| layout |
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basic |
To use the disassembler, click Assemble, then Disassemble. Back to Easy 6502.
{% include start.html %} start: jsr init
loop: jsr drawMap jsr genMap jmp loop
testMemory: rts pha txa pha
lda #0 ldx $10 sta $500,x ldx $78 lda #1 sta $500,x stx $10
lda #0 ldx $11 sta $500,x ldx $79 lda #3 sta $500,x stx $11
lda #0 ldx $12 sta $500,x ldx $7a lda #4 sta $500,x stx $12
lda #0 ldx $13 sta $500,x ldx $7b lda #4 sta $500,x stx $13
pla tax pla
rts
init: ldx #0 lda walls
;draw exactly 256 pixels of wall at top and bottom drawinitialwalls: sta $200,x ;draw the top bit of wall sta $400,x ;draw the bottom bit of wall dex ;count down from 0 cpx #0 ;until we hit 0 bne drawinitialwalls
lda #$10 sta $80 ldx #$0f
;fill $81-$90 with $10 (initial wall offset) setinitialwalloffsets: sta $81,x ; target dex bpl setinitialwalloffsets rts
;--
drawMap: lda #$00 sta $78 lda #$20 sta $79 lda #$c0 sta $7a lda #$e0 sta $7b
ldx #$0f drawLoop: lda $81,x sta $82,x ;shift wall offsets along
tay sty $02 ;store current wall offset in $02 lda pixels,y ;lookup current wall offset in pixels sta $00 ;and store it in $00 iny lda pixels,y ;lookup current wall offset + 1 in pixels sta $01 ;and store it in $01 ;$00 now points to a two-byte pixel memory location
lda walls ldy $78 ;top edge of wall sta ($00),y iny sta ($00),y
ldy $7b sta ($00),y ;bottom edge of wall iny sta ($00),y
ldy $79 ;top edge of tunnel lda #0 ;black for tunnel sta ($00),y iny sta ($00),y
ldy $7a sta ($00),y ;bottom edge of tunnel iny sta ($00),y
; move offsets right two pixels inc $78 inc $79 inc $7a inc $7b inc $78 inc $79 inc $7a inc $7b dex bpl drawLoop rts
;---
genMap: lda $80 ;$80 is next wall inflection point cmp $81 ;$81 is next wall offset beq newinflectionpoint lda $80 clc sbc $81 ;is next wall offset above or below inflection point? bpl raisewalls bmi lowerwalls newinflectionpoint: lda $fe and #$f ;make 4-bit asl ;double (make even number) sta $80 ;set $80 to random value rts lowerwalls: dec $81 dec $81 rts raisewalls: inc $81 inc $81 rts
pixels: dcb $00,$02,$20,$02,$40,$02,$60,$02 dcb $80,$02,$a0,$02,$c0,$02,$e0,$02 dcb $00,$03,$20,$03,$40,$03,$60,$03 dcb $80,$03,$a0,$03,$c0,$03,$e0,$03
walls: dcb $d {% include end.html %}