6502 Emulator Example Code
Revision as of 09:03, 10 January 2020 by Chris Tyler (talk | contribs) (→Place a Graphic on the Screen)
Fill the Bitmapped Display
lda #$00 ; set pointer at $10 to $0200
sta $10
lda #$02
sta $11
ldx #$06 ; max value for $11
ldy #$00 ; index
loop: sta ($10),y ; store colour
iny ; increment index
bne loop ; branch until overflow
inc $11 ; increment hi byte of pointer
lda $11 ; load page number as colour
cpx $11 ; compare with max value
bne loop ; continue if not done
rts ; return
Place a Message on the Character Display
define SCREEN $f000
ldy #$00
char: lda text,y
beq done
sta SCREEN,y
iny
bne char
con e: brk
text:
dcb "6","5","0","2",32,"w","a","s",32,"h","e","r","e",".",00
Type on the Screen
; let the user type on the first page of character screen
; has blinking cursor!
; backspace works (non-destructive), arrows/ENTER don't
next: ldx #$00
idle: inx
cpx #$10
bne check
lda $f000,y
eor #$80
sta $f000,y
check: lda $ff
beq idle
ldx #$00
stx $ff
cmp #$08 ; bs
bne print
lda $f000,y
and #$7f
sta $f000,y
dey
jmp next
print: sta $f000,y
iny
jmp next
Place a Graphic on the Screen
define WIDTH 4 ; width of graphic define HEIGHT 4 ; height of graphic lda #$25 ; create a pointer at $10 sta $10 ; which points to where lda #$02 ; the graphic should be drawn sta $11 lda #$00 ; number of rows we've drawn sta $12 ; is stored in $12 ldx #$00 ; index for data ldy #$00 ; index for screen column draw: lda data,x sta ($10),y inx iny cpy #WIDTH bne draw inc $12 ; increment row counter lda #HEIGHT ; are we done yet? cmp $12 beq done ; ...exit if we are lda $10 ; load pointer clc adc #$20 ; add 32 to drop one row sta $10 lda $11 ; carry to high byte if needed adc #$00 sta $11 ldy #$00 beq draw done: brk ; stop when finished data: dcb 00,03,03,00 dcb 07,00,00,07 dcb 07,00,00,07 dcb 00,03,03,00
Etch-a-Sketchtm Style Drawing
; zero-page variable locations define ROW $20 ; current row define COL $21 ; current column define POINTER $10 ; ptr: start of row define POINTER_H $11 ; constants define DOT $01 ; dot colour define CURSOR $04 ; black colour ldy #$00 ; put help text on screen print: lda help,y beq setup sta $f000,y iny bne print setup: lda #$0f ; set initial ROW,COL sta ROW sta COL draw: lda ROW ; ensure ROW is in range 0:31 and #$1f sta ROW lda COL ; ensure COL is in range 0:31 and #$1f sta COL ldy ROW ; load POINTER with start-of-row lda table_low,y sta POINTER lda table_high,y sta POINTER_H ldy COL ; store CURSOR at POINTER plus COL lda #CURSOR sta (POINTER),y getkey: lda $ff ; get a keystroke ldx #$00 ; clear out the key buffer stx $ff cmp #$43 ; handle C or c beq clear cmp #$63 beq clear cmp #$80 ; if not a cursor key, ignore bmi getkey cmp #$84 bpl getkey pha ; save A lda #DOT ; set current position to DOT sta (POINTER),y pla ; restore A cmp #$80 ; check key == up bne check1 dec ROW ; ... if yes, decrement ROW jmp done check1: cmp #$81 ; check key == right bne check2 inc COL ; ... if yes, increment COL jmp done check2: cmp #$82 ; check if key == down bne check3 inc ROW ; ... if yes, increment ROW jmp done check3: cmp #$83 ; check if key == left bne done dec COL ; ... if yes, decrement COL clc bcc done clear: lda table_low ; clear the screen sta POINTER lda table_high sta POINTER_H ldy #$00 tya c_loop: sta (POINTER),y iny bne c_loop inc POINTER_H ldx POINTER_H cpx #$06 bne c_loop done: clc ; repeat bcc draw ; these two tables contain the high and low bytes ; of the addresses of the start of each row table_high: dcb $02,$02,$02,$02,$02,$02,$02,$02 dcb $03,$03,$03,$03,$03,$03,$03,$03 dcb $04,$04,$04,$04,$04,$04,$04,$04 dcb $05,$05,$05,$05,$05,$05,$05,$05, table_low: dcb $00,$20,$40,$60,$80,$a0,$c0,$e0 dcb $00,$20,$40,$60,$80,$a0,$c0,$e0 dcb $00,$20,$40,$60,$80,$a0,$c0,$e0 dcb $00,$20,$40,$60,$80,$a0,$c0,$e0 ; help message on character screen help: dcb "A","r","r","o","w",32,"k","e","y","s" dcb 32,"d","r","a","w",32,"/",32,"'","C","'" dcb 32,"k","e","y",32,"c","l","e","a","r","s" dcb 00