Let’s Make a Game!

Step 7: Draw the Playfield

By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)

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Original Blog Entry

For this update, we're going to modify the Arena Loop to draw the Arena using the playfield. The new Arena loop has these new changes:

ArenaLoop:                  ;   27 - (currently 7 from bpl ArenaLoop)
        tya                 ; 2 29 - 2LK loop counter in A for testing
        and #%11            ; 2 31 - test for every 4th time through the loop,
        bne SkipX           ; 2 33 (3 34) branch if not 4th time
        inx                 ; 2 35 - if 4th time, increase X so new playfield data is used
SkipX:                      ;   35 - use 35 as it's the longest path here        
...
    ; start of line 1 of the 2LK
        sta GRP1            ; 3  3 - @0-22, update player1 graphics
        lda ArenaPF0,x      ; 4  7 - get current scanline's playfield pattern
        sta PF0             ; 3 10 - @0-22 and update it
        lda ArenaPF1,x      ; 4 14 - get current scanline's playfield pattern
        sta PF1             ; 3 17 - @71-28 and update it
        lda ArenaPF2,x      ; 4 21 - get current scanline's playfield pattern
        sta PF2             ; 3 24 - @60-39
...
    ; start of line 2 of the 2LK
        sta GRP0            ; 3  3 - @0-22, update player0 graphics
        dey                 ; 2  5 - decrease the 2LK loop counter
        bne ArenaLoop       ; 2  7 - (3  8) branch if there's more Arena to draw
        sty PF0             ; 3 10 - Y is 0, blank out playfield
        sty PF1             ; 3 13 - Y is 0, blank out playfield
        sty PF2             ; 3 16 - Y is 0, blank out playfield
        rts                 ; 6 22 - ReTurn from Subroutine

The first change is we're using X as an index into the playfield graphic data. We're changing X every fourth time thru the 2LK, so each byte of playfield data will be used over 8 scanlines. This saves a bit of ROM.

Second change is all 3 playfield registers (PF0, PF1 and PF2) are now updated, and they're only updated on line 1 of our 2LK.

Third change is on line 2, the bpl ArenaLoop is now a bne ArenaLoop else the bottom row of playfield data was only used for 2 scanlines instead of 8. We also blank out the playfield registers when we are done drawing the playfield. The bne change also impacted Overscan—TIM64T was originally set to 32, it's now set to 35.

The playfield data looks like this in jEdit:

And this onscreen:

Lastly we added some collision detection code. Some space was allocated in RAM:

   ;save player locations for playfield collision logic
SavedX:         ds 2    ; stored in $A1-A2
SavedY:         ds 2    ; stored in $A3-A4

Then the Process Joystick routines save the current X and Y values before processing the joystick:

PJloop:    
        ldy ObjectX,x   ; save original X location so the player can be
        sty SavedX,x    ;   bounced back upon colliding with the playfield
        ldy ObjectY,x   ; save original Y location so the player can be
        sty SavedY,x    ;   bounced back upon colliding with the playfield

Finally OverScan was modified to move the players back to their previous X and Y location if a collision was detected:

; Test if player collided with playfield
        bit CXP0FB      ; N = player0/playfield, V=player0/ball
        bpl notP0PF     ; if N is off, then player0 did not collide with playfield
        lda SavedX      ; recall saved X
        sta ObjectX     ; and move player back to it
        lda SavedY      ; recall saved Y
        sta ObjectY     ; and move player back to it
notP0PF:        
        bit CXP1FB      ; N = player1/playfield, V=player1/ball
        bpl notP1PF     ; if N is off, then player1 did not collide with playfield
        lda SavedX+1    ; recall saved X
        sta ObjectX+1   ; and move player back to it
        lda SavedY+1    ; recall saved Y
        sta ObjectY+1   ; and move player back to it
notP1PF:

The ROM and the source are at the bottom of my blog entry.