Technical Reference Guide
4.6 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 ICH2 component and is MC146818-compatible. As shown in the
following figure, the 82801 ICH2 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.
80h
7Fh
0Dh
0Eh
00h
RTC Area
(14 bytes)
Standard Config.
Memory Area
(114 bytes)
Extended Config.
Memory Area
(128 bytes)
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
82801
01h
02h
03h
04h
05h
06h
07h
08h
09h
0Ah
0Bh
0Dh
0Ch
00h
FFh
M
Figure 4-10. 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 on the system board and has a life expectancy of four to eight years. When the battery has
expired it is replaced with a Renata CR2032 or equivalent 3-VDC lithium battery.
4.6.1
CLEARING CMOS
The contents of configuration memory (including the Power-On Password) 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. Press and release the CMOS clear button on the system board.
5. Replace the chassis hood (cover).
6. Reconnect the AC power cord to the outlet and/or system unit.
7. Turn the unit on.
To clear only the Power-On Password refer to section 4.7.1.1.
Compaq Deskpro and Evo Personal Computers
Featuring Intel Celeron and Pentium III Processors
Fifth Edition - March 2002
4-23
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