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From: paul@taniwha.UUCP (Paul Campbell)
Newsgroups: comp.sys.mac.hardware,comp.sys.mac.programmer
Subject: Re: NuBus traffic
Keywords: NuBus, ethernet, video
Message-ID: <871@taniwha.UUCP>
Date: 12 Jun 91 20:10:34 GMT
References: <1991Jun11.160305.7049@hubcap.clemson.edu> <1991Jun11.190925.25293@world.std.com>
Reply-To: paul@taniwha.UUCP (Paul Campbell)
Organization: Taniwha Systems Design, Oakland
Lines: 43

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 ...

>Wipping out the hp.  A 480x640 screen....at 4 bytes per pixel(32bit QD)
>should be... about 1.2 meg of memory.... which will take about
>0.0589 seconds to transfer.  THAT IS FOR A FULL SCREEN UPDATE.

if it were an ideal world, the fastest video cards out there today
take 300ns to transfer 1 24-bit pixel (this is for writes, reads are
often slower), the Mac NuBus interface can take an additional 100-400ns
(depends on which model - for this example let's assume 200ns) plus there
is the actuall time taken to execute the instruction (let's assume 100ns)

This gives us a total time to move a pixel across the bus in a scroll to be
2x(300+200)+100 = 1.1us (on a Mac II with an old video card this number 
can be more like 2.5us), to scroll 640x480 pixels will take 640x480x1.1
which is about 1/3 of a second - which may be almost tolerable in a word
processor but is useless for a terminal emulator - in practice there's enough
other things going on in a Mac/video card that this number is probably on the
fast side (VBLs, refresh cycles on the mother board and the card, VRAM serial
transfer cycles on the card etc etc).

Suffice to say for that 1/3 of a second the NuBus is busy 10/11 of
the time.

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).

	Paul Campbell

-- 
Paul Campbell    UUCP: ..!mtxinu!taniwha!paul     AppleLink: CAMPBELL.P

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