By Andrew Davie (adapted by Duane Alan Hahn)
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Table of Contents
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 '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.
The 6502 clock is derived from the TIA clock through a divide-by-three. That is, for every single clock of 6502 time, three clocks of TIA time have passed. Therefore, there are *exactly* 228/3 = 76 cycles of 6502 time per scanline. The 6502 and TIA perform a complex 'in-step' dance—one cycle of 6502, three cycles of TIA. A side-note: 76 cycles per line x 262 lines per frame x 60 frames per second = the number of 6502 cycles per second for NTSC (roughly equals 1.19MHz).
So, as our 6502 program is executing its instructions, the TIA is also sending data for each scanline. Every cycle of 6502 time we know that the TIA has sent 3 color clocks of information to the TV. If the TIA was in the first 68 color clocks of the scanline, then it was in the horizontal retrace period. If it was in color clock 68-227, then it was drawing pixels on the visible scanline. And so we go, the 6502 program doing its stuff and at the very same time the TIA doing its stuff.
The magic happens when you start changing the 'state' of the TIA, because those changes are reflected immediately in the TIA output to the TV! Since the 6502 is 'locked' to the TIA through their shared timing origin, it is possible for the programmer to know exactly what pixel on a scanline the TIA is currently drawing. And knowing where the TIA 'is at' allows us to change what it is drawing at particular positions on the scanline. We don't have much scope for change, but we do have some. And it is this ability that master '2600 programmers use to achieve all those amazing effects.
Naturally, to achieve this sort of precision timing, programmers have to know exactly how long the 6502 takes to do each instruction. For example, a load/store combination takes a minimum of 5 cycles of 6502 time. How many onscreen pixels is that? Remember, 3 color clocks per 6502 cycle, so that's 3 x 5 = 15 pixels. Essentially, if one were using the quickest possible load/store combinations to change the color of, say, the background, then the absolute quickest this could be done would be every 15 pixels (just on 11 times per scanline).
Here's an updated image of the TV timing, taken from the Stella Programming Guide. Some of the numbers should make sense, now. The ones that don't . . . we'll cover those soon.
Have a good look at this image, and try and understand what it's showing. Your understanding of this will greatly assist your '2600 programming efforts, especially when it comes to designing your kernel.
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 programming—and 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
Sessions 3 & 6: The TIA and the 6502
This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).
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 6502 Instruction Set broken down into 6 groups.
Nice, simple instruction set in little boxes (not made out of ticky-tacky).
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.
An easy-to-read page from The Second Book Of Machine Language.
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.
By John Pickens. Updated by Bruce Clark.
Below are direct links to the most important pages.
Goes over each of the internal registers and their use.
Gives a summary of whole instruction set.
Describes each of the 6502 memory addressing modes.
Describes the complete instruction set in detail.
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.
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.
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 programming specs (HTML version).
Links to useful information, tools, source code, and documentation.
Atari 2600 programming site based on Garon's "The Dig," which is now dead.
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.
Adapted information and charts related to Atari 2600 music and sound.
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.
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.
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.