Interrupts in 8086 microprocessor
An interrupt is a condition that halts the microprocessor temporarily to work on a different task and then return to its previous task. Interrupt is an event or signal that request to attention of CPU. This halt allows peripheral devices to access the microprocessor.
Whenever an interrupt occurs the processor completes the execution of the current instruction and starts the execution of an Interrupt Service Routine (ISR) or Interrupt Handler. ISR is a program that tells the processor what to do when the interrupt occurs. After the execution of ISR, control returns back to the main routine where it was interrupted.
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In 8086 microprocessor following tasks are performed when microprocessor encounters an interrupt:
- The value of flag register is pushed into the stack. It means that first the value of SP (Stack Pointer) is decremented by 2 then the value of flag register is pushed to the memory address of stack segment.
- The value of starting memory address of CS (Code Segment) is pushed into the stack.
- The value of IP (Instruction Pointer) is pushed into the stack.
- IP is loaded from word location (Interrupt type) * 04.
- CS is loaded from the next word location.
- Interrupt and Trap flag are reset to 0.
The different types of interrupts present in 8086 microprocessor are given by:
- Hardware Interrupts –
Hardware interrupts are those interrupts which are caused by any peripheral device by sending a signal through a specified pin to the microprocessor. There are two hardware interrupts in 8086 microprocessor. They are:
- (A) NMI (Non Maskable Interrupt) – It is a single pin non maskable hardware interrupt which cannot be disabled. It is the highest priority interrupt in 8086 microprocessor. After its execution, this interrupt generates a TYPE 2 interrupt. IP is loaded from word location 00008 H and CS is loaded from the word location 0000A H.
- (B) INTR (Interrupt Request) – It provides a single interrupt request and is activated by I/O port. This interrupt can be masked or delayed. It is a level triggered interrupt. It can receive any interrupt type, so the value of IP and CS will change on the interrupt type received.
- Software Interrupts – These are instructions that are inserted within the program to generate interrupts. There are 256 software interrupts in 8086 microprocessor. The instructions are of the format INT type where type ranges from 00 to FF. The starting address ranges from 00000 H to 003FF H. These are 2 byte instructions. IP is loaded from type * 04 H and CS is loaded from the next address give by (type * 04) + 02 H. Some important software interrupts are:
- (A) TYPE 0 corresponds to division by zero(0).
- (B) TYPE 1 is used for single step execution for debugging of program.
- (C) TYPE 2 represents NMI and is used in power failure conditions.
- (D) TYPE 3 represents a break-point interrupt.
- (E) TYPE 4 is the overflow interrupt.
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