cmos -- Device Driver

Device for reading CMOS

The file /dev/cmos the entry via which you can read your system's CMOS.  It
is  a part  of the  driver mem,  which manages memory;  thus, it  has major
number 0 and minor number 3.

The CMOS is a special, non-volatile area of random-access memory (RAM) that
holds information  about your system's configuration.   The following gives
the common meanings assigned to  the various byte positions within the CMOS
area:

Real-time clock:
     0x00           Seconds
     0x01           Alarm, seconds
     0x02           Minutes
     0x03           Alarm, minutes
     0x04           Hours
     0x05           Alarm, hours
     0x06           Day of the week
     0x07           Day of the month
     0x08           Month
     0x09           Year
     0x0A           Update in progress

Diagnostic power byte:
     0x0E           Bit 7 -- Chip lost power
                    Bit 6 -- Bad checksum
                    Bit 5 -- Bad configuration byte
                    Bit 4 -- Bad memory size
                    Bit 3 -- Bad hard-disk byte
                    Bit 2 -- Bad time of day

Restart-status byte:
     0x0F           Reloaded when restarting, e.g., returning from
                    protected mode

Floppy-disk drive, drives A and B:
     0x10           Bits 7-4 -- Drive A:
                            0 = no drive
                            1 = 360-kilobyte drive
                            2 = 1.2-megabyte drive
                            3 = 720-kilobyte drive
                            4 = 1.44-megabyte drive
                    Bits 3-0 -- Drive B:
                            0 = no drive
                            1 = 360-kilobyte drive
                            2 = 1.2-megabyte drive
                            3 = 720-kilobyte drive
                            4 = 1.44-megabyte drive

Floppy-disk drive, drives C and D:
     0x11           Bits 7-4 -- Drive C:
                            0 = no drive
                            1 = 360-kilobyte drive
                            2 = 1.2-megabyte drive
                            3 = 720-kilobyte drive
                            4 = 1.44-megabyte drive
                    Bits 3-0 -- Drive D:
                            0 = no drive
                            1 = 360-kilobyte drive
                            2 = 1.2-megabyte drive
                            3 = 720-kilobyte drive
                            4 = 1.44-megabyte drive

Hard-disk drive:
     0x12           Bits 7-4 -- First hard-disk drive
                            0 = No drive
                            1-3 = Type 1-15
                            F = Use contents of byte 19
                    Bits 3-0 -- Second hard-disk drive
                            0 = No drive
                            1-3 = Type 1-15
                            F = Use contents of byte 1A

Configuration of equipment:
     0x014          Bits 7-6 -- Floppy disks
                            00 = one floppy-disk drive
                            01 = two floppy-disk drives
                            10 = three floppy-disk drives
                            11 = four floppy-disk drives
                    Bits 5-4 -- Type of display
                            00 = EGA/VGA
                            01 = CGA 40×25
                            10 = CGA 80×25
                            11 = monochrome display
                    Bit 1 -- floating-point coprocessor installed
                    Bit 0 -- Floppy-disk drive present

Memory:
     0x15-0x16      Amount of memory below one megabyte
     0x17-0x18      Amount of memory above one megabyte

Type of hard disk:
     0x19           Type of first hard disk.  Read only when
                    bits 7-4 of byte 0x12 equal 0xF.
     0x21           Type of second hard disk.  Read only when
                    bits 3-0 of byte 0x12 equal 0xF.

Miscellaneous:
     0x2E-0x2F      Checksum for bytes 0x10 through 0x2D
     0x30-0x31      Indicate memory size above one megabyte
     0x32           Century byte (BCD)
     0x33           Flag for power-on information:
                    Bit 7 -- Top 128 kilobytes of RAM is installed
                            (shadow RAM is available)
                    Bit 6 -- First boot after running set-up routine

/dev/cmos limits  access to a 256-byte  data area.  Any attempt  to read or
write beyond this limit will fail.

See Also

ATclock,
clock,
device drivers,
RAM

Notes

If  you want  to read  or  set the  real time  clock, then  you  should use
/dev/clock instead of /dev/cmos.

Vendor-specific information,  e.g., your system's  memory configuration, is
often kept  in the  CMOS area at  locations beyond those  documented above.
Therefore, writing  to undocumented regions  of the CMOS  area is extremely
unwise:  your  computer  could subsequently  refuse  to  boot up  properly.
Caveat utilitor.

© 2012 by Stephen A. Ness