8 | |  157  | $35D0 | | $35F8..$35FF |
|  30  | $3980 | |   94  | $39A8 | |  158  | $39D0 | | $39F8..$39FF |
|  31  | $3D80 | |   95  | $3DA8 | |  159  | $3DD0 | | $3DF8..$3DFF |
|**32**| $2200 | | **96**| $2228 | |**160**| $2250 | | $2278..$227F |
|  33  | $2600 | |   97  | $2628 | |  161  | $2650 | | $2678..$267F |
|  34  | $2A00 | |   98  | $2A28 | |  162  | $2A50 | | $2A78..$2A7F |
|  35  | $2E00 | |   99  | $2E28 | |  163  | $2E50 | | $2E78..$2E7F |
|  36  | $3200 | |  100  | $3228 | |  164  | $3250 | | $3278..$327F |
|  37  | $3600 | |  101  | $3628 | |  165  | $3650 | | $3678..$367F |
|  38  | $3A00 | |  102  | $3A28 | |  166  | $3A50 | | $3A78..$3A7F |
|  39  | $3E00 | |  103  | $3E28 | |  167  | $3E50 | | $3E78..$3E7F |
|**40**| $2280 | |**104**| $22A8 | |**168**| $22D0 | | $22F8..$22FF |
|  41  | $2680 | |  105  | $26A8 | |  169  | $26D0 | | $26F8..$26FF |
|  42  | $2A80 | |  106  | $2AA8 | |  170  | $2AD0 | | $2AF8..$2AFF |
|  43  | $2E80 | |  107  | $2EA8 | |  171  | $2ED0 | | $2EF8..$2EFF |
|  44  | $3280 | |  108  | $32A8 | |  172  | $32D0 | | $32F8..$32FF |
|  45  | $3680 | |  109  | $36A8 | |  173  | $36D0 | | $36F8..$36FF |
|  46  | $3A80 | |  110  | $3AA8 | |  174  | $3AD0 | | $3AF8..$3AFF |
|  47  | $3E80 | |  111  | $3EA8 | |  175  | $3ED0 | | $3EF8..$3EFF |
|**48**| $2300 | |**112**| $2328 | |**176**| $2350 | | $2378..$237F |
|  49  | $2700 | |  113  | $2728 | |  177  | $2750 | | $2778..$277F |
|  50  | $2B00 | |  114  | $2B28 | |  178  | $2B50 | | $2B78..$2B7F |
|  51  | $2F00 | |  115  | $2F28 | |  179  | $2F50 | | $2F78..$2F7F |
|  52  | $3300 | |  116  | $3328 | |  180  | $3350 | | $3378..$337F |
|  53  | $3700 | |  117  | $3728 | |  181  | $3750 | | $3778..$377F |
|  54  | $3B00 | |  118  | $3B28 | |  182  | $3B50 | | $3B78..$3B7F |
|  55  | $3F00 | |  119  | $3F28 | |  183  | $3F50 | | $3F78..$3F7F |
|**56**| $2380 | |**120**| $23A8 | |**184**| $23D0 | | $23F8..$23FF |
|  57  | $2780 | |  121  | $27A8 | |  185  | $27D0 | | $27F8..$27FF |
|  58  | $2B80 | |  122  | $2BA8 | |  186  | $2BD0 | | $2BF8..$2BFF |
|  59  | $2F80 | |  123  | $2FA8 | |  187  | $2FD0 | | $2FF8..$2FFF |
|  60  | $3380 | |  124  | $33A8 | |  188  | $33D0 | | $33F8..$33FF |
|  61  | $3780 | |  125  | $37A8 | |  189  | $37D0 | | $37F8..$37FF |
|  62  | $3B80 | |  126  | $3BA8 | |  190  | $3BD0 | | $3BF8..$3BFF |
|  63  | $3F80 | |  127  | $3FA8 | |  191  | $3FD0 | | $3FF8..$3FFF |

<hr>

Don't worry if the address pattern makes no sense right now -- we'll reveal that later -- but if you're curiuous it is this line using integer math:

```JavaScript
        hgr[ y ] = 0x2000 + (y/64)*0x28 + (y%8)*0x400 + ((y/8)&7)*0x80;
```

Here's the [JavaScript source code](list_hgr_table.html) to generate a bare-bones table:

```JavaScript
    function int2pad( n, pad )
    {
        return (Array( pad+1 ).join(' ') + n).slice(-pad);
    }

    function word2hex$( w )
    {
        return "$" + ("    " + w.toString(16).toUpperCase()).slice(-4);
    }

    var hgr = [];
    for( var y = 0; y < 193; ++y ) // Intentional 1 scanline too many!
        hgr[ y ] = 0x2000 + ((y/64)|0)*0x28 + ((y%8)|0)*0x400 + ((y/8)&7)*0x80;

    var text = "", s = " | ";
    for( y = 0; y < 64; ++y )
    {
        var a0 = hgr[ y +   0 ];
        var a1 = hgr[ y +  64 ];
        var a2 = hgr[ y + 128 ];
        text += "| "
        + int2pad( y +   0, 3 ) + s + word2hex$( a0 ) + s
        + int2pad( y +  64, 3 ) + s + word2hex$( a1 ) + s
        + int2pad( y + 128, 3 ) + s + word2hex$( a2 ) + s
        + word2hex$( a2 + 40 ) + ".."
        + word2hex$( a2 + 47 ) + " |\n";
    }
    console.log( text );
```

### No FONT data in ROM

Second, each glyph in the Apple font is in a 7x8 cell -- the leading line on the bottom is usually blank but we'll store that too so that we have a true "underline" and bottom descender on 'j', 'y', etc. How do we know this?

The `TEXT` screen is 40x24 characters. The high resolution graphics `HGR` screen is 280x192.

    Char Width  (px/character) = Screen Width  (px) / Columns (characters) = 280/40 = 7

    Char Height (px/character) = Screen Height (px) / Rows    (characters) = 192/24 = 8

Unfortunately, the data for the TEXT ROM 25123 hardware chip is **not** accessible from the 6502 unlike say the [IBM PC BIOS Character Sets](http://nerdlypleasures.blogspot.com/2015/04/ibm-character-fonts.html). :-/ T his means you will need to manually enter in the 8 bytes/character. :-( The good news is that I've already done this so you can copy / paste. :-)

 You can find a picture of the Apple ]\[ ROM text font on Page 8-9, Figure 8.4 of ["Understanding the Apple \]\["](https://archive.org/stream/understanding_the_apple_ii#page/n203/mode/2up):

![Apple 1 Font](pics/apple1_font.png)

 We're actually going to use the Apple //e ROM text font since it has lower case and the famous "Mouse Text" glyphs. See Page 8-25, Figure 8.8 of ["Understanding the Apple //e"](https://archive.org/stream/Understanding_the_Apple_IIe#page/n233/mode/2up):

![Apple 2e Font](pics/apple2e_font.png)

### HGR bytes are reversed

Third, the video scanner for HGR mode scans bits in reverse. :-/ This means that we need to "flip" the bits in a byte if we want it to appear properly. Not hard, just inconvenient.  We'll store the pre-flipped bits so we don't have to do this at run-time. :-)

 For example, If we want these 4 scan-lines of `\`:

    X____
    _X___
    __X__
    ___X_

 You would _normally_ encode the pixels in binary as:

    %1000_0000 = $80
    %0100_0000 = $40
    %0010_0000 = $20
    %0001_0000 = $10

 And if we tried entering in:

    2100:80
    2500:40
    2900:20
    2D00:10

We would only get:

 * 3 scanlines instead of the expected 4 (see the next point), and
 * the image would be flipped along the left-right (X axis) like this: `/`

![Screenshot 2](pics/hgrfont_02.png?raw=true)

On the Apple we need to flip each byte:

    %0000_0001 = $01
    %0000_0010 = $02
    %0000_0100 = $04
    %0000_1000 = $08

Enter in:

    2200:1
    2600:2
    2A00:4
    2E00:8

And we see the correct: `\`

![Screenshot 3](pics/hgrfont_03.png?raw=true)

### Half-pixel shift

Fourth, we mentioned above that when we entered in $80 that the Apple didn't display any pixels for this byte.  This is because the Apple uses the high-bit as a flag to shift that group of 7 pixels over HALF a pixel. (Yes, half a pixel.) This means the monochrome *effective* resolution is a pseudo 560x192. We can't individually access every 560 pixels, only part of them so it is not a "true" 560 resolution. :-( What this means in practice is that we can use this half-pixel shift / byte to get very smooth slopes for Y, etc. :-)

For example this will give us a "sharp" `Y`:

    2300:22
    2700:22
    2B00:14
    2F00:8
    3300:8
    3700:8
    3B00:8

![Screenshot 4](pics/hgrfont_04.png?raw=true)

If we change the 2nd and 4th scan line to use this half-pixel shift we can't just set the high bit as we won't get quite the correct image:

Enter in: (Again, ignore the beeping.)

    2302:22
    2702:A2 ;
    2B02:14
    2F02:88 ;
    3302:8
    3702:8
    3B02:8

![Screenshot 5](pics/hgrfont_05.png?raw=true)

We actually _also_ need to move the right-edge pixel of these 2 scanlines over left by 1 pixel so it appears in the correct location when shifted.

Enter in:

    2304:22
    2704:92 ;
    2B04:14
    2F04:8C ;
    3304:8
    3704:8
    3B04:8

Ah-ha! We've got a "smooth" `Y`.

![Screenshot 6](pics/hgrfont_06.png?raw=true)

**Note**: The emulators `Virtual ][` and `Apple2js` are *broken* emulators. They do **not** emulate the half-pixel shift of real hardware at all.  This is another reason we won't worry about it for now.

 We're going to ignore the half-pixel shift since it is easy to touch up the font data later if we wish.

At the beginning we said to view the HGR screen in monochrome. Notice how the extra colors make the Hi-Res text much harder to read. If you are running on real hardware the Apple Color Composite Monitor had a push-button on the front to toggle the screen between color and monochrome.  Now we know why!

![Screenshot 7](pics/hgrfont_07.png?raw=true)


## Font Data

Alrighty then, let's get the font data!

Here is a picture of the Apple //e character set:

* ![Apple //e character set](Apple2eFont7x8.png?raw=true)

If we wanted only uppercase ASCII we could get away with 4 rows of 16 characters (symbols, numbers, letters) = 64 glyphs:

    64 glyphs * 8 bytes/glyph = 512 bytes.

Since the font data chews up memory anyways we'll "splurge" and use the full 128 ASCII glyphs:

    128 glyphs * 8 bytes/glyph = 1024 bytes = 1K of data.

Ouch! We're using 1K of our precious 64K.  Now we know why all those font glyphs was in a ROM chip.


### Raw Font Data

I've saved you the trouble of converting all the pixels to hex. :-)

Enter in (or download the raw binary [font.bin](font.bin) and with AppleWin press `F7`, type `bload font.bin,6000`, press `F7`):

```
    6000:10 08 36 7F 3F 3F 7E 36  '@'^
    6008:10 08 36 41 21 21 4A 36  'A'^
    6010:00 00 02 06 0E 1E 36 42  'B'^
    6018:7F 22 14 08 08 14 2A 7F  'C'^
    6020:00 40 20 11 0A 04 04 00  'D'^
    6028:7F 3F 5F 6C 75 7B 7B 7F  'E'^
    6030:70 60 7E 31 79 30 3F 02  'F'^
    6038:00 18 07 00 07 0C 08 70  'G'^
    6040:08 04 02 7F 02 04 08 00  'H'^
    6048:00 00 00 00 00 00 00 2A  'I'^
    6050:08 08 08 08 49 2A 1C 08  'J'^
    6058:08 1C 2A 49 08 08 08 08  'K'^
    6060:7F 00 00 00 00 00 00 00  'L'^
    6068:40 40 40 44 46 7F 06 04  'M'^
    6070:3F 3F 3F 3F 3F 3F 3F 3F  'N'^
    6078:13 18 1C 7E 1C 18 10 6F  'O'^
    6080:64 0C 1C 3F 1C 0C 04 7B  'P'^
    6088:40 48 08 7F 3E 1C 48 40  'Q'^
    6090:40 48 1C 3E 7E 08 48 40  'R'^
    6098:00 00 00 7F 00 00 00 00  'S'^
    60A0:01 01 01 01 01 01 01 7F  'T'^
    60A8:08 10 20 7F 20 10 08 00  'U'^
    60B0:2A 55 2A 55 2A 55 2A 55  'V'^
    60B8:55 2A 55 2A 55 2A 55 2A  'W'^
    60C0:00 3E 41 01 01 01 7F 00  'X'^
    60C8:00 00 3F 40 40 40 7F 00  'Y'^
    60D0:40 40 40 40 40 40 40 40  'Z'^
    60D8:08 1C 3E 7F 3E 1C 08 00  '['^
    60E0:7F 00 00 00 00 00 00 7F  '\'^
    60E8:14 14 77 00 77 14 14 00  ']'^
    60F0:7F 40 40 4C 4C 40 40 7F  '^'^
    60F8:01 01 01 01 01 01 01 01  '_'^
    6100:00 00 00 00 00 00 00 00  ' 
    6108:08 08 08 08 08 00 08 00  '!
    6110:14 14 14 00 00 00 00 00  '"
    6118:14 14 3E 14 3E 14 14 00  '#
    6120:08 3C 0A 1C 28 1E 08 00  '$
    6128:06 26 10 08 04 32 30 00  '%
    6130:04 0A 0A 04 2A 12 2C 00  '&
    6138:08 08 08 00 00 00 00 00  ''
    6140:08 04 02 02 02 04 08 00  '(
    6148:08 10 20 20 20 10 08 00  ')
    6150:08 2A 1C 08 1C 2A 08 00  '*
    6158:00 08 08 3E 08 08 00 00  '+
    6160:00 00 00 00 08 08 04 00  ',
    6168:00 00 00 3E 00 00 00 00  '-
    6170:00 00 00 00 00 00 08 00  '.
    6178:00 20 10 08 04 02 00 00  '/
    6180:1C 22 32 2A 26 22 1C 00  '0
    6188:08 0C 08 08 08 08 1C 00  '1
    6190:1C 22 20 18 04 02 3E 00  '2
    6198:3E 20 10 18 20 22 1C 00  '3
    61A0:10 18 14 12 3E 10 10 00  '4
    61A8:3E 02 1E 20 20 22 1C 00  '5
    61B0:38 04 02 1E 22 22 1C 00  '6
    61B8:3E 20 10 08 04 04 04 00  '7
    61C0:1C 22 22 1C 22 22 1C 00  '8
    61C8:1C 22 22 3C 20 10 0E 00  '9
    61D0:00 00 08 00 08 00 00 00  ':
    61D8:00 00 08 00 08 08 04 00  ';
    61E0:10 08 04 02 04 08 10 00  '<
    61E8:00 00 3E 00 3E 00 00 00  '=
    61F0:04 08 10 20 10 08 04 00  '>
    61F8:1C 22 10 08 08 00 08 00  '?
    6200:1C 22 2A 3A 1A 02 3C 00  '@
    6208:08 14 22 22 3E 22 22 00  'A
    6210:1E 22 22 1E 22 22 1E 00  'B
    6218:1C 22 02 02 02 22 1C 00  'C
    6220:1E 22 22 22 22 22 1E 00  'D
    6228:3E 02 02 1E 02 02 3E 00  'E
    6230:3E 02 02 1E 02 02 02 00  'F
    6238:3C 02 02 02 32 22 3C 00  'G
    6240:22 22 22 3E 22 22 22 00  'H
    6248:1C 08 08 08 08 08 1C 00  'I
    6250:20 20 20 20 20 22 1C 00  'J
    6258:22 12 0A 06 0A 12 22 00  'K
    6260:02 02 02 02 02 02 3E 00  'L
    6268:22 36 2A 2A 22 22 22 00  'M
    6270:22 22 26 2A 32 22 22 00  'N
    6278:1C 22 22 22 22 22 1C 00  'O
    6280:1E 22 22 1E 02 02 02 00  'P
    6288:1C 22 22 22 2A 12 2C 00  'Q
    6290:1E 22 22 1E 0A 12 22 00  'R
    6298:1C 22 02 1C 20 22 1C 00  'S
    62A0:3E 08 08 08 08 08 08 00  'T
    62A8:22 22 22 22 22 22 1C 00  'U
    62B0:22 22 22 22 22 14 08 00  'V
    62B8:22 22 22 2A 2A 36 22 00  'W
    62C0:22 22 14 08 14 22 22 00  'X
    62C8:22 22 14 08 08 08 08 00  'Y
    62D0:3E 20 10 08 04 02 3E 00  'Z
    62D8:3E 06 06 06 06 06 3E 00  '[
    62E0:00 02 04 08 10 20 00 00  '\
    62E8:3E 30 30 30 30 30 3E 00  ']
    62F0:00 00 08 14 22 00 00 00  '^
    62F8:00 00 00 00 00 00 00 7F  '_
    6300:04 08 10 00 00 00 00 00  '`
    6308:00 00 1C 20 3C 22 3C 00  'a
    6310:02 02 1E 22 22 22 1E 00  'b
    6318:00 00 3C 02 02 02 3C 00  'c
    6320:20 20 3C 22 22 22 3C 00  'd
    6328:00 00 1C 22 3E 02 3C 00  'e
    6330:18 24 04 1E 04 04 04 00  'f
    6338:00 00 1C 22 22 3C 20 1C  'g
    6340:02 02 1E 22 22 22 22 00  'h
    6348:08 00 0C 08 08 08 1C 00  'i
    6350:10 00 18 10 10 10 12 0C  'j
    6358:02 02 22 12 0E 12 22 00  'k
    6360:0C 08 08 08 08 08 1C 00  'l
    6368:00 00 36 2A 2A 2A 22 00  'm
    6370:00 00 1E 22 22 22 22 00  'n
    6378:00 00 1C 22 22 22 1C 00  'o
    6380:00 00 1E 22 22 1E 02 02  'p
    6388:00 00 3C 22 22 3C 20 20  'q
    6390:00 00 3A 06 02 02 02 00  'r
    6398:00 00 3C 02 1C 20 1E 00  's
    63A0:04 04 1E 04 04 24 18 00  't
    63A8:00 00 22 22 22 32 2C 00  'u
    63B0:00 00 22 22 22 14 08 00  'v
    63B8:00 00 22 22 2A 2A 36 00  'w
    63C0:00 00 22 14 08 14 22 00  'x
    63C8:00 00 22 22 22 3C 20 1C  'y
    63D0:00 00 3E 10 08 04 3E 00  'z
    63D8:38 0C 0C 06 0C 0C 38 00  '{
    63E0:08 08 08 08 08 08 08 08  '|
    63E8:0E 18 18 30 18 18 0E 00  '}
    63F0:2C 1A 00 00 00 00 00 00  '~
    63F8:00 2A 14 2A 14 2A 00 00  '*
```

