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From: brian@umbc4.umbc.edu (Brian Cuthie)
Newsgroups: comp.sys.mac.hardware,comp.sys.mac.programmer
Subject: Re: NuBus traffic
Keywords: NuBus, ethernet, video
Message-ID: <1991Jun13.183543.12392@umbc3.umbc.edu>
Date: 13 Jun 91 18:35:43 GMT
References: <1991Jun11.160305.7049@hubcap.clemson.edu> <1991Jun11.190925.25293@world.std.com> <871@taniwha.UUCP>
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Organization: Univ. of Maryland Baltimore County, Academic Computing Services
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In article <871@taniwha.UUCP> paul@taniwha.UUCP (Paul Campbell) writes:
>In article <1991Jun11.190925.25293@world.std.com> aep@world.std.com (Andrew E Page) writes:
>>
>>   The Mac implementation of NuBus has a maximum bandwidth of 
>>about 20 MB/second.  Add to that the screen is not really
>>causing all that much taxing of that when you consider that the
>>QuickDraw toolbox does not write to the whole screen at once.  
>
>actually this happens more often than you think - consider a full-screen
>(at 640x480) window scrolling ...
>
>This is why people build quickdraw accelerators, especially for 24-bit cards,
>I know of an existing on-card accelerator can move an entire pixel in <100nS
>so scrolling can be >10x faster (or closer to 20x for a MacII).
>


What I really keep waiting for, is someone to come out with a graphics board
for the MacIIfx that uses the processor direct slot.

The problem with the fx is that is scrolls *slower* than a IIci.  Because
the NuBus is such a pig (at best, on a good day, ~30 Mbytes/sec, block mode)
there is a real bottleneck between the processor and the video ram.  Worse,
the NuBus is EXPENSIVE to design interfaces for.  Even the chipset that
TI has is $50 in quantity.

So, what would be nice, is for someone to introduce a video display board for
the IIfx that used the processor direct slot.  It would be MUCH faster 
and should cost less.  It's not hard to imagine that a 40 Mhz 68030 
running quickdraw would start to approach the speed of a 25 Mhz 29K.

-brian