Apple-2-SW/apple2_hgr_font_tutorial
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Added Table 1 and Table 2

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@ -141,6 +141,10 @@ First, we should notice that video memory is non-linear. :-( You'll want to get
With all the decimal cruft removed:
<hr>
Table 1: HGR Y Address for every scanline
| Y |Address| Y |Address| Y |Address| Screen Hole |
|----:|:-----:|----:|-------|----:|:-----:|:------------:|
| 0 | $2000 | 64 | $2028 | 128 | $2050 | $2078..$207F |
@ -208,6 +212,8 @@ With all the decimal cruft removed:
| 62 | $3B80 | 126 | $3BA8 | 190 | $3BD0 | $3BF8..$3BFF |
| 63 | $3F80 | 127 | $3FA8 | 191 | $3FD0 | $3FF8..$3FFF |
<hr>
Don't worry if the address pattern makes no sense right now -- we'll reveal that later.
Here's the [Javascript source code](list_hgr_table.html) to generate this table:
@ -253,7 +259,7 @@ The `TEXT` screen is 40x24 characters. The high resolution graphics `HGR` screen
Char Height (px/character) = Screen Height (px) / Rows (characters) = 192/24 = 8
Unfortunately, the data for the TEXT ROM 25123 hardware chip is **not** accessible from the 6502 unlike say the [IBM PC BIOS Character Sets](http://nerdlypleasures.blogspot.com/2015/04/ibm-character-fonts.html).:-/ T his means you will need to manually enter in the 8 bytes/character. :-( The good news is that I've already done this so you can copy / paste. :-)
Unfortunately, the data for the TEXT ROM 25123 hardware chip is **not** accessible from the 6502 unlike say the [IBM PC BIOS Character Sets](http://nerdlypleasures.blogspot.com/2015/04/ibm-character-fonts.html). :-/ T his means you will need to manually enter in the 8 bytes/character. :-( The good news is that I've already done this so you can copy / paste. :-)
You can find a picture of the Apple ][ ROM text font on Page 8-9, diagram 8.4 of "Understanding the Apple ]\["
https://archive.org/stream/understanding_the_apple_ii#page/n203/mode/2up
@ -609,7 +615,7 @@ Assuming we want to draw the `A` glyph at the top-left of the screen we would ne
($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. 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.
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.
@ -806,17 +812,37 @@ Since we only care about the high byte:
TmpHi = HgrHi + (8 * $04)
= HgrHi + $20
|Y |TmpHi|Final| (T and $1F) | (T and $1F) or $20 |
|--:|:---:|:---:|:---:|:---:|
| 0| $40 | $20 | $00 | $20 |
| 8| $40 | $20 | $00 | $20 |
| 16| $41 | $21 | $01 | $21 |
| 24| $41 | $21 | $01 | $21 |
| 32| $42 | $22 | $02 | $22 |
| 40| $42 | $22 | $02 | $22 |
| 48| $43 | $23 | $03 | $23 |
| 56| $43 | $23 | $03 | $23 |
| 64| $40 | $20 | $00 | $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| (T and $1F) | (T 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:
@ -826,7 +852,7 @@ Hmm, we would need to replace `SEC SBC` with `AND OR` which we might think would
; Increment the cursor column and move the destination screen pointer back
; up 8 scan lines previously to what it was when DrawChar() was called.
ORG $0370
0370: IncCursorCol
0370: IncCursorCol3
0370:C8 INY
0371:A5 F6 LDA TmpHi
0373:29 1F AND #%00011111 ; Requires an extra OR
@ -860,6 +886,19 @@ The lessons?
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.
```assembly
LDX #0
LDA TmpHi ; new bytes = 2
STA SaveHi ; new bytes = 4
...
CPX #8
BNE _LoadFont
LDA SaveHi ; new bytes = 6
STA TmpHi ; new bytes = 8
```
## DrawChar() version 2
@ -1247,6 +1286,10 @@ The secret to getting high speed graphics rendering on the Apple is to use a loo
The HGR screen address is broken up a triad. Every 64 scan lines the offset change by $28.
<hr>
Table 2: HGR Y Address for evey 8 scanlines
| Y|Address|Hi |Lo |
|---:|------|---|---|
| 0| $2000 |$20|$00|
@ -1276,6 +1319,8 @@ The HGR screen address is broken up a triad. Every 64 scan lines the offset chan
|176| $2350 |$23|$50|
|184| $23D0 |$23|$D0|
<hr>
We'll split this table of 16-bit addresses into Low and High bytes for easier access. We'll also subtract off the hard-coded graphics page 1 high byte = $20 and instead use relative offsets to make it work with either graphics page 1 or 2.
This is our mini HGR Y Address look-up table. "Funny" that it has 24 entries -- the same height as our text screen. :-)
@ -1516,7 +1561,7 @@ Here are all the routines we've entered in so far:
358:F5 18 A5 F6 69 04 85 F6
360:E8 E0 08 D0 EF 60 A5 E5
368:85 F5 A5 E6 85 F6 60
370:C8 18 A5 F6 E9 1F 85 F6
370:C8 38 A5 F6 E9 20 85 F6
378:60 86 F5 B9 00 64 18 65
380:E5 65 F5 85 F5 B9 18 64
388:18 65 E6 85 F6 60 84 F0