Home > Atari Memories > #Assembly > Atari 2600 Programming for Newbies

Atari 2600 Programming for Newbies

Session 12: Initialization

By Andrew Davie (adapted by Duane Alan Hahn, a.k.a. Random Terrain)

As an Amazon Associate I earn from qualifying purchases. (I get commissions for purchases made through certain links on this page.)

Page Table of Contents

Original Session

One of the joys of writing '2600 programs involves the quest for efficiencyboth in processing time used, and in ROM space required for the code. Every now and then, modern-day '2600 programmers will become obsessed with some fairly trivial task and try to see how efficient they can make it.


If you were about to go up on the Space Shuttle, you wouldn't expect them to just put in the key, turn it on, and take off. You'd like the very first thing they do is to make sure that all those switches are set to their correct positions. When our Atari 2600 (which, I might point out in a tenuous link to the previous sentence, is of the same vintage as the Space Shuttle) powers-up, we should assume that the 6502, RAM and TIA (and other systems) are in a fairly unknown state. It is considered good practice to initialize these systems. Unless you really, *really* know what you're doing, it can save you problems later on.


At the end of this session I'll present a highly optimized (and best of all, totally obscure Smile) piece of code which manages to initialize the 6502, all of RAM *and* the TIA using just 9 bytes of code-size. That's quite amazing, really. But first, we're going to do it the 'long' way, and learn a little bit more about the 6502 while we're doing it.





Initializing RAM

We've already been introduced to the three registers of the 6502A, X, and Y. X and Y are known as index registers (we'll see why, very soon) and A is our accumulatorthe register used to do most of the calculations (addition, subtraction, etc).


Let's have a look at the process of clearing (writing 0 to) all of our RAM. Our earlier discussions of the memory architecture of the 6502 showed that the '2600 has just 128 bytes ($80 bytes) of RAM, starting at address $80. So, our RAM occupies memory from $80 - $FF inclusive. Since we know how to write to memory (remember the "stx COLUBK" we used to write a color to the TIA background color register), it should be apparent that we could do this. . .

    lda #0            ; load the value 0 into the accumulator

    sta $80           ; store accumulator to location $80

    sta $81           ; store accumulator to location $81

    sta $82           ; store accumulator to location $82

    sta $83           ; store accumulator to location $83

    sta $84           ; store accumulator to location $84

    sta $85           ; store accumulator to location $85

; 119 more lines to store 0 into location $86 - $FC. . .

    sta $FD           ; store accumulator to location $FD

    sta $FE           ; store accumulator to location $FE

    sta $FF           ; store accumulator to location $FF

You're right, that's ugly! The code above uses 258 bytes of ROM (2 bytes for each store, and 2 for the initial accumulator load). We can't possibly afford thatand especially since I've already told you that it's possible to initialize the 6502 registers, clear RAM, *AND* initialize the TIA in just 9 bytes total!


The index registers have their name for a reason. They are useful in exactly the situation above, where we have a series of values we want to read or write to or from memory. Have a look at this next bit of code, and we'll walk through what it does. . .

    ldx #0

    lda #0

ClearRAM  sta $80,x


    cpx #$80

    bne ClearRAM

Firstly, this code is nowhere-near efficient, but it does do the same job as our first attempt and uses only 11 bytes. It achieves this saving by performing the clear in a loop, writing 0 (the accumulator) to one RAM location every iteration. The key is the "sta $80,x" line. In this "addressing mode", the 6502 adds the destination address ($80 in this exampleremember, this is the start of RAM) to the current value of the X registergiving it a final addressand uses that final address as the source/destination for the operation.









Initializing the TIA

Initializing the TIA is a similar process to initializing the RAMwe just want to write 0 to all memory locations from 0 to $7F (where the TIA lives!). This is safetrust meand we don't really need to know what we're writing to at this stage, just that after doing this the TIA will be nice and happy. We could do this in a second loop, similar to the first, but how about this. . .

    ldx #0

    lda #0


    sta $80,x     ; clear a byte of RAM

    sta 0,x       ; clear a byte of TIA register


    cpx #$80

    bne Clear

That's a perfectly adequate solution. Easy to read and maintain, and reasonably quick. We could, however, take advantage of the fact that RAM and the TIA are consecutive in memory (TIA from 0 - $7F, immediately followed by RAM $80 - $FF) and do the clear in one go. . .






