By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
Original Blog Entry
Let's review the TIA Timing diagram from last time:
We used that to determine when we could safely update the playfield data in order to draw the score and timer. For moveable objects (player0, player1, missile0, missile1 and ball) if you update their graphics during the Visible Screen (cycles 23-76) you run the risk of shearing. For something that's moving fast, like the snowball in Stay Frosty 2, shearing may be an acceptable design compromise:
That snowball should be square, but the left edge has sheared due to the ball object being updated mid-scanline.
To prevent shearing we need to update the objects on cycles 0-22. There's a lot of calculations to be done in the kernel to draw just one player. For Collect I'm using DoDraw, which looks like this for drawing player0:
DoDraw0: lda #HUMAN_HEIGHT-1 ; 2 2 - height of the humanoid graphics, subtract 1 due to starting with 0 dcp HumanDraw ; 5 7 - Decrement HumanDraw and compare with height bcs DoDrawGrp0 ; 2 9 - (3 10) if Carry is Set, then humanoid is on current scanline lda #0 ; 2 11 - otherwise use 0 to turn off player0 .byte $2C ; 4 15 - $2C = BIT with absolute addressing, trick that ; causes the lda (HumanPtr),y to be skipped DoDrawGrp0: ; 10 - from bcs DoDrawGrp0 lda (HumanPtr),y ; 5 15 - load the shape for player0 sta GRP0 ; 3 18 - update player0 to draw Human
That's 18 cycles to draw a single player. One way to make it easier to fit all the code in is to use a 2 Line Kernel (2LK). In a 2LK we update the TIA's registers over 2 scanlines in order to build the display. For Collect, the current routines are updating them like this:
The actual code looks like this:
ldy #ARENA_HEIGHT ; 2 7 - the arena will be 180 scanlines (from 0-89)*2 ArenaLoop: ; 13 - from bpl ArenaLoop ; continuation of line 2 of the 2LK ; this precalculates data that's used on line 1 of the 2LK lda #HUMAN_HEIGHT-1 ; 2 15 - height of the humanoid graphics, subtract 1 due to starting with 0 dcp HumanDraw ; 5 20 - Decrement HumanDraw and compare with height bcs DoDrawGrp0 ; 2 22 - (3 23) if Carry is Set, then humanoid is on current scanline lda #0 ; 2 24 - otherwise use 0 to turn off player0 .byte $2C ; 4 28 - $2C = BIT with absolute addressing, trick that ; causes the lda (HumanPtr),y to be skipped DoDrawGrp0: ; 23 - from bcs DoDrawGrp0 lda (HumanPtr),y ; 5 28 - load the shape for player0 sta WSYNC ; 3 31 ;--------------------------------------- ; start of line 1 of the 2LK sta GRP0 ; 3 3 - @ 0-22, update player0 to draw Human ldx #%11111111 ; 2 5 - playfield pattern for vertical alignment testing stx PF0 ; 3 8 - @ 0-22 ; precalculate data that's needed for line 2 of the 2LK lda #HUMAN_HEIGHT-1 ; 2 10 - height of the humanoid graphics, dcp BoxDraw ; 5 15 - Decrement BoxDraw and compare with height bcs DoDrawGrp1 ; 2 17 - (3 18) if Carry is Set, then box is on current scanline lda #0 ; 2 19 - otherwise use 0 to turn off player1 .byte $2C ; 4 23 - $2C = BIT with absolute addressing, trick that ; causes the lda (BoxPtr),y to be skipped DoDrawGrp1: ; 18 - from bcs DoDrawGRP1 lda (BoxPtr),y ; 5 23 - load the shape for the box sta WSYNC ; 3 26 ;--------------------------------------- ; start of line 2 of the 2LK sta GRP1 ; 3 3 - @0-22, update player1 to draw box ldx #0 ; 2 5 - PF pattern for alignment testing stx PF0 ; 3 8 - @0-22 dey ; 2 10 - decrease the 2LK loop counter bpl ArenaLoop ; 2 12 - (3 13) branch if there's more Arena to draw
If you look at that closely, you'll see I'm splitting DoDraw a bit so that this is how the 2LK works:
By pre-calculating data during the visible portion of the scanline, we'll have more time during the critical 0-22 cycles for when we add the other objects.
