Real-Time Clock And Configuration Memory; Clearing Cmos - HP Compaq D315 Technical Reference Manual

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.
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
4-22
Featuring the AMD Athlon XP Processor
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
Second Edition – April 2003
MCP
FFh
Extended Config.
Memory Area
(128 bytes)
80h
7Fh
Standard Config.
Memory Area
(114 bytes)
0Eh
0Dh
RTC Area
(14 bytes)
00h
CMOS