Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!spool.mu.edu!think.com!mintaka!pogo.ai.mit.edu!dbert From: dbert@pogo.ai.mit.edu (Douglas Siebert) Newsgroups: comp.sys.mac.hardware Subject: Re: LC video vs. SI video... Message-ID: <1991Mar6.015922.6940@mintaka.lcs.mit.edu> Date: 6 Mar 91 01:59:22 GMT References: <1991Feb26.134342.25399@magnus.ircc.ohio-state.edu> <1991Mar6.094536.125@otago.ac.nz> Sender: daemon@mintaka.lcs.mit.edu (Lucifer Maleficius) Organization: The Internet Lines: 36 In article <1991Mar6.094536.125@otago.ac.nz> michael@otago.ac.nz writes: >In article , ingemar@isy.liu.se (Ingemar Ragnemalm) >writes: >> This only occurs when the running program is located in the lowest meg, >> thus sharing that meg with the video. A program running in a higher part >> of the memory runs at full speed even in 8-bit mode. >> > >This doesn't sound like the whole story. Surely the video image can't be >sitting there in the middle of the heap (can it?). I understand Apple are using >the MMU to remap things, so presumably what we have is whatever portion of the >1 meg is left over from the video (716K in 8-bit mode) mapped somewhere into >the main space and whatever other RAM you have in the other bank mapped around >it. Presumably the MMU has the flexibility to put this slow RAM anywhere, so it >would be interesting to know exactly what Apple have done and why. I hope they >have picked a spot based on careful analysis of average memory use... > Nor does this sound like the whole story. The way I understand it, the reason for the slower execution is because when the on-screen image is refreshed the processor cannot access the same block of memory where the screen resides, thus causing the processor to have to wait during each time in which in the video circuitry takes over use of the memory for a cycle or two. Everyone is talking about the "low" megabyte in memory though....WRONG! Unless you have a 2M computer, you will be talking about the low FOUR megabytes of memory, since the 1M simms are banked together in groups of four, all sharing the same block. I know my terminology is probably not correct and likely confusing....perhaps someone else can explain it better than I....but I'm pretty sure this idea should hold. -- ________________________________________________________________________ Doug Siebert dbert@albert.ai.mit.edu MBA Student (2nd year) The University of Iowa