Posted Thu May 22, 2003 5:16 AM
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 colour and intensity information for any scanline. This colour 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 colour 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 colour 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 (= 1.19MHz, roughly).
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 colour clocks of information to the TV. If the TIA was in the first 68 colour clocks of the scanline, then it was in the horizontal retrace period. If it was in colour 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 where on a scanline the TIA is currently drawing (ie: what pixel). 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 colour 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 colour of, say, the background, then the absolute quickest this could be done would be every 15 pixels (ie: just on 11 times per scanline).
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 colour 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.