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CMOS Assignments (BIOS)

The BIOS uses a CMOS to store configuration data. It is currently used as follows:

  First CMOS bank (offsets 0x00 to 0x7f):
    Base memory:
         0x15
         0x16
    Extended memory:
         0x17
         0x18
         0x30
         0x31
    Amount of memory above 16M and below 4GB in 64KB units:
         0x34
         0x35
    Boot device (BOCHS bios specific):
         0x3d
         0x38
         0x3c
    PXE debug:
         0x3f
    Floppy drive type:
         0x10
    Equipment byte:
         0x14
    First HDD:
         0x19
         0x1e - 0x25
    Second HDD:
         0x1a
         0x26 - 0x2d
    Third HDD:
         0x67 - 0x6e
    Fourth HDD:
         0x70 - 0x77
    Extended:
         0x12
    First Sata HDD:
         0x40 - 0x47
    Second Sata HDD:
         0x48 - 0x4f
    Third Sata HDD:
         0x50 - 0x57
    Fourth Sata HDD:
         0x58 - 0x5f
    Number of CPUs:
         0x60
    RAM above 4G in 64KB units:
         0x61 - 0x65

  Second CMOS bank (offsets 0x80 to 0xff):
    Reserved for future use:
         0x80 - 0x81
    First net boot device PCI bus/dev/fn:
         0x82 - 0x83
    Second to third net boot devices:
         0x84 - 0x89

Todo:
Mark which bits are compatible with which BIOSes and which are our own definitions.

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