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Michaelangel007 2016-01-27 09:59:54 -08:00
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@ -1,6 +1,6 @@
#Apple ]\[ //e HGR Font 6502 Assembly Language Tutorial
Revision: 46, Jan 27, 2016.
Revision: 47, Jan 27, 2016.
# Table of Contents
@ -741,7 +741,7 @@ Listing 1:
0350: _DrawChar
0350:A2 00 LDX #0
0352: _LoadFont ; A = font[ offset ]
0352:BD 00 62 LDA Font+#$200,X
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
@ -1002,23 +1002,28 @@ We'll briefly touch upon this topic of optimization again with `bit-shifts` and
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: _DrawChar1
034C:A6 F6 LDX TmpHi
034E:86 FD STX TopHi
; === _DrawChar begin ===
; ORG $0350
;0350: _DrawChar
; ...
;035F:E0 08 CPX #8
;0361:D0 EF BNE _LoadFont
;0363:60 RTS
; === DrawChar end ===
; === _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
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
@ -1027,18 +1032,24 @@ Wait, you say! There IS a way to solve this problem -- and it doesn't take later
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 5:
```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:
@ -1068,6 +1079,45 @@ We are one step closer to printing a string. We have a total of 5 `@` because w
<hr>
# Recap - Listing 4
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 $310
DrawChar:
JMP _DrawChar1
.ORG $34C
_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:
@ -1259,7 +1309,7 @@ Our prefix code to setup the source address becomes:
0350: _DrawChar
0350:A2 00 LDX #0 ; Note: next instruction is self-modified !
0352: _LoadFont ; A = font[ offset ]
0352:BD 00 00 LDA Font+#$200,X
0352:BD 00 00 LDA $0000,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: