Atari 2600 Programming for Newbies

Session 20: Asymmetrical Playfields (Part 3)

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

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Page Table of Contents

Original Session

This session we're going to wrap-up our understanding of playfield graphics.





Full-Screen-Bitmap Tool

It doesn't take long before you get sick of doing data by hand, and often the time spent in creating tools is repaid many-times-over in the increase in productivity and capability those tools deliver. Sometimes a tool is a 'hack' in that it's not professionally produced, it has bugs, and it isn't user-friendly. But until you've tried creating bitmap graphics by hand a bit-at-a-time (and I'm sure that some of you have already done this by now), you won't really appreciate somethinganything!that can make the process easier. Having prepared you for the fairly shocking quality of this, I now point you towards FSB, the Full-Screen-Bitmap tool. It's the tool I use for generating the data for those spiffy Interleaved ChronoColour (tm) Full-Screen-Bitmaps. But it's able to be used for monochrome playfields, too.


The tool (Windows-only, sorryif you're on a non-Windows platform then you may need to write your own) is run from a DOS command-line. It takes three graphics files as input (representing the RED, GREEN, and BLUE components of a color image) and spits-out data which can be used to display the original data on an Atari 2600. For now we're not really at the level of drawing color bitmapsbut we'll get there shortly. First, let's examine how to use FSB to generate data for simple bitmap displays.


As noted, FSB takes three graphics files as input. Let's simplify things, and pass the utility only one file. This equates to having exactly the same data for red, green, and blue components of each pixeland hence the image will be black and white (specifically, it will be two-color). That's the capability of the '2600 playfield display, remember! It's only through trickery that there ever appear to be more than two colors on the screen at any time. That trickery being either time-based or position-based changing of the background and playfield colors to give the impression of more colors.


Actually, I cheated a bitif we pass only one file, the utility will process it, then have a fit when it can't find the others. As I said, it's a bit of a hack. But sometimes, hacking is OK. Sometime, I'll get a round tuit and fix it up.











Until next time, enjoy!







  1. Create a circle as a 40 x 192 image and save it as a .JPG, .PNG or .BMP. Convert it to source-code through FSB to create source-code data. Can you think of good ways to draw circles in such an odd screen-size? Hintmake the size of your image the LAST step in the draw process!
  2. Take one of the asymmetric playfield demos from the last session and convert it to display the data generated in step 1.
  3. Set the playfield color to a RED for one frame, then the next frame set it to a GREEN, and for the third frame set it to a BLUE. What effect do you see? What color does the circle appear to be? Why? If you haven't cottoned-on yet, this is leading towards color-bitmap technologywe may cover that in a future session. By using different colors over time, we can trick the eye to seeing a different color than those we actually use.
  4. How can this temporal color change be used to display a range of colors? This is tricky, so don't worry if you can't understand it. Hint: don't just change the color each frame! What else can you change?
  5. All our discussions about bitmap graphics have revolved around the use of asymmetrical (mirrored) playfields. Yet some (not many!) games use non-mirrored playfields. What timing problems can you see when using non-mirrored playfields for bitmap graphicsand why on earth would you want to do this?




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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