I also recommend "saving" a copy of this font higher in memory.

```
7000<6000.63FFM
```

(For the advanced user, you can save this: `BSAVE FONT7X8,A$6000,L$400`)

<hr>
**AppleWin** users: We can save the state of the virutal machine via `F11`.
<hr>


### Image to Font Data (JavaScript)

If you were wondering how this data was generated, you see the great thing about computers is that they can automate all the tedious and boring crap, er, calculations for us. Here's a HTML + JavaScript program I wrote to convert the [image to HEX](image_2_hex.html):

```javascript
     <!DOCTYPE HTML>
    <html>
    <head>
        <script>
            function byte2hex$( byte )
            {
                return ("0" + byte.toString(16)).toUpperCase().substr(-2)
            }

            function OnLoad()
            {
                var image   = document.getElementById( "Apple2eFont7x8" );
                var canvas  = document.createElement( "Canvas" );
                var context = canvas.getContext( "2d" );

                canvas.width  = image.width;
                canvas.height = image.height;
                context.drawImage( image, 0, 0 );

                var CW = 7, CH = 8; // Cell Width Height
                var address = 0x6000, pixel, rgba, lines = "";
                for( var ty = 0; ty < image.height/CH; ++ty )
                {
                    for( var tx = 0; tx < image.width/CW; ++tx )
                    {
                        var text = "";
                        for( var y = 0; y < CH; ++y )
                        {
                            var hex = 0, mask = 0x1;
                            for( var x = 0; x < CW; ++x, mask <<= 1 )
                            {
                                pixel = context.getImageData( tx*CW+x, ty*CH+y, 1, 1 );
                                rgba  = pixel.data;
                                hex  += rgba[0] ? mask : 0; // assume R=G=B
                            }
                            text += byte2hex$( hex ) + " ";
                        }
                        var c = (16*ty)+tx, d =       String.fromCharCode( c );
                        if (c < 32)         d = "^" + String.fromCharCode( c + 0x40 );
                        text += "; " + d + "\n";
                        lines += "" + address.toString(16).toUpperCase() + ":" + text;
                        address += 8;
                    }
                }
                console.log( lines );
                var pre = document.getElementById( "hexdump" );
                pre.innerHTML = lines;
            }
        </script>
    </head>
    <body onload="OnLoad()">
        <img id="Apple2eFont7x8" src="Apple2eFont7x8.png">
        <hr>
        <pre id="hexdump"></pre>IncCursorCol
    </body>
    </html>
```
Note: If you get a retarded `Uncaught SecurityError: Failed to execute 'getImageData' on 'CanvasRenderingContext2D': The canvas has been tainted by cross-origin data.` with Chrome you need to start it with the command line:

    --allow-file-access-from-files

Another solution is to use a web browser that isn't "broken" such as Firefox, etc. when trying to read _local_ files.


# DrawChar()

## Font -> Screen Memory Trace

OK, so now that we have the font data how the the heck do we _actually_ draw a character "on screen" ?

We need to transfer 8 consecutive bytes (1 byte / scanline) to 8 different scanlines scattered all over memory.

Assuming we want to draw the `A` glyph at the top-left of the screen we would need to transfer bytes from the (source) font glyph memory locations to these (destination) HGR screen memory locations:

    ($6208) -> $2000
    ($6209) -> $2400
    ($620A) -> $2800
    ($620B) -> $2C00
    ($620C) -> $3000
    ($620D) -> $3400
    ($620E) -> $3800
    ($620F) -> $3C00

For simplicity, we're going to "quantize" our destination Y so that we render font glyphs only on the start of every 8 rows and every 7 pixel columns. (See Table 2 down below in section `Y Cursor Position`).  If we then had the starting address we simply could move to the next scan line by successively adding $0400 to our destination screen pointer.

How did I know to use $0400 when going to the next line?  One quirk of the HGR screen is that every 8 successive scan lines start this many bytes away.  Refer back to the `HGR Memory-mapped IO` table listed above.


## DrawChar() version 1

Before we can start a simple `DrawChar(char c)` function, we also first need to assign some zero page memory locations for our static and temporary variables, our 16-bit address of where want to draw to. Since we also have our font data, we need a symbol for that too.

```assembly
                HgrLo   EQU $E5   ; Low  byte Pointer to screen destination
                HgrHi   EQU $E6   ; High byte Pointer to screen destination
                TmpLo   EQU $F5   ; Low  byte Working pointer to screen byte
                TmpHi   EQU $F6   ; High byte Working pointer to screen byte

                Font    EQU $6000
```

Here's the disassembly of our (hard-coded) DrawChar() program:

Listing Demo 1:

```assembly
    ; FUNC: PrintChar()
    ; NOTES: A, X, Y is destroyed
                    ORG $0900
    0900:       PrintChar
    0900:20 0A 09   JSR HgrToTmpPtr
    0903:A9 00      LDA #00         ; glyph 'c' to draw (not used yet)
    0905:A0 00      LDY #00         ; Y = column to draw at (hard-coded)
    0907:4C 10 03   JMP DrawChar

    ; FUNC: HgrToTmpPtr()
    090A:       HgrToTmpPtr
    090A:A5 E5      LDA HgrLo       ; Copy initial screen
    090C:85 F5      STA TmpLo       ; destination pointer
    090E:A5 E6      LDA HgrHi       ; to working pointer
    0910:85 F6      STA TmpHi
    0912:60         RTS
```

Listing 1:

```assembly
    ; FUNC: DrawChar()
    ; PARAM: A = glyph to draw
    ; PARAM: Y = column to draw at; $0 .. $27 (Columns 0 .. 39) (not modified)
    ; INPUT : $F5,$F6 pointer to the destination screen scanline
    ;         Must start at every 8 scanlines.
    ; OUTPUT: The Y-Register (cursor column) is automatically incremented.
                    ORG $0310
    0310:       DrawChar
    0310:4C 50 03   JMP _DrawChar

                     ORG $0350
    0350:       _DrawChar
    0350:A2 00       LDX #0
    0352:       _LoadFont           ; A = font[ offset ]
    0352:BD 00 62    LDA Font+$200,X
    0355:91 F5       STA (TmpLo),Y  ; screen[col] = A
    0357:18          CLC
    0358:A5 F6       LDA TmpHi
    035A:69 04       ADC #4         ; screen += 0x400
    035C:85 F6       STA TmpHi
    035E:E8          INX
    035F:E0 08       CPX #8
    0361:D0 EF       BNE _LoadFont
    0363:60          RTS

```


Enter in:

```
    900:20 0A 09 A9 00 A0 00 4C 10 03
    90A:A5 E5 85 F5 A5 E6 85 F6 60
    310:4C 50 03
    350:A2 00 BD 00 62 91 F5 18
    358:A5 F6 69 04 85 F6 E8 E0
    360:08 D0 EF 60
```

We're almost ready to run this! We just need to initialize one variable -- where to draw the glyph at:

    E5:00 20
    900G

And with any luck you should see the at sign `@` in the top-left.

![Screenshot 8](pics/hgrfont_08.png?raw=true)

<hr>
**AppleWin** users: You can enter these symbols into the debugger to make the disassembly more readable. Press `F7`, then type in (paste with `Ctrl-V`):

    sym HgrLo       = E5
    sym HgrHi       = E6
    sym TmpLo       = F5
    sym TmpHi       = F6
    sym PrintChar   = 0900
    sym HgrToTmpPtr = 090A
    sym DrawChar    = 0310
    sym _DrawChar   = 0350
    sym _LoadFont   = 0352
    sym Font        = 6000
    350L

When you are done with the debugger, press `F7` to return to the emulator.
<hr>

(Screenshot of debugger forthcoming)

## X Cursor Position

If we wanted to draw in columns 1 and 2 instead of column 0 then we need to set the Y register which controls which "column" we'll draw at.

Enter in:

```
    906:1
    900G
    906:2
    900G
```

![Screenshot 9](pics/hgrfont_09.png?raw=true)

By changing the Y register value we can control the column of where to draw the cursor.  This works because we are using the 6502 Indirect Zero-Page Y addressing mode to store the destination pixels with the `STA` instruction.  Since the Y-register must _always_ be used in this addressing mode -- we (effectively) get a column offset "for free." :-)

```assembly
    0355:91 F5       STA (TmpLo),Y  ; screen[col] = A
```

Here's the C pseudo-code of the assembly code:

```c
    char  c      = '@'; // 0x40;
    int   col    = 0;
    char  FONT[] = { ... };      // our font data glyphs starting at 0x6000
    char *screen = 0x2000 + col; // destination
    char *font   = 0x6200;       // eventually want: &FONT[ c*8 ]
    for( y = 0; y < 8; y++, screen += 0x400 )
       *screen = *font++;
```

### CursorCol( col )

Since the Y-register controls the column we can inline this function and have the caller take care of setting the Y-Register before calling DrawChar().

```assembly
                     LDY #column
```

After drawing a character with `DrawChar()` it is handy if we can advance both:

* the column of the cursor
* the pointer to the screen where the next glyph will be drawn

Notice how after 8 scan lines we end up with and `Tmp` address of $4xxx (or $6xxx if we were drawing to HGR page 2.)  This means we need to subtract off $20 from the top byte of the 16-bit address to the temp destination screen pointer.

Listing 2a:

```assembly
    ; FUNC: IncCursorCol1()
    ; OUTPUT: Y-Register (column) is incremented
    ; Increment the cursor column and move the destination screen pointer back
    ; up 8 scan lines previously to what it was when DrawChar() was called.
    ; Version 1
                    ORG $0364
    0364:       IncCursorCol1
    0364:C8         INY
    0365:18         CLC             ; Note:
    0366:A5 F6      LDA TmpHi       ;     (To the astute reader)
    0368:E9 1F      SBC #$1F        ; <-- ??? Shouldn't this be #$20 ?!
    036A:85 F6      STA TmpHi
    036C:60         RTS
```

### Introduction to Optimization

One tip for beginner 6502 assembly programmers. It is tempting just to always clear the carry flag `CLC` before doing any addition or subtraction.  Unfortunately, for subtraction we'll have an off-by-one bug (fence-post error) so we need to subtract _ONE less then the value._  This makes reading the code a little unintuitive. Is there a way to remedy this? Yes.

| Op | Carry | Opcode |
|----|:-----:|:------:|
| +  | Clear | CLC    |
| -  | Set   | SEC    |

We change the carry flag state _before_ we do the operation depending on whether we are `adding` or `subtracting`.  The `Rule of Thumb` is `CLC before ADD` and `SEC before SUB`.  A mnemonic to help you remember is that both `SEC` and `SUB` start with `S`.

Listing 2b:

```assembly
    ; FUNC: IncCursorCol2()
    ; OUTPUT: Y-Register (column) is incremented
    ; Increment the cursor column and move the destination screen pointer back
    ; up 8 scan lines previously to what it was when DrawChar() was called.
    ; Version 2
                     ORG $0364
    0364:       IncCursorCol2
    0364:C8         INY
    0365:38         SEC             ; CLC SBC #1F
    0366:A5 F6      LDA TmpHi       ; was not obvious that we really
    0368:E9 20      SBC #$20        ; meant: TmpHi = A - #$20 !!
    036A:85 F6      STA TmpHi
    036C:60         RTS
```

One thing when writing 6502 assembly is to pay attention to _all_ optimization opportunities due to the slow ~1 MHz of the 6502.  Since we only need to modify the upper few bits instead of doing a bulky subtraction `SEC SBC` we might be tempted to see if there is a faster and/or smaller alternative.  We just need to be careful that our optimization is "shuffling" the bits around _behaves_ in the _exact_ same way at the end of the day. i.e. "The Right Place at the Right Time."

