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Technical Reference Guide
4.6.2 CMOS ARCHIVE AND RESTORE
During the boot process the BIOS saves a copy of CMOS to the flash ROM. If the system
becomes unusable, the last good copy of CMOS can be recalled using the power-override function
as follows:
1. With the unit powered down, press and release the power button to initiate the boot sequence.
2. Immediately after releasing the power button, press it again and hold (typically at least four
seconds) until the unit powers off again. This action will be recorded as a power button
override event.
3. Press and release the power button once more, initiating the boot sequence that should detect
the occurrence of an override event and load the backup copy of CMOS, allowing the system
to boot.
4.6.3 STANDARD CMOS LOCATIONS
Table 4-11 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 4-11. Configuration Memory (CMOS) Map
Table 4-11.
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 D315 and hp d325 Personal Computers
Featuring the AMD Athlon XP Processor
Second Edition - April 2003
4-23