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Atari 2600 Programming for Newbies

Session 15: Playfield Continued

By Andrew Davie (adapted by Duane Alan Hahn)

Table of Contents

Original Session

We've had a bit of time to think about the playfield, and hopefully have a go at some of the exercises. Admittedly I threw you in the deep end with the last sessionso we'll go back a step and walk through exactly what all this playfield oddity is about. We'll also tackle some of the exercises to show that there's more than one way to skin a fish.

 

Last session we learned that the playfield registers PF0 and PF2 are reversed. Specifically, the order of pixels in the playfield registers (one bit per pixel, remember!) is backward, compared to the order for the first playfield register we encounteredPF1. This backward ordering is rather confusing, but that's just the way it is. Have a close look at the diagram presented in the last session and try and understand exactly the "playfield register/bit" to "pixel position on the scanline" correspondence.

 

 

 

 

 

Playfield Mirroring

There's one new playfield-related capability of the '2600 which I'd like to introduce nowplayfield mirroring. I've already introduced this to you when I stated that the right hand side of the playfield was a copy of the left hand side (that is, the left 20 pixels come from the 20 playfield bits held in the TIA registers PF0, PF1 and PF2and the right 20 bits are a copy of the same bits). That copy can be displayed 'normally'or 'mirrored'. When mirrored, the bits are literally a mirrored copy of the left side of the playfield.

 

We're already familiar with two 'types' of TIA register. There's the strobe-type, where a write of any value to the register causes something to happen, independent of the value written (an example is WSYNC, which halts the 6502 until the TIA starts the next scanline). A second type is the normal register to which we write a byte, and the TIA uses that byte for some internal purpose (examples of these are the playfield registers PF0, PF1 and PF2). PF0 was a special-case of this type, wherethough we wrote a byteonly four of the bits were actually used by the TIA. The remaining bits were discarded/ignored (have a look at the PF0 register in the diagram in the last sessionthe X for each bit position in bits D0-D3 indicates those bits are not used).

 

 

 

 
 
 

 

 

 

 

 

Summary

That will do for today's session. We've had an introduction to controlling individual TIA register bits, and seen how to achieve a reflected playfield at next to no cost. We've had a brief introduction to the CTRLPF register, and seen how it has a myriad (well, more than 3) uses. Although some of the previous sessions have asked you to think about tricky subjects like horizontal scrolling, and asymmetrical playfieldsnow is not the time to actually discuss these tricky areas. So until next time (when we'll develop our playfield skills a bit more). . . ciao!

 

 

 

 

 

Exercises

  1. Introduce a RAM shadow of the CTRLPF register, and modify it differently in each section of the kernel. For example turn reflection on and off partway through the midsection of the box, and see what happens.
  2. Have a play with the SCORE bit in the CTRLPF register, and in conjunction with that the COLUP0 and COLUP1 color registers. Note how this SCORE bit changes where the color for the playfield comes from.

 

 

 

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.

 

 

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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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