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From: koll@ernie.NECAM.COM (Michael Goldman)
Newsgroups: comp.unix.xenix,comp.unix.questions,comp.unix.microport,comp.sys.ibm.pc
Subject: Re: *nix performance
Summary: The problem with DMA on the PC/XT/AT machines is that the particular
	 DMA chip used requires TWO cycles for a reasonable mode, but more
	 importantly was never rated above 5 MHz !  Anyone trying to run it
Message-ID: <168@ernie.NECAM.COM>
Date: 19 Oct 88 15:57:21 GMT
References: <9902@ico.ISC.COM> <736@starfish.Convergent.COM> <1901@van-bc.UUCP> <1488@ssc.UUCP>
Distribution: na
Organization: NEC America RATS, San Jose, CA
Lines: 34


As I was saying (I'm just getting used to posting) Any manufacturer
trying to run the DMA chip above 5 MHz risks frying the chip, and
part of the motherboard.  With all these cpus going along at 25 MHz
it is faster to use the cpu.  Before dumping on IBM for using such
a dumb chip, recall that the original PC came with a cassette port
and only 64K on the mother board.  Who needs DMA in that environment ?

This is one more reason to go to the new Microchannel architecture which
has good DMA support and very nice chips.  There are some other problems
with DMA on the PC. One is that DOS is not re-entrant and so you have
to VERRRY Carefully save the state with any program that uses interrupts
which is implicit in any reasonable application with DMA.  With all the
yo-yos trying to be the next Mitch Kapor, IBM wisely left out helping
anyone write DMA programs, for fear of having every one try to save a
few usecs and crashing DOS.  The string transfer assembly instructions
on the 80x86 are as fast as DMA anyway at comparable clock speeds.  IN
a no wait-state system there's no real advantage to DMA for single
threaded OS's like DOS, which is probably why IBM waited to have the
386 in a new bus with a new multi-threaded OS an new DMA chips.
So now one process can wait for a file transfer using DMA while another
process can execute.  This implies that the developers can intelligently
use the DMA chips (don't hold your breath - the operant philosophy
seems to be " If the PC is cheap then I don't have to pay the
programmers much either. " and we get what they pay for (I'm not
bitter, not ME !)).  Finally, recall that the 8088 was still
trying to maintain some compatibility with 8080s and a lot of the
support chips out there at the time hadn't caught up.  The 80386
is what Intel should have designed long ago if they had seen the
future, and now it has good support chips. (Not dumping on Intel,
hindsight is 20-20, and densities didn't allow much earlier.)

Regards,
Michael Goldman