V6/usr/man/man4/mem.4

.th MEM IV 5/27/74
.sh NAME
mem, kmem, null  \*-  core memory
.sh DESCRIPTION
.it Mem
is a special file that is an image of the core memory
of the computer.
It may be used, for example, to examine,
and even to patch the system using the debugger.
.s3
A memory address is
an 18-bit quantity which
is used directly as a UNIBUS address.
References to non-existent locations cause errors to be returned.
.s3
Examining and patching device registers is likely
to lead to unexpected results when read-only or write-only
bits are present.
.s3
The file
.it kmem
is the same as 
.it mem
except that kernel virtual memory
rather than physical memory is accessed.
In particular, the I/O area
of
.it kmem
is located beginning at 160000 (octal)
rather than at 760000.
The 1K region beginning at 140000 (octal)
is the system's data for the current process.
.s3
The file
.it null
returns end-of-file
on
.it read
and ignores
.it write.
.sh FILES
/dev/mem,
/dev/kmem,
/dev/null