Initializing RAM and the TIA

    ldx #0

    lda #0


    sta 0,x


    bne Clear

The above example uses 9 bytes, again, but now clears RAM and TIA in one 'go' by iterating the index register (which is the effective address when used in "sta 0,x") from 0 to 0 (ie: increments 256 times and then wraps back to 0 and the loop halts). This is starting to get into "elegant" territory, something the experienced guys strive for!


Furthermore, after this code has completed, X = 0 and A = 0a nice known state for two of the 3 6502 registers.


That's all I'm going to explain for the initialization at this stagewe should insert this code just after the "Reset" label and before the "StartOfFrame" label. This would cause the code to be executed only on a system reset, not every frame (as, every frame, the code branches back to the "StartOfFrame" for the beginning of the next frame).







Before we end today's session, though, I thought I'd share the "magical" 9-byte system clear with you. There's simply no way that I would expect you to understand this bit of code at the momentit pulls every trick in the bookbut this should give you some taste of just how obscure a bit of code CAN be, and how beautifully elegant clever coding can do amazing things.





    LDX #0


    PHA           ; BEST WAY TO GET SP=$FF, X=0







Though the above was a truly magical piece of code, I've since developed an EIGHT byte solution to the problem of clearing RAM and initializing the stack and registers. Smile

        ldx #0


Clear   dex



        bne Clear

After the above, X=A=0, and all of RAM and the TIA has been initialized to 0, and the stack pointer is initialized to $FF. Amazing!


See you next time!




Other Assembly Language Tutorials

Be sure to check out the other assembly language tutorials and the general programming pages on this web site.


Amazon Stuff


< Previous Session



Next Session >





Session Links

Session 1: Start Here

Session 2: Television Display Basics

Sessions 3 & 6: The TIA and the 6502

Session 4: The TIA

Session 5: Memory Architecture

Session 7: The TV and our Kernel

Session 8: Our First Kernel

Session 9: 6502 and DASM - Assembling the Basics

Session 10: Orgasm

Session 11: Colorful Colors

Session 12: Initialization

Session 13: Playfield Basics

Session 14: Playfield Weirdness

Session 15: Playfield Continued

Session 16: Letting the Assembler do the Work

Sessions 17 & 18: Asymmetrical Playfields (Parts 1 & 2)

Session 19: Addressing Modes

Session 20: Asymmetrical Playfields (Part 3)

Session 21: Sprites

Session 22: Sprites, Horizontal Positioning (Part 1)

Session 22: Sprites, Horizontal Positioning (Part 2)

Session 23: Moving Sprites Vertically

Session 24: Some Nice Code

Session 25: Advanced Timeslicing





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 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 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.



Guide to 6502 Assembly Language Programming by Andrew Jacobs

Below are direct links to the most important pages.



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.




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.




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.



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.



Atari 2600 BASIC

If assembly language is too hard for you, try batari Basic. It's a BASIC-like language for creating Atari 2600 games. It's the faster, easier way to make Atari 2600 games.

Try batari Basic

Back to Top



In Case You Didn't Know


Trump's Jab = Bad

Did you know that Trump's rushed experimental rona jab has less than one percent overall benefit? It also has many possible horrible side effects. 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 or a family member on web sites such as Facebook and Twitter 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:

Best of Sped Up and Edited Tennessee House of Representatives Health Subcommittee Hearing Room 2 (March 1, 2022)

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 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 those two supplements since summer of 2020 in the hopes that they would scare away the flu and other viruses (or at least make them less severe).



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 #ad Jarrow B-Right for many years. It makes me much easier to live with.



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.



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: #ad Undoctored, #ad Wheat Belly, and #ad Eat Rich, Live Long.



Negative Ions = Good

Negative ions are good for us. You might want to avoid positive ion generators and ozone generators. Whenever I need a new air cleaner (with negative ion generator), I buy it from 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.


Watch these two YouTube videos for more information:

CO2 is Greening The Earth

The Climate Agenda


View this page and any external web sites at your own risk. I am not responsible for any possible spiritual, emotional, physical, financial or any other damage to you, your friends, family, ancestors, or descendants in the past, present, or future, living or dead, in this dimension or any other.


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.


Home Inventions Quotations Game Design Atari Memories Personal Pages About Site Map Contact Privacy Policy Tip Jar