# Step 5 - automate Vertical Delay

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

ROM

Source

COLLECT TUTORIAL NAVIGATION

## 18 Comments

## Recommended Comments