RE-POST February: Lab 2

This post will be about an interesting mini-project and its implementation.

6502 Math and String

Criteria to be met

Must work in 6502 Emulator.
Must output to the character screen as well as the graphics screen.
Must accept user input from the keyboard.
Must use some arithmetic/math instructions.

Simple Game Chosen: Maze

I have always been interested in games and have previously coded in C++ and JavaScript a simple maze game and wanted to try doing it in 6502. This was harder than I expected, especially since I wasn't familiar with the assembly language. There was a lot of time when I had to refer to the documentation to understand what the code does, outside of what we had gone through in the first lab.

The following links are the documentation that I used to understand the language:

I have also leveraged these examples to create my game.

At first, when I saw the code for "Place a Graphic on the Screen" and copy paste the provided code to the 6502 emulator, I thought this would be a good way to create my maze and that I would just need to create a collision for the walls. Therefore, while playing around with the code and deciding how my maze would look like, my code for the maze has become:


    define HEIGHT 8
    define WIDTH 8
    drawMaze:
        lda #$21 
        sta POINT_MAZE_DRAWN_LOW 
        lda #$02
        sta POINT_MAZE_DRAWN_HIGH
        lda #$00
        sta DRAW_MAZE
        ldx #$00
        ldy #$00
    draw:
        lda maze_data,x
        sta (POINT_MAZE_DRAWN_LOW), y
        inx
        iny
        cpy #WIDTH 
        bne draw 
        inc DRAW_MAZE
        lda #HEIGHT
        cmp DRAW_MAZE
        beq done
        lda POINT_MAZE_DRAWN_LOW
        clc
        adc #$20
        sta POINT_MAZE_DRAWN_LOW
        lda POINT_MAZE_DRAWN_HIGH
        adc #$00
        sta POINT_MAZE_DRAWN_HIGH
        ldy #$00
        beq draw
    maze_data:
        dcb 01,01,00,01,01,01,10,00
        dcb 00,01,00,01,00,00,00,00
        dcb 01,01,00,01,01,00,01,00
        dcb 01,00,00,00,01,01,01,00
        dcb 01,01,01,00,00,00,01,00
        dcb 00,00,01,00,01,01,01,00
        dcb 00,01,01,01,01,00,01,01
        dcb 01,01,00,00,00,00,00,01

Afterward, I tried the "Etch-a-SketchTM Style Drawing" code and saw that I could use this logic to create my player path:


    define CURRENT_ROW $20
    define CURRENT_COL $21
    define DRAW_MAZE $22 
    define POINT_MAZE_DRAWN_LOW $14
    define POINT_MAZE_DRAWN_HIGH $15
    define POINT_PLAYER_LOW $10
    define POINT_PLAYER_HIGH $11
    define POINT_TARGET_LOW $12
    define POINT_TARGET_HIGH $13
    define PATH $04
    define PLAYER $0d
    

    gameInstruction: 
        ldy #$00
    pHelpLoop: 
        lda help,y
        beq done
        sta $f000,y
        iny
        bne pHelpLoop
    gameInitialization: 
        lda #$01
        sta CURRENT_ROW
        sta CURRENT_COL
        rts
    gameLoop: 
        jsr updatePosition
        jsr getInput
        jsr checkCollision
        ldx #$00
        stx $ff
        jmp gameLoop
    

    updatePosition: 
        ldy CURRENT_ROW
        lda table_low,y
        sta POINT_PLAYER_LOW
        lda table_high,y
        sta POINT_PLAYER_HIGH
        ldy CURRENT_COL     
        lda #PLAYER
        sta (POINT_PLAYER_LOW),y
        rts
    getInput: 
        lda $ff
        cmp #$80   
        bmi getInput
        cmp #$84
        bpl getInput
        pha     
        lda #PATH   
        sta (POINT_PLAYER_LOW),y
        pla     
        cmp #$80    
        bne checkRight
        dec CURRENT_ROW     
        rts
    checkRight: 
        cmp #$81 
        bne checkDown
        inc CURRENT_COL
        rts
    checkDown: 
        cmp #$82
        bne checkLeft
        inc CURRENT_ROW 
        rts
    checkLeft: 
        cmp #$83
        bne done
        dec CURRENT_COL
        rts
    done: 
        rts
    help:
        dcb "T","o",32,"p","l","a","y",44,32,"u","s","e",32
        dcb "t","h","e",32,"a","r","r","o","w",32,"k","e","y","s"
        dcb 00


    table_high:
        dcb $02,$02,$02,$02,$02,$02,$02,$02
        dcb $03,$03,$03,$03,$03,$03,$03,$03
        dcb $04,$04,$04,$04,$04,$04,$04,$04
        dcb $05,$05,$05,$05,$05,$05,$05,$05
    table_low:
        dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
        dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
        dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
        dcb $00,$20,$40,$60,$80,$a0,$c0,$e0

