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Path: sserve!manuel.anu.edu.au!munnari.oz.au!uunet!math.fu-berlin.de!mailgzrz.TU-Berlin.DE!ceres.fokus.gmd.de!gmdtub!bigfoot!tmh From: tmh@bigfoot.first.gmd.de (Thomas Hoberg) Newsgroups: comp.unix.bsd Subject: Re: ISA/DMA in 386BSD Message-ID: <TMH.92Oct26024615@bigfoot.first.gmd.de> Date: 26 Oct 92 01:46:15 GMT References: <BILL.92Oct24005633@kepler.ucsd.edu> <1992Oct25.044504.29339@ntuix.ntu.ac.sg> Sender: news@bigfoot.first.gmd.de Organization: GMD-FIRST, Berlin Lines: 35 In-reply-to: eoahmad@ntuix.ntu.ac.sg's message of 25 Oct 92 04:45:04 GMT In article <1992Oct25.044504.29339@ntuix.ntu.ac.sg> eoahmad@ntuix.ntu.ac.sg (Othman Ahmad) writes: There used to be a long discussion on this topic in comp.os.minix because the minix driver does not use DMA? Personally I do not like DMA. With 486/25 , memory copy is faster than DMA, and it does not have the memory limitation problem. The reason why Adaptec for example chose DMA for their SCSI host adapters wasn't transfer speed but "intelligence", the intelligence being the host adapters ability to "batch" multi sector i/o without having to resort to extra memory on the controller. The "batch scripts" or "channel programs" (IBM mainframe blue speak) are read via DMA and data is transferred that way, too. The main gain is a lower interrupt rate. With WD1002 compatible devices (MFM/RLL/ESDI/IDE) there is an interrupt for every 512 bytes transferred. Modern IDE drives do up to 1.6MB/sec which translates to 3200 interrupts/sec. Under DOS this doesn't matter (single tasking, little interrupt overhead). Under a multi-tasking OS things are a bit different (esp. UNIX). DMA on a PC motherboard is done with chips dating back to the 8080 era (1976/8-bit) and the throughput is likewise, which is why companies like Adaptec chose to implement their own DMA on the adapter (bus mastering DMA). The throughput there is limited only by ISA bus bandwidth and that bandwidth isn't easily saturated by a disk drive. I have never tested the penalty involved with the extra copy for disk transfers beyond 16MB on an ISA system (I only have 16MB), but it's a penalty added only to disk transfers. Disk transfers should be avoided as much as possible anyway. Todays disks simply are no match for today's processors any more and if your system starts paging, you better get more memory. --- Thomas M. Hoberg | Internet: tmh@first.gmd.de 1000 Berlin 41 | tmh@cs.tu-berlin.de Wielandstr. 4 | Germany | BITNET: tmh@tub.bitnet +49-30-851-50-21 |