Lab 3 - Pong pt.1

For this Lab we have begun to build off of our knowledge in 6502 assembly in order to make a more robust program. We were given five options in tasks to do and with very little extra help had to figure out how to achieve an effective result. The five options were to create a bouncing graphic (think dvd logo), to create a numeric display which displayed two digits, to create the game pong, to create a kaleidoscope where one quadrant is mirrored in the other three, lastly and most challenging to draw a line between to points that can be moved around in real time.

Our group chose to work on Pong since it seemed like an enjoyable app to create and at least a couple of us had a bit of a grasp on how they wanted to tackle the problem. We started off by looking at some example code that was provided for us for a fairly unrelated program but it allowed us to get some good ideas for how to create out code (Link to Example Code). This code specifically helped us with Three things.

1. How to turn a screen made of pages into coordinates
2. How to use those coordinates to draw on the screen
3. How to take keyboard input

Using this code we began to experiment and attempt to get a ball moving across the screen this resulted in the following code.

 ; zero-page variable locations
 define DOTROW        $20    ; current row
 define    DOTCOL        $21    ; current column
 define DOTDELTAX    $30    ; current Delta X
 define DOTDELTAY    $31    ; current Delta Y
 define    POINTER        $10    ; ptr: start of row
 define    POINTER_H    $11

 ; constants
 define    DOT        $01    ; dot colour
 define    CURSOR        $04    ; black colour


     ldy #$00    ; put help text on screen
 print:    lda help,y
     beq setup
     sta $f000,y
     iny
     bne print

 setup:    lda #$0f    ; set initial ROW,COL
     sta DOTROW
    lda #$02
     sta DOTCOL
    lda #$20    ;set angle to 45
    sta DOTDELTAX
    sta DOTDELTAY
   


 game:    lda DOTROW        ; ensure ROW is in range 0:31
     and #$1f
     sta DOTROW

     lda DOTCOL        ; ensure COL is in range 0:31
     and #$1f
     sta DOTCOL

     ldy DOTROW        ; load POINTER with start-of-row
     lda table_low,y
     sta POINTER
     lda table_high,y
     sta POINTER_H


    pha        ; save A

     lda #DOT    ; set current position to DOT
     sta (POINTER),y

     pla        ; restore A

DotMovA:lda DOTCOL
    inc DOTCOL
    lda DOTCOL

DotMovB:lda DOTROW
    inc DOTROW
    lda DOTROW



 done:    clc        ; repeat
     bcc game


 ; these two tables contain the high and low bytes
 ; of the addresses of the start of each row

 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

 ; help message on character screen

 help:
 dcb "A","r","r","o","w",32,"k","e","y","s"
 dcb 32,"d","r","a","w",32,"/",32,"'","C","'"
 dcb 32,"k","e","y",32,"c","l","e","a","r","s"
 dcb 00  

The code above is greatly unaltered from the etch-a-sketch code. All it does is use that code to draw a line across the screen continuously but It allowed us to learn quite a lot about how to get a ball moving across the screen since it practically is that just without the previous position being removed thus a line is drawn. So from that base it is quite simple to begin to work out how a ball will move properly such as in pong which will be talked about in the next Blog.