Now that I have both my maze map and my player, I have to add the wall collision and I will be using the ROM routines to clean and initialize the screen:


  define SCINIT $ff81
  jsr gameInstruction
  jsr drawMaze
  jsr gameInitialization
  jsr gameLoop
  checkCollision:
      ldy CURRENT_ROW
      lda table_low,y
      sta POINT_TARGET_LOW
    lda table_high,y
      sta POINT_TARGET_HIGH
      ldy CURRENT_COL
      lda (POINT_TARGET_LOW),y
      cmp #$01
      beq done
      cmp #$04
      beq done
      cmp #$0a
      beq gameComplete
      lda #$00
      sta (POINT_TARGET_LOW),y
      lda $ff
      cmp #$80
      bne ifRight
      inc CURRENT_ROW
      rts
  ifRight:
      cmp #$81
      bne ifDown
      dec CURRENT_COL
      rts
  ifDown:
      cmp #$82
      bne ifLeft
      dec CURRENT_ROW
      rts
  ifLeft:
      cmp #$83
      bne done
      inc CURRENT_COL
      rts
  gameComplete:
      jsr SCINIT
      ldy #$00
  pGameComplete:
      lda complete,y
      beq done
      sta $f000,y
      iny
      bne pGameComplete
      brk
  complete:
      dcb "C","o","n","g","r","a","t", "u", "l", "a", "t", "i", "o", "n", "!","!","!",32
      dcb "T","h","e",32,"g","a","m","e",32,"i","s",32,
      dcb "c","o","m","p","l","e","t","e","d","."
      dcb 00

Furthermore, I have leveraged the code in "Place a Message on the Character Display" to instruct the player how to play the game and a message at the end, when the game is completed.

After coding the maze map and player path separately because the code started to get too big, and scrolling up and down was inconvenient in the 6502 emulator. Now that I have the maze map with collision and the player path working individually, I need to combine them together. After, a couple of trial and error, I have realized that there is an order of how the code has to be placed. Such as when I put the maze map at the beginning, nothing is being shown, however leaving it at the end, would work correctly. Here is the final code for my simple maze game, using the 6502 emulator:


  define CURRENT_ROW $20
  define CURRENT_COL $21
  define DRAW_MAZE $22 
  define POINT_MAZE_DRAWN_LOW $14
  define POINT_MAZE_DRAWN_HIGH $15
  define POINT_PLAYER_LOW $10
  define POINT_PLAYER_HIGH $11
  define POINT_TARGET_LOW $12
  define POINT_TARGET_HIGH $13
  define PATH $04
  define PLAYER $0d
  define HEIGHT 8
  define WIDTH 8
  define SCINIT $ff81
  

      jsr gameInstruction
      jsr drawMaze
      jsr gameInitialization
      jsr gameLoop
  

  gameInstruction: 
  	ldy #$00
  pHelpLoop: 
  	lda help,y
  	beq done
  	sta $f000,y
  	iny
  	bne pHelpLoop
  

  gameInitialization: 
  	lda #$01
  	sta CURRENT_ROW
  	sta CURRENT_COL
  	rts


  gameLoop: 
  	jsr updatePosition
  	jsr getInput
  	jsr checkCollision
  	ldx #$00
  	stx $ff
  	jmp gameLoop
  
updatePosition: 
  	ldy CURRENT_ROW
  	lda table_low,y
  	sta POINT_PLAYER_LOW
  	lda table_high,y
  	sta POINT_PLAYER_HIGH
  	ldy CURRENT_COL     
      	lda #PLAYER
      	sta (POINT_PLAYER_LOW),y
      	rts


