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Atari 2600 Programming for Newbies

Sessions 3 & 6: The TIA and the 6502

By Andrew Davie (adapted by Duane Alan Hahn)

Table of Contents

Original Sessions

Let's spend this session having a look at how some of the hardware generates a scanline for the TV. Remember in session 2, we had a good look at how a TV works, and in particular how a TV frame is composed of 262 scanlines (NTSC) or 312 scanlines (PAL). It's the programmer's job to control how many scanlines are sent to the TV, but it is the '2600 which builds the actual signal comprising the color and intensity information for any scanline. This color and intensity information is derived from the internal 'state' of the TIA (Television Interface Adaptor) chip inside the '2600. The TIA is responsible for creating the signal for a single scanline for the TV.

 

 

 

 

 

The TIA

The TIA 'draws' the pixels on the screen 'on-the-fly'. Each pixel is one 'clock' of the TIA's processing time, and there are exactly 228 color clocks of TIA time on each scanline. But a scanline consists of not only the time it takes to scan the electron beam across the picture tube, but also the time it takes for the beam to return to the start of the next line (the horizontal blank, or retrace). Of the 228 color clocks, 160 are used to draw the pixels on the screen (giving us our maximum horizontal resolution of 160 pixels per line), and 68 are consumed during the retrace period.

 

 

 

 

 

 

 

Summary

Don't despair! It is not necessary for you to learn how to count 6502 cycles at this stage. Those sort of tricks are for more advanced '2600 programmingand the original design of the TIA hardware made this unnecessary. It's only when you need to push the hardware (TIA) beyond its original design, that you will come to appreciate the benefit inherent in the way that the 6502 and TIA are intricately tied together.

 

Next session we'll have a closer look at the TIA and how it determines what color to use for each pixel of the scanline it is drawing. In particular, we'll start to look at background, playfield, sprite, missile and ball graphics.

 

 

 

Other Assembly Language Tutorials

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

 

 

< 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 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.

 

 

6502.org

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.

 

 

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.

 

 

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.

 

 

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

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Disclaimer

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.

 

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