#Apple ]\[ //e HGR Font 6502 Assembly Language Tutorial By: Michael Pohoreski Revision: 60, Mar 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 * Fat Font: S * Fat Font: X * Fat Font: 2 * Fat Font: j * Fat Font: s * Fat Font: l * Fat Font: M * Fat Font: W * Fat Font: Q * Fat Font: 7 * 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 ![Screenshot TEXT flash A](pics/text_flash_a.png?raw=true) We used the ASCII character `A` which has a hex value 0x41. Hmm, OK, so we see an `A` but why is it flashing??
**Note**: If you use AppleWin, select the lines, copy, switch back to the emulator, and press Shift-Insert to paste.
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 ![Screenshot TEXT inverse A](pics/text_inverse_a.png?raw=true) 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.
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 characters that help make a text based UI. This is what the full 32 Mouse Text glyphs look like: ![Screenshot ASCII Font 7x8](pics/ascii_table_1_font7x8.png?raw=true) Anyways, enter in: ``` C00F:1 ``` ![Screenshot TEXT mousetext A](pics/text_mousetext_a.png?raw=true) 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.
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 ![Screenshot 0](pics/hgrfont_00.png?raw=true) 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:
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..$3FFF |
Don't worry if the address pattern makes no sense right now -- we'll reveal that later -- but if you're curiuous it is this line using integer math: ```JavaScript hgr[ y ] = 0x2000 + (y/64)*0x28 + (y%8)*0x400 + ((y/8)&7)*0x80; ``` Here's the [JavaScript source code](list_hgr_table.html) to generate a bare-bones table: ```JavaScript function int2pad( n, pad ) { return (Array( pad+1 ).join(' ') + n).slice(-pad); } function word2hex$( 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). :-/ This 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`)
**AppleWin** users: We can save the state of the virutal machine via `F11`.
### 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
IncCursorCol
    
    
```
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)


**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.
(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)
**AppleWin** users: Press `F7`, copy & paste the below, press `F7` when done. SYM TopHgr = FD SYM _DrawChar1 = 34C SYM IncCursorCol = 363 34CL
## 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 ; += 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 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!
**AppleWin** users: Press `F7`, copy & paste the below, press `F7` when done. SYM DrawCharCol = 33A 33AL (Debugger screenshot forthcoming.)
## 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)
**AppleWin** users: Press `F7`, copy & paste the below, press `F7` when done. SYM glyph = FE 1000L
## 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)
**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
## 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`)
**AppleWin** users: Press `F7`, copy & paste the below, press `F7` when done. sym patch2 = 1037
## 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.
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|
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
**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
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 $1100 1100: PrintAYX 1100:A9 41 LDA #41 ; A-register = char 1102:A0 01 LDY #1 ; Y-register = col 1 (2nd column) 1104:A2 02 LDX #2 ; X-register = row 2 (3rd row) 1106:4C 35 03 JMP DrawCharColRow ``` Enter in: 1100:A9 41 A0 01 A2 02 4C 35 03 1100G ![Screenshot 16](pics/hgrfont_16.png?raw=true)
**AppleWin** users: You know the drill ... SYM SetCursorRow = 313 SYM DrawCharColRow = 335
## Natural Params SetCursorColRow() Unfortunately, our usage of the X and Y registers are not intuitive. This is due to the limited addressing modes of the 6502. :-/ If the 6502 had a symmetrical indirect zero-page X addressing mode: LDA ($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 ```
**AppleWin**: SYM SetCursorColRow = 321 SYM DrawString = 37E ASC Msg 120E:1219
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 #= 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 ```
