By Andrew Davie (adapted by Duane Alan Hahn)
Table of Contents
Time to complete our understanding of what constitutes a TV frame—exactly what has to be sent to the TV to make it display a picture correctly.
Let's take another look at the diagram with the timing information and the Pitfall! image inside.
Your understanding of the numbers across the top should be good, but we'll briefly revisit what they mean, just to make sure.
There are 228 TIA color clocks on each scanline. 160 of those are spent drawing pixels, and 68 of them are the horizontal retrace period for the TV's scanning of the electron beam back to the start of the next line. In the diagram we see the horizontal blank (retrace) at the left side, so our very first color clock for the TIA's first visible pixel on the screen is cycle #68. We should understand this timing fairly well by now.
What we're going to finalize this session is our understanding of the numbers down the right-hand side—which represent the scanlines sent to the TV. The diagram shows a valid NTSC TV frame—and thus it consists of 262 scanlines. A PAL diagram would consist of 312 scanlines—and the inner 'picture' area would increase from 192 lines to 242 lines.
Let's go from the top. The first thing that the TV needs is a 'reset signal' to indicate to it that a new frame is starting. This is the 3-scanline section at the very top of the frame. There are special ways to trigger the TIA to send this signal, but we're not going to have to worry too much about understanding that—just about every game does it exactly the same way—all we need to remember is that the first thing to send is that reset trigger (called VSYNC).
TVs are not all made the same. Some cut off more of the picture than others, some show wider pictures, some show taller pictures, etc. To 'standardize' the picture, the diagram shows the recommended spread of valid picture lines, surrounded by blank (or 'overscan') lines. In this case, there are 192 lines of actual picture. We don't *HAVE* to stick to this—we could steal some of the lines from the vertical blank section, and some from the overscan section, and increase our picture section appropriately.
As long as our total number of scanlines adds up to 262 for NTSC TVs (or 312 for PAL TVs), then the TV will be able to display the frame. But remember, the further we get 'out of specs' with this method, the less likely it is that ALL TVs will show the picture section in its entirety.
OK, let's march through the numbers on the right side of the diagram.
3 Scanlines devoted to the vertical synchronization.
37 scanlines of vertical blank time.
192 (NTSC) or 242 (PAL) lines of actual picture.
30 scanlines of overscan.
Total: 262 scanlines (NTSC) or 312 scanlines (PAL), constituting a valid TV frame. You send the TV this, and it will be a rock-solid display.
One interesting aside: if you send a PAL TV an *odd* number of scanlines, it will only display in black and white. I don't know the exact reason for this, but it must be to do with where/when the color signal is encoded in the TV image, and where the TV looks for it. So remember, always send an even number of scanlines to a PAL TV.
You *can* send frames with different numbers of scanlines. That is, 262 and 312 are not totally immutable values. But if you do vary these numbers, it is highly likely that an increasing number of TVs—the further you deviate from these standards—will simply not be able to display your image. So, although you *can* . . . you shouldn't.
Fortunately, emulators available to us today are able to show us the actual number of scanlines which are being generated on each frame. This must have been quite a challenging task for early '2600 programmers—nowadays its quite easy to make sure we get it right.
Well, now we have all the knowledge we need about the composition of a TV frame. Once we know how to make the TIA generate its reset signal at the top of the frame, and how to wait the correct amount of time to allow us to correctly generate the right number of scanlines for those other sections, we will be able to design our first 'kernel'—the bit that actually 'draws' the frame.
When we have our kernel working, there's not much more to a '2600 game other than moving sprites around, changing colors, etc. See you next time.
Other Assembly Language Tutorials
Session 7: The TV and our Kernel
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.