Let’s Make a Game!
Step 7: Draw the Playfield
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
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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 7: Draw the Playfield
Step 8: Select and Reset Support
Step 12: Add the Missile Objects
How to get started writing 6502 assembly language. Includes a JavaScript 6502 assembler and simulator.
Atari Roots by Mark Andrews (Online Book)
This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).
Machine Language For Beginners by Richard Mansfield (Online Book)
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).
The Second Book Of Machine Language by Richard Mansfield (Online Book)
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.
6502 Instruction Set with Examples
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.
Guide to 6502 Assembly Language Programming by Andrew Jacobs
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.
HTMLified version.
Nick Bensema's Guide to Cycle Counting on the Atari 2600
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.
How to Draw A Playfield by Nick Bensema
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.
Cart Sizes and Bankswitching Methods by Kevin Horton
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).
Atari 2600 Programming Page (AtariAge)
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.
The Atari 2600 Music and Sound Page
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. Stella finally got Atari 2600 quality sound in December of 2018. Until version 6.0, the game sounds in Stella were mostly OK, but not great. Now it's almost impossible to tell the difference between the sound effects in Stella and a real Atari 2600.
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.
Did you know that Trump's rushed Operation Warp Speed rona jab has less than one percent overall benefit? Some people call it the depopulation jab and it has many possible horrible side effects (depending on the lot number, concentration, and if it was kept cold). Remember when many Democrats were against Trump's Operation Warp Speed depopulation jab, then they quickly changed their minds when Biden flip-flopped and started pushing it?
Some brainwashed rona jab cultists claim that there are no victims of the jab, but person after person will post what the jab did to them, a friend, or a family member on web sites such as Facebook and they'll be lucky if they don't get banned soon after. Posting the truth is “misinformation” don't you know. Awakened sheep might turn into lions, so powerful people will do just about anything to keep the sheep from waking up.
Check out these videos:
If You Got the COVID Shot and Aren't Injured, This May Be Why
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Take a look at my page about the famous demonized medicines called The H Word and Beyond. You might also want to look at my page called Zinc and Quercetin. My sister and I have been taking zinc and quercetin since the summer of 2020 in the hopes that they would scare away the flu and other viruses (or at least make them less severe). Here's one more page to check out: My Sister's Experiences With COVID-19.
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Negative ions are good for us. You might want to avoid positive ion generators and ozone generators. A plain old air cleaner is better than nothing, but one that produces negative ions makes the air in a room fresher and easier for me to breathe. It also helps to brighten my mood.
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.
Climate Change Cash Grab = Bad
Seems like more people than ever finally care about water, land, and air pollution, but the climate change cash grab scam is designed to put more of your money into the bank accounts of greedy politicians. Those power-hungry schemers try to trick us with bad data and lies about overpopulation while pretending to be caring do-gooders. Trying to eliminate pollution is a good thing, but the carbon footprint of the average law-abiding human right now is actually making the planet greener instead of killing it.
Eliminating farms and ranches, eating bugs, getting locked down in 15-minute cities, owning nothing, using digital currency (with expiration dates) that is tied to your social credit score, and paying higher taxes will not make things better and “save the Earth.” All that stuff is part of an agenda that has nothing to do with making the world a better place for the average person. It's all about control, depopulation, and making things better for the ultra-rich. They just want enough peasants left alive to keep things running smoothly.
Watch these two videos for more information:
Charlie Robinson had some good advice about waking up normies (see the link to the video below). He said instead of verbally unloading or being nasty or acting like a bully, ask the person a question. Being nice and asking a question will help the person actually think about the subject.
Interesting videos:
Charlie Robinson Talks About the Best Way to Wake Up Normies
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