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Path: sserve!newshost.anu.edu.au!harbinger.cc.monash.edu.au!yeshua.marcam.com!usc!howland.reston.ans.net!noc.near.net!news.delphi.com!usenet From: John Dyson <dysonj@delphi.com> Newsgroups: comp.os.386bsd.development Subject: Re: DMA bounce-buffers now implemented in FreeBSD-current Date: Sun, 27 Mar 94 00:25:02 -0500 Organization: Delphi (info@delphi.com email, 800-695-4005 voice) Lines: 24 Message-ID: <Z0xMZsu.dysonj@delphi.com> References: <JKH.94Mar25161128@sentnl.ilo.dec.com> <UWP.94Mar25194257@maelaren.cs.tu-berlin.de> NNTP-Posting-Host: delphi.com X-To: Udo Wolter <uwp@cs.tu-berlin.de> Udo Wolter <uwp@cs.tu-berlin.de> writes: >Can anyone describe what DMA bounce buffers exactly do besides of allowing >more than 16MB in an ISA machine ? Well, the bounce buffers allow the use of a SCSI controller like the ISA 1542 or BL545S on a machine with greater than 16MB. If you do not have such an interface you have not been significantly limited in use of memory above 16MB. For example, I have a 20MB machine with IDE drives and a 545S SCSI interface with a tape drive. Before the bounce buffers, I needed to boot a special kernel that limited the use of memory to below 16MB when using the SCSI tape drive. Now, with bounce buffer support, the inability of the ISA DMA SCSI interface to support memory above 16MB is no longer consequential. The OS takes care of the problem for me, and I can use the SCSI subsystem freely on my 20MB system. >What does this exactly mean ? Does this mean that I really can use upto 32MB or >will the system just ignore them and that I'll get no more errors if I have >more than 16MB installed ? It means that you can really use a system with more than 16MB without concerning yourself with the limitations of the ISA-based SCSI adaptor. Therefore, if you want to use 64MB of memory, the system will work and use all of it.