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Date: Tue, 23 Jan 1996 01:59:35 +1100
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199601221459.BAA03551@godzilla.zeta.org.au>
To: bugs@FreeBSD.ORG, luigi@labinfo.iet.unipi.it
Subject: Re: dd /dev/mem ...
Sender: owner-bugs@FreeBSD.ORG
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>Running 2.1R, I tried the following running as root:

>    dd if=/dev/mem bs=32k skip=512 count=1 | dd bs=32 count=1 | hexdump

>and my P100 w/ 16MB was frozen: no keyboard, no mouse, everything
>still on the X screen.

>the manpage says "Only offsets within the bounds of /dev/mem are
>allowed.", and 512*32K is right beyond the physical RAM. However, I
>have run C programs which would read beyond the limit with no problems.

>Is it a bug in /dev/mem, "dd" or me ?

Probably in /dev/mem, the manpage, and you :-).  /dev/mem doesn't bound
memory in any way.  However, the man page may be too restrictive - you
may want to access memory not known to the system for some reason.
However, you shouldn't be surprised when accessing it does something
bad.  dd should fail if the memory has holes.  There might be special
hardware.  There might be real memory with uninitialzied parity...

Reading beyond the end of physical memory on my 16M system works up to
150MB but is very slow (1.5MB/sec average).

Bruce