Path: utzoo!mnetor!uunet!husc6!uwvax!dogie!uwmcsd1!ig!agate!ucbvax!proteon.COM!jas
From: jas@proteon.COM ("John A. Shriver")
Newsgroups: comp.protocols.tcp-ip.ibmpc
Subject: Western Digital board
Message-ID: <8805101529.AA25308@monk.proteon.com>
Date: 10 May 88 15:29:29 GMT
References: <21523@amdcad.AMD.COM>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 14

Yes, AT DMA is slow as a dog.  The glue logic between the 8237 DMAC,
which is an 8 bit device, and the 16-bit bus, is a very slow kludge.
16-bit DMA using the second DMAC is somewhat better, and boards that
take over the bus and do their own DMA are much better.  However,
doing either of these limits your board to use in AT's, which dealers
don't like.

What is nice is that an AT can move memory from one place to anther
incredibly fast using string moves.  (These are 286-only
instructions).  It blows away most DMA implmentations (no arbitration
delays).  Since the CPU is essentially halted during DMA anyways, why
not let it do the copy.

The tradeoffs are probably different on the MicroChannel in PS/2's.