Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!mtxinu!taniwha!paul
From: paul@taniwha.UUCP (Paul Campbell)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Programming and the 8/24 GC card
Message-ID: <551@taniwha.UUCP>
Date: 26 Mar 90 19:02:25 GMT
References: <2344@husc6.harvard.edu>
Reply-To: paul@taniwha.UUCP (Paul Campbell)
Organization: Taniwha Systems Design, Oakland
Lines: 39

In article <2344@husc6.harvard.edu> fry@brauer.harvard.edu (DSF2) writes:
>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?

The answer is that there is nothing to prevent an application from
doing this - if they break the rules. Rules? you ask, what rules?
Apple was very smart, they announced how to do this a year ago
when they released the 32-bit QD documentation - but didn't draw
atttention to it. The description for the "GetPixBaseAddr" routine
sais:

"GetPixBaseAddr waits until all drawing to the PixMap is completed
and returns a 32-bit pointer to the beginning of its pixels ..."

As an accelerator designer (you can see why I was reading between
the lines back then) I would like to stress that this is VERY
important. One of the best ways to get more QD performance out of
the system is to overlap QD operations with application processing
(otherwise perceived QD acceleration is ultimately limited to how fast
applications can run) - ideally an app should be able to keep an
accelerator stoked to the point that it never has to wait for display.


	Paul Campbell
	SuperMac
-- 
Paul Campbell    UUCP: ..!mtxinu!taniwha!paul     AppleLink: CAMPBELL.P
Remember 1990? that was the year the US government funded a Communist election
victory in Nicaragua and claimed it a victory for Capitalism.