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.

0Dh

Register D

0Ch

Register C

0Bh

Register B

0Ah

Register A

09h

Year

08h

Month

07h

Date of Month

06h

Day of Week

05h

Hours (Alarm)

04h

Hours (Timer)

03h

Minutes (Alarm)

02h

Minutes (Timer)

01h

Seconds (Alarm)

00h

Seconds (Timer)

82801

Extended Config.

Memory Area

(128 bytes)

Standard Config.

Memory Area

(114 bytes)

RTC Area (14 bytes)

FFh

80h

7Fh

0Eh

0Dh

00h

CMOS

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 (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

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HP 8100 manual Real-Time Clock and Configuration Memory, Clearing Cmos

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