6502.org

Posted: **Sat Apr 14, 2018 11:53 pm**

Hi. I've only recently started to learn 6502 code and decided to start off by breaking down the classic snake game found herehttps://skilldrick.github.io/easy6502/#snake . I'm trying to break it down into simple parts first by just making two pixels go down the assembler and then stop without any user input but am having trouble making it work as I described. Can anybody assist me in making it work at this basic level? Here is what i have so far which only creates 4 pixels and then stops.

Code: Select all

define snakeLength $00
define snakeHeadL $10
define snakeHeadH $11
define snakeBodyStart $12 

init:  ;initialize the values of the lo and hi bytes
       ;of the snake head and its length
  lda #$04
  sta snakeLength
  
  lda #$21
  sta snakeHeadL
  
  lda #$01
  sta snakeBodyStart
  
  lda #$14
  sta $02
  
  lda #$22
  sta snakeHeadH
  sta $13
  sta $15
  
loop:  ;this is the main game loop
  jsr shiftValues
  jsr updateHead
  jsr incHead
  jsr gameCheck
  jsr drawSnake
  jmp loop
  
gameCheck:
  ldx #2
  lda snakeHeadL, x
  cmp snakeHeadL
  lda snakeHeadH,x
  cmp snakeHeadH
  beq gameOver
  inx
  inx
  cpx snakeLength
  beq drawSnake
  jmp gameCheck
  
incHead:
  inc snakeHeadH
  lda #$6
  cmp snakeHeadH
  beq gameOver
  rts
  
updateHead:  ;this will move the head one row down
             ;the assembler
  LDA snakeHeadL
  CLC
  ADC #$20
  STA snakeHeadL
  rts
  
shiftValues:    ;this will shift the values of the 
                ;snakeHead to the body
  LDX snakeLength
  DEX
  LDA snakeHeadL, X ;will give body pixel
  STA snakeBodyStart, X
  rts
  
drawSnake:  ;this will draw the snake at the current
            ;position after the updates
  ldx snakeLength
  lda #$01
  sta (snakeHeadL,x)

  ldx #$00
  lda #$01
  sta (snakeHeadL,x)
  rts
  
gameOver:

Posted: **Sun Apr 15, 2018 3:49 am**

One first-glance problem is that your shiftValues routine needs to be a loop.

Posted: **Sun Apr 15, 2018 6:26 am**

Welcome braker!

A bit of single-stepping can help you figure out what's going on. With experience you can simulate things in your head, or, easier, on paper. But at this level of programming you can't afford to skim-read a program you're debugging - every instruction counts, including the ones you forgot to write.

Another common debugging technique is to divide-and-conquer: put some kind of stop part way through the program, run up to that point, and then inspect the state of the machine and the state of memory to make sure everything is as it should be. Depending on the result, move your stop point earlier or later, and repeat.

There's no substitute for rolling up you sleeves and working through the program carefully. There are plenty of people here to help, but don't ask before you've tried pretty hard, or you'll just get the answer, and you won't get the valuable experience of finding the answer yourself.

Good luck!

Posted: **Tue Apr 17, 2018 4:39 am**

White Flame wrote:

One first-glance problem is that your shiftValues routine needs to be a loop.

Ok so after some time working the code I now have this but still doesn't seem to be working right. Any advice on what to do next? i think maybe my gameCheck isnt what it should be but I would like some advice on what to do next.

Code: Select all

define snakeLength $00
define snakeHeadL $10
define snakeHeadH $11
define snakeBodyStart $12 

init:  ;initialize the values of the lo and hi bytes
       ;of the snake head and its length
  lda #4   
  sta snakeLength
  
  lda #$00 
  sta snakeHeadL  

  lda #$02
  sta snakeBodyStart
  lda #$01
  sta $05 
  
  lda #$02
  sta snakeHeadH
  sta $03
  sta $04
  
loop:  ;this is the main game loop
  jsr shiftValues
  jsr updateHead
  jsr gameCheck
  jsr drawSnake
  jmp loop
  
gameCheck:
  ldx #2
  lda snakeHeadL, x
  cmp snakeHeadL
  lda snakeHeadH,x
  cmp snakeHeadH
  beq gameOver
  inx
  inx
  cpx snakeLength
  beq drawSnake
  jmp gameCheck
  
  
updateHead:  ;this will move the head one row down
             ;the assembler
  lda snakeHeadL
  clc
  adc #$20
  sta snakeHeadL
  
incHeadH:
  lda snakeHeadH
  clc
  adc #$01
  sta snakeHeadH
  cmp snakeHeadH
  rts
  
shiftValues:
  lda snakeHeadL, X 
  sta snakeBodyStart, X
  dex
  bpl shiftValues
  rts
  
drawSnake:  ;this will draw the snake at the current
            ;position after the updates
  ldx snakeLength
  lda #0
  sta (snakeHeadL,x)

  ldx #0
  lda #1
  sta (snakeHeadL,x)
  rts
  
gameOver:

Posted: **Tue Apr 17, 2018 9:59 pm**

Where do you suspect the problem lies? How far have you traced into it where it stops working? Have you verified that each subroutine works as expected in isolation?

Posted: **Sat Apr 21, 2018 8:37 am**

I have added a few comments to your code that appears strange to me - I admit that I actually don't understand how the game works. Nevertheless it may help you somehow.

Code: Select all

....
loop:  ;this is the main game loop
  jsr shiftValues
  jsr updateHead
  jsr gameCheck
  jsr drawSnake
  jmp loop
  
gameCheck:
  ldx #2
GAMECHKLOOP:
  lda snakeHeadL, x
  cmp snakeHeadL
  lda snakeHeadH,x
  cmp snakeHeadH      -- this is not a valid 16 bit compare if you intended that
  beq gameOver
  inx
  inx
  cpx snakeLength
  beq drawSnake       -- bne GAMECHKLOOP
  jmp gameCheck       -- rts
  
  
updateHead:  ;this will move the head one row down
             ;the assembler
  lda snakeHeadL
  clc
  adc #$20
  sta snakeHeadL
  
incHeadH:
  lda snakeHeadH
  clc                     -- I don't know how the snake creeps, but if this should be
  adc #$01                -- snakeHead = snakeHead+$20 then the clc is wrong
  sta snakeHeadH
  cmp snakeHeadH          -- cmp ? this would only set the Z flag, but you don't use this anyway
  rts
  
shiftValues:
  lda snakeHeadL, X 
  sta snakeBodyStart, X
  dex
  bpl shiftValues
  rts
  
drawSnake:  ;this will draw the snake at the current
            ;position after the updates
  ldx snakeLength
  lda #0
  sta (snakeHeadL,x)

  ldx #0
  lda #1
  sta (snakeHeadL,x)
  rts
  
gameOver:

Edit: I inserted Garths very important hint (see below).

Posted: **Sat Apr 21, 2018 7:01 pm**

Should your "jmp gameCheck" be "JMP gameCheck+2"? As you have it, X won't advance; because although you increment it, it keeps getting re-initialized to 2. Unless interrupts are changing the variables, the exit condition will never be met. If you do the +2 on the jump address, you'll skip the LDX #2 part that's re-initializing it every time, so it can advance.

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