Compaq Deskpro EP 6233 - Real-Time Clock and Configuration Memory

Technical Reference Guide
Compaq Deskpro EP Series of Personal Computers
First Edition–- April 1998
4-29
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.
RTC Area
(14 bytes)
Lower Config.
Memory Area
(114 bytes)
Upper Config.
Memory Area
(128 bytes)
00h
0Dh
0Eh
00h (80h)
7Fh
(
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
82371
CMOS