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From: mrb1@cbnewsh.ATT.COM (maurice.r.baker)
Newsgroups: comp.unix.i386,comp.sys.att,comp.unix.questions,comp.unix.wizards
Subject: Read/write at absolute memory address on Sys V/3.2 6386 UNIX
Keywords: /dev/mem, device drivers, absolute addresses, memory-mapped I/O
Message-ID: <304@cbnewsh.ATT.COM>
Date: 27 Apr 89 19:34:28 GMT
Organization: AT&T Bell Laboratories
Lines: 31

Hello ----

One more question for the net:

	Within the confines of System V/3.2 UNIX on an AT&T 6386E, I need
to read from and write to a particular range of memory locations......to
be specific, 0xC6000 to 0xC63FF (they are actually on a Matrox graphics  
board).

	Short of writing a device driver which gets linked into the kernel
and runs in privileged mode, is there any other way of doing the job?  Spe-
cifically, through /dev/mem ???  The manuals I have access to are very terse 
when it comes to describing much about /dev/mem.  I did try an intuitive test
program or two, without any luck.  (The /dev/mem permissions were set to
rw-rw-rw-, so that shouldn't have been a problem). 
Perhaps there is a neat way to do this,
although the ISDG does sound an ominous note ---

	"Because user programs and the operating system execute in different
address spaces, the I/O cannot take place directly from the device to the
user program ..."

	Which is kind of what I feared, but it certainly is worth a try
posting the question anyhow.  Maybe someone can give some interesting
historical view of /dev/mem, or how to use it, if nothing else.

Thanks for the help on this, and previous inquiries.

M. Baker
homxc!mrb1
201-949-3455