  getInput: 
  	lda $ff
      	cmp #$80   
      	bmi getInput
      	cmp #$84
      	bpl getInput
  	pha     
      	lda #PATH   
      	sta (POINT_PLAYER_LOW),y
      	pla     
      	cmp #$80    
      	bne checkRight
  	dec CURRENT_ROW     
      	rts
  

  checkRight: 
  	cmp #$81 
  	bne checkDown
  	inc CURRENT_COL
  	rts
  

  checkDown: 
  	cmp #$82
  	bne checkLeft
  	inc CURRENT_ROW 
  	rts


  checkLeft: 
  	cmp #$83
  	bne done
  	dec CURRENT_COL
  	rts
  

  done: 
  	rts
  

  checkCollision: 
  	ldy CURRENT_ROW 
  	lda table_low,y
  	sta POINT_TARGET_LOW
  	lda table_high,y
  	sta POINT_TARGET_HIGH
  	ldy CURRENT_COL     
      	lda (POINT_TARGET_LOW),y
      	cmp #$01
      	beq done
      	cmp #$04
      	beq done
      	cmp #$0a
      	beq gameComplete
      	lda #$00
      	sta (POINT_TARGET_LOW),y
      	lda $ff
      	cmp #$80    
      	bne ifRight
      	inc CURRENT_ROW     
      	rts
  

  ifRight: 
  	cmp #$81 
  	bne ifDown
      	dec CURRENT_COL     
      	rts
  

  ifDown: 
  	cmp #$82 
  	bne ifLeft
      	dec CURRENT_ROW     
      	rts
  ifLeft:
  	cmp #$83 
  	bne done
      	inc CURRENT_COL     
      	rts
  

  gameComplete: 
  	jsr SCINIT
  	ldy #$00
   
  pGameComplete: 
  	lda complete,y
  	beq done
  	sta $f000,y
  	iny
  	bne pGameComplete
  	brk
  

  drawMaze: 
  	lda #$21 
  	sta POINT_MAZE_DRAWN_LOW 
  	lda #$02
  	sta POINT_MAZE_DRAWN_HIGH
      	lda #$00    
      	sta DRAW_MAZE
      	ldx #$00    
      	ldy #$00
      
  draw: 
  	lda maze_data,x
  	sta (POINT_MAZE_DRAWN_LOW), y
  	inx
  	iny
  	cpy #WIDTH 
  	bne draw 
      	inc DRAW_MAZE   
      	lda #HEIGHT
      	cmp DRAW_MAZE   
      	beq done
      	lda POINT_MAZE_DRAWN_LOW
      	clc
      	adc #$20    
      	sta POINT_MAZE_DRAWN_LOW  
      	lda POINT_MAZE_DRAWN_HIGH
      	adc #$00
      	sta POINT_MAZE_DRAWN_HIGH
      	ldy #$00    
      	beq draw            
  

  help:
  	dcb "T","o",32,"p","l","a","y",44,32,"u","s","e",32
  	dcb "t","h","e",32,"a","r","r","o","w",32,"k","e","y","s"
  	dcb 00
  

  complete:
  	dcb "C","o","n","g","r","a","t", "u", "l", "a", "t", "i", "o", "n", "!","!","!",32
  	dcb "T","h","e",32,"g","a","m","e",32,"i","s",32,
  	dcb "c","o","m","p","l","e","t","e","d","."
  	dcb 00
  

  maze_data:
  	dcb 01,01,00,01,01,01,10,00
  	dcb 00,01,00,01,00,00,00,00
  	dcb 01,01,00,01,01,00,01,00
  	dcb 01,00,00,00,01,01,01,00
  	dcb 01,01,01,00,00,00,01,00
  	dcb 00,00,01,00,01,01,01,00
  	dcb 00,01,01,01,01,00,01,01
  	dcb 01,01,00,00,00,00,00,01
  

  table_high:
  	dcb $02,$02,$02,$02,$02,$02,$02,$02
  	dcb $03,$03,$03,$03,$03,$03,$03,$03
  	dcb $04,$04,$04,$04,$04,$04,$04,$04
  	dcb $05,$05,$05,$05,$05,$05,$05,$05
  

  table_low:
  	dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
  	dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
  	dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
  	dcb $00,$20,$40,$60,$80,$a0,$c0,$e0

After assembling the code and pressing on "Run":

When the player has reached their goal:

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