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Technical Reference Guide
Compaq iPAQ Family of Internet Devices
First Edition - March 2000
4-17
4.7 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 ICH component and is MC146818-compatible. As shown in the
following figure, the 82801 ICH 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. Also note
that CMOS locations above 3Fh are used for the control and status of features that should be
handled through BIOS function INT15h, AX=E845h.
Figure 4-7.
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)
Standard Config.
Memory Area
(114 bytes)
Extended 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
82801
CMOS