Re: [stella] What would you do with more RAM?

Subject: Re: [stella] What would you do with more RAM?
From: Nick Bensema <nickb@xxxxxx>
Date: Sun, 04 May 2003 22:57:17 -0700
Glenn Saunders wrote:

What you are describing is what was done in the Atari 8-bit home computer, which is at its core a single-scanline kernel-based machine, it's just that the kernel is arbitrated by ANTIC which gets its direction from RAM. You can still drive an Atari8 without ANTIC on as if it were a 2600, and some early games do that.

I'm curious; which Atari 8-bit games forego the ANTIC chip? As far as I know, they all use it, though they might use custom display lists in their own way.

Though, I think you may be misunderstanding what a kernel is. And maybe we've all been misusing it for all these years on the list, but in essence, the Atari 8-bit home computers don't really use a kernel as a 2600 programmer would know it.

The Atari computers have a GTIA chip, which behaves as you described, spitting out a scanline full of registers at any given time. It's more advanced in that it supports more players and missiles, and a wider variety of bit-mapping engines, allowing for multiple playfield colors.

While on a VCS, the TIA must be spoon-fed each scanline by a program executed by the 6502 CPU, the Atari home computers don't burden the 6502 in that way. Instead, they have the ANTIC chip, which does all the spoon-feeding of memory to the GTIA. For players and missiles alike, it does pretty much what was posted earlier:
L1 LDA pm_area,y

It can do this every scanline, or every other scanline, or never, depending on whether player/missile graphics are in hi-res, lo-res, or off mode. And that's pretty much all it does with player/missile graphics. This means you have to put aside a whole block of 128 or 256 bytes if you want to use player/missile graphics. There are no other options, unless you know some hacking tricks I don't.

It's a little more flexible with playfield graphics, though you're now limited to using bitmaps or tile-based systems, and can't use nifty compression tricks most of the time. And you still have to do all the mid-screen color changes by yourself. Flexibility sacrificed for convenience, I suppose.

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