.\" $NetBSD: cmos.4,v 1.9 2014/03/18 18:20:40 riastradh Exp $
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.Dd April 21, 2010
.Dt CMOS 4 i386
.Os
.Sh NAME
.Nm cmos
.Nd Read/write access to IBM PC/AT CMOS RAM
.Sh SYNOPSIS
.Cd pseudo-device cmos
.Sh DESCRIPTION
The
.Nm
pseudo-device can be used to read the real-time clock and
.Tn ISA
configuration data from an ISA-compatible
.Tn CMOS
.Tn RAM ,
and to write the
.Tn ISA
configuration data.
.Pp
A program reads between 0 and 48 bytes from the
.Tn CMOS
.Tn RAM , starting at
byte 0 of the
.Tn RAM ,
using a single call to
.Xr read 2 .
Likewise, a program writes between 0 and 48 bytes to the CMOS RAM,
starting at byte 0 of the RAM, using a single call to
.Xr write 2 .
.Pp
.Nm
does not allow programs to overwrite the real-time clock data
(bytes 0 through 9), the status registers (10 through 13),
the diagnostic status or
.Tn CMOS
shutdown status (bytes 14 and 15),
or the
.Tn CMOS
checksum (bytes 46 and 47).
Writes to those bytes are ignored.
.Pp
On writes,
.Nm
recomputes the
.Tn CMOS
checksum and writes it to the
.Tn CMOS
.Tn RAM .
.Sh EXAMPLES
Display entire contents of CMOS RAM:
.Bd -literal -offset 2n
# dd if=/dev/cmos bs=48 count=1 | od -t x1
0000000   37  00  09  00  22  00  06  13  04  80  26  02  50  80  00  00
0000020   00  51  f0  00  01  80  02  00  fc  0f  2f  00  00  00  00  00
0000040   00  80  81  f0  ff  00  00  00  00  00  00  00  00  00  05  ee
0000060
.Ed
.Pp
Change boot order on Soekris net4521 to
.Tn PXE
.Tn ROM ,
Primary
.Tn HDD ,
Secondary
.Tn HDD :
.Bd -literal -offset 2n
# dd if=/dev/cmos of=/tmp/cmos0 bs=48 count=1
1+0 records in
1+0 records out
48 bytes transferred in 0.001 secs (48000 bytes/sec)
# cp /tmp/cmos0 /tmp/cmos
# printf '\exf0\ex80\ex81\exff' | dd bs=1 seek=33 conv=notrunc of=/tmp/cmos
4+0 records in
4+0 records out
4 bytes transferred in 0.001 secs (4000 bytes/sec)
# dd if=/tmp/cmos of=/dev/cmos
0+1 records in
0+1 records out
48 bytes transferred in 0.001 secs (48000 bytes/sec)
.Ed
.Sh ERRORS
A program can read or write no more than 48 bytes to
.Nm .
Both
.Xr read 2
and
.Xr write 2
will return
.Er EINVAL
if more than 48 bytes are read or written at once.
.Sh AUTHORS
The original
.Nm
driver was written by
.An Takahiro Kambe Aq Mt taca@back-street.net .
.An David Young Aq Mt dyoung@NetBSD.org
modified the original and added it to
.Nx .
.\" .Sh BUGS