IBM 770 manual System Board, Real-Time, Clock, 000-00D, month, Month

Real-Time

Clock

Bytes

(Hex

000–00D):

Bit

definitions

and

addresses for the real-time clock

bytes

are

shown

in

Figure 2-15.

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

Figure

2-15. Real-Time

Clock

Bytes

(Hex

000–00D)

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)

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

Figure

2-16. Status

Register

A

(Hex

00A)

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

forµs. at

least

244

2-22 System Board