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Path: sserve!newshost.anu.edu.au!munnari.oz.au!network.ucsd.edu!usc!howland.reston.ans.net!xlink.net!fauern!lrz-muenchen.de!colin.muc.de!ars.muc.de!rommel From: rommel@ars.muc.de (Kai Uwe Rommel) Newsgroups: comp.os.386bsd.questions Subject: Re: > 16 MB of RAM Distribution: world Message-ID: <741742311rommel.root@ars.muc.de> Sender: root@ars.muc.de Date: Sun, 04 Jul 93 00:31:51 MET References: <20vaj1$8el@hrd769.brooks.af.mil> <210g8n$3pr@pdq.coe.montana.edu> <210uds$d3n@dsapp1.hmi.de> Organization: Private Lines: 38 In article <210uds$d3n@dsapp1.hmi.de> pfh@dsapp1.hmi.de (Frank Hoffmann) writes: >nate@bsd.coe.montana.edu (Nate Williams) writes: > >>When you can't run more than 16MB of memory is when > >>1) You have an ISA box - limits you to 24 data lines == 16MB > >Wrong. The CPU doesn't use the ISA bus to access memory. >If your board supports more than 16 MB , the CPU can use it. >I have Linux on a 20 MB isa system, and I'm sure all the memory is used . > >>2) You use a disk controller that does DMA (Direct Memory Access) >> (Most decent SCSI boards use DMA to get better performance, so >> that is why SCSI comes up so often. IDE drives do not, and >> as such don't suffer from having more than 16MB, but also >> don't have the same performance) > >The bottleneck ist the ISA DMA chip, which can only address 16MB >(24Bit Adresse ) . Memory above 16 MB can be used, but you have to >use double-buffering : DMA transfer to a place below 16MB , then >the CPU has to move the data to the desired place above 16MB . I think Nate meant that you are in trouble if *both* conditions are true. Further more, bus-master DMA SCSI controllers like the 1542B don't use the main-board DMA controller but have their own. Unfortunately, it's on the wrong side of the bus. :-) Conclusion: the problem IS the ISA bus with only 24 address lines. Kai Uwe Rommel -- /* Kai Uwe Rommel Muenchen, Germany * * rommel@ars.muc.de Phone +49 89 723 4101 * * rommel@informatik.tu-muenchen.de Fax +49 89 324 4524 */ DOS ... is still a real mode only non-reentrant interrupt handler, and always will be. -Russell Williams