The problem is we want to see if we can _simply_ the transform of TmpHi after 8 scanlines (basically reset the cursor back to the original scanline before we drew all 8 scanlines of the glyph):

    Tmp   = Hgr + (8 * $0400)

Since we only care about the high byte:

    TmpHi = HgrHi + (8 * $04)
          = HgrHi + $20

    Legend:
        Initial = destination HGR address before we draw the glyph
        TmpHi   = destination HGR address after drawing all 8 lines
        Final   = destination HGR address set back to initial value

|Y  |Initial|TmpHi|Final| (TmpHi and $1F) | (TmpHi and $1F) or $20 |
|--:|:-----:|:---:|:---:|:---:|:---:|
|  0| $2000 | $40 | $20 | $00 | $20 |
|  8| $2080 | $40 | $20 | $00 | $20 |
| 16| $2100 | $41 | $21 | $01 | $21 |
| 24| $2180 | $41 | $21 | $01 | $21 |
| 32| $2200 | $42 | $22 | $02 | $22 |
| 40| $2280 | $42 | $22 | $02 | $22 |
| 48| $2300 | $43 | $23 | $03 | $23 |
| 56| $2380 | $43 | $23 | $03 | $23 |
| 64| $2028 | $40 | $20 | $00 | $20 |
| 72| $20A8 | $40 | $20 | $00 | $20 |
| 80| $2128 | $41 | $21 | $01 | $21 |
| 88| $21A8 | $41 | $21 | $01 | $21 |
| 96| $2228 | $42 | $22 | $02 | $22 |
|104| $22A8 | $42 | $22 | $02 | $22 |
|112| $2328 | $43 | $23 | $03 | $23 |
|120| $23A8 | $43 | $23 | $03 | $23 |
|128| $2050 | $40 | $20 | $00 | $20 |
|136| $20D0 | $40 | $20 | $00 | $20 |
|144| $2150 | $41 | $21 | $01 | $21 |
|152| $21D0 | $41 | $21 | $01 | $21 |
|160| $2250 | $42 | $22 | $02 | $22 |
|168| $22D0 | $42 | $22 | $02 | $22 |
|176| $2350 | $43 | $23 | $03 | $23 |
|184| $23D0 | $43 | $23 | $03 | $23 |

Hmm, we would need to replace `SEC SBC` with `AND OR` which we might think would be a littler faster and takes less code to boot but let's verify our assumption:

Listing 3a:

```assembly
    ; FUNC: IncCursorCol3()
    ; OUTPUT: Y-Register (column) is incremented
    ; Increment the cursor column and move the destination screen pointer back
    ; up 8 scan lines previously to what it was when DrawChar() was called.
    ; Version 3a
                     ORG $0364
    0364:       IncCursorCol3
    0364:C8         INY
    0365:A5 F6      LDA TmpHi
    0367:29 1F      AND #%00011111  ; Requires an extra OR
    0369:09 20      ORA #$20        ; Hard-code to HGR page 1 (high byte)
    036B:85 F6      STA TmpHi
    036D:60         RTS
```


Hmm, so the code _isn't_ any smaller **on a 6502 CPU**.  It _might_ be on _other_ CPUs.

Second, is it any faster?

```assembly
    SEC    ; 2 cycles
    SBC #n ; 2 cycles
```

vs

```assembly
    AND #n ; 2 cycles
    ORA #m ; 2 cycles
```
Nope. Bummer. :-(

We probably should remove that hard-coded HGR page 1.

Listing 3b:

```assembly
    ; FUNC: IncCursorCol3()
    ; OUTPUT: Y-Register (column) is incremented
    ; Increment the cursor column and move the destination screen pointer back
    ; up 8 scan lines previously to what it was when DrawChar() was called.
    ; Version 3b
                     ORG $0364
    0364:       IncCursorCol3
    0364:C8         INY
    0365:A5 F6      LDA TmpHi
    0367:29 1F      AND #%00011111  ; Requires an extra OR
    0369:05 E6      ORA HgrHi       ; user specified Page 1 or Page 2
    036B:85 F6      STA TmpHi
    036D:60         RTS
```

At least the timing for `ORA HgrHi` is still 2 clock cycles. :-)

The lessons?

* Verify our assumptions and Profile!
* Even though we "failed" _this_ time, we shouldn't be afraid to experiment with "out-the-box" thinking using the 6502 instructions; sometimes there are clear "wins" but you won't know unless you try!  Also, it isn't always obvious if we should optimize to minimize space (with the potential to run slower) or to optimize for higher performance (at the cost of more code.) The "proper" solution depends on the context of your needs.    With 8-bit CPU's we tend to focus on `code density` -- cram as much code in as little space as possible.  Graphics / Rendering unfortunately "needs" to run as fast as possible so this means unrolling loops, etc., to run "flat out" even though we lose valuable memory.

We'll briefly touch upon this topic of optimization again with `bit-shifts` and `memcpy()`.

Wait, you say! There IS a way to solve this problem -- and it doesn't take lateral thinking.  What we _really_ are doing is just _restoring_ TmpHi back to its previous value!  We need to **save** TmpHi when we set the `rows to draw` to 0, and **restore** it after drawing 8 rows.

Listing 4a:

```assembly
                TopHi   EQU $FD

                    ORG $034C
    034C:       _DrawChar1
    034C:A6 F6      LDX TmpHi
    034E:86 FD      STX TopHi

                ; === _DrawChar begin ===
                ;   ORG $0350
    ;350:       _DrawChar
    ;...:
    ;35F:E0 08      CPX #8
    ;361:D0 EF      BNE _LoadFont
    ;363:60         RTS
                ; === DrawChar end ===

    ; FUNC: IncCursorCol()
                     ORG $0363      ; intentional extend _DrawChar by
    0363:       IncCursorCol        ; over-writing RTS
    0363:C8          INY
    0364:A6  FD      LDX TopHi      ; Move cursor back to top of scanline
    0366:86  F6      STX TmpHi
    0368:60          RTS
```

We just need to touch up our entry point `PrintChar` at $0310 instead of calling `_DrawChar` ($0350) we need to call our new `_DrawChar1` ($034C):

Listing 4b:

```assembly
                    ORG $0310
    0310:       DrawChar
    0310:4C 4C 03   JMP _DrawChar1  ; NEW entry point
```

Listing Demo 2:

```assembly
                    ORG $0A00
    0A00:       DemoDraw3Char
    0A00:20 00 09   JSR PrintChar
    0A03:20 10 03   JSR DrawChar
    0A06:20 10 03   JMP DrawChar
    0A09:60         RTS
```

Enter in:

```
    310:4C 4C 03
    34C:A6 F6 86 FD
    363:C8 A6 FD 86 F6 60
```

Let's try it out:

    A00: 20 00 09 20 10 03 20 10 03 60
    A00G

We are one step closer to printing a string.  We have a total of 5 `@` because we didn't change our initial column from above.  We are only printing 3 chars, the previous 2 are "left over" from the previous demo.

(Screenshot showing 5 `@` forthcoming)

<hr>
**AppleWin** users: Press `F7`, copy & paste the below, press `F7` when done.

    SYM TopHgr       = FD
    SYM _DrawChar1   = 34C
    SYM IncCursorCol = 363
    34CL
<hr>


## Recap - Listing 5

Here is what our render glyph code looks like so far:

```assembly
        HgrLo   = $E5   ; Low  byte Pointer to screen destination
        HgrHi   = $E6   ; High byte Pointer to screen destination
        TmpLo   = $F5   ; Low  byte Working pointer to screen byte
        TmpHi   = $F6   ; High byte Working pointer to screen byte
        Font    = $6000

        ORG $0310
DrawChar:
        JMP _DrawChar1

        ORG $034C
_DrawChar1
        LDX TmpHi
        STX TopHi
_DrawChar:
        LDX #0
_LoadFont:              ; A = font[ offset ]
        LDA Font+$200,X
        STA (TmpLo),Y   ; screen[col] = A
        CLC
        LDA TmpHi       ;
        ADC #4          ; screen += 0x400
        STA TmpHi
        INX
        CPX #8
        BNE _LoadFont
IncCursorCol:
        INY
        LDX TopHi       ; Move cursor back to top of scanline
        STX TmpHi
        RTS
```


## DrawChar() version 2

The glyph to draw is currently hard-coded to $40 (`@`). The pointer to the start of this glyph is located at:

    source = $6000 + ($40*8) = $6000 + $200 = $6200

If we wanted to draw a different glyph, say `D` we would need to modify the source pointer of the font glyph data.

Recall that our font has this memory layout:

|Char|Index|Address|
|---:|----:|-------|
| ^@ | $00 | $6000 |
| ^A | $01 | $6008 |
| ^B | $02 | $6010 |
| ^C | $03 | $6018 |
|    |   : |     : |
|Spc | $20 | $6100 |
| !  | $21 | $6108 |
|    |   : |     : |
| 0  | $30 | $6180 |
| 1  | $31 | $6188 |
| 2  | $32 | $6190 |
| 3  | $33 | $6198 |
|    |   : |     : |
| ?  | $3F | $61F8 |
| @  | $40 | $6200 |
| A  | $41 | $6208 |
| B  | $42 | $6210 |
| C  | $43 | $6218 |
| D  | $44 | $6220 |
| :  |   : |     : |
| _  | $5F | $62F8 |
| :  |   : |     : |
| ~  | $7E | $63F0 |
|    | $7F | $63F8 |

The 6502 stores and loads 16-bit addresses in `Little-Endian` format so for glyph `D` we need to store the bytes of the address `$6220` in **reverse** order.

Enter in:

    353:20 62

And to draw the new glyph, enter in:

    900G

We should see the third character of `@` change to `D`.

![Screenshot 10](pics/hgrfont_10.png?raw=true)


## DrawChar() version 3

Let's remove the hard-coded printing of the glyph and use the character data we really want to draw.  This means we need to "fix-up" the temporary source pointer to the font glyph data.  Since we have 8 bytes/glyph we need to manually calculate the array offset.

Our array offset for the source glyph data is:

    address = $6000 + (glyph * 8)

Technically the C pseudo-code would be the more elegant:

```c
    char* GetGlyphAddress( char c )
    {
        static char FONT[]  = { ... };      // our font data glyphs starting at 0x6000
        return &FONT[ c * 8 ];
    }
```

A more equivalent 6502 version would be:

```c
    int GetAddress( char c )
    {
               int  offset  = c * 8;
        static char FONT[]  = { ... };      // our font data glyphs starting at 0x6000
               int  address = 0x6000 + offset;

        return address;
    }
```

Since we are dealing with a 16-bit address offset it is simpler to break this down into a low-byte and high-byte calculation for the 6502 since it can't natively do 16-bit offsets. Every 32 characters we need to offset 256 bytes.

```c
    int AddressHi = FontAddressHi + (c / 32)
```

But since the 6502 doesn't have a division instruction we need to use bit-shifts instead. The calculation `c / 32` = `c / 2^5` = `c / (1 << 5)` == `c >> 5`.

```c
    char c        = 'D'; // 0x44

    int Font      = 0x6000;
    int FontHi    = (Font >> 8) & 0xFF;
    int FontLo    = (Font >> 0) & 0xFF;
    int AddressHi = FontHi + ((c >> 5) & 0x07);
    int AddressLo = FontLo + ((c << 3) & 0xF8);
```

We'll assign unique letters to each bit of `c`:

    +-------------------------------+
    | P | Q | R | s | t | u | v | w | glyph to draw
    +-------------------------------+
      7   6   5   4   3   2   1   0   bit position

You see what I did there? :-) I put the low bits in `lowercase` and the High bits in `Uppercase` as a **visual mnemonic** to help remember which bits belong to which part of the address.

IF this is confusing, remember, we are calculating a 16-bit offset:

    Offset =       %PQRstuvw * 8
    Offset =       %PQRstuvw * 2^3;
    Offset =       %PRQstuvw << 3
    Offset = |%00000PQR|stuvw000|
             |High Byte|Low Byte|

A naive 6502 glyph/32 calculation would be to use 5 shift right bit-shifts:

```assembly
    48      PHA     ; save c
    --      --      ; calc low byte offset
    68      PLA     ; pop c  = %PQRstuvw to draw
    4A      LSR     ; c /  2 = %0PQRstuv C=w
    4A      LSR     ; c /  4 = %w0PQRstu C=V
    4A      LSR     ; c /  8 = %wv0PQRst C=u
    4A      LSR     ; c / 16 = %uvw0PQRs C=t
    4A      LSR     ; c / 32 = %tUvw0PQR C=s
    29 07   AND #07 ;        = %00000PQR
    18      CLC     ;
```

We can optimize the `CLC` out by clearing the bottom bits and _then_ doing the shift:

```assembly
    48      PHA     ; save c
    --      --      ; calc low byte offset
    68      PLA     ; pop c  = %PQRstuvw to draw
    29 E0   AND #E0 ;        = %PQR00000 s=0, Optimization: implicit CLC
    4A      LSR     ; c /  2 = %0PQR0000
    4A      LSR     ; c /  4 = %00PQR000
    4A      LSR     ; c /  8 = %000PQR00
    4A      LSR     ; c / 16 = %0000PQR0
    4A      LSR     ; c / 32 = %00000PQR
```

However we can save one instruction (and 2 cycles) if we optimize `c/32` to use the counter-intuitive 6502's `ROL` instruction -- which only requires 4 instructions instead:

```assembly
    48      PHA     ; save c
    --      --      ; calc low byte offset
    68      PLA     ; pop c  = %PQRstuvw to draw
    29 E0   AND #E0 ;        = %PQR00000 s=0, Optimization: implicit CLC
    2A      ROL     ;        = %QR000000 C=P
    2A      ROL     ;        = %R000000P C=Q
    2A      ROL     ;        = %000000PQ C=R
    2A      ROL     ; c / 32 = %00000PQR C=0
```

Our prefix code to setup the source address becomes:

Listing 6:

```assembly
    ; FUNC: _DrawChar2a( c, col )
    ; PARAM: A = glyph to draw
    ; PARAM: Y = column to draw at; $0 .. $27 (Columns 0 .. 39) (not modified)
    ; NOTES: X is destroyed
                    ORG $0335
    0335:       _DrawChar2a
    0335:48         PHA             ; push c = %PQRstuvw to draw
    0336:29 1F      AND #1F         ;        = %000stuvw R=0, implicit CLC
    0338:0A         ASL             ; c * 2    %00stuvw0
    0339:0A         ASL             ; c * 4    %0stuvw00
    033A:0A         ASL             ; c * 8    %stuvw000
    033B:69 00      ADC #<Font      ; += FontLo; Carry = 0 since R=0 from above
    033D:8D 53 03   STA _LoadFont+1 ; AddressLo = FontLo + (c*8)
    0340:68         PLA             ; pop c  = %PQRstuvw to draw
    0341:29 E0      AND #E0         ;        = %PQR00000 s=0, implicit CLC
    0343:2A         ROL             ;        = %QR000000 C=P
    0344:2A         ROL             ;        = %R000000P C=Q
    0345:2A         ROL             ;        = %000000PQ C=R need one more
    0346:2A         ROL             ; c / 32 = %00000PQR C=0 shift to get R
    0347:69 60      ADC #>Font      ; += FontHi; Carry = 0 since S=0 from above
    0349:8D 54 03   STA _LoadFont+2; AddressHi = FontHi + (c/32)
                                    ; intentional fall into _DrawChar1
    034C:       _DrawChar1
    034C:A6 F6      LDX TmpHi
    034E:86 FD      STX TopHi
    0350:       _DrawChar
    0350:A2 00      LDX #0          ; Note: next instruction is self-modified !
    0352:       _LoadFont           ; A = font[ offset ]
    0352:BD 00 60   LDA Font,X
```

Did you catch that **note** ?  One popular trick on the 6502 was `self-modifying code`.  Instead of wasting memory with yet-another-variable we directly change the load/store instructions themselves!  This actually has 2 advantages:

 * It lets us avoid an expensive indirection pointer access, and
 * It is the fastest way to load/store/copy an array. The 6502 addressing mode is `LDA address,X` or `LDA address,Y`.

However, there are still 2 more optimizations we can make:

1\. If we assume our font is "page aligned", that is, starts at a multiple of 256 bytes -- we could remove the redundant `AddressLo += (FontLo + (c*8))` and replace with the direct `AddressLo = (c*8)`.  Technically, in C you would keep only the bottom 8-bits by masking off the the other bits with `& 0xFF` but since the 6502 registers are only 8-bit and we're storing a byte the `& 0xFF` is not needed.

2\. It is "funny" how we end up shifting and rotating in the **same** direction: Left! :-)  It would nice to _leverage this work_ for both the high and low byte address calculation. That is the technical term for _"Don't do dumb (redundant) work"_ or in the immortal words of Back to the Future: "Think, McFly!" :-)

    Given: glyph c     in binary is:    %PQRstuvw
    The 16-bit address in binary is: %PQRstuvw000

| Glyph     | AddressHi | AddressLo |
|:---------:|:---------:|:---------:|
| %PQRstuvw | 00000PQR  | stuvw000  |

Our original glyph fits in one byte.  Our 16-bit address offset technically _could also_ fit in one byte -- we just have shifted it over 3 bits with zeroes.

