[[Category:6502]][[Category:Assembly Language]][[Category:SPO600]]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 ©2020-2022 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.'''

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 colourto the value of the pointer + Y iny ; increment index- prepare to fill next pixel bne loop ; branch until overflowpage done - stops when Y==0

inc $11 ; increment hi high byte of pointer lda $11 ; load page number as colour

rts brk ; done - returnto debugger

=== 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

con edone: brk ; when we're done, break (stop the program)

text: ; this is the text message

lda $11 ; carry to high byte if needed

data: ; graphic to be displayed

; 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 on 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