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4-8 www.hp.com Technical Reference Guide
System Support
4.4 Real-Time Clock and Configuration Memory
The Real-time clock (RTC) and configuration memory (also referred to as “CMOS”) functions
are provided by the 82801 component and is MC146818-compatible. As shown in the following
figure, the 82801 PCH component provides 256 bytes of battery-backed RAM divided into two
128-byte configuration memory areas. The RTC uses the first 14 bytes (00-0Dh) of the standard
memory area. All locations of the standard memory area (00-7Fh) can be directly accessed using
conventional OUT and IN assembly language instructions through I/O ports 70h/71h, although
the suggested method is to use the INT15 AX=E823h BIOS call.
Figure 4-4. Configuration Memory Map
A lithium 3-VDC battery is used for maintaining the RTC and configuration memory while the
system is powered down. During system operation a wire-Ored circuit allows the RTC and
configuration memory to draw power from the power supply. The battery is located in a battery
holder (XBT1) on the system board and has a life expectancy of three or more years. When the
battery has expired it is replaced with a CR2032 or equivalent 3-VDC lithium battery.
4.4.1 Clearing CMOS
The contents of configuration memory can be cleared by the following procedure:
1. Turn off the unit.
2. Disconnect the AC power cord from the outlet and/or system unit.
3. Remove the chassis hood (cover) and insure that no LEDs on the system board are
illuminated.
4. On the system board, press and hold the CMOS clear button (switch SW50, colored yellow)
for at least 5 seconds.
5. Replace the chassis hood (cover).
6. Reconnect the AC power cord to the outlet and/or system unit.
7. Turn the unit on.
✎
The above procedure does not clear the Power On Password. To clear the Power-On Password
refer to section 4.5.1.
RTC Area
(14 bytes)
Standard Config.
Memory Area
(114 bytes)
Extended Config.
Memory Area
(128 bytes)
00h
0Dh
0Eh
80h
FFh
7Fh
Seconds (Timer)
Minutes (Timer)
Seconds (Alarm)
Hours (Alarm)
Hours (Timer)
Minutes (Alarm)
Day of Week
Date of Month
Month
Year
Register A
Register B
Register C
Register D
00h
0Bh
01h
02h
03h
04h
05h
06h
07h
08h
09h
0Ah
0Dh
0Ch
82801
CMOS