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Newsgroups: comp.unix.bsd Path: sserve!manuel!munnari.oz.au!spool.mu.edu!agate!tfs.com!tfs.com!julian From: julian@tfs.com (Julian Elischer) Subject: Re: 386BSD Questions (16+MB ram) Message-ID: <1992Oct3.214059.20248@tfs.com> Organization: TRW Financial Systems References: <53950@dime.cs.umass.edu> <65158@stanton.UUCP> <1992Oct3.150747.11580@Urmel.Informatik.RWTH-Aachen.DE> Date: Sat, 3 Oct 1992 21:40:59 GMT Lines: 42 In article <1992Oct3.150747.11580@Urmel.Informatik.RWTH-Aachen.DE> kuku@acds.physik.rwth-aachen.de writes: > >I'm also wondering about this ongoing discussion on "16MB limit on the ISA bus". >Recently I had a 486/33 motherboard which could be populated with 32 MB of >4MB SIMMs. > >Is 386bsd bound to a 16MB limit? I cannot believe that. > Well, the problem occurs with devices that use DMA. They only have 24 address bits if they use the ISA dma (8bit) or have their own DMA systems. 386BSD has in it a 'bounce buffer' system to allow such devices to dma stuff down to the bounce buffer from which it is copied to the high memory. The problem with this is it only really works well for devices that do single section (non scatter-gather) DMA. For this reason the adaptec 1542 scsi driver will not support more than 16MB on any machine. The bustec 742a and adaptec 1742 drivers are EISA board drivers and DO support upto 4GB ram. I'm sure some system could be worked out where every section of a scatter-gather operation could be checked and those above 16MB could be relaced with multiple 'bounce buffers' but I don't think it's worth it.. if you want >16MB you probably want performance too and EISA boards are getting cheaper. incidentally, on an EISA machine there is a register that supplies the top 8 bits of a motherboard DMA with address bits so if the option EISA is true, we should use that rather than the bounce buffer. julian .