If we had the byte: `stuvwPQR` that would be _extremely_ convenient as it would be trivial to calculate the offset:

```assembly
    PHA             ; push A = %stuvwPQR% initial glyph
    AND #$F8        ;      A = %stuvw000
    STA AddressLo
    PLA             ; pop  A = %stuvwPQR
    AND #$07        ;      A = %00000PQR
    CLC
    ADC FontHi
    STA AddressHi
```

Let us trace the rotate left `ROL` instruction 4 times paying attention to the Carry register `C` and the Accumulator `A`:

    ROL   C   A         Comment
    0     ?   PQRstuvw  initial glyph to draw
    1     P   QRstuvw?
    2     Q   Rstuvw?P
    3     R   stuvw?PQ

Hmm, that is _suspiciously_ close to what we want! Is there any way we can end up with?

    ROL   C   A
    ?     ?   stuvwPQR

We would need to start with the carry **pre-loaded** with P, and the QR already shifted over one to the left:

    ROL   C   A
    n-3   P   QR?stuvw
    n-2   Q   R?stuvwP
    n-1   R   ?stuvwPQ
    n     ?   stuvwPQR

That might look like something like this:

```assembly
    PHA             ; push c
    AND #$E0
    TAX             ; save %PQR00000
    PLA             ; pop c
    AND #$1F
    STA Temp        ; save %000stuvw
    TXA             ;     A=PQR00000
    ASL             ; C=P A=QR000000  <- stuvw not taking advantage of shift :-(
    OR  Temp        ; C=P A=QR0stuvw
    ROL             ; C=Q A=R0stuvwP
    ROL             ; C=R A=0stuvwPQ
    ROL             ; C=0 A=stuvwPQR
    PHA
    AND #$1F        ;        = %000stuvw
    STA _LoadFont+1 ; AddressLo = FontLo + (c*8)
    PLA
    AND #$E0
    CLC
    ADC AddressHi
    STA _LoadFont+2
```

Hmm, that seems like an **awful lot of work** just for some `bit-shuffling`!! For one thing we're doing a shift and `stuvw` is not taking advantage of it.  Can we not we make use of the fact that we will eventually be doing `AND #1F` -> `AND #$F8` and `AND #E0` -> `AND #07` ?

The **lateral** thinking is to _use partial results_.

    ROL   C   A = glyph to draw
    0     ?   PQRstuvw
    1     P   QRstuvw?
    2     Q   Rstuvw?P
    3     R   stuvw?PQ
              --push A--
              stuvw000 <- A & #$F8
              --store low byte offset--
              --pop A--
    4     s   tuvw?PQR
              00000PQR <- A & #$07
              --CLC--
              --add FontHi to A
              --store high byte offset--

Let's code this up:

Listing 7:

```Assembly
    ; FUNC: DrawCharCol( c, col ) alias _DrawChar2
    ; PARAM: A = glyph to draw
    ; PARAM: Y = column to draw at; $0 .. $27 (Columns 0 .. 39) (not modified)
    ; NOTES: X is destroyed
                    ORG $033A
    033A:       DrawCharCol         ;     A=%PQRstuvw
    033A:2A         ROL             ; C=P A=%QRstuvw?
    033B:2A         ROL             ; C=Q A=%Rstuvw?P
    033C:2A         ROL             ; C=R A=%stuvw?PQ
    033D:AA         TAX             ;     X=%stuvw?PQ push glyph
    033E:29 F8      AND #$F8        ;     A=%stuvw000
    0340:8D 53 03   STA _LoadFont+1 ; AddressLo = (c*8)
    0343:8A         TXA             ;     A=%stuvw?PQ pop glyph
    0344:29 03      AND #3          ; Optimization: s=0 implicit CLC !
    0346:2A         ROL             ; C=s A=%00000PQR and 1 last ROL to get R
    0347:69 60      ADC #>Font      ; += FontHi; Carry=0 since s=0 from above
    0349:8D 54 03   STA _LoadFont+2 ; AddressHi = FontHi + (c/32)
                                    ; intentional fall into _DrawChar1 @ $034C
```

Since we'll re-use our existing font drawing code `_DrawChar1` at $034C it is always a good idea to document why there is no `RTS` at the end.

Here is a comparison between the original and final version (clock cycle timings are the #'s):

    ORG $0335 (old)   ORG $033A (new)
    3 PHA             2 ROL              
    2 AND #1F         2 ROL              
    2 ASL             2 ROL              
    2 ASL             2 TAX              
    2 ASL             2 AND #$F8         
    2 ADC #<Font      4 STA _LoadFont+1  
    4 STA _LoadFont+1 2 TXA              
    4 PLA             2 AND #3           
    2 AND #E0         2 ROL              
    2 ROL             3 ADC #>Font       
    2 ROL             4 STA _LoadFont+2  
    2 ROL             -
    2 ROL             -
    2 ADC #>Font      -
    4 STA _LoadFont+2 -

           Original   Final
    Bytes  23         18
    Cycles 37         27

Much better!!!

We need to (again) touch up our `DrawChar` entry point at $0310 calling `_DrawChar2` ($034C) to call `DrawCharCol` ($033A):

```assembly
    310:4C 3A 03         JMP DrawCharCol
```

Enter in:

```
    B00:20 0A 09 A9 00 A0 00 4C 10 03
    310:4C 3A 03
    33A:2A 2A 2A AA 29 F8
    340:8D 53 03 8A 29 03 2A 69
    348:60 8D 54 03
    B00G
```

We should now see an closed apple glyph!

![Screenshot 11](pics/hgrfont_11.png?raw=true)

To change which glyph is printed:

    B04:41
    B00G

And we should see an `A` printed.

![Screenshot 12](pics/hgrfont_12.png?raw=true)

We now have the ability to print any of the 128 ASCII characters!


<hr>
**AppleWin** users: Press `F7`, copy & paste the below, press `F7` when done.

    SYM DrawCharCol = 33A
    33AL

(Debugger screenshot forthcoming.)

<hr>


## Character Inspector

Let's verify this by writing a character inspector. We'll use the arrow keys to select the glyph and ESC to exit.

Listing Demo 3a:

```assembly
    ; FUNC: DemoCharInspect()
                KEYBOARD    EQU $C000
                KEYSTROBE   EQU $C010

                HgrLo       EQU $F5
                HgrHi       EQU $F6
                glyph       EQU $FE
                DrawChar    EQU $310

                     ORG $1000
    1000:         DemoCharInspect
    1000:A9 00       LDA #0         ; glyph=0
    1002:85 FE       STA glyph      ; save which glyph to draw
    1004:A9 00    .1 LDA #0         ; screen = 0x2000
    1006:85 F5       STA HgrLo      ;
    1008:A9 20       LDA #$20       ; HGR Page 1
    100A:85 F6       STA HgrHi      ;
    100C:A5 FE       LDA glyph      ; A = glyph
    100E:A0 00       LDY #00        ; Y = col
    1010:20 10 03    JSR DrawChar
    1013:AD 00 C0 .2 LDA KEYBOARD   ; read A=key
    1016:10 FB       BPL .2         ; no key?
    1018:8D 10 C0    STA KEYSTROBE  ; debounce key
    101B:C9 88       CMP #$88       ; key == <-- ? CTRL-H
    101D:D0 0A       BNE .4         ;
    101F:C6 FE       DEC glyph      ; yes, --glyph
    1021:A5 FE    .3 LDA glyph      ; glyph &= 0x7F
    1023:29 7F       AND #$7F       ;
    1025:85 FE       STA glyph      ;
    1027:10 DB       BPL .1         ; always branch, draw prev char
    1029:C9 95    .4 CMP #$95       ; key == --> ? CTRL-U
    102B:D0 05       BNE .5         ;
    102D:E6 FE       INC glyph      ; yes, ++glyph
    102F:18          CLC            ; always branch
    1030:90 EF       BCC .3         ;   draw prev char
    1032:C9 9B    .5 CMP #$9B       ; key == ESC ?
    1034:D0 DD       BNE .2         ;
    1036:60          RTS            ; yes, exit
```

Enter in this code:

    1000:A9 00 85 FE A9 00 85 F5
    1008:A9 20 85 F6 A5 FE A0 00
    1010:20 10 03 AD 00 C0 10 FB
    1018:8D 10 C0 C9 88 D0 0A C6
    1020:FE A5 FE 29 7F 85 FE 10
    1028:DB C9 95 D0 05 E6 FE 18
    1030:90 EF C9 9B D0 DD 60
    1000G

We now have an ASCII char inspector!

![Screenshot 13](pics/hgrfont_13.png?raw=true)



<hr>
**AppleWin** users: Press `F7`, copy & paste the below, press `F7` when done.

    SYM glyph = FE
    1000L

<hr>


## Character Inspector version 2

Let's fix it up to print the hex value of the current character we are inspecting:

Listing Demo 3b:

```assembly
                    ORG $1010
    1010:20 3C 10   JSR Patch1

                    ORG $1037
    103C:       Patch1
    103C:48         PHA             ; save c
    103D:20 10 03   JSR DrawChar
    1040:68         PLA             ; restore c so we can print it in hex
    1041:4C 01 03   JMP DrawHexByte
```

Listing 8:

```assembly
                    ORG $0301
    ; FUNC: DrawHexByte( c ) = $0301
    ; PARAM: A = byte to print in hex
    0301:       DrawHexByte
    0301:48         PHA             ; save low nibble
    0302:6A         ROR             ; shift high nibble
    0303:6A         ROR             ; to low nibble
    0304:6A         ROR             ;
    0305:6A         ROR             ;
    0306:20 0A 03   JSR DrawHexNib  ; print high nib in hex
    0309:68         PLA             ; pritn low  nib in hex

    ; FUNC: DrawHexNib() = $030C
    ; PARAM: A = nibble to print as hex char
    030A:       DrawHexNib
    030A:29 0F      AND #$F         ; base 16
    030C:AA         TAX             ;
    030D:BD 90 03   LDA NIB2HEX,X   ; nibble to ASCII
                                    ; intentional fall into PrintChar
                    ORG $0390
    0390:30 31 32 33    NIB2HEX ASC "0123456789ABCDEF"
    0394:34 35 36 37
    0398:38 39 41 42
    039C:43 44 45 46
```

Q. Can you see a way to make DrawHexNib smaller?


Enter in the changes:

    1010:20 3C 10
    103C:48 20 10 03 68 4C 01 03
    0301:   48 6A 6A 6A 6A 20 0A
    0308:03 68 29 0F AA BD 90 03
    0390:30 31 32 33 34 35 36 37
    0398:38 39 41 42 43 44 45 46
    1000G

And now we have our own DrawHexByte() function.

![Screenshot 14](pics/hgrfont_14.png?raw=true)

<hr>
**AppleWin** users: Press `F7`, copy & paste the below, press `F7` when done.

    sym patch1      = 103C
    sym DrawHexByte = 0301
    sym DrawHexNib  = 030A
    db  Nib2HeX     390:39F
    1000L

<hr>


## Character Inspector version 3

Let's add a space after the character but before the hex value to improve readability of the output.  The new code is:

Listing Demo 3c:

```assembly
                    ORG $1010
    1010:20 37 10   JSR Patch2

                    ORG $1037
    1037:       Patch2
    1037:48         PHA             ; save c
    1038:20 10 03   JSR PrintChar
    103B:A9 20      LDA #' '        ; Draw whitespace
```

Enter in these changes:

    1010:20 37 10
    1037:48 20 10 03 A9 20
    1000G

![Screenshot 15](pics/hgrfont_15.png?raw=true)

Our final version is:

    1000:A9 00 85 FE A9 00 85 F5
    1008:A9 20 85 F6 A5 FE A0 00
    1010:20 37 10 AD 00 C0 10 FB
    1018:8D 10 C0 C9 88 D0 0A C6
    1020:FE A5 FE 29 7F 85 FE 10
    1028:DB C9 95 D0 05 E6 FE 18
    1030:90 EF C9 9B D0 DD 60 48
    1038:20 10 03 A9 20 20 10 03
    1040:68 4C 01 03
    1000G

(To save this, `BSAVE CHAR_INSPECT3.BIN,A$1000,L$68`)

<hr>
**AppleWin** users: Press `F7`, copy & paste the below, press `F7` when done.

    sym patch2      = 1037

<hr>

## Y Cursor Position

Right now the line we "print" to is hard-coded since we are using a screen address of $2000 with the pointer at $E5, $E6.

We're going to digress slightly before we fix this.

The secret to getting high speed graphics rendering on the Apple is to use a look-up table.  We're going to have a 16-bit address lookup table for Y=0, Y=8, Y=16, .. Y = 184

The HGR screen address is broken up a triad. Every 64 scan lines the offset change by $28.

<hr>

    Table 2: HGR Y Address for every 8 scanlines

|  Y|Address|Hi |Lo |
|---:|------|---|---|
|  0| $2000 |$20|$00|
|  8| $2080 |$20|$80|
| 16| $2100 |$21|$00|
| 24| $2180 |$21|$80|
| 32| $2200 |$22|$00|
| 40| $2280 |$22|$80|
| 48| $2300 |$23|$00|
| 56| $2380 |$23|$80|
| - | ----- | - | - |
| 64| $2028 |$20|$28|
| 72| $20A8 |$20|$A8|
| 80| $2128 |$21|$28|
| 88| $21A8 |$21|$A8|
| 96| $2228 |$22|$28|
|104| $22A8 |$22|$A8|
|112| $2328 |$23|$28|
|120| $23A8 |$23|$A8|
| - | ----- | - | - |
|128| $2050 |$20|$50|
|136| $20D0 |$20|$D0|
|144| $2150 |$21|$50|
|152| $21D0 |$21|$D0|
|160| $2250 |$22|$50|
|168| $22D0 |$22|$D0|
|176| $2350 |$23|$50|
|184| $23D0 |$23|$D0|

<hr>

We'll split this table of 16-bit addresses into Low and High bytes for easier access. We'll also subtract off the hard-coded graphics page 1 high byte = $20 and instead use relative offsets to make it work with either graphics page 1 or 2.

This is our mini HGR Y Address look-up table. "Funny" that it has 24 entries -- the same height as our text screen. :-)

Listing 9a:

```assembly
                    ORG $03A0
                HgrLoY
                    DFB $00,$80,$00,$80,$00,$80,$00,$80
                    DFB $28,$A8,$28,$A8,$28,$A8,$28,$A8
                    DFB $50,$D0,$50,$D0,$50,$D0,$50,$D0
                HgrHiY
                    DFB $00,$00,$01,$01,$02,$02,$03,$03
                    DFB $00,$00,$01,$01,$02,$02,$03,$03
                    DFB $00,$00,$01,$01,$02,$02,$03,$03
```

Enter these bytes (or download [hgrtable.bin](hgrtable.bin) and `BLOAD HGRTABLE.BIN,3A0`):

Our `HgrLoY` table:

    03A0:00 80 00 80 00 80 00 80
    03A8:28 A8 28 A8 28 A8 28 A8
    03B0:50 D0 50 D0 50 D0 50 D0

Our `HgrHiY` table:

    03B8:00 00 01 01 02 02 03 03
    03C0:00 00 01 01 02 02 03 03
    03C8:00 00 01 01 02 02 03 03

<hr>
**AppleWin** users: To save this press `F7`, at the debugger console `bsave "hgrtable.bin",3A0:3CF`, press `F7`.

