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
STA GRP0
INY
BEQ L1
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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