2.9BSD/usr/man/man4/mem.4

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.TH MEM 4
.UC
.SH NAME
mem, kmem  \-  main memory
.SH DESCRIPTION
.I Mem
is a special file that is an image of the main memory
of the computer.
It may be used, for example, to examine,
and even to patch the system.
.I Kmem
is the same as 
.I mem
except that kernel virtual memory
rather than physical memory is accessed.
.I Kmem
is faster than \fImem\fP when the address,
count, and base are all even.
.PP
References to nonexistent locations return errors.
.PP
Examining and patching device registers is likely
to lead to unexpected results when read-only or write-only
bits are present.
.PP
On PDP-11s, the I/O page
begins at location 0160000 of
.I kmem
and the per-process data segment for the current process
begins at 0140000.
.SH FILES
/dev/mem
.br
/dev/kmem
.SH BUGS
.PP
On PDP-11's, memory files are accessed one byte
at a time, an inapproriate method for some
device registers.
.PP
On machines with ENABLE/34(tm) memory mapping boards
the I/O page can be accessed only through \fIkmem\fP.