Interrupt. Interrupt – to break the flow of speech or action of (someone) by saying or doing something (Longman dictionary). Examples. When your phone rings during a lecture, what will happen? When you are studying then your cell phone rings – what will you do?
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Interrrupt is a procedure that interrupts whatever program
is currently executing by the CPU.
Interrupts are particularly useful when interfacing
I/O devices that provide or require data at relatively
low data-transfer rates , eg a keyboard.
During an interrupt, the CPU will perform pre-defined
operations according to the interrupt nature so the
microprocessor can execute other software before the
How to get key typed in the keyboard or a keypad?
The CPU executes a program that check for the available of data
If a key is pressed then read the data, otherwise keep waiting
Just like the mechanism used in LAB for sensing the keypad
The CPU executes other program, as soon as a key is pressed, the
Keyboard generates an interrupt. The CPU will response to the
interrupt – read the data. After that returns to the original program.
So by proper use of interrupt, the CPU can serve many devices at the
No key pressed
Do key pressed action
Stop or turn
Interrupt service routine
Return to the original
CS + IP
Vector 0 = interrupt type 0
This represent the interrupt level 50
Starting from 0 type 50 should be in 50x4 since each pointer
requires 4 bytes
In HEX, 200 is C8 to CB
C8 stores the IP value, CA stores the CS
Note: /INTA is issued twice
The IF flag must be 1
Type number is input at the 2nd
Read Interrupt type
Address of ISR is obtained by
x4 the Interrupt type number
and address the Interrupt pointer
IRET is reached
Return to interrupted program
input is used
the Type No
Resolve by storing
The ISR at the
Only IR0 active
IR5 and IR0 active
This is a PAL
From the CPU’s point of
View, the 8259 is also a
How the 8259 is enabled?