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From: johnmac@fawlty.towers.oz (John MacLean)
Newsgroups: comp.sys.apple2
Subject: Re: Stellar 7 re-release
Message-ID: <547@fawlty.towers.oz>
Date: 16 Jan 91 04:48:49 GMT
Reply-To: johnmac@fawlty.ips.oz (John MacLean)
Organization: Tower Technology, Lane Cove, NSW, Australia
Lines: 25

>>picture, so I can see the problem there, but an ordinary display program
>>should be able to get all 3200 colors.
>You don't need to use an interrupt on every scanline...your program
>can track the vertical scanline counter register (documented in
>an Apple IIGS technote I don't have handy) to figure out when to change.
>One could also use all 16 hardware-supported palettes and update them
>every 16 scanlines, which might buy you some extra CPU to do something
>useful other than synchronize to the electron beam.
>Dave Seah 

Most display programs use all 16 palettes - although some use 8.
You need to use at least 2 and possibly at least 3.
Most recent code that I have seen does not use interrupts, but rather
scans the counters as you suggested.
However, better synchronization should be possible by disabling
interrupts and using the WAI instruction.
It "hangs" the CPU when interrupts are disabled, and falls through at
the instant of the interrupt - without going through any vectors.
I never got round to getting this working, but it should be the best way
to do it.
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