Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!husc6!brauer!fry
From: fry@brauer.harvard.edu (DSF2)
Newsgroups: comp.sys.mac.programmer
Subject: Programming and the 8/24 GC card
Message-ID: <2344@husc6.harvard.edu>
Date: 26 Mar 90 08:17:51 GMT
Sender: news@husc6.harvard.edu
Organization: Harvard Math Department
Lines: 40


Reading through Apple Direct, I was thrilled to see that some
level of multiprocessing is afforded by the new 8/24 GC accelerated 
graphics card.  If a program calls a QuickDraw routine,
the GC's AMD 29000 is given responsibility for executing it.
The 29000 tells the CPU that the routine has already finished
(which is a lie) and the CPU goes on to the next instructions
while the 29000 performs the QD routine.  Should the
CPU encounter another QD instruction before the 29000 finishes, 
it will wait for the 29000. 

My worry is what level of compatibility is required from a
program to make this work.  Certainly Apple has always warned
us that we should user their routines to do QuickDraw, but
for some applications this is just too slow.  What happens if
you try to do this with the GC installed?

For instance, suppose a program calls EraseRect (substitute
your favorite QD trap here) in an offscreen PixMap and then tries to 
access a pixel directly as an offset from the baseAddr immediately 
afterwards.  We assume the 29000 hasn't yet finished, so the
PixMap won't be in the state the program expects it to be.
Or, more likely, a program will try to set the value of a
pixel.

Is there some handshaking going on that will prevent a program
from doing this?  I imagine that would be very simple if the
PixMap were on the GC's onboard DRAM, but what if the PixMap
is in the main memory?

If this really is a problem, is there a way for a program to
know a GC is installed?  At least then it could assume the
board is fast enough that it's not necessary to go behind
QuickDraw's back.


David Fry				fry@huma1.harvard.EDU
Department of Mathematics		fry@huma1.bitnet
Harvard University			...!harvard!huma1!fry
Cambridge, MA  02138