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From: romkey@kaos.UUCP (John Romkey)
Newsgroups: comp.dcom.lans,comp.sys.ibm.pc
Subject: IBM PC/AT DMA loses (was Re: PC LAN Comparison)
Message-ID: <261@kaos.UUCP>
Date: Fri, 20-Nov-87 23:24:33 EST
Article-I.D.: kaos.261
Posted: Fri Nov 20 23:24:33 1987
Date-Received: Mon, 23-Nov-87 03:12:25 EST
References: <2070@killer.UUCP> <1020@kodak.UUCP> <155@tic.UUCP>
Reply-To: romkey@kaos.UUCP (John Romkey)
Organization: Chaos; Somverille, MA
Lines: 26
Keywords: IBM PC, hardware, DMA loses
Xref: mnetor comp.dcom.lans:949 comp.sys.ibm.pc:10385

In article <155@tic.UUCP> ruiu@tic.UUCP (Dragos Ruiu) writes:
>   However, I had great difficulty believing their hardware section, which
>repeatedly says that DMA interface boards are slower than others. Unless there 
>is something I don't know, DMA is much faster than interrupt driven hardware.
>I have not had the time or the resources to research their claims, but
>common sense would indicate that this claim of theirs is way off base.

First off, interrupts and DMA are orthogonal. It's really a choice of DMA
versus programmed I/O. On the PC/AT, the DMA controller runs about 10%
slower than the standard PC DMA controller. On top of this, the 80286 has
string IN and OUT instructions in addition to the 8086 string MOV instruction.
Both string IN/OUT and string MOV instructions push data around a lot faster
than the DMA on that system controller can. You basically end up with the
processor sitting in a tight loop shoveling bytes across the bus.

Under MS-DOS this is fine, since MS-DOS is single tasking. You'll never have
another process waiting to use computes that are being used up by having
the processor move data that other hardware could move (more slowly). Under
a multitasking system you'd want to examine the tradeoff; in some situations
you'd get more aggregate computes out of the system if the I/O was performed
by the (slow) DMA controller than the processor, because other processes could
run while the system waited for the DMA to complete.
-- 
			- john romkey
		...mit-eddie!blblbl!kaos!romkey
		    romkey@xx.lcs.mit.edu