Real-Time Clock And Configuration Memory; Clearing Cmos - HP Compaq dc7800 Series Technical Reference Manual
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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 ICH9 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 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 (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. 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.
To clear only the Power-On Password refer to section 4.5.1.
Technical Reference Guide
Register D
0Dh
Register C
0Ch
0Bh
Register B
Register A
0Ah
Year
09h
Month
08h
Date of Month
07h
Day of Week
06h
Hours (Alarm)
05h
Hours (Timer)
04h
Minutes (Alarm)
03h
Minutes (Timer)
02h
Seconds (Alarm)
01h
Seconds (Timer)
00h
www.hp.com
82801
FFh
Extended Config.
Memory Area
(128 bytes)
80h
7Fh
Standard Config.
Memory Area
(114 bytes)
0Eh
0Dh
RTC Area
(14 bytes)
00h
CMOS
System Support
4-9
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