Since we're updating the players on every other scanline, each byte of graphic data is displayed twice (compare the thickness of the humanoid pixels with the red lines drawn with the playfield). Also, the players never line up as they're never updated on the same scanlines:
The designers of the TIA planned for this by adding a Vertical Delay feature to the players and ball (though sadly not the missiles). The TIA registers for this are VDELP0, VDELP1 and VDELBL. For this update to Collect, I've tied the Vertical Delay to the difficulty switches, putting the switch in position A will turn on the delay for that player so we can experiment with how that works. For the next update I'll set the Vertical Delay based on the Y position of the player (this also means the maximum Y value will be double that of this build).
Left Difficulty A, Right Difficulty B so VDELP0 = 1 and VDELP1 = 0. Sprites line up with the same Y:
Left Difficulty B, Right Difficulty A so VDELP0 = 0 and VDELP1 = 1. Sprites line up when player1's Y = player0's Y + 1:
The code that preps the data used by DoDraw looks like this:
; HumanDraw = ARENA_HEIGHT + HUMAN_HEIGHT - Y position lda #(ARENA_HEIGHT + HUMAN_HEIGHT) sec sbc ObjectY sta HumanDraw ; HumanPtr = HumanGfx + HUMAN_HEIGHT - 1 - Y position lda #<(HumanGfx + HUMAN_HEIGHT - 1) sec sbc ObjectY sta HumanPtr lda #>(HumanGfx + HUMAN_HEIGHT - 1) sbc #0 sta HumanPtr+1 ; BoxDraw = ARENA_HEIGHT + HUMAN_HEIGHT - Y position lda #(ARENA_HEIGHT + HUMAN_HEIGHT) sec sbc ObjectY+1 sta BoxDraw ; BoxPtr = HumanGfx + HUMAN_HEIGHT - 1 - Y position lda #<(HumanGfx + HUMAN_HEIGHT - 1) sec sbc ObjectY+1 sta BoxPtr lda #>(HumanGfx + HUMAN_HEIGHT - 1) sbc #0 sta BoxPtr+1 ... HumanGfx: .byte %00011100 .byte %00011000 .byte %00011000 .byte %00011000 .byte %01011010 .byte %01011010 .byte %00111100 .byte %00000000 .byte %00011000 .byte %00011000 HUMAN_HEIGHT = * - HumanGfx
I'm sure some of you are wondering why the human graphics are upside down. If you wanted to loop through something 10 times, you'd normally think to write the code like this:
ldy #0 Loop: ; do some work iny cpy #10 bne Loop
But the 6507 does an automatic check for 0 (as well as positive/negative) which lets you save 2 cycles of processing time by eliminating the CPY command:
ldy #10 Loop: ; do some work dey bne Loop
Alternatively, if your initial value is less than 128, you can use this:
ldy #(10-1) Loop: ; do some work dey bpl Loop
Making the loop count down instead of up saves 2 cycles, but doing so requires the graphics to be upside down. 2 cycles doesn't sound like much, but in a scanline that's 2.6% of your processing time and saving it might be what allows you to update everything you want. In Kernels I've written, I often use every cycle—and that includes eliminating the sta WSYNC to buy back 3 cycles of processing time. See the reposition kernels in this post about Draconian.
I've also added joystick support that will let you move around the players. Pressing FIRE will slow down the movement, making it easier to line things up. The score (on the left) is used to display player0's Y position, and the timer is used for player1. As an added bonus, I'm showing how you can save ROM space by creating graphics that only face in one direction by using REFP0 and REFP1 (REFlect Player) to make the graphics face the other way. The routine's fairly sizable, so I'm not posting it here. Download the source code and check it out!
The ROM and the source are at the bottom of my blog entry.
Other Assembly Language Tutorials
Step 4: 2 Line Kernel
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Cycle counting is an important aspect of Atari 2600 programming. It makes possible the positioning of sprites, the drawing of six-digit scores, non-mirrored playfield graphics and many other cool TIA tricks that keep every game from looking like Combat.
Atari 2600 programming is different from any other kind of programming in many ways. Just one of these ways is the flow of the program.
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If assembly language seems a little too hard, don't worry. You can always try to make Atari 2600 games the faster, easier way with batari Basic.
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