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Chapter 4 System Support
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 MCP component and is MC146818-compatible. As shown in the following
figure, the MCP 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.
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
MCP
0Bh
0Dh
0Ch
00h
01h
02h
03h
04h
05h
06h
07h
08h
09h
0Ah
FFh
80h
7Fh
0Dh
0Eh
00h
C
M
OS
Figure 4-11. Configuration Memory Map
A lithium 3-VDC battery is used for maintaining the RTC and configuration memory while the
system is powered down. The battery is located in a battery holder on the system board and has a
life expectancy of about three 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 and disconnect the AC power cord from the outlet and/or system unit.
2. Remove the chassis hood (cover) and insure that no LEDs on the system board are
illuminated.
3. On the JBAT1 header, move the jumper from pins 1 and 2 to pins 2 and 3. Leave the jumper
on pins 2 and 3 for about 5 seconds. This action will ground the battery input to the CMOS
circuitry.
4. Replace the jumper onto pins 1 and 2.
5. Replace the chassis hood (cover).
6. Reconnect the AC power cord to the outlet and/or system unit and reboot the system.
To clear only the Power-On Password refer to section 4.7.1.1.
Compaq D315 and hp d325 Personal Computers
Featuring the AMD Athlon XP Processor
Second Edition – April 2003
4-22