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Now you should be ready to learn a bit more about what you're doing.
First off, you should consider strongly consider reading through "Assembly In One Step" if you don't know 6502 Assembly, and at least skim through the Stella Programmer's Guide (See our Information Resources section for those.) There's a lot that I'm not going to explain here--there's a lot I don't know enough to explain here!--and you'll likely find your self turning to these resources again and again.
Our Friend The Television
(This section is mostly just a rehash of the first few pages of the Stella Programmer's Guide)
A television works by aiming a stream of electrons at the screen, moving it quickly across the glass in front. (Bear with me here, Mr. Wizard I ain't.) It fires that stream left to right (from the viewer's perspective) across the screen for one "scan line", zips back to the left side, moves down, and does the next scan line. There are 192 visible scan lines on a TV. And your atari program has to tell the TIA (Television Interface Adaptor) what to put on each line, a splitsecond before it starts drawing that line. (Well, you can skip telling TIA what to do every other line to give your program more time to think...it will just make a copy of the line before. More on that later.)
But there's more to this process then that visible picture...thankfully! Otherwise you'd be spending almost all your meager computing power telling TIA what to draw, without the time to make the calculations for the rest of your game. There are four distinct "intervals" of what the electron beam (sounds very scifi, don't it?) is doing...five, in fact, when you realize there's a bit of time before the electron beam starts drawing each individual scanline. (that time is called the "horizontal blank")
So, life in the Atari/Television world is divided into "clock counts", 228 per scanline. Each "Machine Cycle" takes up 3 clock counts...and the actual assembly instructions take 2-6 machine cycles,so you don't have a lot of time to do computations.
Let me do my own version of that Stella Programmer's Guide diagram:
Here's what's going on. First, the CPU has to send the VSYNC. This message gets to the TV and tells it to get ready to start a new "frame", or picture. But you can't just hit it and quit it...the CPU has to wait for 3 whole scanlines for the Television to get the message. Luckily, waiting for a scanline to finish drawing is one of the things the Atari does best...you'll learn that when you "do a WSYNC", the CPU halts, and waits for the current scan line to finish. So an Atari Program turns on the VSYNC, does WSYNCs three times, the turns the VSYNC off. Now the TV's good to go.
Now the Atari enters the time known as the "Vertical Blank". The TV hasn't really begun to draw the picture yet...it won't for another 37 scanlines. In fact, the CPU needs to turn on a special "VBLANK" register that says there's no picture here. In general, there are two strategies for this time. The most obvious strategy is to just do another 37 WSYNCs. A better strategy (we'll learn the details of this in a bit) is to set one of the Atari's timers. That way we can do alot of the game logic we want (move things, check collisions, fiddle with memory, all the jazz) without having to know if a series of instructions fits in a particular scanline or not. We set a calculated amount into a timer, then once our program logic is done logic'ing, we do a "do-nothing" loop until the timer equals zero. Then we turn off VBLANK. And then we move on.
Time to start drawing...almost. For each line, you get about 22 Machine Cycles of "Horizontal Blank" to do some work...usually spent telling the TIA if the various players and missiles and playfields (oy, you'll find out them in a bit) are "on" for the upcoming line and then the electron starts drawing. Now, the Stella Programmer's Guide talks about how you can get extra time to do game calculations by only setting TIA every other line...and programmers who are really slick can do all sorts of logic on the fly, have everything ready to go for the TIA just in time, and still be home in time for dinner.
Now, from what I've seen, most programmers will count the # of scanlines, rather than use the timer trick I mentioned for the vertical blank...that's because of how important it is to tell the TIA "is this object (player, missile, ball) 'on' for this scan line".
So, it's a very different world from most computer programming, where you can just set pixels on the screen and they stay there. One saving grace is that the TIA holds the last values you set, so you don't have to redo every damn thing every damn line. Still, it's pretty challenging.
Once you've done the 192 lines (or so...sometimes the math gets a little crooked) you get another respite, the "oversan" period. This is just like the vertical blank, except its only 30 lines, not 37. So you should set the VBLANK it might make sense to do the timer trick again, and do some more program logic.
Once the overscan is finished, it's time to start over with the VSYNC. (Note that if you've turn on VBLANK at the start of the the overscan, you might as well leave it on throughout the vertical sync, since the vertical blank period needs it to.)
So that was a lot to get through, but believe me, we need to get a handle on this to have any idea what's going on as we start to look at even the simplest Atari programs.
By The Way: all of the above applies only to NTSC televisions, the video standard in the United States (and Canada, Mexico and Japan). There's also a video standard called "PAL" used in Europe and some other places. PAL has 312 scanlines, and refreshes a bit slower. (50 framers per second for PAL, 60 for NTSC). And the colors are a bit different. There are techniques for programming around these differences, but I'm not going to get into that anywhere in this tutorial.
Oh, and the diagram above is a bit of a lie...really, there's some "horizontal blank" time on either side of the visible image, the part you can see isn't really on the right side like that. But in practice, the view in the illustration is good enough.
Next: My First Program