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 modify the Arena Loop to draw the Arena using the playfield. The new Arena loop has these new changes:
ArenaLoop: ; 27 - (currently 7 from bpl ArenaLoop) tya ; 2 29 - 2LK loop counter in A for testing and #%11 ; 2 31 - test for every 4th time through the loop, bne SkipX ; 2 33 (3 34) branch if not 4th time inx ; 2 35 - if 4th time, increase X so new playfield data is used SkipX: ; 35 - use 35 as it's the longest path here ... ; start of line 1 of the 2LK sta GRP1 ; 3 3 - @0-22, update player1 graphics lda ArenaPF0,x ; 4 7 - get current scanline's playfield pattern sta PF0 ; 3 10 - @0-22 and update it lda ArenaPF1,x ; 4 14 - get current scanline's playfield pattern sta PF1 ; 3 17 - @71-28 and update it lda ArenaPF2,x ; 4 21 - get current scanline's playfield pattern sta PF2 ; 3 24 - @60-39 ... ; start of line 2 of the 2LK sta GRP0 ; 3 3 - @0-22, update player0 graphics dey ; 2 5 - decrease the 2LK loop counter bne ArenaLoop ; 2 7 - (3 8) branch if there's more Arena to draw sty PF0 ; 3 10 - Y is 0, blank out playfield sty PF1 ; 3 13 - Y is 0, blank out playfield sty PF2 ; 3 16 - Y is 0, blank out playfield rts ; 6 22 - ReTurn from Subroutine
The first change is we're using X as an index into the playfield graphic data. We're changing X every fourth time thru the 2LK, so each byte of playfield data will be used over 8 scanlines. This saves a bit of ROM.
Second change is all 3 playfield registers (PF0, PF1 and PF2) are now updated, and they're only updated on line 1 of our 2LK.
Third change is on line 2, the bpl ArenaLoop is now a bne ArenaLoop else the bottom row of playfield data was only used for 2 scanlines instead of 8. We also blank out the playfield registers when we are done drawing the playfield. The bne change also impacted Overscan—TIM64T was originally set to 32, it's now set to 35.
The playfield data looks like this in jEdit:
And this onscreen:
Lastly we added some collision detection code. Some space was allocated in RAM:
;save player locations for playfield collision logic SavedX: ds 2 ; stored in $A1-A2 SavedY: ds 2 ; stored in $A3-A4
Then the Process Joystick routines save the current X and Y values before processing the joystick:
PJloop: ldy ObjectX,x ; save original X location so the player can be sty SavedX,x ; bounced back upon colliding with the playfield ldy ObjectY,x ; save original Y location so the player can be sty SavedY,x ; bounced back upon colliding with the playfield
Finally OverScan was modified to move the players back to their previous X and Y location if a collision was detected:
; Test if player collided with playfield bit CXP0FB ; N = player0/playfield, V=player0/ball bpl notP0PF ; if N is off, then player0 did not collide with playfield lda SavedX ; recall saved X sta ObjectX ; and move player back to it lda SavedY ; recall saved Y sta ObjectY ; and move player back to it notP0PF: bit CXP1FB ; N = player1/playfield, V=player1/ball bpl notP1PF ; if N is off, then player1 did not collide with playfield lda SavedX+1 ; recall saved X sta ObjectX+1 ; and move player back to it lda SavedY+1 ; recall saved Y sta ObjectY+1 ; and move player back to it notP1PF:
The ROM and the source are at the bottom of my blog entry.
Other Assembly Language Tutorials
Step 7: Draw the Playfield
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.
A useful page from Assembly Language Programming for the Atari Computers.
Continually strives to remain the largest and most complete source for 6502-related information in the world.
By John Pickens. Updated by Bruce Clark.
Below are direct links to the most important pages.
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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Never litter. Toss it in the trash or take it home. Do not throw it on the ground. Also remember that good people clean up after themselves at home, out in public, at a campsite and so on. Leave it better than you found it.
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