    DB HgrLoY 3A0:3B7
    DB HgrHiY 3B8:3CF

<hr>

To select which row to draw at we'll pass that in the X register to our DrawCharColRow() routine:

Listing 9b:

```assembly
    ; FUNC: SetCursorRow( row )
    ; PARAM: X = row    to draw at; $0 .. $17 (Rows 0 .. 23) (not modified)
    ; INPUT : $E5,$E6 initial pointer to the destination screen scanline
    ;         Note: Must start at every 8 scanlines.
    ; OUTPUT: $F5,$F5 working pointer to the destination screen scanline
                    ORG $0313
    0313:       SetCursorRow
    0313:BD A0 03   LDA HgrLoY,X    ; HgrLoY[ row ]
    0316:85 F5      STA TmpLo
    0318:BD B8 03   LDA HgrHiY,X    ; HgrHiY[ row ]
    031B:18         CLC
    031C:65 E6      ADC HgrHi
    031E:85 F6      STA TmpHi
    0320:60         RTS

    ; FUNC: DrawCharColRow()
    ; PARAM: A = glyph to draw
    ; PARAM: Y = column to draw at  ; $0 .. $27 (Columns 0 .. 39) (not modified)
    ; PARAM: X = row    to draw at  ; $0 .. $17 (Rows 0 .. 23) (destroyed)
                    ORG $0335
    0335:       DrawCharColRow
    0335:48         PHA
    0336:20 13 03   JSR SetCursorRow
    0339:68         PLA
                                    ; intentional fall into _DrawChar2
```

Enter in:

    313:         BD A0 03 85 F5
    318:BD B8 03 18 65 E6 85 F6
    320:60
    335:48 20 13 03 68

Now we can print a char at any location:

Listing Demo 4:

```assembly
                    ORG $1100
    1100:       PrintAYX
    1100:A9 41      LDA #41         ; A-register = char
    1102:A0 01      LDY #1          ; Y-register = col 1 (2nd column)
    1104:A2 02      LDX #2          ; X-register = row 2 (3rd row)
    1106:4C 35 03   JMP DrawCharColRow
```

Enter in:

    1100:A9 41 A0 01 A2 02 4C 35 03
    1100G

![Screenshot 16](pics/hgrfont_16.png?raw=true)


<hr>
**AppleWin** users: You know the drill ...

    SYM SetCursorRow   = 313
    SYM DrawCharColRow = 335

<hr>


## Natural Params SetCursorColRow()

Unfortunately, our usage of the X and Y registers are not intuitive. This is due to the limited addressing modes of the 6502. :-/ If the 6502 had a symmetrical indirect zero-page X addressing mode:

     LDA ($ZP),X

We could map the X-register to the natural column (x-axis), and the Y-register to the natural row (y-axis).  Alas, we're stuck with the X=row and Y=col unless we wanted to add extra code to "swap" the two.

Listing 10:

```assembly
    ; FUNC: SetCursorColRowYX()
    ; PARAM: Y = col
    ; PARAM: X = row
                    ORG $0369
    369:        SetCursorColRowYX
    369:20 13 03    JSR SetCursorRow
    36C:18          CLC
    36D:98          TYA
    36E:65 F5       ADC TmpLo
    371:85 F5       STA TmpLo
    373:60          RTS
```
Or are we stuck? Since we're using a function to calculate the destination address let's fix the order.

We'll need to change the `X` offset in SetCursorColRowXY() to `Y`;

```assembly
    ; FUNC: SetCursorColRow2a( row )
    ; PARAM: Y = row
    ; NOTES: Version 2a !
    0928:           ORG $0928
    0928:       SetCursorColRow2a
    0928:B9 A0 03   LDA HgrLoY,Y    ; changed from: ,X
    092B:18         CLC
    092C:65 E5      ADC HgrLo
    092E:85 F5      STA TmpLo
    0930:B9 B8 03   LDA HgrHiY,Y    ; changed from: ,X
    0933:18         CLC
    0934:65 E6      ADC HgrHi
    0936:85 F6      STA TmpHi
    0938:60         RTS
```

And change the low byte to add `X` instead:

```assembly
    ; FUNC: SetCursorColRow2b( col, row ) = $0379
    ; PARAM: X = col
    ; PARAM: Y = row
    ; NOTES: Version 2b !
                    ORG $0979
    979:        SetCursorColRow2b
    979:20 13 03    JSR SetCursorRow
    97C:18          CLC
    37D:88          TXA             ; changed from: TYA
    97E:65 F5       ADC TmpLo
    981:85 F5       STA TmpLo
    983:60
```

This is a little clunky but it is progress. Let's write the new SetCursorColRow() version with the SetCursorRow() inlined so we don't have to use a JSR.

Listing 11:

```assembly
    ; FUNC: SetCursorColRow( col, row )
    ; PARAM: X = column to draw at; $0 .. $27 (Columns 0 .. 39) (not modified)
    ; PARAM: Y = row    to draw at; $0 .. $17 (Rows 0 .. 23) (not modified)
    ; NOTES: Version 3! X and Y is swapped from earlier version!
    ; [$F5] = HgrLoY[ Y ] + ScreenLo + X
                    ORG $0321
    0321:       SetCursorColRow
    0321:86 F5      STX TmpLo
    0323:B9 A0 03   LDA HgrLoY,Y    ; HgrLoY[ row ]
    0326:18         CLC
    0327:65 F5      ADC TmpLo       ; add column
    0329:85 F5      STA TmpLo
    032B:B9 B8 03   LDA HgrHiY,Y    ; HgrHiY[ row ]
    032E:18         CLC             ; \ could optimize this into
    032F:65 E6      ADC HgrHi       ; / single ORA HgrHi
    0331:85 F6      STA TmpHi
    0333:60         RTS
    0334:EA         NOP             ; pad
```

Enter in:

```
    321:   86 F5 B9 A0 03 18 65
    328:F5 85 F5 B9 B8 03 18 65
    330:E6 85 F6 60
```

# DrawString()

Now that we have the basic print char working lets extend it to print a C-style string (one that is zero terminated.)

Listing 12:

```assembly
                String  EQU $F0

    ; FUNC: DrawString( *text )
    ; PARAM: X = High byte of string address
    ; PARAM: Y = Low  byte of string address
    037E:           ORG $037E
    037E:       DrawString
    037E:84 F0      STY String+0
    0380:86 F1      STX String+1
    0382:A0 00      LDY #0
    0384:B1 F0  .1  LDA (String),Y
    0386:F0 07      BEQ .2         ; null byte? Done
    0388:20 10 03   JSR DrawChar   ; or DrawCharCol for speed
    038B:C0 28      CPY #40        ; col < 40?
    038D:90 F5      BCC .1
    038F:60     .2  RTS
```
<hr>
**AppleWin**:

    SYM SetCursorColRow = 321
    SYM DrawString      = 37E
    ASC Msg 120E:1219
<hr>


And our example to verify that it works:

Listing Demo 5:

```assembly
    ; FUNC: DemoDrawString()
                      ORG $1200
    1200:          DemoDrawString
    1200:A2 03        LDX #3        ; col = 3
    1202:A0 02        LDY #2        ; row = 2
    1204:20 21 03     JSR SetCursorColRow
    1207:A2 12        LDX #>Tx      ; High
    1209:A0 0E        LDY #<Tx      ; Low
    120B:4C 7E 03     JMP DrawString
    120E:          Tx ASC "Hello World",0
    120E:48 65 6C 6C 6F 20 57 6F 72 6C 64 00
```

Enter in:

```
    37E:84 F0 86 F1 A0 00 B1 F0
    386:F0 07 20 10 03 C0 28 90 F5 60

    1200:A2 03 A0 02 20 21 03
    1207:A2 12 A0 0E 4C 7E 03
    120E:48 65 6C 6C 6F 20 57 6F 72 6C 64 00
    1200G
```

![Screenshot 17](pics/hgrfont_17.png?raw=true)

Note: An easy way to get the hex bytes for a string is to use this tiny JavaScript snippet to convert a text string to hex:

```JavaScript
    var txt = "Hello World";
    for( var i=0; i < txt.length; ++i )
        console.log( txt.charCodeAt(i).toString(16) );
```


# Copy text screen to HGR

For our final trick we are going to copy the characters off the text screen onto the HGR screen.  More magic?  Nah, just bit-shuffling.

The text screen, like the HGR screen, is also non-linear, and also broken up into a triad:

|Row|Text Address|HGR Address|HGR Scanline|
|--:|:----:|:-----:|---:|
| 0 | $400 | $2000 |   0|
| 1 | $480 | $2080 |   8|
| 2 | $500 | $2100 |  16|
| 3 | $580 | $2180 |  24|
| 4 | $600 | $2200 |  32|
| 5 | $600 | $2280 |  40|
| 6 | $700 | $2300 |  48|
| 7 | $780 | $2380 |  56|
| - | ---- | ----- | ---|
| 8 | $428 | $2028 |  64|
| 9 | $4A8 | $20A8 |  72|
|10 | $528 | $2128 |  80|
|11 | $5A8 | $21A8 |  88|
|12 | $628 | $2228 |  96|
|13 | $6A8 | $22A8 | 104|
|14 | $728 | $2328 | 112|
|15 | $7A8 | $23A8 | 120|
| - | ---- | ----- | ---|
|16 | $450 | $2050 | 128|
|17 | $4D0 | $20D0 | 136|
|18 | $550 | $2150 | 144|
|19 | $5D0 | $21D0 | 152|
|20 | $650 | $2250 | 160|
|21 | $6D0 | $22D0 | 168|
|22 | $750 | $2350 | 176|
|23 | $7D0 | $23D0 | 184|


While the Apple's memory layout seems esoteric it has beautiful symmetry. For any given text row notice that:

* the low  byte of the text address is the same low byte of the corresponding HGR scanline address, and
* the high byte of the text address is 0x1C less then the high byte of the HGR address

Technically, to convert the HGR high byte address to a Text high byte address, we only need to map these 4 high bytes:

| HGR High Byte    | Text High Byte  |
|:----------------:|:---------------:|
| $20 = %0010_0000 | $4 = %0000_0100 |
| $21 = %0010_0001 | $5 = %0000_0101 |
| $22 = %0010_0010 | $6 = %0000_0110 |
| $23 = %0010_0011 | $7 = %0000_0111 |

Which we could do via:

```assembly
    LDA HgrHiY, Y     ; Y is row
    AND #7            ; strip off top 6 bits
    OR  #4            ; Set text page 1 = $0400
````

But we'll save a byte and use the normal subtraction instead:

```assembly
    LDA HgrHiY, Y     ; Y is row
    SEC               ; Convert HgrHiY to TextHiY byte
    SBC #$1C          ; A -= 0x1C
```

If we care about absolute speed we could see which one takes the fewer clock cycles.

There is also the reverse conversion -- to convert a Text address to a HGR address which could be done with the same `AND #3, OR #20`, but since we don't have a Text Y table address and already have a HGR 16-bit address table we will  re-use that.

Here's the Pseudo-code to copy the text screen to the HGR Screen:

```c
    for( row = 0; row < 24; row++ )
    {
       SrcTextLo = HgrLoY[ row ];
       SrcTextHi = HgrHiY[ row ] - 0x1C;
    // SetCursorColRow( 0, row ) which does:
       DstHgrLo  = HgrLoY[ row ]
       DstHgrHi  = HgrHiY[ row ]

       for( col = 0; col < 40; col++ )
       {
           c = SrcText[ col ]
           DrawCharCol( c );
       }
    }
```

And here is the assembly:

```assembly
    ; FUNC: CopyTextToHGR()
    ; DATA:
    ;    $6000.$63FF  Font 7x8 Data
    ;    $6400.$642F  HgrLoY, HgrHiY table for every 8 scanlines
    1300:       Txt  EQU $F7
                     ORG $1300
    1300:       CopyTextToHgr
    1300:A9 00       LDA #0
    1302:85 F3       STA row
    1304:85 E5       STA $E5
    1306:A9 20       LDA #$20       ; Dest = HGR1 = $2000
    1308:85 E6       STA $E6
    130A:A4 F3    .1 LDY row        ; Y = row
    130C:C0 18       CPY #24        ; 24 rows is #$18
    130E:B0 22       BCS .3         ; Y >= 24
    1310:A2 00       LDX #0
    1312:86 F2       STX col        ; X = col
    1314:20 21 03    JSR SetCursorColRow
    1317:38          SEC            ; A = HgrHiY[ row ]
    1318:E9 1C       SBC #$1C       ; Convert HgrHiY to TextHiY byte
    131A:85 F8       STA Txt+1      ; A -= 0x1C -> TxtHi
    131C:B9 A0 03    LDA HgrLoY, Y  ; A = HgrLoY[ row ]
    131F:85 F7       STA Txt+0      ;           -> TxtLo
    1321:A4 F2       LDY col
    1323:B1 F7    .2 LDA (Txt),Y
    1325:29 7F       AND #$7F
    1327:20 3A 03    JSR DrawCharCol
    132A:C0 28       CPY #$28       ; 40 cols is #$28
    132C:90 F5       BCC .2         ; Y < 40
    132E:E6 F3       INC row
    1330:D0 D8       BNE .1         ; always
    1332:60       .3 RTS
```

<hr>
**AppleWin** users:

    sym Txt2Hgr.1 = 130A
    sym Txt2Hgr.2 = 1323
    sym Txt2Hgr.3 = 1332

<hr>

Enter in:

```
    1300:A9 00 85 F3 85 E5 A9 20
    1308:85 E6 A4 F3 C0 18 B0 22
    1310:A2 00 86 F2 20 21 03 38
    1318:E9 1C 85 F8 B9 A0 03 85
    1320:F7 A4 F2 B1 F7 29 7F 20
    1328:3A 03 C0 28 90 F5 E6 F3
    1330:D0 D8 60
```

And now for the moment of truth! Don't worry if you can't see what you are typing.

```
    FC58G
    1300L
    1300G
```

Voila!

![Screenshot 18](pics/hgrfont_18.png?raw=true)

In case you were wondering why I turned 50% scanlines `on` this is how the HGR screen would normally look like in color:

![Screenshot 19](pics/hgrfont_19.png?raw=true)

That's why I turned 50% scanlines on, for better readability:

![Screenshot 20](pics/hgrfont_20.png?raw=true)



Using one of the newer emulators with NTSC emulation, unfortunately, doesn't help with readability: :-/

* NTSC Alpha (with tweaked Palette):

![Screenshot Tweaked](pics/ntsc_tweaked.png?raw=true)

 * NTSC Sheldon -- which unfortunately has WAY too much ghosting: :-(

![Screenshot Sheldon](pics/ntsc_sheldon.png?raw=true)


And just to prove that it copied the bottom 4 text rows as well:

```
    C052
```

And to restore the bottom 4 text rows

```
    C053
```

# Exercises

## Exercise 1: ScrollHgrUpPixel()

Hey!  Homework?  Yes, the only (true) way to demonstrate you understand the theory is with implementation:

    Write a function to "scroll" the HGR screen up:

    * one "text line" (8 pixels), and

    Hint: This is basically a gloried and specialized `memcpy()`.


## Exercise 2: ScrollHgrUpLine()

    Write a function to "scroll" the HGR screen up:

    * one scan line (1 pixel)

    Hint: For scrolling up one pixel we can spot the pattern if we inspect
    the memory flow of how pixels get shuffled around:

        40 bytes from $2400.$2427 -> $2000.$2027
        40 bytes from $2800.$2827 -> $2400.$2427
        etc

    Don't forget that you only need to copy 191 rows, not 192, since the
    very bottom scanline should be "blank."


# Recap - Font Draw

Here are all the (core) font rendering routines we've entered in so far:

```
    0301:   48 6A 6A 6A 6A 20 0A
    0308:03 68 29 0F AA BD 90 03
    0310:4C 3A 03 BD A0 03 85 F5
    0318:BD B8 03 18 65 E6 85 F6
    0320:60 86 F5 B9 A0 03 18 65
    0328:F5 85 F5 B9 B8 03 18 65
    0330:E6 85 F6 60 EA 48 20 13
    0338:03 68 2A 2A 2A AA 29 F8
    0340:8D 53 03 8A 29 03 2A 69
    0348:60 8D 54 03 A6 F6 86 FD
    0350:A2 00 BD 00 60 91 F5 18
    0358:A5 F6 69 04 85 F6 E8 E0
    0360:08 D0 EF C8 A6 FD 86 F6
    0368:60
    0390:30 31 32 33 34 35 36 37
    0398:38 39 41 42 43 44 45 46
    03A0:00 80 00 80 00 80 00 80
    03A8:28 A8 28 A8 28 A8 28 A8
    03B0:50 D0 50 D0 50 D0 50 D0
    03B8:00 00 01 01 02 02 03 03
    03C0:00 00 01 01 02 02 03 03
    03C8:00 00 01 01 02 02 03 03
```

(To save this: `BSAVE FONTDRAW.BIN,A$300,L$D0`)

# Other Fonts

## Fat Stroke Fonts

[Moon Patrol](https://www.google.com/search?q=apple+2+moon+patrol&tbm=isch) on the Apple 2 used what I call a "Fat Stroke Font" -- each stroke was 2 pixels when possible.

