Let’s Make a Game!

Step 5: Automate Vertical Delay

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 double the Y range of the player objects. To use the new Y value for the 2LK we just need to divide it in half using the LSR command. The remainder of the divide, which ends up in the Carry flag, will conveniently tell us if we need to turn on Vertical Delay.

This routine preps the 2LK data for player0 and turns on VDELP0 if required (if you're wondering, VDELP0 is turned off in VerticalSync):

    ; prep Humanoid's Y position for 2LK 
        ldx #1              ; preload X for setting VDELPx
        lda ObjectY         ; get the human's Y position
        lsr                 ; divide by 2 for the 2LK position
        sta Temp            ; save for position calculations
        bcs NoDelay0        ; if carry is set we don't need Vertical Delay
        stx VDELP0          ; carry was clear, so set Vertical Delay
NoDelay0:        
    ; HumanDraw = ARENA_HEIGHT + HUMAN_HEIGHT - Y position
        lda #(ARENA_HEIGHT + HUMAN_HEIGHT)
        sec
        sbc Temp
        sta HumanDraw
        
    ; HumanPtr = HumanGfx + HUMAN_HEIGHT - 1 - Y position
        lda #<(HumanGfx + HUMAN_HEIGHT - 1)
        sec
        sbc Temp
        sta HumanPtr
        lda #>(HumanGfx + HUMAN_HEIGHT - 1)
        sbc #0
        sta HumanPtr+1

One minor problem with the prior 2LK was that player1 could not show up on the topmost scanline of the Arena:

Closeup:

To fix this, we'll modify the kernel to prime GRP1 before it enters the loop that draws the Arena:

        ldy #ARENA_HEIGHT+1 ; 2  7 - the arena will be 180 scanlines (from 0-89)*2
        
    ; prime GRP1 so player1 can appear on topmost scanline of the Arena        
        lda #BOX_HEIGHT-1   ; 2  9 - height of the box graphics, 
        dcp BoxDraw         ; 5 14 - Decrement BoxDraw and compare with height
        bcs DoDrawGrp1pre   ; 2 16 - (3 17) if Carry is Set, then box is on current scanline
        lda #0              ; 2 18 - otherwise use 0 to turn off player1
        .byte $2C           ; 4 22 - $2C = BIT with absolute addressing, trick that
                            ;        causes the lda (BoxPtr),y to be skipped
DoDrawGrp1pre:              ;   17 - from bcs DoDrawGRP1pre
        lda (BoxPtr),y      ; 5 22 - load the shape for the box
        sta GRP1            ; 3 25 - @0-22, update player1 to draw box
        dey                 ; 2 27
        
ArenaLoop:                  ;   13 - from bpl ArenaLoop

The 2LK calculations for player1 used to be the same as for player0, but now must be modified to compensate for the priming of GRP1:

    ; prep box's Y position for 2LK
        lda ObjectY+1       ; get the box's Y position
        clc
        adc #1              ; add 1 to compensate for priming of GRP1
        lsr                 ; divide by 2 for the 2LK position
        sta Temp            ; save for position calculations
        bcs NoDelay1        ; if carry is set we don't need Vertical Delay
        stx VDELP1          ; carry was clear, so set Vertical Delay
NoDelay1:        
    ; BoxDraw = ARENA_HEIGHT + BOX_HEIGHT - Y position + 1
    ; the + 1 compensates for priming of GRP1
        lda #(ARENA_HEIGHT + BOX_HEIGHT +1)
        sec

        sbc Temp
        sta BoxDraw
        
    ; BoxPtr = BoxGfx + BOX_HEIGHT - 1 - Y position
        lda #<(BoxGfx + BOX_HEIGHT - 1)
        sec
        sbc Temp
        sta BoxPtr
        lda #>(BoxGfx + BOX_HEIGHT - 1)
        sbc #0
        sta BoxPtr+1

Added GRP1 priming which allows player1 to cover full Arena:

Closeup:

Lastly, I added a new Box graphic for player1:

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