Date: Sun, 15 Dec 1996 04:46:16 +0100 From: Eivind Eklund <eivind@dimaga.com> To: Tony Overfield <tony@dell.com> Cc: hackers@freebsd.org Subject: Re: MAXMEM was: Re: 2.1.6 on Compaq Prosignia 500 (2.1.5 worked) Message-ID: <3.0.32.19961215044615.00925dd0@dimaga.com>
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At 08:24 PM 12/14/96 -0600, you wrote: >At 07:32 PM 12/12/96 +0100, Eivind Eklund wrote: >>>options "MAXMEM=65536" # 64 MB memory >>due to Compaq (and Dell) using an extremely irritating aspect of the EISA >>standard, and saying that they have 16MB in RTC-memory. > >I think you're irritated about the wrong thing. The RTC-memory >cannot (in any standardized way) indicate more than ~65MB of >system memory. This means that bypassing the BIOS and groping >around in the RTC-memory is a dead-end solution. > >There are standardized BIOS calls to obtain the correct amount of >memory, even when it exceeds 65 MB. I think the boot loader should >make these BIOS calls and pass the correct information to the kernel. The problem is that these BIOS calls are (to my knowledge; I'm no BIOS/PC hardware guru) only are guaranteed available on EISA computers, and only work from protected mode. To do this "properly" requires both a check to see if the computer has an EISA bus (which is in no way guaranteed - EISA seems to be dying), a switch to protected mode, and a fetch of the value. Few operating systems seems to do this "correctly". As examples, both Novell and SCO need this to be specified as an option. The reason I called it extremely irritating wasn't that I consider a BIOS call for getting amount of memory a bad idea - it was the fact that this is implemented as a protected mode call which only work if you've got EISA, instead of an extension that could be easily detected and called for any clone. I certainly agree that it would be nice for the boot code to detect this. I even looked at implementing it when the problem came up, but found it to be beyond what I wanted to learn about PC hardware. -- Eivind Eklund gopher://nic.follonett.no:79/0eivind Work: eivind@dimaga.com http://www.dimaga.com/ Home: perhaps@yes.no http://maybes.yes.no/perhaps/ All of the above is a product of either your or my imagination, and not official.
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