6502 SNAKE

Play a game of Snake written in 6502 assembly!

Snake

W: UP A: LEFT S: DOWN D: RIGHT

;6502 asm SNAKE code define snakeLength $00 define snakeHeadL $10 define snakeHeadH $11 define snakeBodyStartL $12 define snakeBodyStartH $13 define snakeTailL $14 define snakeTailH $15 define lastKeyPressed $ff define snakeDir $01 define ASCII_w $77 define ASCII_a $61 define ASCII_s $73 define ASCII_d $64 define UP $01 define RIGHT $02 define DOWN $03 define LEFT $04 define appleLoPos $02 define appleHiPos $03 define randomByte $FE init: ;Load up the snake pieces. LDA #$04 STA snakeLength LDA #$40 STA snakeHeadL LDA #$02 STA snakeHeadH LDA #$20 STA snakeBodyStartL LDA #$02 STA snakeBodyStartH LDA #$00 STA snakeTailL LDA #$02 STA snakeTailH ;Start of the game board. LDA #$01 LDX #$00 STA snakeHeadL, x STA snakeBodyStartL, x LDA #DOWN STA snakeDir ;Initializing the Apple JSR setApplePos JSR drawApple ;Main Game Loop loop: JSR readKeys JSR checkAppleCollision JSR updateSnake JSR checkSnakeCollision JSR drawSnake JSR drawApple JSR slowDown JMP loop ;Read in keys from the user. readKeys: LDA lastKeyPressed CMP #ASCII_w BEQ upDir CMP #ASCII_d BEQ rightDir CMP #ASCII_s BEQ downDir CMP #ASCII_a BEQ leftDir RTS checkAppleCollision: ;With apple LDA snakeHeadL CMP appleLoPos ;If the lo bites match, then check the high bites. BEQ checkCollisionAppleHi RTS checkCollisionAppleHi: LDA snakeHeadH CMP appleHiPos BEQ collisionWithApple RTS collisionWithApple: LDA #$01 LDX #$00 STA (appleLoPos, x) ;Cover up the apple with a white pixel when eaten. LDA snakeLength CLC ADC #$02 STA snakeLength ;brk JSR setApplePos RTS upDir: LDA snakeDir CMP #DOWN BEQ earlyReturn LDA #UP STA snakeDir RTS rightDir: LDA snakeDir CMP #LEFT BEQ earlyReturn LDA #RIGHT STA snakeDir RTS downDir: LDA snakeDir CMP #UP BEQ earlyReturn LDA #DOWN STA snakeDir RTS leftDir: LDA snakeDir CMP #RIGHT BEQ earlyReturn LDA #LEFT STA snakeDir RTS updateSnake: ;Shift values, PART 1 LDX snakeLength DEX updateLoop: ;brk LDA snakeHeadL, x STA snakeBodyStartL, x DEX BPL updateLoop ;update the head, PART 2 LDA snakeDir CMP #UP BEQ updateUpSnake CMP #RIGHT BEQ updateRightSnake CMP #DOWN BEQ updateDownSnake CMP #LEFT BEQ updateLeftSnake RTS checkSnakeCollision: LDX #$00 LDA (snakeHeadL,x) CMP #$01 ;If there is a white pixel already there, then end the game. BEQ endGame RTS earlyReturn: RTS updateUpSnake: SEC LDA snakeHeadL SBC #$20 STA snakeHeadL BCC updateHiPosSubtract RTS updateRightSnake: CLC LDA snakeHeadL ADC #$01 STA snakeHeadL LDA snakeHeadL AND #$1f BEQ endGame RTS updateDownSnake: CLC LDA snakeHeadL ADC #$20 STA snakeHeadL BCS updateHiPos ;If it did carry then update the hiByte so we can get to the next quadrant. RTS updateLeftSnake: SEC LDA snakeHeadL SBC #$01 STA snakeHeadL LDA snakeHeadL AND #$1f CMP #$1f BEQ endGame RTS drawSnake: ;Drawing the black tail pixel LDA #$00 LDX snakeLength STA (snakeHeadL, x) ;Drawing the white head pixel LDA #$01 LDX #$00 STA (snakeHeadL, x) RTS ;Draws the apple in a random location flashing different colors. drawApple: LDA randomByte AND #$0D CLC ADC #$02 LDX #$00 STA (appleLoPos, x) RTS ;Sets the apple a random position. setApplePos: LDA randomByte AND #$03 CLC ADC #$02 STA appleHiPos LDA randomByte STA appleLoPos RTS slowDown: LDX #$0 slowLoop: NOP DEX BNE slowLoop RTS updateHiPos: INC snakeHeadH LDA #$06 CMP snakeHeadH ;Making sure that we don't go past the edge of the box. BEQ endGame ;BNE drawSnake ;If we are still in the box then go back to updating the loByte. RTS updateHiPosSubtract: DEC snakeHeadH LDA #$01 CMP snakeHeadH ;Making sure that we dont go past the top of the screen. BEQ endGame ;BNE drawSnake ;If we are still on the screen then go back to drawing the snake. RTS ;Ends the game and creates a pixel skull and bones. endGame: LDA #$01 ;LDA #$02 STA $028a STA $0295 STA $02a9 STA $02aa STA $02ac STA $02ad STA $02ae STA $02af STA $02b0 STA $02b1 STA $02b2 STA $02b3 STA $02b5 STA $02b6 ;LDA #$02 STA $02cb STA $02cc STA $02cd STA $02ce STA $02cf STA $02d0 STA $02d1 STA $02d2 STA $02d3 STA $02d4 ;LDA #$02 STA $02ea STA $02eb STA $02ec STA $02ed STA $02ee STA $02ef STA $02f0 STA $02f1 STA $02f2 STA $02f3 STA $02f4 STA $02f5 ;LDA #$02 STA $030a STA $030b STA $030c ;STA $030d STA $030e STA $030f STA $0310 STA $0311 ;STA $0312 STA $0313 STA $0314 STA $0315 ;LDA #$02 STA $032a STA $032b ;STA $032c ;STA $032d ;STA $032e STA $032f STA $0330 ;STA $0331 ;STA $0332 ;STA $0333 STA $0334 STA $0335 ;LDA #$02 STA $034a STA $034b ;STA $034c ;STA $034d STA $034e STA $034f STA $0350 STA $0351 ;STA $0352 ;STA $0353 STA $0354 STA $0355 ;LDA #$02 ;STA $036a STA $036b STA $036c STA $036d STA $036e ;STA $036f ;STA $0370 STA $0371 STA $0372 STA $0373 STA $0374 ;STA $0375 ;LDA #$02 STA $0389 STA $038a ;STA $038b ;STA $038c STA $038d STA $038e STA $038f STA $0390 STA $0391 STA $0392 ;STA $0393 ;STA $0394 STA $0395 STA $0396 ;LDA #$01 ;STA $03a9 STA $03aa ;STA $03ab ;STA $03ac STA $03ad ;STA $03ae STA $03af STA $03b0 ;STA $03b1 STA $03b2 ;STA $03b3 ;STA $03b4 STA $03b5 ;STA $03b6 BRK

6502 JS Assembler GitHub: skilldrick, licensed under CC BY 4.0