Apple-2-SW/apple2_hgr_font_tutorial
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Michaelangel007 2016-01-10 10:16:24 -08:00
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@ -108,7 +108,7 @@ Magic? :-)
Nah, just Computer Science. :-)
The first question you probalby have is "How did I know what bytes to use?" We'll get to that in a second.
The first question you probably have is "How did I know what bytes to use?" We'll get to that in a second.
## Quirks of the Apple HGR screen
@ -181,7 +181,7 @@ Third, the video scanner for HGR mode scans bits in reverse. :-/ This means that
We would only get:
* 3 scanlines intead of the expected 4 (see the next point), and
* 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: `/`
On the Apple we need to flip each byte:
@ -673,7 +673,7 @@ A naive glyph/32 calculation would be to use 5 shift right bit-shifts:
4A LSR ; c / 32 = %00000PQR
```
However we can save one instruction (and 2 cycles) if we optimize `c/32` to use the counter-intutive 6502's `ROL` instruction -- which only requires 4 instructions instead:
However we can save one instruction (and 2 cycles) if we optimize `c/32` to use the counterintuitive 6502's `ROL` instruction -- which only requires 4 instructions instead:
```assembly
68 PLA ; pop c = %PQRSTUVW to draw
@ -776,12 +776,12 @@ Let's verify this by writing a character inspector. We'll use the arrow keys to
1021:A5 FE .3 LDA $FE ; c &= 0x7F
1023:29 7F AND #7F
1025:85 FE STA $FE
1027:10 DB BPL .1 ; allways branch, draw prev char
1027:10 DB BPL .1 ; always branch, draw prev char
1029:C9 95 .4 CMP #95 ; key == --> ?
102B:D0 05 BNE .5 ;
102D:E6 FE INC $FE ; yes, ++c
102F:18 CLC
1030:90 EF BCC .3 ; allways branch, draw prev char
1030:90 EF BCC .3 ; always branch, draw prev char
1032:C9 9B .5 CMP #9B ; key == ESC ?
1034:D0 DD BNE .2 ;
1036:60 RTS ; yes, exit
@ -1041,7 +1041,7 @@ Enter:
379:20 28 03 18 98 65 F5 85 F5 60
Or are? Since we're using a function to calculate the destinatin address let's fix the order.
Or are? 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 CursorRow() to `Y`;
@ -1227,7 +1227,7 @@ The text screen, like the HGR screen, is also non-linear, and also broken up int
While the Apple's memory layout seems esoteric it has beautiful symmetry. For any given text row notice that:
* the low byte of the text address is the same low byte of the HGR address
* the high byte of the text addres is 0x1C less then the high byte of the HGR address
* the high byte of the text address is 0x1C less then the high byte of the HGR address
Since we already have a HGR 16-bit address table we can re-use it.
@ -1280,7 +1280,7 @@ And here is the assembly:
1328:C0 28 CPY #$28 ; 40 is #$18
132A:90 F7 BCC .2 ; Y < 40
132C:E6 F3 INC row
133E:D0 DA BNE .1 ; allways
133E:D0 DA BNE .1 ; always
1330:60 .3 RTS
```
@ -1817,7 +1817,7 @@ The bulk of the ScrollHgrUpPixel() was generated with this Javascript program [s
And who said Javascript was a useless language? :-)
That's all folks! No go write some cool font blitter code.
That's all folks! Now go write some cool font blitter code.
## References