By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
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Original Blog Entry
The 2LK has been revised to display the ball object. Like player0, the ball needs to be primed before the ArenaLoop begins:
; prime ENABL so ball can appear on topmost scanline of Arena ldx #1 ; 2 2 - D1=0, so ball will be off lda #BOX_HEIGHT-1 ; 2 4 - height of box graphic dcp BallDraw ; 5 9 - Decrement BallDraw and compare with height bcs DoEnablPre ; 2 11 - (3 12) if Carry is Set, then ball is on current scanline .byte $24 ; 3 14 - $24 = BIT with zero page addressing, trick that ; causes the inx to be skipped DoEnablPre: ; 12 - from bcs DoEnablPre inx ; 2 14 - D1=1, so ball will be ON stx ENABL ; 3 17 ; prime GRP0 so player0 can appear on topmost scanline of the Arena lda #HUMAN_HEIGHT-1 ; 2 19 - height of player0 graphics, dcp Player0Draw ; 5 24 - Decrement Player0Draw and compare with height bcs DoDrawGrp0pre ; 2 26 - (3 27) if Carry is Set, then player0 is on current scanline lda #0 ; 2 28 - otherwise use 0 to turn off player0 .byte $2C ; 4 32 - $2C = BIT with absolute addressing, trick that ; causes the lda (Player0Ptr),y to be skipped DoDrawGrp0pre: ; 27 - from bcs DoDrawGRP0pre lda (Player0Ptr),y ; 5 32 - load the shape for player0 sta GRP0 ; 3 35 - @0-22, update player0 graphics dey ; 2 37 ArenaLoop: ; 37 - (currently 11 from bpl ArenaLoop) tya ; 2 39 - 2LK loop counter in A for testing and #%11 ; 2 41 - test for every 4th time through the loop, bne SkipX ; 2 43 - (3 44) branch if not 4th time inc ArenaIndex ; 5 48 - if 4th time, increase index so new playfield data is used SkipX: ; 48 - use 48 as it's the longest path here ; continuation of line 2 of the 2LK ; this precalculates data that's used on line 1 of the 2LK lda #HUMAN_HEIGHT-1 ; 2 50 - height of the humanoid graphics, subtract 1 due to starting with 0 dcp Player1Draw ; 5 55 - Decrement Player1Draw and compare with height bcs DoDrawGrp1 ; 2 57 - (3 58) if Carry is Set, then player1 is on current scanline lda #0 ; 2 59 - otherwise use 0 to turn off player1 .byte $2C ; 4 63 - $2C = BIT with absolute addressing, trick that ; causes the lda (Player1Ptr),y to be skipped DoDrawGrp1: ; 58 - from bcs DoDrawGrp1 lda (Player1Ptr),y ; 5 63 - load the shape for player1 sta WSYNC ; 3 66 ;--------------------------------------- ; start of line 1 of the 2LK sta GRP1 ; 3 3 - @0-22, update player1 graphics ldx ArenaIndex ; 3 6 lda ArenaPF0,x ; 4 10 - get current scanline's playfield pattern sta PF0 ; 3 13 - @0-22 and update it lda ArenaPF1,x ; 4 17 - get current scanline's playfield pattern sta PF1 ; 3 20 - @71-28 and update it lda ArenaPF2,x ; 4 24 - get current scanline's playfield pattern sta PF2 ; 3 27 - @60-39 ; precalculate data that's needed for line 2 of the 2LK ldx #1 ; 2 29 - D1=0, so ball will be off lda #BOX_HEIGHT-1 ; 2 31 - height of box graphic dcp BallDraw ; 5 36 - Decrement BallDraw and compare with height bcs DoEnabl ; 2 38 - (3 39) if Carry is Set, then ball is on current scanline .byte $24 ; 3 41 - $24 = BIT with zero page addressing, trick that ; causes the inx to be skipped DoEnabl: ; 39 - from bcs DoEnablPre inx ; 2 41 - D1=1, so ball will be ON lda #HUMAN_HEIGHT-1 ; 2 43 - height of the box graphics, dcp Player0Draw ; 5 48 - Decrement Player0Draw and compare with height bcs DoDrawGrp0 ; 2 50 - (3 51) if Carry is Set then player0 is on current scanline lda #0 ; 2 52 - otherwise use 0 to turn off player0 .byte $2C ; 4 56 - $2C = BIT with absolute addressing, trick that ; causes the lda (Player0Ptr),y to be skipped DoDrawGrp0: ; 51 - from bcs DoDrawGRP0 lda (Player0Ptr),y ; 5 56 - load the shape for player0 sta WSYNC ; 3 59 ;--------------------------------------- ; start of line 2 of the 2LK sta GRP0 ; 3 3 - @0-22, update player0 graphics stx ENABL ; 3 6 - @0-22, update ball graphics dey ; 2 8 - decrease the 2LK loop counter bne ArenaLoop ; 2 10 - (3 11) branch if there's more Arena to draw sty PF0 ; 3 13 - Y is 0, blank out playfield sty PF1 ; 3 16 - Y is 0, blank out playfield sty PF2 ; 3 19 - Y is 0, blank out playfield rts ; 6 25 - ReTurn from Subroutine
I then modified RandomLocation to set all objects to the same location for comparison:
RandomLocation: ... ; for alignment test, set to (100, 100) lda #100 sta ObjectX,x sta ObjectY,x rts
This revealed a minor quirk with the TIA—namely that when objects are set to the same X position, missiles and the ball end up 1 pixel to the left of where a player ends up (player1 is the square and it's directly on top of the red ball).
