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.\"     from: @(#)mem.4	6.3 (Berkeley) 3/27/91
.\"	$Id: mem.4,v 1.2 1993/08/01 07:36:47 mycroft Exp $
.\"
.Dd March 27, 1991
.Dt MEM 4 tahoe
.Os
.Sh NAME
.Nm mem ,
.Nm kmem ,
.Nm vmem
.Nd main memory
.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/vmem
allows access to the mapped portion of the
.Tn VERSAbus
.Tn I/O
space.  On the Tahoe the upper megabyte
of the physical address space is accessible through
this file (0xfff00000 through 0xffffffff). 
Accesses to the upper 64 kilobytes of
the
.Tn I/O
space result in
.Tn VERSAbus
transfers with a
16-bit address (the offset in this region) and a
.Dq No non-privileged short Tn I/O
.Tn VERSAbus
address modifier.
Accesses to the remainder of the mapped region,
result in
.Tn VERSAbus
transfers with a 24-bit address
and a ``non-privileged standard''
.Tn VERSAbus
address
modifier.  This region is actually part of the region
between 0xfeff0000 and 0xffff0000
which generates
.Tn VERSAbus
transfers with a 24-bit address.
Accesses to the remainder of the one gigabyte
.Tn I/O
space
generate transfers that utilize a 32-bit address with
a ``non-privileged extended'' address modifier.  Any
32-bit address generated by a cpu access to this part
of the
.Tn I/O
space have the upper two bits zero; thus,
for example, an access to physical address 0xfe000000
causes the address 0x3e000000 to be supplied in the
resultant
.Tn VERSAbus
read/write cycle.
.Pp
On the Tahoe, the 
base address for the
per-process data of the current process
is virtual address 0xbffff000.
.Sh FILES
.Bl -tag -width Pa -compact
.It Pa /dev/mem
.It Pa /dev/kmem
.It Pa /dev/vmm
.El
.Sh HISTORY
The
.Nm mem ,
.Nm kmem
files appeared in
.At v6 .
The file
.Nm vmem
appeared in
.Bx 4.3 tahoe .
.Sh BUGS
Memory files are accessed one byte
at a time, an inappropiate method for some
device registers.