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Path: sserve!newshost.anu.edu.au!munnari.oz.au!constellation!convex!convex!convex!cs.utexas.edu!uunet!math.fu-berlin.de!dsmail.hmi.de!news From: pfh@dsapp1.hmi.de (Frank Hoffmann) Newsgroups: comp.os.386bsd.questions Subject: Re: > 16 MB of RAM Date: 2 Jul 1993 11:16:44 +0200 Organization: Hahn-Meitner-Institut Berlin Lines: 33 Message-ID: <210uds$d3n@dsapp1.hmi.de> References: <1993Jun30.184624.15579@leland.Stanford.EDU> <20ub8n$ngn@germany.eu.net> <20vaj1$8el@hrd769.brooks.af.mil> <210g8n$3pr@pdq.coe.montana.edu> NNTP-Posting-Host: dsapp1.hmi.de 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 . Might be slow down the system, but it works ( a least with the Linux kernel..) Frank -- ###################################################### # Frank Hoffmann (Hoffmann-f@vax.hmi.dbp.de) # # Hahn Meitner Institut Berlin # ######################################################