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From: truett@cup.portal.com
Newsgroups: comp.dcom.lans,comp.sys.ibm.pc
Subject: Re: IBM PC/AT DMA loses (was Re: PC LAN Comparison)
Message-ID: <1560@cup.portal.com>
Date: Tue, 24-Nov-87 00:10:12 EST
Article-I.D.: cup.1560
Posted: Tue Nov 24 00:10:12 1987
Date-Received: Fri, 27-Nov-87 06:04:30 EST
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On the IBM bus, the DMA loses yet again!  Remember that on that bus, the
CPU unconditionally grants the bus to any DMA request (it almost has to,
that's how the PC and XT do their dynamic memory refresh).  Thus, several
DMA devices can capture the bus and lock the CPU out.  If PIO is used,
though, the CPU has a choice.

On a bus that allows a DMA request to be blocked by a higher priority
compute task this problem probably does not occur.  Also, note that the
assumption that a CPU only needs the bus every n-th cycle is very dependent
on the design of the particular system being considered.  There are, I
believe, processors out there that can do a fetch and an operation on every
cycle.  I know some DSPs do and some RISCs probably do, not to memtion
highly pipelined microprocessors of more traditional type.

Truett Smith, Sunnyvale, CA
UUCP:  truett@cup.portal.com