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Atari 2600 Programming for Newbies

Session 25: Advanced Timeslicing

By Andrew Davie (adapted by Duane Alan Hahn)

Table of Contents

Original Session

Time is tight. Really tight! The general approach has been to think of the TV frame as the limiting factor for the capabilities of the machine. Whatever you can do in 'one frame' (i.e, nominally @60Hz on NTSC or @50Hz on PAL) . . . that's IT. So in fact you can work out exactly how much time you have to do stuff. As we've seen in earlier tutorials, the '2600 programmer has to pump data out to the TIA in sync with the TV as it's drawing scanlines. You need to feed the TV scanlines to draw a proper picture. There are 76 cycles per scanline, and 262 scanlines per standard TV frame (312 for PAL). So 76 * 262 = 19912 cycles per frame. Multiply that by the NTSC frame rate (actually 59.94Hz) and you get . . . 1193525.28 (i.e., there's our 1.19MHz CPU clock speed). It all makes sense.

 

So, just 262 lines. The visible screen is smaller than that, of course (usually 192 scanlines of actual graphics)so we only need to pump data to the screen for a smaller number of lines. The rest is black, nothing to see. Below is a good visual diagram of where the time goes.

 

 

So, during those blank lines, the CPU doesn't have to pump data to the screen. In fact these two major areas of 'blackness' (that is, the vertical blank, and the overscan) account for 37 scanlines (*76 = 2812 cycles) and 30 scanlines (*76 = 2280 cycles). Now that's not exactly swimming in available CPU capacity but it's better than nothing. So the general usage of these blank areas has been to whack in 'stuff' that takes a fair bit of time to do.

 

The problem is, you can't whack in too MUCH stuff. Because when those 37 scanlines of time have elapsed, you MUST be writing to the TIA again to make sure the next frame is displaying properly. Same for the 30 lines of overscan. There's no getting around it; you take too much time, and you stuff up the timing, and consequently the TV picture will roll, judderjitter and basically look horrible. The hard and fast rule has been to simply stay within the limitation, or to reduce the number of visible scanlines to give more processing time for doing more complex STUFF. Each scanline of visible data you sacrificed, you got 76 scanlines of available time to do your stuff. A compromise.

 

 

 

 

 

Timer Registers

Fortunately, we have the timer registers. These are single countdown registers that will regularly decrement a value written to them. I only use TIM64Tthis one counts 64 cycle blocks. If I write 10 to it, then I would expect it to reach 0 some 640 cycles later. So, the usage has been to calculate the amount of time before the screen drawing has to (re)commence, divide by 64, and put that value in TIM64T. By reading INTIM and waiting until that reaches 0, you effectively wait the right number of cycles. You can do your (variable time) 'stuff' and not really care about how long it takes (as long as it doesn't take TOO long), and after it's finish you enter a tight loop just reading INTIM and waiting for it to go to 0. When it goes to 0, fire off a WSYNC and then begin the TV frame drawing once again.

 

That's how it's BEEN done, but that's not how I did it in Boulder Dash!

 

 

 

 

 

INTIM

The INTIM register effectively tells you not only if you're out of time, but also exactly how MUCH time you have remaining (in blocks of 64 cycles if you're using TIM64T). So, if you think about it, you can actually make decisions about if you should call a subroutine based on this value. For example, say you had a small routine which you know takes (say) 1000 cycles to run. That's 1000/64 units (= 15.625). So, if INTIM was reading 16 or greater you KNOW you can call that subroutine and not run out of time!  This gets rather nice. Given a guaranteed maximum run-time for any subroutine (and you get this by cycle-counting the subroutine very very carefully), you can use this knowledge to determine if/when it's appropriate to call that subroutine. Furthermore, after you HAVE called the subroutine, you can repeat the processlook at INTIM and determine if there's enough time to run OTHER subroutines.

 

 

 

 

 

 

 

 

Summary

So the above is the secret to making much more complex games than have heretofore been produced on the machine. You CAN keep the TV display going full speed (60Hz) while doing processing-intensive game logic. And you CAN do very very very complex game logic taking absolutely heaps of processing time. The trick, as noted, is to separate out the two so they are not synchronousand to divide the complex logic into discrete, very quick, sub-components.

 

Divide and conquer!

 

 

 

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

 

 

Back to the Beginning >

 

 

 

 

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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