NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/comp/os/os2/misc/35409

Internet Message Format | 1992-11-05 | 2.7 KB

Path: sparky!uunet!usc!cs.utexas.edu!rutgers!njitgw.njit.edu!hertz.njit.edu!dic5340
From: [e-mail] (David Charlap)
Newsgroups: comp.os.os2.misc
Subject: Re: Does VESA Local Bus put a huge strain on a CPU?
Message-ID: <[e-mail]>
Date: 5 Nov 92 08:38:32 GMT
References: <[e-mail]> <[e-mail]>
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Organization: New Jersey Institute of Technology, Newark, N.J.
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In article <[e-mail]> [e-mail] (Nishith A. Patel) writes:
>[e-mail] (David Charlap) writes:
>>ISA runs at a theoretical 8MHz at a 16-bit datapath, making an
>>effective 16MB per second if every bus-cycle would be used.  This is
>>almost never the case, though.  Most ISA machines max out at about
>>5-10MB/s.  The fastest I've ever seen an ISA card do was 4MB/s.  I
>>think a bus-mastering card can approach the maximum speed, since it
>>can shut the CPU out of the bus.

One minor correction: ISA can only access the bus every other clock
cycle, so an 8MB computer has a theoretical maximum at 8MB/s, not 16.

>>LocalBus, I believe, is 32-bits wide, and runs at the CPU's internal
>>speed.  This would deliver a theoretical 100MB/s if every clock was
>>used for transfer.  This, just like with MCA and ISA, is unrealistic.
>>I think you can expect about 20-50MB/s with an LB slot and an LB card
>>that keeps the CPU and other cards off the bus.
>
>Now one thing I don't understand: If MCA can come close to its theoretical
>peformance of 20, 40, or 80 MB/s, depending on the card, then why can't LB
>do the same if it does the ideal things that MCA does to reach the limit?
>if LB can theoreticsally get 100 MB/s, why is the expected only 20-50 if it
>does everything right? What is it wasting all those clock cycles on

It probably could, but the CPU would be doing nothing but servicing
the bus every clock cycle.  And there's the other problem that your
RAM won't be able to keep up with 100MB/s, since most RAM is about
60-80ns.  With bus-mastering devices, it is theoretically possible for
two cards to talk to each other at full speed while the CPU goes off
and does something else.

>And if LB is running at 33 MHz, isn't the max more than 100 MB/s?

LB runs at processor speed.  I was assuming a 25MHz system, such as
the one I have.  (No, my system is not LB).


--
David Charlap         |"there aren't 50,000 things for which it's worth writing
[e-mail]| software; and the computer industry doesn't have enough
----------------------+ programmers to create that much good software.
Therefore, most of it must be worthless" -- Boris Beizer "The Frozen Keyboard"