> Yes, I recall this as well. Which makes sense as NES code is much less
> coupled to the structure of the playfield than Atari code (yes, that's a
> good way to say it...), although it can be made to generate interrupts
when
> a given scanline is started.
No, you could NOT make the NES generate scan line interrupts on particular
scan lines. Sorry, no no no!! The only way to do this was to position
sprite 0 on the scan line you wanted, and then poll the collision bit to
wait for it to be set by the hardware. And you could only do this once per
frame - that is, you could use sprite 0 only once, and for the rest of the
frame it had to be timed.
If you wanted to do mid-screen changes of screen registers, you had to
pretty much couple your code to the display - not so much as the 2600 does,
but in a similar fashion. No interrupts to help you out, though you did
have the ability to wait for the HBLANK period on any line.
The Atari 400/800, on the other hand, had display list interrupts (from
memory), which would effectively allow you to interrupt on any scan line on
the screen. Although memory has faded, and I concede that I may in fact be
wrong - I have programmed all of these machines (2600, Atari 400/800 and
NES) - so I'm pretty sure about my information.
Cheers
A
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