IBM AT 5170 Technical Reference page 73

Real-Time Clock Information
The following figure describes real-time clock bytes and specifies
their addresses.
Byte Function
Address
0
Seconds
00
1
Second Alarm
01
2
Minutes
02
3
Minute Alarm
03
4
Hours
04
5
Hour Alarm
05
6
Day of Week
06
7
Date of Month
07
8
Month
08
9
Year
09
10
Status Register A OA
11
Status Register B
OB
12 Status Register C OC
13
Status Register D
aD
Real-Time Clock Information (Addresses 00 - 00'
Note: The setup program initializes registers A, B, C, and
D when the time and date are set. Also Interrupt lA is the
.~
BIOS interface to read/set the time and date.
It
initializes
the status bytes the same as the Setup program.
Status Register A
Bit 7 Update in Progress (UIP)-A 1 indicates the
time update cycle is in progress. A 0 indicates
the current date and time are available to read.
Bit 6-Bit 4 22-Stage Divider (DV2 through DVO)-These
three divider-selection bits identify which
time-base frequency is being used. The system
initializes the stage divider to OlD, which selects a
32.768-kHz time base.
System Board 1-57