Compaq 4000N - Manual (Page 66)

Chapter 4 System Support
Compaq Deskpro 4000N and 4000S Personal Computers
First Edition - September 1997
4-24
4.5 REAL-TIME CLOCK AND CONFIGURATION MEMORY
The Real-time clock (RTC) and configuration memory functions are provided by the PC87307
I/O controller. The RTC uses the first 14 of 256 bytes of configuration memory and is
MC146818-compatible. As shown in the following figure, the 87307 controller provides 256
bytes of configuration memory, divided into two 128-byte banks. The RTC/configuration memory
can be accessed using conventional OUT and IN assembly language instructions using I/O ports
70h/71h, although the suggested method is to use the INT15 AX=E823h BIOS call.
Figure 4–8. Configuration Memory Map
NOTE: Non-volatile (NVRAM) storage of PCI, ESCD, and Environmental Variable (EV) data
is provided by portions of the 256-KB system BIOS ROM component.
A 3-VDC battery is used for maintaining the RTC and configuration memory while the system is
powered down. This battery is soldered on the system board and is designed to last from 5-7
years. Once expired, the soldered battery is by-passed by connecting a replacement battery
(Compaq p/n 160274-001 or equivalent 4.5 VDC @ 660 ma alkaline battery) to header P14 pins
9-12. On-board logic regulates the external battery voltage to 3 VDC.
The configuration memory (including the password) can be cleared by moving the jumper from
P14 pins 1 and 2 to pins 2 and 3 for at least one minute while unit power ids off. The password
can be disabled by switching DIP SW1-1 on.
RTC Area
(14 bytes)
Lower Config.
Memory Area
(114 bytes)
Upper Config.
Memory Area
(128 bytes)
00h
0Dh
0Eh
80h
FFh
7Fh
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
00h
0Bh
01h
02h
03h
04h
05h
06h
07h
08h
09h
0Ah
0Dh
0Ch
87307
CMOS