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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 OverscanTIM64T was originally set to 32, it's now set to 35.

 

The playfield data looks like this in jEdit:

Playfield Data

And this onscreen:

Playfield Data On Screen

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.

 

 

 

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Table of Contents for Let’s Make a Game!

Introduction

Step 1: Generate a Stable Display

Step 2: Timers

Step 3: Score and Timer Display

Step 4: 2 Line Kernel

Step 5: Automate Vertical Delay

Step 6: Spec Change

Step 7: Draw the Playfield

Step 8: Select and Reset Support

Step 9: Game Variations

Step 10: “Random Numbers”

Step 11: Add the Ball Object

Step 12: Add the Missile Objects

Step 13: Add Sound Effects

Step 14: Add Animation

 

 

 

 

Useful Links

Easy 6502 by Nick Morgan

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 Six Instruction Groups

The 6502 Instruction Set broken down into 6 groups.

6502 Instruction Set

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.

6502 Instruction Set

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.

 

 

6502.org

Continually strives to remain the largest and most complete source for 6502-related information in the world.

NMOS 6502 Opcodes

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.

Registers

Goes over each of the internal registers and their use.

Instruction Set

Gives a summary of whole instruction set.

Addressing Modes

Describes each of the 6502 memory addressing modes.

Instruction Reference

Describes the complete instruction set in detail.

 

 

Stella Programmer's Guide

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 Specifications

Atari 2600 programming specs (HTML version).

 

 

Atari 2600 Programming Page (AtariAge)

Links to useful information, tools, source code, and documentation.

 

 

MiniDig

Atari 2600 programming site based on Garon's "The Dig," which is now dead.

 

 

TIA Color Charts and Tools

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.

 

 

Game Standards and Procedures

A guide and a check list for finished carts.

 

 

Stella

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.

 

 

JAVATARI

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.

 

 

batari Basic Commands

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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In Case You Didn't Know

 

Trump's Jab = Bad

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

Thought Experiment: What Happens After the Jab?

The Truth About Polio and Vaccines

What Is Causing the Mysterious Self-Assembling Non-Organic Clots and Sudden Deaths?

Full Video of Tennessee House of Representatives Health Subcommittee Hearing Room 2 (The Doctors Start Talking at 33:28)

 

 

H Word and I Word = Good

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.

 

 

B Vitamins = Good

Some people appear to have a mental illness because they have a vitamin B deficiency. For example, the wife of a guy I used to chat with online had severe mood swings which seemed to be caused by food allergies or intolerances. She would became irrational, obnoxious, throw tantrums, and generally act like she had a mental illness. The horrid behavior stopped after she started taking a vitamin B complex. I've been taking Jarrow B-Right (#ad) for many years. It makes me much easier to live with. I wonder how many people with schizophrenia and other mental mental illnesses could be helped by taking a B complex once or twice a day with meals (depending on their weight)?

 

 

Soy = Bad

Unfermented soy is bad!When she stopped eating soy, the mental problems went away.” Fermented soy doesn't bother me, but the various versions of unfermented soy (soy flour, soybean oil, and so on) that are used in all kinds of products these days causes a negative mental health reaction in me that a vitamin B complex can't tame. The sinister encroachment of soy has made the careful reading of ingredients a necessity.

 

I started taking AyaLife (99% Pure CBD oil) as needed in April of 2020. So far it's the only thing that helps my mood when I've mistakenly eaten something that contains soy. AyaLife is THC-free (non-psychoactive) and is made in the USA. I also put a couple dropper fulls under my tongue before leaving the house or if I just need to calm down.

 

It's supposedly common knowledge that constantly angry Antifa-types basically live on soy products. What would happen if they stopped eating and drinking soy sludge and also took a B complex every day? Would a significant number of them become less angry? Would AyaLife CBD oil also help?

 

 

Wheat = Bad

If you are overweight, have type II diabetes, or are worried about the condition of your heart, check out the videos by Ken D Berry, William Davis, and Ivor Cummins. It seems that most people should avoid wheat, not just those who have a wheat allergy or celiac disease. Check out these books: Undoctored (#ad), Wheat Belly (#ad), and Eat Rich, Live Long (#ad).

 

 

Negative Ions = Good

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.

 

 

Litterbugs = Bad

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:

CO2 is Greening The Earth

The Climate Agenda

 

 

How to Wake Up Normies

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

Georgia Guidestones Explained

The Men Who Own Everything

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Use any example programs at your own risk. I am not responsible if they blow up your computer or melt your Atari 2600. Use assembly language at your own risk. I am not responsible if assembly language makes you cry or gives you brain damage.

 

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