Chapter 4 System Support
4.6.2 CMOS ARCHIVE AND RESTORE
During the boot sequence the BIOS saves a copy of NVRAM (CMOS contents, password(s) and
other system variables) in a portion of the flash ROM. Should the system become un-usable, the
last good copy of NVRAM data can be restored with the Power Button Override function. This
function is invoked with the following procedure:
1. With the unit powered down, press and release the power button.
2. Immediately after releasing the power button in step 1, press and hold the power button until
the unit powers down. This action will be recorded as a Power Button Override event.
With the next startup sequence the BIOS will detect the occurrence of the Power Button Override
event and will load the backup copy of NVRAM from the ROM to the CMOS.
NOTE: The Power Button Override feature does not allow quick cycling of the system
(turning on then off). If the power cord is disconnected during the POST routine, the
splash screen image may become corrupted, requiring a re-flashing of the ROM (refer to
chapter 8, BIOS ROM).
4.6.3 STANDARD CMOS LOCATIONS
Table 4-14 and the following paragraphs describe standard configuration memory locations 0Ah-
3Fh. These locations are accessible through using OUT/IN assembly language instructions using
port 70/71h or BIOS function INT15, AX=E823h.
Table 4-14. Configuration Memory (CMOS) Map
Table 4-14.
Configuration Memory (CMOS) Map
Location Function Location Function
00-0Dh Real-time clock 24h System board ID
0Eh Diagnostic status 25h System architecture data
0Fh System reset code 26h Auxiliary peripheral configuration
10h Diskette drive type 27h Speed control external drive
11h Reserved 28h Expanded/base mem. size, IRQ12
12h Hard drive type 29h Miscellaneous configuration
13h Security functions 2Ah Hard drive timeout
14h Equipment installed 2Bh System inactivity timeout
15h Base memory size, low byte/KB 2Ch Monitor timeout, Num Lock Cntrl
16h Base memory size, high byte/KB 2Dh Additional flags
17h Extended memory, low byte/KB 2Eh-2Fh Checksum of locations 10h-2Dh
18h Extended memory, high byte/KB 30h-31h Total extended memory tested
19h Hard drive 1, primary controller 32h Century
1Ah Hard drive 2, primary controller 33h Miscellaneous flags set by BIOS
1Bh Hard drive 1, secondary controller 34h International language
1Ch Hard drive 2, secondary controller 35h APM status flags
1Dh Enhanced hard drive support 36h ECC POST test single bit
1Eh Reserved 37h-3Fh Power-on password
1Fh Power management functions 40-FFh Feature Control/Status
NOTES:
Assume unmarked gaps are reserved.
Higher locations (>3Fh) contain information that should be accessed using the INT15, AX=E845h
BIOS function (refer to Chapter 8 for BIOS function descriptions).
Compaq Deskpro and Evo Personal Computers
Featuring Intel Celeron and Pentium III Processors
Fifth Edition – March 2002
4-24
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