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Real-Time Clock Bytes (Hex 000–00D):
Bit definitions and
addresses for the real-time clock bytes are shown in Figure 2-15.
Figure 2-15. Real-Time Clock Bytes (Hex 000–00D)
Address
(Hex)
Function
Byte Number
000 Seconds 0
001 Second alarm 1
002 Minutes 2
003 Minute alarm 3
004 Hours 4
005 Hour alarm 5
006 Day of week 6
007 Date of month 7
008 Month 8
009 Year 9
00A Status register A 10
00B Status register B 11
00C Status register C 12
00D Status register D 13
Note: The setup program initializes status registers A and B when
the time and date are set. Interrupt 1AH is the BIOS
interface to read and set the time and date; it initializes the
registers in the same way that the setup program does.
Status Register A (Hex 00A)
Figure 2-16. Status Register A (Hex 00A)
Bit Function
7 Update in progress (UIP)
6 Countdown chain
1 - resets countdown chain
0 - countdown chain enabled
5 Oscillator enable
0 - oscillator off
1 - oscillator on
4 Bank select
3–0 Rate-selection bits
Bit 7 This bit is a status flag that can be monitored. If this bit
is 1, the update transfer will soon occur. If this bit 0, the
update transfer will not occur for at least 244 µs.
2-22 System Board