Specifications

Legacy BIOS References
163
5.3 Fixed CMOS Locations
The table below lists the fixed CMOS locations.
Table 15 Fixed CMOS Locations
Start
Location
Length
in
bytes
Description Modified by
Traditional
BIOS
Comments
0x00 10 RTC Yes INT1A
0x0A 6 CMOS Status No
0x10 1 Floppy drive type No
0x12 1 Hard Disk No
0x14 1 Equipment byte No
0x15 2 Base memory size No
0x17 2 Extended memory size No
0x19 Hard disk C drive type No
0x1A Hard Disk D drive type No
0x2E 2 Standard CMOS
checksum
Yes If any location 0x10 through
0x2D is changed
0x30 2 Extended memory found
by BIOS
No
0x32 1 Century byte Yes On Roll over, IN 1A
0x33 1 Information Flag No Bit 0 =1 - Cache good. Not
check summed.
0x3E 2 Extended CMOS
checksum
Yes If any location 0x30 through
0x7F is changed and in check
summed region.
Notes:
If CMOS is not supported by standard EFI, then the bytes that are labeled as not modified by the
traditional BIOS must be initialized by EfiCompatibility but are not modified at runtime.
The CSM may use other CMOS bytes. If standard EFI supports CMOS, then the CMOS usage most not
conflict.
CMOS locations greater than 0x33 are up to the implementer as far as inclusion/exclusion in a checksum
range.