Path: utzoo!attcan!uunet!icom!xwkg.Icom.Com!andy From: andy@xwkg.Icom.Com (Andrew H. Marrinson) Newsgroups: comp.unix.sysv386 Subject: Re: Looking for i386 versions that support >16Meg memory Keywords: i386 OS Message-ID: Date: 12 Nov 90 05:11:07 GMT References: <728@bulus3.BMA.COM> <273C17A5.2621@telly.on.ca> Sender: news@icom.icom.com (News Feed) Organization: Icom Systems, Inc. Lines: 30 There are some things to watch out for when going beyond 16Meg even if -- like the person running ESIX -- it seems to work. In particular, the AT bus DMA channels and all ISA-BUS cards support only 24-bit addresses. Now Unix since (at least) 3.2 has a mechanism to deal with this a file (mentioned in a previous posting) allows you to indicate which portions of memory support DMA and which don't. When a DMA would be performed to one of these addresses, a copy is performed instead. So end of problem, right? Wrong. Some drivers may not know about this scheme and blindly truncate the address to twenty-four bits or otherwise fail. In particular, third party DMA controllers such as Adaptec 1540 or WD7000 are more likely to suffer from this than others, since they won't be using the supplied dma library routines. This is a problem if you try to read or write raw partitions (e.g. with a DBMS) from a user program that could be located above 16Meg. I am not saying any particular OS/driver combination suffers from this problem, just that you should be aware of it and find out for your particular combination whether it works correctly or not. This may be the reason ISC and/or ESIX claim there OS's do not work with > 16 Meg. Just another reason why AT/ISA architecture is gimping our hardware... (Uh oh, stand by for flames on that one...) -- Andrew H. Marrinson Icom Systems, Inc. Wheeling, IL, USA (andy@icom.icom.com)