gistfile1.txt

; Written by Renato Araujo, 2023
; renatorraraujo@gmail.com
; @renatoexpert

; Assembly demo for 6502
; I am playing this at the following app
; https://play.google.com/store/apps/details?id=com.redlee90.learn6502assembly
; Addresses may vary from device to device,
; eg. atari, nes, motorola...

define input_addr $ff
define random_addr $fe

define black $00
define white $01
define blue $03

JSR init
JSR loop

init: 
    LDA #$11
    STA $00
    LDA #$03
    STA $01
    RTS 

loop: 
    JSR clear
    JSR getkey
    JSR draw
    JSR sleep
    JMP loop 

clear: ; clear current position
    LDY #$00
    LDA random_addr
    STA ($00),y
    RTS 

getkey: 
    CLC 
    LDA input_addr 
    CMP #$77
    BEQ upkey
    CMP #$64 
    BEQ rightkey
    CMP #$73
    BEQ downkey
    CMP #$61
    BEQ leftkey
    RTS 

rightkey:
    CLC 
    LDA $00
    ADC #$01
    AND #$1F
    CMP #$00 
    BNE goright 
    RTS 

goright: 
    INC $00
    RTS 

leftkey: 
    CLC 
    LDA $00
    AND #$1F
    CMP #$00
    BNE goleft 
    RTS 

goleft: 
    DEC $00 
    RTS 

upkey:
    LDA $00
    AND #$E0 
    CMP #$00 
    BNE goup
    LDA $01
    CMP #$02
    BNE goup 
    RTS 

goup:
    LDA $00
    SBC #$20
    STA $00
    BCC dec_area
    RTS 

downkey: 
    LDA $00 
    AND #$E0
    CMP #$E0
    BNE  godown 
    LDA $01
    CMP #$05
    BNE godown 
    RTS 

godown:
    CLC 
    LDA $00
    ADC #$20
    STA $00
    BCS pre_inc_area 
    RTS 

pre_inc_area: 
    LDA $01
    CMP #$05
    BNE inc_area 
    RTS 

inc_area:
    INC $01
    RTS 

dec_area: 
    DEC $01
    RTS 

draw:
    LDY  #$00
    LDA #$01
    STA ($00),y
    RTS 

sleep:
    LDX #$00
    LDY #$00
    JSR sleepcycle 
    RTS 

sleepcycle:    
    NOP
    NOP
    DEX 
    BNE sleepcycle 
    DEY 
    BNE sleepcycle  
    RTS