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Newsgroups: comp.os.386bsd.development Path: sserve!newshost.anu.edu.au!munnari.oz.au!news.Hawaii.Edu!ames!elroy.jpl.nasa.gov!usc.edu!howland.reston.ans.net!usenet.ins.cwru.edu!eff!news.kei.com!news.oc.com!utacfd.uta.edu!rwsys!hammy!gordon From: gordon@sneaky.lonestar.org (Gordon Burditt) Subject: Re: [FreeBSD 1.0R] DMA Problems? References: <CHCErs.G5w@genesis.nred.ma.us> <2dj25i$1ga@u.cc.utah.edu> <2encotINN3sq@bonnie.sax.de> Organization: /usr/lib/news/organi[sz]ation Date: Tue, 21 Dec 1993 10:28:27 GMT Message-ID: <CIDrrv.3MJ@sneaky.lonestar.org> Lines: 40 >In fact, i'm also experiencing lotta DMA overruns when attempting to do >some floppy IO while some heavy compile job is running. This is in a box >with an Adaptec SCSI, and i didn't track it down whether it's the CPU >load that causes the trouble (would really look strange to me), or if >it's the heavy disk IO that happens while compiling. > >Assuming it's the latter, so it could be some problem with the host adaptor >DMA cycles - does the adaptec really care for other DMA requests floating >around on the bus while it is considering to take over the bus? The default Adaptec 154x bus-on (11 us) and bus-off (4 us) times can starve floppy DMA, which needs one access every 16 microseconds. This causes DMA overruns (reading) and underruns (writing). It gets a little more complicated as the CPU and refresh insist on getting in a shot, also. From the Adaptec EZ-SCSI manual, page D-9: /f<bus off time> This option sets the bus-off time for the AHA-1540/1542 host adapter only. (An AHA-1740 host adapter running in Standard Mode will accept this option without giving an error message, but the setting will have no physical effect on the board.) Valid bus-off times are between 1 and 64 microseconds. The default time is 4 microseconds. If it takes a very long time to back up your system with a floppy tape drive or a floppy disk backup program, you probably need to change the bus-off time to 12 microseconds and the bus-on time to 4 microseconds. If you change one setting be sure to change the other setting as well. ... The default bus-on time is 11 microseconds. END QUOTE Gordon L. Burditt sneaky.lonestar.org!gordon