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Atari 2600 Programming for Newbies

Sessions 17 & 18: Asymmetrical Playfields

(Parts 1 & 2)

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

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Page Table of Contents

Original Sessions

By now you should be familiar with how the '2600 playfield works. In summary, there are three playfield registers (PF0, PF1, PF2) and these hold 20 bits of playfield data. The '2600 displays this data twice on every scanline, and you can have the second half mirrored, if you wish. Playfield is a single-color, but each half of the screen may be set to use the colors of the players (more about those, later!). In short, though, we have a fairly versatile system just great for PONG-style games.


Pretty soon, though, programmers started doing much more sophisticated things with the TIAand especially with the playfield registersthan just displaying symmetrical (or mirrored) playfields.


Since writes to TIA immediately change the internal 'state' of the TIA, and since the TIA and 6502 work in tandem during the display of a TIA frame, there's no reason why the 6502 can't modify things on-the-fly in the middle of scanlines. For example, any write to playfield registers will IMMEDIATELY reflect in changes to the data that the TIA is sending for a particular scanline. I qualify this slightly by my non-knowledge if these immediate changes are on a per-pixel basis, or on a per-byte basis. Something for us all to play with!


In any case, as will probably have become obvious to you by now, it is possible to display a different 'shape' on the left and right of any scanline. As stated, if we left the TIA alone then it would display the same (or a mirrored version) data on the left and right halves of the screencoming from its 20 pixel playfield data. But if we modify any of the playfield registers on-the-fly (that is, mid-scanline) then we will see the results of that modification straight away when the TIA draws the rest of the scanline.





The TIA and Frame Timing

Let's revisit briefly our understanding of the TIA and frame timing. Please refer to the earlier sessions where the timing of the TIA and 6502 were covered. In summary, there are exactly 228 color-clocks of TIA 'time' on any scanline160 of those clocks are actual visible pixels on the screen and 68 of them are the time it takes for the horizontal retrace to occur.


Our 'zero point' of any scanline is the beginning of horizontal retrace. This is the point at which the TIA re-enables the 6502 if it has been halted by a WSYNC write. At the beginning of any scanline, then, we know that we have exactly 68 color clocks (68/3 = 22.667 cycles) before the TIA starts 'drawing' the line itself.


You should already be familiar with the horizontal resolution of '2600 playfieldexactly 40 pixels per scanline. I use the term 'pixels' interchangeably hereto mean a minimum unit of graphic resolution. For the playfield, there are 40 pixels a line. But the TIA has 160 color-clocks per line, and in fact sprite resolution is also 160 pixels per line. Another way of looking at this is that each playfield pixel is 4 color-clocks wide, and each sprite pixel is 1 color clock wide (as a minimum, anywaythis can be adjusted to give double-wide and quadruple-wide sprites. We'll get to sprites soon, I promise!)









Timing Diagram

The following diagram shows the timing relationship between the TIA, the 6502, and playfield pixels. Further, it shows the times at which it is safe to write the playfield registers for both left and right-sides of the screen.

Timing Diagram







Rather than give you a code sample this session, I'd like you to grab the last playfield code and convert it to display an asymmetrical playfield. Doesn't have to be fancyjust demonstrate a consistent change between left and right halves of the screen, writing PF0, PF1 and PF2 twice each on each scanline. Once you've mastered this concept you can truly say you're on the way to programming a '2600 game!


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.


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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 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.



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