Interrupt Fundamentals in Microprocessors

An interrupt is a signal that tells the microprocessor to stop its current task temporarily and attend to a more urgent event. This mechanism allows the microprocessor to handle multiple tasks efficiently without constantly checking the status of various devices or programs.

When an interrupt occurs, the microprocessor typically saves its current state (like the program counter and registers) and jumps to a specific routine called the Interrupt Service Routine (ISR) or interrupt handler. After the ISR finishes its task, the microprocessor restores its saved state and resumes the main program execution from where it was interrupted. Let's analyze each statement regarding interrupts.

Interrupt Handling and Execution Flow

• Option 1: With an interrupt, the microprocessor completes current instruction execution and services the request.

  This statement is true. When an interrupt signal is received, the microprocessor does not immediately stop in the middle of an instruction. Instead, it completes the execution of the instruction it is currently processing. Once the current instruction is finished, the microprocessor then acknowledges the interrupt request and proceeds to save its context and jump to the corresponding interrupt service routine (ISR).

• Option 2: Non maskable interrupt cannot be disabled by any instruction.

  This statement is also true. A Non-Maskable Interrupt (NMI) is a high-priority interrupt that cannot be ignored or disabled by software instructions or masking bits. NMIs are typically reserved for critical events like memory errors, power failures, or hardware malfunctions where immediate attention is required to prevent data loss or system damage.

• Option 3: After servicing the interrupt, the main program is suspended.

  This statement is not true. The purpose of an interrupt is to temporarily divert the microprocessor's attention. After the interrupt service routine (ISR) has completed its task, the microprocessor restores the saved context of the main program (program counter, registers, flags) and resumes its execution from the exact point where it was interrupted. The main program is not suspended; rather, its execution is temporarily paused and then continued.

• Option 4: Microprocessor can have external hardware and/or internal software interrupts.

  This statement is true. Microprocessors are designed to handle various types of interrupts:

  • External Hardware Interrupts: These are generated by peripheral devices (e.g., keyboard, mouse, disk drive, timer, network card) that need the CPU's attention. These signals arrive on dedicated pins of the microprocessor.
  • Internal Software Interrupts (or Traps/Exceptions): These are generated by software instructions (e.g., an `INT` instruction in x86 architecture used for system calls) or by exceptional conditions arising during program execution (e.g., division by zero, page fault, invalid opcode).

Conclusion on Interrupt Behavior

Based on the analysis, the statement that is not true about an interrupt is: "After servicing the interrupt, the main program is suspended." The main program actually resumes its execution.