A similiar font is the one used by "Beautiful Boot".  Let's examine it.


### Beautiful Boot

Here is the font used by Beautiful Boot

```
    6100:80 80 80 80 80 80 80 80  ' 
    6108:80 87 87 80 87 87 87 87  '!
    6110:80 80 80 80 B6 A4 B6 B6  '"
    6118:80 9E 9E BF 9E BF 9E 9E  '#
    6120:80 8C 9F B0 9E 83 BE 8C  '$
    6128:80 BB BB 86 8C 98 B7 B7  '%
    6130:80 AE 9B BB 8F 86 8F 86  '&
    6138:80 80 80 80 8C 88 8C 8C  ''
    6140:80 B8 9C 8E 8E 8E 9C B8  '(
    6148:80 87 8E 9C 9C 9C 8E 87  ')
    6150:80 88 AA 9C BE 9C AA 88  '*
    6158:80 80 8C 8C BF BF 8C 8C  '+
    6160:83 86 87 80 80 80 80 80  ',
    6168:80 80 80 80 BE BE 80 80  '-
    6170:80 87 87 80 80 80 80 80  '.
    6178:80 83 87 8E 9C B8 F0 E0  '/
    6180:80 9E B3 B3 B3 B3 B3 9E  '0
    6188:80 BF 8C 8C 8C 8F 8E 8C  '1
    6190:80 BF BF 8E B8 B3 BF 9E  '2
    6198:80 9E BF B0 BE B0 BF 9E  '3
    61A0:80 B0 B0 BF B3 B6 BC B8  '4
    61A8:80 9E BF B0 9F 83 BF BF  '5
    61B0:80 9E BF B3 9F 83 BF 9E  '6
    61B8:80 8C 8C 8C 98 B0 BF BF  '7
    61C0:80 9E BF B3 9E B3 BF 9E  '8
    61C8:80 9E BF B0 BE B3 BF 9E  '9
    61D0:80 9C 9C 80 80 9C 9C 80  ':
    61D8:83 86 87 80 80 87 87 80  ';
    61E0:80 F0 B8 9C 8E 9C B8 F0  '<
    61E8:80 80 80 BE 80 BE 80 80  '=
    61F0:80 87 8E 9C B8 9C 8E 87  '>
    61F8:80 8C 80 8C 98 B3 BF 9E  '?
    6200:80 BE 83 BB BB B3 B3 9E  '@
    6208:80 B3 B3 BF BF B3 BF 9E  'A
    6210:80 9F BF B3 9F B3 BF 9F  'B
    6218:80 9E BF B3 83 B3 BF 9E  'C
    6220:80 9F BF B3 B3 B3 BF 9F  'D
    6228:80 BF BF 83 9F 83 BF BF  'E
    6230:80 83 83 9F 9F 83 BF BF  'F
    6238:80 9E BF B3 BB 83 BF 9E  'G
    6240:80 B3 B3 BF BF B3 B3 B3  'H
    6248:80 BF BF 8C 8C 8C BF BF  'I
    6250:80 9E BF B3 B0 B0 B0 B0  'J
    6258:80 B3 BB 9F 8F 9F BB B3  'K
    6260:80 BF BF 83 83 83 83 83  'L
    6268:80 B3 B3 B3 B3 B3 BF B3  'M
    6270:80 B3 B3 BB BF B7 B3 B3  'N
    6278:80 9E BF B3 B3 B3 BF 9E  'O
    6280:80 83 83 9F BF B3 BF 9F  'P
    6288:80 AE 93 AB A3 A3 BF 9E  'Q
    6290:80 B3 BB 9F BF B3 BF 9F  'R
    6298:80 9E B3 B0 9E 83 B3 9E  'S
    62A0:80 8C 8C 8C 8C 8C BF BF  'T
    62A8:80 9E BF B3 B3 B3 B3 B3  'U
    62B0:80 8C 9E B3 B3 B3 B3 B3  'V
    62B8:80 B3 BF B3 B3 B3 B3 B3  'W
    62C0:80 B3 B3 9E 8C 9E B3 B3  'X
    62C8:80 8C 8C 8C 9E B3 B3 B3  'Y
    62D0:80 BF BF 86 8C 98 BF BF  'Z
    62D8:80 BC BC 8C 8C 8C BC BC  '[
    62E0:80 E0 F0 B8 9C 8E 87 83  '\
    62E8:80 8F 8F 8C 8C 8C 8F 8F  ']
    62F0:80 80 80 80 80 BF 9E 8C  '^
    62F8:80 FF FF 80 80 80 80 80  '_
    6300:80 80 80 80 80 98 8C 86  '`
    6308:80 BE B3 BE B0 9E 80 80  'a
    6310:80 9F B3 B3 B3 9F 83 83  'b
    6318:80 9E B3 83 B3 9E 80 80  'c
    6320:80 BE B3 B3 B3 BE B0 B0  'd
    6328:80 9E 83 9F B3 9E 80 80  'e
    6330:80 86 86 86 9F 86 B6 9C  'f
    6338:9E B0 BE B3 B3 9E 80 80  'g
    6340:80 B3 B3 B3 B3 9F 83 83  'h
    6348:80 8C 8C 8C 8C 80 8C 80  'i
    6350:9E B3 B3 B0 B0 B0 80 B0  'j
    6358:80 B3 9B 8F 9B B3 83 83  'k
    6360:80 9E 8C 8C 8C 8C 8C 8E  'l
    6368:80 B3 B3 B3 BF B3 80 80  'm
    6370:80 B3 B3 B3 B3 9F 80 80  'n
    6378:80 9E B3 B3 B3 9E 80 80  'o
    6380:83 83 9F B3 B3 9F 80 80  'p
    6388:B0 B0 BE B3 B3 BE 80 80  'q
    6390:80 83 83 83 B3 9F 80 80  'r
    6398:80 9E B0 9E 83 9E 80 80  's
    63A0:80 9C B6 86 86 9F 86 86  't
    63A8:80 BE B3 B3 B3 B3 80 80  'u
    63B0:80 8C 9E B3 B3 B3 80 80  'v
    63B8:80 B3 BF B3 B3 B3 80 80  'w
    63C0:80 B3 9E 8C 9E B3 80 80  'x
    63C8:9E B0 BE B3 B3 B3 80 80  'y
    63D0:80 BF 86 8C 98 BF 80 80  'z
    63D8:80 9C 9E 86 87 86 9E 9C  '{
    63E0:8C 8C 8C 8C 8C 8C 8C 8C  '|
    63E8:80 8E 9E 98 B8 98 9E 8E  '}
    63F0:80 80 80 80 80 98 BF 86  '~
    63F8:00 00 00 00 00 00 00 00  '#
```

Again copy this to higher memory

```
8000<6000.63FFM
```

Fire up our ASCII Character Inspector. Recall:

```
    1000:A9 00 85 FE A9 00 85 F5
    1008:A9 20 85 F6 A5 FE A0 00
    1010:20 37 10 AD 00 C0 10 FB
    1018:8D 10 C0 C9 88 D0 0A C6
    1020:FE A5 FE 29 7F 85 FE 10
    1028:DB C9 95 D0 05 E6 FE 18
    1030:90 EF C9 9B D0 DD 60 48
    1038:20 10 03 A9 20 20 10 03
    1040:68 4C 01 03
    1000G
```

Press `ESC` when done viewing.

Argh, those glyphs are upside down!

Here's our 5 byte patch to fix our rendering code.

```assembly
    0350:         _DrawChar
    0350:A2 07       LDX #7         ; <- #0
    :
    035E:CA          DEX            ; <- INX
    035F:EA EA       NOP NOP        ; <- CPX #8
    0361:10 EF       BPL _LoadFont  ; <- BNE _LoadFont
```

Enter in:

```
    351:7
    35E:CA EA EA 10
```

Hmm, some of those glyphs are badly designed (inconsistent.) :-/ That's the biggest problem amateur artists have; they haven't yet internalized a "consistent style" -- their's is all over the place.  This comes with experience of knowing:

 * When to follow the "rules", and
 * When to bend/break the "rules".

Let's write a program to view all the ASCII characters.

```Assembly
                ORG $1050
                ; <<Forthcoming!>>
```

Here is a table of all the glyphs that we'll eventually fix:

|Glyph|
|:---:|
| 2 |
| 7 |
| M |
| Q |
| S |
| W |
| Y |
| X |
| ^ |
| j |
| m |
| s |

Alrighty then.

The first glyph that stands out is the `S`. Our rule is:

* Horizontal and Vertical lines are 2 pixels when possible.


Change the `S` = $53 @ `6298`:

From:

```
    6298:80 9E B3 B0 9E 83 B3 9E  'S
```

To:

```
    6298:80 9E BF B8 9E 87 BF 9E  'S
```

While that matches the style of `5` it doesn't match the style of `A`.
Let's fix the and bottom rows.

```
    6298:80 9F BF B8 9E 87 BF BE  'S
```

There we go!

Q. How did I know what bits/bytes to use?

A. The "Beautiful Boot" font is 7x8, with the last pixel being a spacer. This makes each glyph effectively by 6x8. The font also sets the high bit so we'll need to preserve that. Why? (Answer below.)

a) Remember, bits are reversed on the HGR screen:

|Bits|HGR|Hex|
|:--:|--:|--:|
|0000| $0| $0|
|1000| $8| $1|
|0100| $4| $2|
|1100| $C| $3|
|0010| $2| $4|
|1010| $A| $5|
|0110| $6| $6|
|1110| $E| $7|
|0001| $1| $8|
|1001| $9| $9|
|0101| $5| $A|
|1101| $D| $B|
|0011| $3| $C|
|1011| $B| $D|
|0111| $7| $E|
|1111| $F| $F|

 Let's sort that table by HGR hex value:

|Bits|HGR|Hex|
|:--:|--:|--:|
|0000| $0| $0|
|0001| $1| $8|
|0010| $2| $4|
|0011| $3| $C|
|0100| $4| $2|
|0101| $5| $A|
|0110| $6| $6|
|0111| $7| $E|
|1000| $8| $1|
|1001| $9| $9|
|1010| $A| $5|
|1011| $B| $D|
|1100| $C| $3|
|1101| $D| $B|
|1110| $E| $7|
|1111| $F| $F|

 With our "flip bits table" we can list the `old` and `new` bytes:

|HGR Bits |HGR|Hex|Row|
|:-------:|--:|--:|:--|
|0111_1001|$79|$9E| 0 |
|0111_1101|$7D|$BE| 0 |
|    -    | --| --| - |
|1100_1101|$CD|$B3| 1 |
|1111_1101|$CF|$BF| 1 |
|    -    | --| --| - |
|0000_0110|$C1|$83| 2 |
|0000_1110|$E1|$87| 2 |
|    -    | --| --| - |
|0000_1101|$0D|$B0| 4 |
|0001_1101|$1D|$B8| 4 |
|    -    | --| --| - |
|0111_1001|$79|$9E| 0 |
|1111_1001|$F9|$9F| 0 |

b) Did I manually enter in those hex values?

`<<Forthcoming!>>`



# What's next?

What's left? Quite a few things actually:

 * Copy the 80-Column text screen to DHGR (Double High Resolution)
 * Hook into the COUT so all text appears onto the HGR or DHGR screen
 * Other 7x8 fonts
 * Other non-7x8 fonts


# Conclusion

Hope this HGR font tutorial helped you understand the inner workings of a font blitter!

Happy (Apple ]\[ //e //c) Hacking!

Michael "AppleWin Debug Dev"


# Solutions

## Solution 1: ScrollHgrUpLine()

Figure it out !  You have all the tools and knowledge.


Seriously though, start with the data flow. Let's look at the HGR "text" lines we need to copy from/to:

    Src Dst
     1    0
     2    1
     3    2
     4    3
     5    4
     6    5
     7    6
     8    7
     9    8
    10    9
    11   10
    12   11
    13   12
    14   13
    15   14
    16   15
    17   16
    18   17
    19   18
    20   19
    21   20
    22   21
    23   22
    --   23

We need to copy 23 HGR "text" lines to the one above; we also need to clear the bottom row.  Pseudo-code would be:

```C
    for( int line = 0; line < 24-1; line++
        copy_line( line+1, line );
    clear_line( 23 );
```

Each HGR "text" line takes up 40 columns

```C
    for( int row = 0; row < 23; row++
        char *src = HGRAddressY[ row + 1 ]
        char *dst = HGRAddressY[ row + 0 ]
        for( int col = 0; col < 40; col++ )
            *dst++ = *src++;
    clear_line( 23 );
```

However each HGR "text" line takes 8 scanlines (since our font cell is 7x8).

```C
    for( row = 0; row < 23; row++
        char *src = HGRAddressY[ row + 1 ]
        char *dst = HGRAddressY[ row + 0 ]
        for( line = 0; line < 8; line++ )
        {
            for( col = 0; col < 40; col++ )
                *dst++ = *src++;
            src += 0x400;
            dst += 0x400;
        }
```

We could code this up as:

```assembly
                Src     EQU $F8
                Dst     EQU $FA
                Tmp     EQU $FC

    ; NOTE: Requires $E6 to be initialized with $20 or $40 for graphics page

                     ORG $1900
    1900:         ScrollHgrUpLine
    1900:A2 00       LDX #0         ; row
    ; Copy Top 184 rows
    1902:BD A0 03 .1 LDA HgrLoY,X
    1905:85 FA       STA Dst+0
    1907:BD B8 03    LDA HgrHiY,X
    190A:05 E6       ORA $E6
    190C:85 FB       STA Dst+1
    190E:E8          INX
    190F:BD A0 03    LDA HgrLoY,X
    1912:85 F8       STA Src+0
    1914:BD B8 03    LDA HgrHiY,X
    1917:05 E6       ORA $E6
    1919:85 F9       STA Src+1
    191B:A9 07       LDA #7         ; 8 scanlines/text line
    191D:85 FC       STA Tmp
    191F:A0 27    .2 LDY #40-1      ; 40 columns to copy
    1921:B1 F8    .3 LDA (Src),Y
    1923:91 FA       STA (Dst),Y
    1925:88          DEY
    1926:10 F9       BPL .3         ; while (y-- > 0)
    1928:18          CLC
    1929:A5 F9       LDA Src+1
    192B:69 04       ADC #4
    192D:85 F9       STA Src+1
    192F:18          CLC
    1930:A5 FB       LDA Dst+1
    1932:69 04       ADC #4
    1934:85 FB       STA Dst+1
    1936:C6 FC       DEC Tmp
    1938:10 E5       BPL .2
    193A:E0 17       CPX #23        ; 24 rows is #$18
    193C:90 C4       BCC .1         ; Y >= 23
    ; Zero Bottom 8 scanlines       ; $F8,$F9 = $D0,$43
    193E:A2 08       LDX #8         ; bottom 8 scanlines
    1940:38          CLC
    1941:A5 F9       LDA Src+1
    1943:E9 20       SBC #20
    1945:85 F9       STA Src+1
    1947:A9 00    .4 LDA #00        ; color = black
    1949:A0 27       LDY #40-1      ; 40 columns to copy
    194B:91 F8    .5 STA (Src),Y
    194D:88          DEY
    194E:10 FB       BPL .5         ; while (y-- > 0)
    1950:18          CLC
    1951:A5 F9       LDA Src+1
    1953:69 04       ADC #4
    1955:85 F9       STA Src+1
    1957:CA          DEX
    1958:D0 ED       BNE .4         ; BNZ ; Try this fun bug: D0 EF
    195A:60          RTS
```

Enter in:

```
    1900:A2 00 BD A0 03 85 FA BD
    1908:B8 03 05 E6 85 FB E8 BD
    1910:A0 03 85 F8 BD B8 03 05
    1918:E6 85 F9 A9 07 85 FC A0
    1920:27 B1 F8 91 FA 88 10 F9
    1928:18 A5 F9 69 04 85 F9 18
    1930:A5 FB 69 04 85 FB C6 FC
    1938:10 E5 E0 17 90 C4 A2 08
    1940:38 A5 F9 E9 20 85 F9 A9
    1948:00 A0 27 91 F8 88 10 FB
    1950:18 A5 F9 69 04 85 F9 CA
    1958:D0 ED 60
```

Excellent.


