6502 snake (destructive edition)

;  ___           _        __ ___  __ ___
; / __|_ _  __ _| |_____ / /| __|/  \_  )
; \__ \ ' \/ _` | / / -_) _ \__ \ () / /
; |___/_||_\__,_|_\_\___\___/___/\__/___|
; Change direction: W A S D
define appleL         $00 ; screen location of apple, low byte
define appleH         $01 ; screen location of apple, high byte
define snakeHeadL     $10 ; screen location of snake head, low byte
define snakeHeadH     $11 ; screen location of snake head, high byte
define snakeBodyStart $12 ; start of snake body byte pairs
define snakeDirection $02 ; direction (possible values are below)
define snakeLength    $03 ; snake length, in bytes
; Directions (each using a separate bit)
define movingUp      1
define movingRight   2
define movingDown    4
define movingLeft    8
; ASCII values of keys controlling the snake
define ASCII_w      $77
define ASCII_a      $61
define ASCII_s      $73
define ASCII_d      $64
; System variables
define sysRandom    $fe
define sysLastKey   $ff
  jsr init
  jsr loop
init:
  jsr initSnake
  jsr generateApplePosition
  rts
initSnake:
  lda #movingRight  ;start direction
  sta snakeDirection
  lda #4  ;start length (2 segments)
  sta snakeLength
  lda #$11
  sta snakeHeadL
  lda #$10
  sta snakeBodyStart
  lda #$0f
  sta $14 ; body segment 1
  lda #$04
  sta snakeHeadH
  sta $13 ; body segment 1
  sta $15 ; body segment 2
  rts
generateApplePosition:
  ;load a new random byte into $00
  lda sysRandom
  sta appleL
  ;load a new random number from 2 to 5 into $01
  lda sysRandom
  and #$03 ;mask out lowest 2 bits
  clc
  adc #2
  sta appleH
  rts
loop:
  jsr readKeys
  jsr checkCollision
  jsr updateSnake
  jsr drawApple
  jsr drawSnake
  jsr spinWheels
  jmp loop
readKeys:
  lda sysLastKey
  cmp #ASCII_w
  beq upKey
  cmp #ASCII_d
  beq rightKey
  cmp #ASCII_s
  beq downKey
  cmp #ASCII_a
  beq leftKey
  rts
upKey:
  lda #movingDown
  bit snakeDirection
  bne illegalMove
  lda #movingUp
  sta snakeDirection
  rts
rightKey:
  lda #movingLeft
  bit snakeDirection
  bne illegalMove
  lda #movingRight
  sta snakeDirection
  rts
downKey:
  lda #movingUp
  bit snakeDirection
  bne illegalMove
  lda #movingDown
  sta snakeDirection
  rts
leftKey:
  lda #movingRight
  bit snakeDirection
  bne illegalMove
  lda #movingLeft
  sta snakeDirection
  rts
illegalMove:
  rts
checkCollision:
  jsr checkAppleCollision
  jsr checkSnakeCollision
  rts
checkAppleCollision:
  lda appleL
  cmp snakeHeadL
  bne doneCheckingAppleCollision
  lda appleH
  cmp snakeHeadH
  bne doneCheckingAppleCollision
  ;eat apple
  inc snakeLength
  inc snakeLength ;increase length
  jsr generateApplePosition
doneCheckingAppleCollision:
  rts
checkSnakeCollision:
  ldx #2 ;start with second segment
snakeCollisionLoop:
  lda snakeHeadL,x
  cmp snakeHeadL
  bne continueCollisionLoop
maybeCollided:
  lda snakeHeadH,x
  cmp snakeHeadH
  beq didCollide
continueCollisionLoop:
  inx
  inx
  cpx snakeLength          ;got to last section with no collision
  beq didntCollide
  jmp snakeCollisionLoop
didCollide:
  jmp gameOver
didntCollide:
  rts
updateSnake:
  ldx snakeLength
  dex
  txa
updateloop:
  lda snakeHeadL,x
  sta snakeBodyStart,x
  dex
  bpl updateloop
  lda snakeDirection
  lsr
  bcs up
  lsr
  bcs right
  lsr
  bcs down
  lsr
  bcs left
up:
  lda snakeHeadL
  sec
  sbc #$20
  sta snakeHeadL
  bcc upup
  rts
upup:
  dec snakeHeadH
  lda #$1
  cmp snakeHeadH
  beq collision
  rts
right:
  inc snakeHeadL
  lda #$1f
  bit snakeHeadL
  beq collision
  rts
down:
  lda snakeHeadL
  clc
  adc #$20
  sta snakeHeadL
  bcs downdown
  rts
downdown:
  inc snakeHeadH
  lda #$7
  cmp snakeHeadH
  beq collision
  rts
left:
  dec snakeHeadL
  lda snakeHeadL
  and #$1f
  cmp #$1f
  beq collision
  rts
collision:
  jmp gameOver
drawApple:
  ldy #0
  lda sysRandom
  sta (appleL),y
  rts
drawSnake:
  ldx #0
  lda #1
  sta (snakeHeadL,x) ; paint head
  ldx snakeLength
  lda #0
  sta (snakeHeadL,x) ; erase end of tail
  rts
spinWheels:
  ldx #0
spinloop:
  nop
  nop
  dex
  bne spinloop
  rts
gameOver:
Debugger
Monitor Start: $ Length: $