**AppleWin** users: sym Txt2Hgr.1 = 130A sym Txt2Hgr.2 = 1323 sym Txt2Hgr.3 = 1332
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. First, let's write a program to display an ASCII table of the various glyphs. Listing [Ascii Table:](asm/asciitable.s) ```assembly KEYBOARD = $C000 KEYSTROBE = $C010 TXTCLR = $C050 ; Mode Graphics MIXCLR = $C052 ; Full screen MIXSET = $C053 ; Split screen PAGE1 = $C054 HIRES = $C057 ; Mode HGR HgrLo = $F5 HgrHi = $F6 glyph = $FE row = $FF DrawChar = $310 HgrLoY = $3A0 HgrHiY = $3B8 START_ROW = 0 ORG $1080 AsciiTable LDY #(START_ROW-1) & $FF STY row LDA #0 ; glyph=0 STA glyph ; save which glyph to draw BIT PAGE1 ; Page 1 BIT TXTCLR ; not text, but graphics BIT MIXSET ; Split screen text/graphics BIT HIRES ; HGR, no GR _NextRow INC row LDY row LDA HgrLoY,Y STA HgrLo ; Screen Address Lo LDA HgrHiY,Y ORA #$20 ; HGR Page 1 STA HgrHi ; Screen Address Hi LDY #00 ; Y = col _NextCol LDA glyph ; A = glyph JSR DrawChar INC glyph ; yes, ++glyph LDA glyph ; CMP #$20 ; done 16 chars? BEQ _NextRow CMP #$40 BEQ _NextRow CMP #$60 BEQ _NextRow CMP #$80 BNE _NextCol _Done RTS ; Optimization: BEQ _NextRow ``` Enter in: ``` 1080:A0 FF 84 FF A9 00 85 FE 1088:2C 54 C0 2C 50 C0 2C 53 1090:C0 2C 57 C0 E6 FF A4 FF 1098:B9 A0 03 85 F5 B9 B8 03 10A0:09 20 85 F6 A0 00 A5 FE 10A8:20 10 03 E6 FE A5 FE C9 10B0:20 F0 E1 C9 40 F0 DD C9 10B8:60 F0 D9 C9 80 D0 E7 60 1080G ``` ![Screenshot ASCII Font 7x8](pics/ascii_table_1_font7x8.png?raw=true) ### Beautiful Boot Here is the font used by Beautiful Boot ``` 6100:80 80 80 80 80 80 80 80 ' 6108:80 87 87 80 87 87 87 87 '! 6110:80 80 80 80 B6 A4 B6 B6 '" 6118:80 9E 9E BF 9E BF 9E 9E '# 6120:80 8C 9F B0 9E 83 BE 8C '$ 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 ``` Let's display this just a few lines down at row 6. We need to use `row-1` because of the pre-increment at `_NextRow` @ `$1094`. ``` 1081:5 1080G ``` ![Screenshot ASCII Table Font BB Upside Down](pics/ascii_table_2_fontbb.png?raw=true) 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 ``` Let's try that again: ``` 1080G ``` OK, that's much better. ![Screenshot ASCII Table Font BB Upside Down](pics/ascii_table_3_fontbb.png?raw=true) Hmmm, some of those glyphs are badly designed (inconsistent.) :-/ That's the biggest problem amateur artists have; they haven't yet internalized a "consistent style" -- theirs is all over the place. This comes with experience of knowing: * When to follow the "rules", and * When to bend/break the "rules". Here is a table of all the glyphs that we'll eventually fix: |Glyph| |:---:| | 2 | | 7 | | M | | Q | | S | | W | | Y | | X | | ^ | | j | | l | | m | | s | Alrighty then. ## Quick Review of Typography Don't bother with [typeface anatomy](https://en.wikipedia.org/wiki/Typeface_anatomy) on Wikipedia. It is not comprehensive, nor detailed enough. Here are 2 pictures that will help us understand the aesthetics and language of typography: ![Typography](http://lunaweb.com/blog/wp-content/uploads/2013/06/typography.gif) ![Anatomy of Typography](http://pamelaplatt.com/digital_course_resources/typography/anatomy_of_typography.jpg) ### Fat Font: S 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 ``` To: ``` 6298:80 9E BF B8 9E 87 BF 9E ``` Let's display our new glyphs at row 12. ``` 1081:0B 1080G ``` ![Screenshot ASCII Table Font BB S2](pics/ascii_table_s2_fontbb.png?raw=true) While that matches the style of `5` it doesn't match the style of `A`. Let's fix the top and bottom rows. ``` 6298:80 9F BF B8 9E 87 BF BE ``` ![Screenshot ASCII Table Font BB S3](pics/ascii_table_s3_fontbb.png?raw=true) And just in case you had trouble seeing the difference ... ![Screenshot ASCII Table Font BB S4](pics/ascii_table_s4_fontbb.png?raw=true) 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? `<>` ### Fat Font: X The next glyph that stands out is the `X`. Change the `X` = $53 @ `6298`: From: ``` 62C0:80 B3 B3 9E 8C 9E B3 B3 ``` To: ``` 62C0:80 33 B3 9E 8C 9E B3 33 ``` ![Screenshot ASCII Table Font BB