## Solution 2: ScrollHgrUpPixel()

There are many different ways to solve this depending if we want to prioritize space or speed.

We could manually unroll every loop such as this monstrosity (we trade space for speed):

Enter this (or download [hgr_scroll_up.bin](hgr_scroll_up.bin) or `BRUN HGR_SCROLL_UP`):

```
    1400:A2 27
    1402:BD 00 24 9D 00 20
    1408:BD 00 28 9D 00 24
    140E:BD 00 2C 9D 00 28
    1414:BD 00 30 9D 00 2C
    141A:BD 00 34 9D 00 30
    1420:BD 00 38 9D 00 34
    1426:BD 00 3C 9D 00 38
    142C:BD 80 20 9D 00 3C
    1432:BD 80 24 9D 80 20
    1438:BD 80 28 9D 80 24
    143E:BD 80 2C 9D 80 28
    1444:BD 80 30 9D 80 2C
    144A:BD 80 34 9D 80 30
    1450:BD 80 38 9D 80 34
    1456:BD 80 3C 9D 80 38
    145C:BD 00 21 9D 80 3C
    1462:BD 00 25 9D 00 21
    1468:BD 00 29 9D 00 25
    146E:BD 00 2D 9D 00 29
    1474:BD 00 31 9D 00 2D
    147A:BD 00 35 9D 00 31
    1480:BD 00 39 9D 00 35
    1486:BD 00 3D 9D 00 39
    148C:BD 80 21 9D 00 3D
    1492:BD 80 25 9D 80 21
    1498:BD 80 29 9D 80 25
    149E:BD 80 2D 9D 80 29
    14A4:BD 80 31 9D 80 2D
    14AA:BD 80 35 9D 80 31
    14B0:BD 80 39 9D 80 35
    14B6:BD 80 3D 9D 80 39
    14BC:BD 00 22 9D 80 3D
    14C2:BD 00 26 9D 00 22
    14C8:BD 00 2A 9D 00 26
    14CE:BD 00 2E 9D 00 2A
    14D4:BD 00 32 9D 00 2E
    14DA:BD 00 36 9D 00 32
    14E0:BD 00 3A 9D 00 36
    14E6:BD 00 3E 9D 00 3A
    14EC:BD 80 22 9D 00 3E
    14F2:BD 80 26 9D 80 22
    14F8:BD 80 2A 9D 80 26
    14FE:BD 80 2E 9D 80 2A
    1504:BD 80 32 9D 80 2E
    150A:BD 80 36 9D 80 32
    1510:BD 80 3A 9D 80 36
    1516:BD 80 3E 9D 80 3A
    151C:BD 00 23 9D 80 3E
    1522:BD 00 27 9D 00 23
    1528:BD 00 2B 9D 00 27
    152E:BD 00 2F 9D 00 2B
    1534:BD 00 33 9D 00 2F
    153A:BD 00 37 9D 00 33
    1540:BD 00 3B 9D 00 37
    1546:BD 00 3F 9D 00 3B
    154C:BD 80 23 9D 00 3F
    1552:BD 80 27 9D 80 23
    1558:BD 80 2B 9D 80 27
    155E:BD 80 2F 9D 80 2B
    1564:BD 80 33 9D 80 2F
    156A:BD 80 37 9D 80 33
    1570:BD 80 3B 9D 80 37
    1576:BD 80 3F 9D 80 3B
    157C:BD 28 20 9D 80 3F
    1582:BD 28 24 9D 28 20
    1588:BD 28 28 9D 28 24
    158E:BD 28 2C 9D 28 28
    1594:BD 28 30 9D 28 2C
    159A:BD 28 34 9D 28 30
    15A0:BD 28 38 9D 28 34
    15A6:BD 28 3C 9D 28 38
    15AC:BD A8 20 9D 28 3C
    15B2:BD A8 24 9D A8 20
    15B8:BD A8 28 9D A8 24
    15BE:BD A8 2C 9D A8 28
    15C4:BD A8 30 9D A8 2C
    15CA:BD A8 34 9D A8 30
    15D0:BD A8 38 9D A8 34
    15D6:BD A8 3C 9D A8 38
    15DC:BD 28 21 9D A8 3C
    15E2:BD 28 25 9D 28 21
    15E8:BD 28 29 9D 28 25
    15EE:BD 28 2D 9D 28 29
    15F4:BD 28 31 9D 28 2D
    15FA:BD 28 35 9D 28 31
    1600:BD 28 39 9D 28 35
    1606:BD 28 3D 9D 28 39
    160C:BD A8 21 9D 28 3D
    1612:BD A8 25 9D A8 21
    1618:BD A8 29 9D A8 25
    161E:BD A8 2D 9D A8 29
    1624:BD A8 31 9D A8 2D
    162A:BD A8 35 9D A8 31
    1630:BD A8 39 9D A8 35
    1636:BD A8 3D 9D A8 39
    163C:BD 28 22 9D A8 3D
    1642:BD 28 26 9D 28 22
    1648:BD 28 2A 9D 28 26
    164E:BD 28 2E 9D 28 2A
    1654:BD 28 32 9D 28 2E
    165A:BD 28 36 9D 28 32
    1660:BD 28 3A 9D 28 36
    1666:BD 28 3E 9D 28 3A
    166C:BD A8 22 9D 28 3E
    1672:BD A8 26 9D A8 22
    1678:BD A8 2A 9D A8 26
    167E:BD A8 2E 9D A8 2A
    1684:BD A8 32 9D A8 2E
    168A:BD A8 36 9D A8 32
    1690:BD A8 3A 9D A8 36
    1696:BD A8 3E 9D A8 3A
    169C:BD 28 23 9D A8 3E
    16A2:BD 28 27 9D 28 23
    16A8:BD 28 2B 9D 28 27
    16AE:BD 28 2F 9D 28 2B
    16B4:BD 28 33 9D 28 2F
    16BA:BD 28 37 9D 28 33
    16C0:BD 28 3B 9D 28 37
    16C6:BD 28 3F 9D 28 3B
    16CC:BD A8 23 9D 28 3F
    16D2:BD A8 27 9D A8 23
    16D8:BD A8 2B 9D A8 27
    16DE:BD A8 2F 9D A8 2B
    16E4:BD A8 33 9D A8 2F
    16EA:BD A8 37 9D A8 33
    16F0:BD A8 3B 9D A8 37
    16F6:BD A8 3F 9D A8 3B
    16FC:BD 50 20 9D A8 3F
    1702:BD 50 24 9D 50 20
    1708:BD 50 28 9D 50 24
    170E:BD 50 2C 9D 50 28
    1714:BD 50 30 9D 50 2C
    171A:BD 50 34 9D 50 30
    1720:BD 50 38 9D 50 34
    1726:BD 50 3C 9D 50 38
    172C:BD D0 20 9D 50 3C
    1732:BD D0 24 9D D0 20
    1738:BD D0 28 9D D0 24
    173E:BD D0 2C 9D D0 28
    1744:BD D0 30 9D D0 2C
    174A:BD D0 34 9D D0 30
    1750:BD D0 38 9D D0 34
    1756:BD D0 3C 9D D0 38
    175C:BD 50 21 9D D0 3C
    1762:BD 50 25 9D 50 21
    1768:BD 50 29 9D 50 25
    176E:BD 50 2D 9D 50 29
    1774:BD 50 31 9D 50 2D
    177A:BD 50 35 9D 50 31
    1780:BD 50 39 9D 50 35
    1786:BD 50 3D 9D 50 39
    178C:BD D0 21 9D 50 3D
    1792:BD D0 25 9D D0 21
    1798:BD D0 29 9D D0 25
    179E:BD D0 2D 9D D0 29
    17A4:BD D0 31 9D D0 2D
    17AA:BD D0 35 9D D0 31
    17B0:BD D0 39 9D D0 35
    17B6:BD D0 3D 9D D0 39
    17BC:BD 50 22 9D D0 3D
    17C2:BD 50 26 9D 50 22
    17C8:BD 50 2A 9D 50 26
    17CE:BD 50 2E 9D 50 2A
    17D4:BD 50 32 9D 50 2E
    17DA:BD 50 36 9D 50 32
    17E0:BD 50 3A 9D 50 36
    17E6:BD 50 3E 9D 50 3A
    17EC:BD D0 22 9D 50 3E
    17F2:BD D0 26 9D D0 22
    17F8:BD D0 2A 9D D0 26
    17FE:BD D0 2E 9D D0 2A
    1804:BD D0 32 9D D0 2E
    180A:BD D0 36 9D D0 32
    1810:BD D0 3A 9D D0 36
    1816:BD D0 3E 9D D0 3A
    181C:BD 50 23 9D D0 3E
    1822:BD 50 27 9D 50 23
    1828:BD 50 2B 9D 50 27
    182E:BD 50 2F 9D 50 2B
    1834:BD 50 33 9D 50 2F
    183A:BD 50 37 9D 50 33
    1840:BD 50 3B 9D 50 37
    1846:BD 50 3F 9D 50 3B
    184C:BD D0 23 9D 50 3F
    1852:BD D0 27 9D D0 23
    1858:BD D0 2B 9D D0 27
    185E:BD D0 2F 9D D0 2B
    1864:BD D0 33 9D D0 2F
    186A:BD D0 37 9D D0 33
    1870:BD D0 3B 9D D0 37
    1876:BD D0 3F 9D D0 3B
    187C:A9 00    9D D0 3F
    1881:CA 30 03 4C 02 14
    1887:60
```

And let's write a little demo ...

```assembly
    13F7:A0 C0        LDY #C0
    13F9:20 00 14  .1 JSR ScrollHgrUpPixel
    13FC:88           DEY
    13FD:D0 FA        BNE .1
    13FF:60           RTS
```

Enter in:

```
    13F7:A0 C0 20 00 14 88 D0 FA 60
```

(To save to disk type `BSAVE HGR_SCROLL_UP.BIN,A$13F7,L$490`)

And let's try it out:

```
    1300L
    1300G
    1400G
    1400G
    1400G
```

![Screenshot 21](pics/hgrfont_21.png?raw=true)

And for the grand finale:

```
    13F7G
```

Sweet !

