6502 Emulator Example Code
Revision as of 15:03, 10 January 2020 by Chris Tyler (talk | contribs) (→Place a Message on the Character Display)
This is a collection of simple examples of 6502 code which will run in the 6502 Emulator. To use this code, copy and paste one of these programs into the text box of the emulator.
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 page done
inc $11 ; increment high byte of pointer
lda $11 ; load page number as colour
cpx $11 ; compare with max value
bne loop ; continue if not done
brk ; done - return to debugger
Place a Message on the Character Display
define SCREEN $f000
ldy #$00
char: lda text,y
beq done
sta SCREEN,y
iny
bne char
done: 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