X2](pics/ascii_table_x2_fontbb.png?raw=true) Hmm. Close, but no cigar. We need to shift the top and bottom row's right edge pixels over by 1 pixel: ``` XxXx....XxXx xXxX....xXxX xXxXxXxX xXxX xXxXxXxX xXxX....xXxX XxXx....XxXx ``` ``` XxXx......XxXx xXxX....xXxX xXxXxXxX xXxX xXxXxXxX xXxX....xXxX XxXx......XxXx ``` OK, this should fix it: ``` 62C0:80 63 B3 9E 8C 9E B3 63 ``` ![Screenshot ASCII Table Font BB X3](pics/ascii_table_x3_fontbb.png?raw=true) Hmm, that's not really an improvement -- we lost our symmetry. Let's review with-out the half-pixel shift. ``` XX..XX 1 XX..XX 2 XXXX 3 XX 4 XXXX 5 XX..XX 6 XX..XX 7 ``` We really want the top and rows "anchored" and nudge rows 2 and 6 in half a pixel From: ``` 62C0:80 B3 B3 9E 8C 9E B3 63 ``` To: ``` 62C0:80 B3 9B 9E 8C 9E 9B B3 ``` ![Screenshot ASCII Table Font BB X4](pics/ascii_table_x4_fontbb.png?raw=true) Ugh! That's even worse! Why? Because all the _other_ rows are **already** half-pixel shifted over -- let's "undo" that for this glyph: ``` 62C0:80 33 9B 1E 0C 1E 9B 33 ``` ![Screenshot ASCII Table Font BB X5](pics/ascii_table_x5_fontbb.png?raw=true) Almost! We just need to add 1/2 a pixel on the left edge and 1/2 a pixel on the right edge of the middle row 4. ``` 62C0:80 33 9B 1E 8E 1E 9B 33 ``` ![Screenshot ASCII Table Font BB X6](pics/ascii_table_x6_fontbb.png?raw=true) Looking good! ![Screenshot ASCII Table Font BB X7](pics/ascii_table_x7_fontbb.png?raw=true) Except we have one minor problem -- the "kerning" between the `W` and `X` versus the `X` and `Y` is no longer consistent. Why? This is because all the glyphs in the font has the high-bit ON which means every glyph is half-pixel shifted to the right. In order to fix this we need to "undo" ALL these half pixel shifts -- all we need to do is subtract `$80`, or better yet `XOR $80` from every byte. We can either do this now, or fix the other glyphs and do this later. Let's touch up some more glyphs first though. ### Fat Font: 2 We want to fix row 4 of the `2`: 6190:80 BF BF 8E 38 B3 BF 9E ![Screenshot ASCII Table Font BB 2b](pics/ascii_table_2b_fontbb.png?raw=true) Excellent! ![Screenshot ASCII Table Font BB 2c](pics/ascii_table_2c_fontbb.png?raw=true) ### Fat Font: j The serif/terminal on the `f` is one pixel (tall) while the serif/tail on the `j` is two pixels (tall). From: ``` 6350:9E B3 B3 B0 B0 B0 80 B0 ``` To: ``` 6350:9E B3 B0 B0 B0 B0 80 B0 ``` ![Screenshot ASCII Table Font BB j2](pics/ascii_table_j2_fontbb.png?raw=true) The dot / tittle on the `j` is inconsistent with the `i`. :-/ Thankfully, another easy fix: ``` 6350:9E B3 B0 B0 B0 80 B0 80 ``` ![Screenshot ASCII Table Font BB j3](pics/ascii_table_j3_fontbb.png?raw=true) ![Screenshot ASCII Table Font BB j4](pics/ascii_table_j4_fontbb.png?raw=true) ### Fat Font: l The top serif on the lowercase `l` doesn't also match `f`: From: ``` 6360:80 9E 8C 8C 8C 8C 8C 8E ``` To: ``` 6360:80 9E 8C 8C 8C 8C 8C 87 ``` ![Screenshot ASCII Table Font BB l2](pics/ascii_table_l2_fontbb.png?raw=true) Hmm, the terminal is _too_ long. ``` 6360:80 9E 8C 8C 8C 8C 8C 86 ``` ![Screenshot ASCII Table Font BB l3](pics/ascii_table_l3_fontbb.png?raw=true) Nope, we need the top-right "hard" edge. The real problem is that the **bottom** of the `l` has serifs while `i` doesn't. Let's make this a little more _cursive_. ``` 6360:80 98 8C 8C 8C 8C 8C 86 ``` ![Screenshot ASCII Table Font BB l4](pics/ascii_table_l4_fontbb.png?raw=true) ![Screenshot ASCII Table Font BB l5](pics/ascii_table_l5_fontbb.png?raw=true) ### Fat Font: s The lowercase `s` should have the left and right edges aligned on the top and bottom rows: From: ``` 6398:80 9E B0 9E 83 9E 80 80 ``` To: ``` 6398:80 9F B0 9E 83 BE 80 80 ``` ![Screenshot ASCII Table Font BB s12](pics/ascii_table_s12_fontbb.png?raw=true) ![Screenshot ASCII Table Font BB s13](pics/ascii_table_s13_fontbb.png?raw=true) ### Fat Font: M The round corners of `A` are **sans serif**. Let's make the `M` consistent: ### Fat Font: W ### Fat Font: Q ### Fat Font: 7 While the `7` is not bad, we could do a very minor touchup. # 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 # 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)