Apple-2-SW/apple2_hgr_font_tutorial
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Name hard-coded load font self modifying code

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michaelangel007 2016-01-11 02:03:04 -08:00
parent 7ee71cafff
commit 3066eadaf2
1 changed files with 18 additions and 18 deletions
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README.md
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@ -843,22 +843,22 @@ Our prefix code to setup the source address becomes:
; PARAM: Y = column to draw at; $0 .. $27 (Columns 0 .. 39) (not modified)
; NOTES: X is destroyed
ORG $033B
33B:48 DrawCharCol
33B: PHA ; push c = %PQRSTUVW to draw
33C:29 1F AND #1F ; = %000STUVW R=0, Optimization: implicit CLC
33E:0A ASL ; c * 2 %00STUVW0
33F:0A ASL ; c * 4 %0STUVW00
340:0A ASL ; c * 8 %STUVW000
341:69 00 ADC #00 ; += FontLo; Carry = 0 since R=0 from above
343:8D 55 03 STA $355 ; AddressLo = FontLo + (c*8)
346:68 PLA ; pop c = %PQRSTUVW to draw
347:29 60 AND #60 ; = %PQR00000 S=0, Optimization: implicit CLC
349:2A ROL ; = %QR000000 C=P
34A:2A ROL ; = %R000000P C=Q
34B:2A ROL ; = %000000PQ C=R
34C:2A ROL ; c / 32 = %00000PQR C=0 and one more to get R
34D:69 60 ADC #60 ; += FontHi; Carry = 0 since S=0 from above
34F:8D 56 03 STA $356 ; AddressHi = FontHi + (c/32)
33B: DrawCharCol
33B:48 PHA ; push c = %PQRSTUVW to draw
33C:29 1F AND #1F ; = %000STUVW R=0, Optimization: implicit CLC
33E:0A ASL ; c * 2 %00STUVW0
33F:0A ASL ; c * 4 %0STUVW00
340:0A ASL ; c * 8 %STUVW000
341:69 00 ADC #00 ; += FontLo; Carry = 0 since R=0 from above
343:8D 55 03 STA _Font+1 ; AddressLo = FontLo + (c*8)
346:68 PLA ; pop c = %PQRSTUVW to draw
347:29 60 AND #60 ; = %PQR00000 S=0, Optimization: implicit CLC
349:2A ROL ; = %QR000000 C=P
34A:2A ROL ; = %R000000P C=Q
34B:2A ROL ; = %000000PQ C=R
34C:2A ROL ; c / 32 = %00000PQR C=0 and one more to get R
34D:69 60 ADC #60 ; += FontHi; Carry = 0 since S=0 from above
34F:8D 56 03 STA _Font+2 ; AddressHi = FontHi + (c/32)
```
Recall we'll re-use our existing font drawing code `_DrawChar` at $0352:
@ -867,7 +867,7 @@ Recall we'll re-use our existing font drawing code `_DrawChar` at $0352:
ORG $0352
352: _DrawChar
352:A2 00 LDX #0 ; next instruction is Self-Modifying!
354:BD 00 00 _Draw LDA $0000,X ; A = font[ offset + i ]
354:BD 00 00 _Font LDA $0000,X ; A = font[ offset + i ]
357:91 F5 STA (TmpLo),Y ; screen[col] = A
359:18 CLC
35A:A5 F6 LDA TmpHi
@ -875,7 +875,7 @@ Recall we'll re-use our existing font drawing code `_DrawChar` at $0352:
35E:85 F6 STA TmpHi
360:E8 INX
361:E0 08 CPX #8
363:D0 EF BNE _Draw
363:D0 EF BNE _Font
365:60 RTS
```