COHERENT manpages
This page displays the COHERENT manpage for cmos [Device for reading CMOS].
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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.