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Technical Reference Guide
Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition - September 1998
4-29
4.6 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 4–11.
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 four to
eight years. When the battery has expired it is replaced with a Renata CR2032 or equivalent 3-
VDC lithium battery.
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