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From: kjb@cs.vu.nl (Kees J Bot)
Subject: Re: 32B RAM with Minix 1.7.4?
Nntp-Posting-Host: hornet.cs.vu.nl
References: <51aipj$j4@eng-ser1.erg.cuhk.hk> <ufd8zpizrc.fsf@pc3135.cs.cuhk.edu.hk>
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Organization: Fac. Wiskunde & Informatica, VU, Amsterdam
Date: Sat, 14 Sep 1996 16:25:46 GMT
Message-ID: <DxqDMz.8nJ@cs.vu.nl>
Lines: 49

arthur@pc3135.cs.cuhk.edu.hk (Arthur S.L. Hsieh) writes:

>I can boot 1.7.4 in a Pentium-166 system with 32M ram.  Minix just
>ignores anything above 16M as it only supports 16M extended memory
>according to MINIX INFORMATION SHEET
>(http://www.cs.vu.nl/~ast/minix_17.html).

There are two ways to ask a machine how much extended memory it has,
with an old and trusty BIOS call, but than you only get a number <=
65535, or with one of several new BIOS calls.  What seems to be the
most supported new call returns the number of kilobytes between 1M and
16M, and the number of megabytes above the 16M border.

I haven't added the new call to the boot monitor yet, because I'm not
sure it works well in all cases, or returns an error correctly when it
is not supported.

Alas some new Pentiums have started giving the number of kilobytes
between 1M and 16M for the old BIOS call.  My 486 doesn't do that, it
says 19M extended memory for my 20M machine.

Anyway, by setting 'emssize=31744' you can tell the monitor how much
memory your system really has.  (31744 = 31 * 1024).

>The BUGS section of usage(8) does mention that you might encounter
>some problem when it does DMA above 16M and you've more than 16M
>physical memory.

This is no problem for normal I/O that goes through FS' block cache.
Only if you do something like

	vol /dev/nrst4 | tar xvfp -

to extract a SCSI tape, as I often do, you run the risk that 'vol' is
sitting above 16M in memory.  Minix allocates memory from low addresses
up, so you need to have very big processes already running for things to
go wrong.

BTW, the 'p' option on tar tells tar to extract files with their
original file modes.  It normally applies the file creation mask.  For
'root' the 'p' is the default, that is why people who extract /usr with
'tar xvf' still get a /usr/tmp/ that is mode 777.

Now that I've put Minix 2.0 together I finally have some time.  I have
posted so little that the mail spammers have forgotten me.  :-)  
--
long:  http://www.cs.vu.nl/~ast/minix.html         Kees J. Bot  (kjb@cs.vu.nl)
short: http://www.cs.vu.nl/ftp/minix/README.html        Systems Programmer
ftp:   ftp://ftp.cs.vu.nl/pub/minix/		   Vrije Universiteit Amsterdam