Here's the (non-standard) assembly to scroll the HGR screen up one pixel:  (I'm using the non-conventional `:` as an assembler end-of-statement sepearator to logically group the byte copies together.)

```assembly
    ; FUNC: ScrollHgrUpPixel()
              ORG $1400
    1400:     LDX #27 ; 39 columns       ; Src Y    Dst Y
    1402:  .1 LDA $2400,X : STA $2000,X  ; [  1] -> [  0]
    1408:     LDA $2800,X : STA $2400,X  ; [  2] -> [  1]
    140E:     LDA $2C00,X : STA $2800,X  ; [  3] -> [  2]
    1414:     LDA $3000,X : STA $2C00,X  ; [  4] -> [  3]
    141A:     LDA $3400,X : STA $3000,X  ; [  5] -> [  4]
    1420:     LDA $3800,X : STA $3400,X  ; [  6] -> [  5]
    1426:     LDA $3C00,X : STA $3800,X  ; [  7] -> [  6]
    142C:     LDA $2080,X : STA $3C00,X  ; [  8] -> [  7]
    1432:     LDA $2480,X : STA $2080,X  ; [  9] -> [  8]
    1438:     LDA $2880,X : STA $2480,X  ; [ 10] -> [  9]
    143E:     LDA $2C80,X : STA $2880,X  ; [ 11] -> [ 10]
    1444:     LDA $3080,X : STA $2C80,X  ; [ 12] -> [ 11]
    144A:     LDA $3480,X : STA $3080,X  ; [ 13] -> [ 12]
    1450:     LDA $3880,X : STA $3480,X  ; [ 14] -> [ 13]
    1456:     LDA $3C80,X : STA $3880,X  ; [ 15] -> [ 14]
    145C:     LDA $2100,X : STA $3C80,X  ; [ 16] -> [ 15]
    1462:     LDA $2500,X : STA $2100,X  ; [ 17] -> [ 16]
    1468:     LDA $2900,X : STA $2500,X  ; [ 18] -> [ 17]
    146E:     LDA $2D00,X : STA $2900,X  ; [ 19] -> [ 18]
    1474:     LDA $3100,X : STA $2D00,X  ; [ 20] -> [ 19]
    147A:     LDA $3500,X : STA $3100,X  ; [ 21] -> [ 20]
    1480:     LDA $3900,X : STA $3500,X  ; [ 22] -> [ 21]
    1486:     LDA $3D00,X : STA $3900,X  ; [ 23] -> [ 22]
    148C:     LDA $2180,X : STA $3D00,X  ; [ 24] -> [ 23]
    1492:     LDA $2580,X : STA $2180,X  ; [ 25] -> [ 24]
    1498:     LDA $2980,X : STA $2580,X  ; [ 26] -> [ 25]
    149E:     LDA $2D80,X : STA $2980,X  ; [ 27] -> [ 26]
    14A4:     LDA $3180,X : STA $2D80,X  ; [ 28] -> [ 27]
    14AA:     LDA $3580,X : STA $3180,X  ; [ 29] -> [ 28]
    14B0:     LDA $3980,X : STA $3580,X  ; [ 30] -> [ 29]
    14B6:     LDA $3D80,X : STA $3980,X  ; [ 31] -> [ 30]
    14BC:     LDA $2200,X : STA $3D80,X  ; [ 32] -> [ 31]
    14C2:     LDA $2600,X : STA $2200,X  ; [ 33] -> [ 32]
    14C8:     LDA $2A00,X : STA $2600,X  ; [ 34] -> [ 33]
    14CE:     LDA $2E00,X : STA $2A00,X  ; [ 35] -> [ 34]
    14D4:     LDA $3200,X : STA $2E00,X  ; [ 36] -> [ 35]
    14DA:     LDA $3600,X : STA $3200,X  ; [ 37] -> [ 36]
    14E0:     LDA $3A00,X : STA $3600,X  ; [ 38] -> [ 37]
    14E6:     LDA $3E00,X : STA $3A00,X  ; [ 39] -> [ 38]
    14EC:     LDA $2280,X : STA $3E00,X  ; [ 40] -> [ 39]
    14F2:     LDA $2680,X : STA $2280,X  ; [ 41] -> [ 40]
    14F8:     LDA $2A80,X : STA $2680,X  ; [ 42] -> [ 41]
    14FE:     LDA $2E80,X : STA $2A80,X  ; [ 43] -> [ 42]
    1504:     LDA $3280,X : STA $2E80,X  ; [ 44] -> [ 43]
    150A:     LDA $3680,X : STA $3280,X  ; [ 45] -> [ 44]
    1510:     LDA $3A80,X : STA $3680,X  ; [ 46] -> [ 45]
    1516:     LDA $3E80,X : STA $3A80,X  ; [ 47] -> [ 46]
    151C:     LDA $2300,X : STA $3E80,X  ; [ 48] -> [ 47]
    1522:     LDA $2700,X : STA $2300,X  ; [ 49] -> [ 48]
    1528:     LDA $2B00,X : STA $2700,X  ; [ 50] -> [ 49]
    152E:     LDA $2F00,X : STA $2B00,X  ; [ 51] -> [ 50]
    1534:     LDA $3300,X : STA $2F00,X  ; [ 52] -> [ 51]
    153A:     LDA $3700,X : STA $3300,X  ; [ 53] -> [ 52]
    1540:     LDA $3B00,X : STA $3700,X  ; [ 54] -> [ 53]
    1546:     LDA $3F00,X : STA $3B00,X  ; [ 55] -> [ 54]
    154C:     LDA $2380,X : STA $3F00,X  ; [ 56] -> [ 55]
    1552:     LDA $2780,X : STA $2380,X  ; [ 57] -> [ 56]
    1558:     LDA $2B80,X : STA $2780,X  ; [ 58] -> [ 57]
    155E:     LDA $2F80,X : STA $2B80,X  ; [ 59] -> [ 58]
    1564:     LDA $3380,X : STA $2F80,X  ; [ 60] -> [ 59]
    156A:     LDA $3780,X : STA $3380,X  ; [ 61] -> [ 60]
    1570:     LDA $3B80,X : STA $3780,X  ; [ 62] -> [ 61]
    1576:     LDA $3F80,X : STA $3B80,X  ; [ 63] -> [ 62]
    157C:     LDA $2028,X : STA $3F80,X  ; [ 64] -> [ 63]
    1582:     LDA $2428,X : STA $2028,X  ; [ 65] -> [ 64]
    1588:     LDA $2828,X : STA $2428,X  ; [ 66] -> [ 65]
    158E:     LDA $2C28,X : STA $2828,X  ; [ 67] -> [ 66]
    1594:     LDA $3028,X : STA $2C28,X  ; [ 68] -> [ 67]
    159A:     LDA $3428,X : STA $3028,X  ; [ 69] -> [ 68]
    15A0:     LDA $3828,X : STA $3428,X  ; [ 70] -> [ 69]
    15A6:     LDA $3C28,X : STA $3828,X  ; [ 71] -> [ 70]
    15AC:     LDA $20A8,X : STA $3C28,X  ; [ 72] -> [ 71]
    15B2:     LDA $24A8,X : STA $20A8,X  ; [ 73] -> [ 72]
    15B8:     LDA $28A8,X : STA $24A8,X  ; [ 74] -> [ 73]
    15BE:     LDA $2CA8,X : STA $28A8,X  ; [ 75] -> [ 74]
    15C4:     LDA $30A8,X : STA $2CA8,X  ; [ 76] -> [ 75]
    15CA:     LDA $34A8,X : STA $30A8,X  ; [ 77] -> [ 76]
    15D0:     LDA $38A8,X : STA $34A8,X  ; [ 78] -> [ 77]
    15D6:     LDA $3CA8,X : STA $38A8,X  ; [ 79] -> [ 78]
    15DC:     LDA $2128,X : STA $3CA8,X  ; [ 80] -> [ 79]
    15E2:     LDA $2528,X : STA $2128,X  ; [ 81] -> [ 80]
    15E8:     LDA $2928,X : STA $2528,X  ; [ 82] -> [ 81]
    15EE:     LDA $2D28,X : STA $2928,X  ; [ 83] -> [ 82]
    15F4:     LDA $3128,X : STA $2D28,X  ; [ 84] -> [ 83]
    15FA:     LDA $3528,X : STA $3128,X  ; [ 85] -> [ 84]
    1600:     LDA $3928,X : STA $3528,X  ; [ 86] -> [ 85]
    1606:     LDA $3D28,X : STA $3928,X  ; [ 87] -> [ 86]
    160C:     LDA $21A8,X : STA $3D28,X  ; [ 88] -> [ 87]
    1612:     LDA $25A8,X : STA $21A8,X  ; [ 89] -> [ 88]
    1618:     LDA $29A8,X : STA $25A8,X  ; [ 90] -> [ 89]
    161E:     LDA $2DA8,X : STA $29A8,X  ; [ 91] -> [ 90]
    1624:     LDA $31A8,X : STA $2DA8,X  ; [ 92] -> [ 91]
    162A:     LDA $35A8,X : STA $31A8,X  ; [ 93] -> [ 92]
    1630:     LDA $39A8,X : STA $35A8,X  ; [ 94] -> [ 93]
    1636:     LDA $3DA8,X : STA $39A8,X  ; [ 95] -> [ 94]
    163C:     LDA $2228,X : STA $3DA8,X  ; [ 96] -> [ 95]
    1642:     LDA $2628,X : STA $2228,X  ; [ 97] -> [ 96]
    1648:     LDA $2A28,X : STA $2628,X  ; [ 98] -> [ 97]
    164E:     LDA $2E28,X : STA $2A28,X  ; [ 99] -> [ 98]
    1654:     LDA $3228,X : STA $2E28,X  ; [100] -> [ 99]
    165A:     LDA $3628,X : STA $3228,X  ; [101] -> [100]
    1660:     LDA $3A28,X : STA $3628,X  ; [102] -> [101]
    1666:     LDA $3E28,X : STA $3A28,X  ; [103] -> [102]
    166C:     LDA $22A8,X : STA $3E28,X  ; [104] -> [103]
    1672:     LDA $26A8,X : STA $22A8,X  ; [105] -> [104]
    1678:     LDA $2AA8,X : STA $26A8,X  ; [106] -> [105]
    167E:     LDA $2EA8,X : STA $2AA8,X  ; [107] -> [106]
    1684:     LDA $32A8,X : STA $2EA8,X  ; [108] -> [107]
    168A:     LDA $36A8,X : STA $32A8,X  ; [109] -> [108]
    1690:     LDA $3AA8,X : STA $36A8,X  ; [110] -> [109]
    1696:     LDA $3EA8,X : STA $3AA8,X  ; [111] -> [110]
    169C:     LDA $2328,X : STA $3EA8,X  ; [112] -> [111]
    16A2:     LDA $2728,X : STA $2328,X  ; [113] -> [112]
    16A8:     LDA $2B28,X : STA $2728,X  ; [114] -> [113]
    16AE:     LDA $2F28,X : STA $2B28,X  ; [115] -> [114]
    16B4:     LDA $3328,X : STA $2F28,X  ; [116] -> [115]
    16BA:     LDA $3728,X : STA $3328,X  ; [117] -> [116]
    16C0:     LDA $3B28,X : STA $3728,X  ; [118] -> [117]
    16C6:     LDA $3F28,X : STA $3B28,X  ; [119] -> [118]
    16CC:     LDA $23A8,X : STA $3F28,X  ; [120] -> [119]
    16D2:     LDA $27A8,X : STA $23A8,X  ; [121] -> [120]
    16D8:     LDA $2BA8,X : STA $27A8,X  ; [122] -> [121]
    16DE:     LDA $2FA8,X : STA $2BA8,X  ; [123] -> [122]
    16E4:     LDA $33A8,X : STA $2FA8,X  ; [124] -> [123]
    16EA:     LDA $37A8,X : STA $33A8,X  ; [125] -> [124]
    16F0:     LDA $3BA8,X : STA $37A8,X  ; [126] -> [125]
    16F6:     LDA $3FA8,X : STA $3BA8,X  ; [127] -> [126]
    16FC:     LDA $2050,X : STA $3FA8,X  ; [128] -> [127]
    1702:     LDA $2450,X : STA $2050,X  ; [129] -> [128]
    1708:     LDA $2850,X : STA $2450,X  ; [130] -> [129]
    170E:     LDA $2C50,X : STA $2850,X  ; [131] -> [130]
    1714:     LDA $3050,X : STA $2C50,X  ; [132] -> [131]
    171A:     LDA $3450,X : STA $3050,X  ; [133] -> [132]
    1720:     LDA $3850,X : STA $3450,X  ; [134] -> [133]
    1726:     LDA $3C50,X : STA $3850,X  ; [135] -> [134]
    172C:     LDA $20D0,X : STA $3C50,X  ; [136] -> [135]
    1732:     LDA $24D0,X : STA $20D0,X  ; [137] -> [136]
    1738:     LDA $28D0,X : STA $24D0,X  ; [138] -> [137]
    173E:     LDA $2CD0,X : STA $28D0,X  ; [139] -> [138]
    1744:     LDA $30D0,X : STA $2CD0,X  ; [140] -> [139]
    174A:     LDA $34D0,X : STA $30D0,X  ; [141] -> [140]
    1750:     LDA $38D0,X : STA $34D0,X  ; [142] -> [141]
    1756:     LDA $3CD0,X : STA $38D0,X  ; [143] -> [142]
    175C:     LDA $2150,X : STA $3CD0,X  ; [144] -> [143]
    1762:     LDA $2550,X : STA $2150,X  ; [145] -> [144]
    1768:     LDA $2950,X : STA $2550,X  ; [146] -> [145]
    176E:     LDA $2D50,X : STA $2950,X  ; [147] -> [146]
    1774:     LDA $3150,X : STA $2D50,X  ; [148] -> [147]
    177A:     LDA $3550,X : STA $3150,X  ; [149] -> [148]
    1780:     LDA $3950,X : STA $3550,X  ; [150] -> [149]
    1786:     LDA $3D50,X : STA $3950,X  ; [151] -> [150]
    178C:     LDA $21D0,X : STA $3D50,X  ; [152] -> [151]
    1792:     LDA $25D0,X : STA $21D0,X  ; [153] -> [152]
    1798:     LDA $29D0,X : STA $25D0,X  ; [154] -> [153]
    179E:     LDA $2DD0,X : STA $29D0,X  ; [155] -> [154]
    17A4:     LDA $31D0,X : STA $2DD0,X  ; [156] -> [155]
    17AA:     LDA $35D0,X : STA $31D0,X  ; [157] -> [156]
    17B0:     LDA $39D0,X : STA $35D0,X  ; [158] -> [157]
    17B6:     LDA $3DD0,X : STA $39D0,X  ; [159] -> [158]
    17BC:     LDA $2250,X : STA $3DD0,X  ; [160] -> [159]
    17C2:     LDA $2650,X : STA $2250,X  ; [161] -> [160]
    17C8:     LDA $2A50,X : STA $2650,X  ; [162] -> [161]
    17CE:     LDA $2E50,X : STA $2A50,X  ; [163] -> [162]
    17D4:     LDA $3250,X : STA $2E50,X  ; [164] -> [163]
    17DA:     LDA $3650,X : STA $3250,X  ; [165] -> [164]
    17E0:     LDA $3A50,X : STA $3650,X  ; [166] -> [165]
    17E6:     LDA $3E50,X : STA $3A50,X  ; [167] -> [166]
    17EC:     LDA $22D0,X : STA $3E50,X  ; [168] -> [167]
    17F2:     LDA $26D0,X : STA $22D0,X  ; [169] -> [168]
    17F8:     LDA $2AD0,X : STA $26D0,X  ; [170] -> [169]
    17FE:     LDA $2ED0,X : STA $2AD0,X  ; [171] -> [170]
    1804:     LDA $32D0,X : STA $2ED0,X  ; [172] -> [171]
    180A:     LDA $36D0,X : STA $32D0,X  ; [173] -> [172]
    1810:     LDA $3AD0,X : STA $36D0,X  ; [174] -> [173]
    1816:     LDA $3ED0,X : STA $3AD0,X  ; [175] -> [174]
    181C:     LDA $2350,X : STA $3ED0,X  ; [176] -> [175]
    1822:     LDA $2750,X : STA $2350,X  ; [177] -> [176]
    1828:     LDA $2B50,X : STA $2750,X  ; [178] -> [177]
    182E:     LDA $2F50,X : STA $2B50,X  ; [179] -> [178]
    1834:     LDA $3350,X : STA $2F50,X  ; [180] -> [179]
    183A:     LDA $3750,X : STA $3350,X  ; [181] -> [180]
    1840:     LDA $3B50,X : STA $3750,X  ; [182] -> [181]
    1846:     LDA $3F50,X : STA $3B50,X  ; [183] -> [182]
    184C:     LDA $23D0,X : STA $3F50,X  ; [184] -> [183]
    1852:     LDA $27D0,X : STA $23D0,X  ; [185] -> [184]
    1858:     LDA $2BD0,X : STA $27D0,X  ; [186] -> [185]
    185E:     LDA $2FD0,X : STA $2BD0,X  ; [187] -> [186]
    1864:     LDA $33D0,X : STA $2FD0,X  ; [188] -> [187]
    186A:     LDA $37D0,X : STA $33D0,X  ; [189] -> [188]
    1870:     LDA $3BD0,X : STA $37D0,X  ; [190] -> [189]
    1876:     LDA $3FD0,X : STA $3BD0,X  ; [191] -> [190]
    187C:     LDA #00     : STA $3FD0,X  ; zero  -> [191]
    1881:     DEX
    1882:     BMI .2   ; x < 0 ?
    1884:     JMP .1
    1887:  .2 RTS
```

The bulk of the ScrollHgrUpPixel() was generated with this JavaScript program [scroll_hgr_up_pixel.html](scroll_hgr_up_pixel.html):

```JavaScript
    var hgr = [];
    for( var y = 0; y < 193; ++y ) // Intentional 1 scanline too many!
        hgr[ y ] = 0x2000 + ((y/64)|0)*0x28 + ((y%8)|0)*0x400 + ((y/8)&7)*0x80;

    for( var y = 0; y < 192; ++y )
        console.log( "[" + y + "]: " + hgr[y].toString(16).toUpperCase() );

    function byte2hex$( byte )
    {
        return ("0" + byte.toString(16)).toUpperCase().substr(-2)
    }

    var address = 0x1402, out = "";
    for( var y = 0; y < 192/8; ++y )
        for( var x = 0; x < 8; ++x )
        {
            var row = y*8 + x; // Assumes hgr[] has a dummy 193rd scanline!
            var src = hgr[ row + 1 ];
            var dst = hgr[ row + 0 ];
            var mem = "BD "
                    + byte2hex$( (src >> 0) & 0xFF ) + " "
                    + byte2hex$( (src >> 8) & 0xFF ) + " "
                    + "9D "
                    + byte2hex$( (dst >> 0) & 0xFF ) + " "
                    + byte2hex$( (dst >> 8) & 0xFF ) + " ";
            var txt = "  "
                 + "  LDA $" + src.toString(16).toUpperCase() + ",Y "
                 + ": STA $" + dst.toString(16).toUpperCase() + ",Y "
                 + " ; ["   + ("  " + (row+1)).substr(-3) 
                 + "] -> [" + ("  " + (row  )).substr(-3) + "]"  + "\n";
            if (row != 191)
                out += address.toString(16).toUpperCase() + ":" + mem + txt;
            address += 6; // 6 bytes per line
        }
    console.log( out );
```

And who said JavaScript was a useless language? :-)

That's all folks!  Now go write some cool font blitter code.


# References

* https://en.wikibooks.org/wiki/6502_Assembly
* http://www.6502.org/tutorials/compare_instructions.html
* http://www.6502.org/tutorials/6502opcodes.html
* https://skilldrick.github.io/easy6502/

# Misc. Utilities and Files

* Convert [font image to C array](image_2_c.html)
* Convert [C array to binary font](c_2_fontbin.c)
* [Raw Binary Font](font.bin) Within AppleWin's debugger: `bload font.bin,6000`

# TODO:

- [x] Screenshots!
- [x] Cleanup all assembly for consistent indentation and alignment
- [x] Count cycles of old and new DrawCharCol Address Calculation
- [x] Alternative `Beautiful Boot` Font
- [x] Binary code at Font Drawing @ $301 `DRAWFONT` and Char Inspect @ $1000 `CHARINSPECT` so you can load it directly into the emulator
- [x] Disk image: `HGRFONT.DSK` (DRAWFONT, CHARINSPECT, FONT7X8, FONTBB)
- [ ] Fix Fat Font glyphs (In progress)
- [ ] Re-engineer Codepage 437 Font to 7x8 cells:
 ![font_codepage_437_8x8.png](font_codepage_437_8x8.png) (In progress)
- [ ] Double Hi-Res
- [ ] PDF of this document (As a work-around use Chrome and Print to PDF)