IBM 770 manual System Board, BitFunction

Bit

2

This

bit

indicates

whether the binary-coded-decimal

(BCD)

or binary format is used for time-and-date calendar

updates. If

set

to

1,

this

bit

indicates

binary

format. The

system

initializes

this bit

to 0.

Bit

1

This

bit

indicates

whether

the hours

byte

is

in 12-hour or

24-hour

mode. If

set

to 1, this bit indicates the 24-hour

mode. The system initializes this

bit

to

1.

Bit

0

If

set

to

1,

this

bit

enables

the

daylight-saving-time mode.

If

set

to

0, this bit

disables

the

daylight-saving-time mode,

and the clock reverts to standard time. The system

initializes this bit to 0.

Status

Register

C

(Hex

00C)

Bit

Function

7

Interrupt

request

flag

6

Periodic

interrupt

flag

5

Alarm

interrupt

flag

4

Update-ended interrupt flag

3–0

Reserved

Figure

2-18. Status

Register

C

(Hex

00C)

Note:

Interrupts

are

enabled

by

bits

6,

5, and

4

in

status register B.

Bit

7

If set to 1, this bit indicates that an interrupt has

occurred;

bits

6,

5,

and

4

indicate

the

type of

interrupt.

Bit

6

If set to 1, this bit indicates that a periodic interrupt has

occurred.

Bit

5

If set to 1, this bit indicates that an alarm interrupt has

occurred.

Bit

4

If set to 1, this bit indicates that an update-ended

interrupt

has

occurred.

Bits 3–0These bits are reserved.

Status Register D (Hex 00D)

BitFunction

7Valid RAM

6–0Reserved

Figure 2-19. Status Register D (Hex 00D)

2-24 System Board