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From: romkey@kaos.UUCP (John Romkey)
Newsgroups: comp.dcom.lans,comp.sys.ibm.pc
Subject: Re: IBM PC/AT DMA loses (was Re: PC LAN Comparison)
Message-ID: <269@kaos.UUCP>
Date: Thu, 26-Nov-87 16:34:06 EST
Article-I.D.: kaos.269
Posted: Thu Nov 26 16:34:06 1987
Date-Received: Mon, 30-Nov-87 03:22:54 EST
References: <2070@killer.UUCP> <1020@kodak.UUCP> <155@tic.UUCP> <261@kaos.UUCP> <1560@cup.portal.com> <162@tic.UUCP>
Reply-To: romkey@kaos.UUCP (John Romkey)
Organization: Chaos; Somerville, MA
Lines: 45
Xref: mnetor comp.dcom.lans:971 comp.sys.ibm.pc:10580

In article <162@tic.UUCP> ruiu@tic.UUCP (Dragos Ruiu) writes:
>An aquaintance who is designing a major PC based hardware project has chosen
>to use double-ported memory. Truett Smith has already suggested this as the 
>solution.
>
>So, in light of the dropping cost of such devices, they are the preferred way
>to go. Right ?

Right. Many of the recent network interfaces for the PC and AT in fact use
dual-ported memory with the LAN controller hardware on one side and the
PC or AT bus on the other. In fact, the best network interfaces on the market
right now all take this approach.

But there's still a catch. Most of these network interfaces only provide
8K or 16K bytes of RAM. To get really good performance out of them, you
want their memory available to receive data from the net as soon as is
possible. So you end up copying the data into the PC's main memory. You can
actually program the DMA controller to do that, but who'd want to? Using an
8086 MOVS instruction is so much faster...it should be faster even on the PC,
but I don't have the books here to check it out and make sure.

So you still end up copying, rather than using the data in place.

You could put lots of memory on the network interface, like 256Kbytes, but
then you'd have two problems. The hardware would have a hard time mapping in
all that memory into the PC address space, so it would probably have to be
bank-switched. The software would have problems managing it and figuring out
who had buffers where and then trying to reclaim them later on.

The boards which are memory mapped include the Micom-Interlan NI5210,
the Western Digital WD8003, the Univation NIC and the Excelan EXOS205, all
of which are ethernet interfaces. Proteon also sells a memory-mapped IEEE 802.5
token ring card, which is either the P1340 or the P1344.

I'm sure I've left out a couple, but I just woke up...

>-- 
>Dragos Ruiu          Disclaimer: My opinons are my employer's, I'm unemployed!
>            UUCP:{ubc-vision,mnetor,vax135,ihnp4}!alberta!edson!tic!dragos!work
>(403) 432-0090         #1705, 8515 112th Street, Edmonton, Alta. Canada T6G 1K7 
>Never play leapfrog with Unicorns...
-- 
			- john romkey
		...mit-eddie!blblbl!kaos!romkey
		    romkey@xx.lcs.mit.edu