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Path: sserve!manuel.anu.edu.au!munnari.oz.au!spool.mu.edu!hri.com!noc.near.net!uunet!optigfx!optigfx.com From: mrm@optigfx.com (Mike Murphy) Newsgroups: comp.unix.bsd Subject: DMA disk controllers Message-ID: <1732@optigfx.optigfx.com> Date: 23 Oct 92 21:30:05 GMT Sender: mrm@optigfx.optigfx.com Lines: 26 Reading news, I forget which group, I came upon an article that said "ISA bus DMA is so slow, nobody oughtta use it". Being a skeptic, I get out an AT(tm) Tech. Ref. manual, and go through the numbers. DMA channels 5-7 are 16-bit and should be able to do about 1.6MB/sec. That's fast enough with respect to drives for MFM, RLL, and IDE, since they can at best do about 1.2MB/sec bulk throughput. Neat, I think, since I know that offloading the CPU by doing DMA in the disk driver rather than CPU transfers, thereby freeing up the CPU to do good stuff rather than just I/O is a Good Thing in a multi-programming system. It is at least worthy of investigation. Does anybody have a system that becomes CPU sluggish when doing a lot of I/O? Maybe DMA would help. Yes, I know that insw and outsw are faster with a super-zorch CPU, but the DMA ought to be fast enough, and it would offload the CPU if the DMA setup didn't take more CPU than the DMA saved. Then I look further at the circuit diagram for the IBM(tm) hard disk adapter. No hard disk DMA. Only floppy disk DMA on channel 2 (8-bit, BTW). Awww, nuts, cheapskates, I think again, so I go get some disk controllers off the shelf and look at 'em. Not a one has DMA for the hard drive. Most don't even have foil on the connector for DRQ5-7/DACK5-7. Then I try and remember, have I ever seen a controller with a jumper for selecting DMA channel? Nope. Does anyone know of a hard disk controller, MFM, RLL, or IDE, that supports DMA? Include a smiley in any response suggesting SCSI. Thanks, Mike