mem.4

早期freebsd实现

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.\"     @(#)mem.4	8.1 (Berkeley) 6/5/93
.\"
.Dd June 5, 1993
.Dt MEM 4 vax
.Os BSD 4
.Sh NAME
.Nm mem ,
.Nm kmem ,
.Nm kUmem
.Nd memory files
.Sh DESCRIPTION
The special file
.Nm /dev/mem
is an interface to the physical memory of the computer.
Byte offsets in this file are interpreted as physical memory addresses.
Reading and writing this file is equivalent to reading and writing
memory itself.
Only offsets within the bounds of
.Nm /dev/mem
are allowed.
.Pp
Kernel virtual memory is accessed through the interface
.Nm /dev/kmem
in the same manner as
.Nm /dev/mem .
Only kernel virtual addresses that are currently mapped to memory are allowed.
.Pp
The file
.Nm /dev/kUmem
also refers to kernel virtual memory,
but may be used to access areas mapped to
.Tn UNIBUS
address space
and other
.Tn I/O
areas.
It forces all accesses to use word (short integer) accesses.
.Pp
On the
.Tn VAX
11/780,
the
.Tn I/O
space base address is
20000000(16);
on an 11/750
the
.Tn I/O space addresses are of the form fxxxxx(16).
On all
.Tn VAX Ns 'en
the
per-process data
size
for the current process
is
.Dv UPAGES
long and ends at the virtual address 80000000(16).
.Sh FILES
.Bl -tag -width Pa -compact
.It Pa /dev/mem
.It Pa /dev/kmem
.It Pa /dev/kUmem
.El
.Sh HISTORY
The
.Nm mem ,
.Nm kmem
files appeared in
.At v6 .
The file
.Nm kUmem
appeared in
.Bx 3.0 .