Atari 2600 Programming for Newbies
Sessions 3 & 6: The TIA and the 6502
By Andrew Davie (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
As an Amazon Associate I earn from qualifying purchases.
Page Table of Contents
Original Sessions
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
Be sure to check out the other assembly language tutorials and the general programming pages on this web site.
Amazon: Atari 2600 Programming (#ad)
Amazon: 6502 Assembly Language Programming (#ad)
Atari 2600 Programming for Newbies (#ad)
|
|
|
Session 2: Television Display Basics
Sessions 3 & 6: The TIA and the 6502
Session 5: Memory Architecture
Session 7: The TV and our Kernel
Session 9: 6502 and DASM - Assembling the 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 20: Asymmetrical Playfields (Part 3)
Session 22: Sprites, Horizontal Positioning (Part 1)
Session 22: Sprites, Horizontal Positioning (Part 2)
Session 23: Moving Sprites Vertically
Session 25: Advanced Timeslicing
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.
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.