Atari 2600 Programming for Newbies

Session 11: Colorful Colors

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

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Page Table of Contents

Original Session

Even our language treats 'color' differentlyhere in Oz we write 'colour' and in the USA they write 'color'. Likewise, '2600 units in different countries don't quite speak the same language when it comes to color.


We have already seen why there are 3 variants of '2600 unitsthese variations (PAL, NTSC, SECAM) exist because of the differences in TV standards in various countries. Specifically, the color information is encoded in different ways into the analogue TV signal for each system, and the '2600 hardware is responsible for inserting that color information in the data sent to the TV.






3 Different Color Palettes

Not only do these different '2600 systems write the color information in different ways, they also write totally different colors! What is one color on a NTSC system is probably NOT the same color on PAL, and almost certainly not the same color on SECAM!


Here are some wonderful color charts that show the colors used by each of the systems. . .


Below are adapted versions of those color charts. . .





Colors are represented on the '2600 by numbers. How else could it be? The color to number correspondence is essentially an arbitrary associationso, for example on a NTSC machine the value $1A is yellowish, on PAL the same color is gray, and on SECAM it is aqua (!). If the same color values were used on a game converted between a NTSC and PAL system, then everything would look very weird indeed! To read the color charts on the page linked to above, form a 2-digit hex number from the hue and the lum values (ie: hue 2, lum 5 -> $25 value -> brown(ish) on NTSC, and as it happens, a very similar brown(ish) on PAL.


[Instead of getting your color values from static charts, you can do it the easy way and use the interactive palettes on the TIA Color Charts and Tools page. It includes an NTSC/PAL color conversion tool and Atari 2600 color compatibility tools that can help you quickly find colors that go great together (possibly saving you a lot of time and energy).]






COLUBK  (Color-Luminosity Background)

We've already played with colors in our first kernel! In the picture section (the 192 scanlines) we had the following code. . .

       ; 192 scanlines of picture... 

        ldx #0 

        REPEAT 192; scanlines 


          stx COLUBK 

          sta WSYNC 


We should know by now what that 'sta WSYNC' doesand now it's time to understand the rest of it. Remember the picture that the kernel shows? A very pretty rainbow effect, with color stripes across the screen. It's the TIA producing those colors, but it's our kernel telling the TIA what color to show on each line. And it's done with the 'stx COLUBK' line.


Remember how the TIA maps to memory in locations 0 - $7F, and that WSYNC is a label representing the memory location of the TIA register (which happens, of course, to be called WSYNC). In similar fashion, COLUBK is a label which corresponds to the TIA register of the same name. This particular register allows us to set the color of the background that the TIA sends to the TV!


A quick peek at the symbol table shows. . .

 COLUBK          0009       (R ) 

In fact, the very best place to look is in the Stella Programmer's guidefor here you will be able to see the exact location and usage of this TIA register. This is a pretty simple one, thoughall we do is write a number representing the color we want (selected from the color charts linked to, above) and the TIA will display this color as the background.







Other Assembly Language Tutorials

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



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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 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.



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