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.
Other Assembly Language Tutorials
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Table of Contents for Let’s Make a Game!
Step 1: Generate a Stable Display
Step 3: Score and Timer Display
Step 5: Automate Vertical Delay
Step 8: Select and Reset Support
Step 12: Add the Missile Objects
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The "bankswitching bible." Also check out the Atari 2600 Fun Facts and Information Guide and this post about bankswitching by SeaGtGruff at AtariAge.
Atari 2600 programming specs (HTML version).
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Links to useful information, tools, source code, and documentation.
Atari 2600 programming site based on Garon's "The Dig," which is now dead.
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