Real-Time Clock And Configuration Memory; Figure 4-11. Configurationm - Compaq Deskpro EP 6233 Technical Reference Manual

4.6

REAL-TIME CLOCK AND CONFIGURATION MEMORY

The Real-time clock (RTC) and configuration memory functions are provided by the 82371 South
Bridge component. The RTC uses the first 14 of 256 bytes of configuration memory and is
MC146818-compatible. As shown in the following figure, the 82371 provides 256 bytes of
configuration memory (CMOS), divided into two 128-byte banks. The lower configuration
memory can be accessed using conventional OUT and IN assembly language instructions using
I/O ports 70h (index) and 71h (data). The upper configuration memory area is accessible using
ports 72h/73h. When using the OUT/IN instructions only addresses 00h-7Fh are valid.
Software should access CMOS using the INT15 AX=E823h BIOS call. This system supports the
Client Management BIOS function INT15 AX=E845h, which accesses CMOS feature bits data.
These functions are described in more detail in Chapter 7, "BIOS ROM."
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 from
four to eight years. When the battery has expired it is replaced with a Renata CR2032 or
equivalent 3-VDC lithium battery.
0Dh Register D
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
Compaq Deskpro EP Series of Personal Computers
First Edition–- April 1998
Technical Reference Guide
82371
7Fh (FFh)
Upper Config.
Memory Area
(128 bytes)
00h (80h)
7Fh
Lower Config.
Memory Area
(114 bytes)
0Eh
0Dh
RTC Area
(14 bytes)
00h
CMOS
4-29