This is a known issue and the solution is to increase the X value by 1 to compensate. I did so by adding this to the end of RandomLocation:
RandomLocation: ... cpx #2 bcc RLdone inc ObjectX,x ; missile and ball objects need their X adjusted RLdone: rts
And now player1 and the ball line up:
The NewGame routine revised last time already sets the ball to a random X-Y location, so all that's left to make it show up is to revise PositionObjects. Two changes are needed, first is to set the X position of the ball object by starting the POloop with X=4 (in the prior build X was initialized to 1), then prep a new variable BallDraw that's used by the 2LK to draw the ball object on the correct scanlines.
PositionObjects: ldx #4 ; position all objects POloop lda ObjectX,x ; get the object's X position jsr PosObject ; set coarse X position and fine-tune amount dex ; DEcrement X bpl POloop ; Branch PLus so we position all objects sta WSYNC ; wait for end of scanline sta HMOVE ; use fine-tune values to set final X positions ... ; prep ball's Y position for 2LK ldx #1 ; preload X for setting VDELBL lda ObjectY+4 ; get the balls's Y position clc adc #1 ; add 1 to compensate for priming of ball lsr ; divide by 2 for the 2LK position sta Temp ; save for position calculations bcs NoDelayBL ; if carry is set we don't need Vertical Delay stx VDELBL ; carry was clear, so set Vertical Delay NoDelayBL: ; BallDraw = ARENA_HEIGHT + BOX_HEIGHT - Y position + 1 ; the + 1 compensates for priming of ENABL lda #(ARENA_HEIGHT + BOX_HEIGHT + 1) sec sbc Temp sta BallDraw rts
Lastly, Overscan's been updated to process collisions with the ball.
OverScan: ... TestCollisions: ; Test left player collisions ... bit CXP0FB ; N = player0/playfield, V=player0/ball ... notP0PF: ; oVerflow flag is not affected by lda or sta bvc notP0BL ; if V is off, then player0 did not collide with ball ldx #4 ; which box was collected jsr CollectBox ; update score and reposition box notP0BL: ... RightPlayer: ; Test right player collisions ... bit CXP1FB ; N = player1/playfield, V=player1/ball ... notP1PF: ; oVerflow flag is not affected by lda or sta bvc notP1BL ; if V is off, then player1 did not collide with ball ldx #4 ; which box was collected jsr CollectBox ; update score and reposition box notP1BL:
2 player games are now playable:
And 1 player games have 2 boxes to collect:
The ROM and the source are at the bottom of my blog entry.
Other Assembly Language Tutorials
Step 11: Add the Ball Object
This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).
This book only assumes a working knowledge of BASIC. It was designed to speak directly to the amateur programmer, the part-time computerist. It should help you make the transition from BASIC to machine language with relative ease.
The 6502 Instruction Set broken down into 6 groups.
Nice, simple instruction set in little boxes (not made out of ticky-tacky).
This book shows how to put together a large machine language program. All of the fundamentals were covered in Machine Language for Beginners. What remains is to put the rules to use by constructing a working program, to take the theory into the field and show how machine language is done.
An easy-to-read page from The Second Book Of Machine Language.
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Goes over each of the internal registers and their use.
Gives a summary of whole instruction set.
Describes each of the 6502 memory addressing modes.
Describes the complete instruction set in detail.
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.
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).
Links to useful information, tools, source code, and documentation.
Atari 2600 programming site based on Garon's "The Dig," which is now dead.
Includes interactive color charts, an NTSC/PAL color conversion tool, and Atari 2600 color compatibility tools that can help you quickly find colors that go great together.
Adapted information and charts related to Atari 2600 music and sound.
A guide and a check list for finished carts.
A multi-platform Atari 2600 VCS emulator. It has a built-in debugger to help you with your works in progress or you can use it to study classic games.
A very good emulator that can also be embedded on your own web site so people can play the games you make online. It's much better than JStella.
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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