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From: bruner@sp15.csrd.uiuc.edu (John Bruner)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Who cares about sync? (was Re: Animation)
Message-ID: <BRUNER.91Feb11154618@sp15.csrd.uiuc.edu>
Date: 11 Feb 91 23:46:18 GMT
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	<1991Feb11.142307.38779@eagle.wesleyan.edu>
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In-Reply-To: rcook@eagle.wesleyan.edu's message of 11 Feb 91 19:23:06 GMT

Just for curiosity's sake, is there an alternate screen buffer on the
Classic?

I don't believe that an 8MHz 68000 can redraw the entire Mac screen in
less than 1/60 second.  Considering memory traffic alone (ignoring any
other instruction execution overheads), there aren't enough bus cycles
to fetch the instructions and move the data around.

[1/60 second at 8MHz is 133,333 cycles.  At four cycles per bus
transaction, that's 33,333 word reads or writes.  A 512*342 1-bit
image has 21,888 bytes, so updating it requires 10,944 reads and
10,944 writes.  That leaves only 33333-21888 = 11445 bus transactions
for instruction fetches, and this simple analysis doesn't consider the
other factors that will slow you down.]

Of course, most animation sequences don't involve changing all of the
pixels at once.  Thus, a CopyBits of a large image from an offscreen
bitmap may not be the best way to update an animated image.
--
John Bruner	Center for Supercomputing R&D, University of Illinois
	bruner@csrd.uiuc.edu		(217) 244-4476