Computer Interfacing

Computer interfacing

Priority interrupt controllers (PICs)

It

is

possible

to

establish interrupt priorities

by

means

of hardware,

the

priority interrupt controller being

an

example

of

this.

The

basic

PIC

logic

is

shown

in

Figure

4.7,

which shows

how

either interrupt lines, number

0 to

7

are

handled. Each

of

these lines

is

ANDed with

the ap-

propriate

bit of an 8-bit

mask register.

The

contents

of

the mask register

are

determined

by the

program which

loads

a

byte into this register

as

required from time

to

time.

Thus,

to

enable

all

eight interrupts

to be

effective,

the mask register would

be

loaded with

FFH,

that

is all

ones; loading

00H

int