===== 6502 Emulator Example Code =====
This is a collection of simple examples of [[6502]] [[assembly language|assembly language]] 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 [[http://6502.cdot.systems|emulator]].
**This code is Copyright©2020-2024 Seneca College of Applied Arts and Technology. Each of these programs is free software; you can redistribute them and/or modify them under the terms of the [[https://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html|GNU|General Public License]] as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.**
===== Fill the Bitmapped Display =====
lda #$00 ; set pointer at $10 to $0200
sta $10
lda #$02
sta $11
ldx #$06 ; max value for $11, the high byte of the pointer
ldy #$00 ; index - this value is added to the pointer
lda #$07 ; colour code to be used to fill the display
loop: sta ($10),y ; store colour to the value of the pointer + Y
iny ; increment index - prepare to fill next pixel
bne loop ; branch until page done - stops when Y==0
inc $11 ; increment high byte of pointer
cpx $11 ; compare with max value
bne loop ; continue if not done
brk ; done - return to debugger
===== Place a Message on the Character Display =====
==== Without using the ROM routines ====
define SCREEN $f000 ; location of screen memory
ldy #$00 ; index value (character we're currently processing)
char: lda text,y ; get a character from address (text + Y)
beq done ; if the character is NULL, branch to done
sta SCREEN,y ; store character at (SCREEN + Y)
iny ; increment Y (go to next character)
bne char ; repeat loop
done: brk ; when we're done, break (stop the program)
text: ; this is the text message
dcb "6","5","0","2",32,"w","a","s",32,"h","e","r","e",".",00
==== Using the ROM routines ====
; ROM routines
define SCINIT $ff81 ; initialize/clear screen
define CHRIN $ffcf ; input character from keyboard
define CHROUT $ffd2 ; output character to screen
define SCREEN $ffed ; get screen size
define PLOT $fff0 ; get/set cursor coordinates
jsr SCINIT ; initialize and clear the screen
ldy #$00
char: lda text,y
beq done
jsr CHROUT ; put the character in A on to the screen
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!
; does not use ROM routines
; 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: ; graphic to be displayed
dcb 00,03,03,00
dcb 07,00,00,07
dcb 07,00,00,07
dcb 00,03,03,00
===== Etch-a-Sketch™ 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
beq getkey
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 for the 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
===== Additional Examples =====
Additional examples, as well as the source code for the emulator's ROM routines, are in a repository at https://github.com/ctyler/6502js-code