Interrupts Interrupt is a process where an external device can get the

attention of the microprocessor. The process starts from the I/O device The process is asynchronous.

Classification of Interrupts Interrupts can be classified into two types:

Maskable Interrupts (Can be delayed or Rejected) Non-Maskable Interrupts (Can not be delayed or Rejected)

Interrupts can also be classified into: Vectored (the address of the service routine is hard-wired) Non-vectored (the address of the service routine needs to

be supplied externally by the device)

Interrupts An interrupt is considered to be an emergency signal that may

be serviced.

The Microprocessor may respond to it as soon as possible.

What happens when MP is interrupted ?

When the Microprocessor receives an interrupt signal, it suspends the currently executing program and jumps to an Interrupt Service Routine (ISR) to respond to the incoming interrupt.

Each interrupt will most probably have its own ISR.

Responding to Interrupts

Responding to an interrupt may be immediate or delayed depending on whether the interrupt is maskable or non-maskable and whether interrupts are being masked or not.

There are two ways of redirecting the execution to the ISR depending on whether the interrupt is vectored or non-vectored.

Vectored: The address of the subroutine is already known to the Microprocessor

Non Vectored: The device will have to supply the address of the subroutine to the Microprocessor

Interrupt Vectors and the Vector Table

An interrupt vector is a pointer to where the ISR is stored in memory.

In the 8088/8086 processor as well as in the 80386/80486/Pentium processors operating in Real Mode (16-bit operation), the interrupt vector is a pointer to the Interrupt Vector Table.

The Interrupt Vector Table occupies the address range from 00000H to 003FFH (the first 1024 bytes in the memory map).

Each entry in the Interrupt Vector Table is 4 bytes long:

The first two represent the offset address and the last two the segment address of the ISR.

The first 5 vectors are reserved by Intel to be used by the processor.

The vectors 5 to 255 are free to be used by the user.

5

The Intel x86 Vector Interrupts:- Protected Mode (32-bit) In the 80386/80486/Pentium processors operating in the

Protected Mode (32-bit operation), the interrupt vector is a pointer to the Interrupt Descriptor Table.

The Interrupt Descriptor Table can be located anywhere in the memory.

Its starting address is pointed by the Interrupt Descriptor Table Register (IDTR).

Each entry in the Interrupt Vector Table is 8 bytes long:

Four bytes represent the 32-bit offset address, two the segment selector and the rest information such as the privilege level.

The first 32 vectors are reserved by Intel to be used by the processor.

The vectors 33 to 255 are free to be used by the user.

ACOE2550

2

4

6

1

3

5

7 Offset (A31 - A16)

Offset (A15 - A0)

Segment Selector

00H01110PFThe protected modeinterrupt descriptor

IVT

ISR

While the CPU is executing a program, an interrupt breaks the normal sequence of execution of instructions, diverts its execution to some other program called “Interrupt Service Routine (ISR).

After executing ISR, the control is transferred back again to the main program which was being executed at the time of interruption.

Interrupt Response

ISR