Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!husc6!necntc!gordon!blblbl!kaos!romkey 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