Understanding CPU Interrupts: Types, Handling, and Addressing Modes

Posted on Feb 13, 2025 in Computers

Understanding CPU Interrupts

What is an Interrupt?

In computing, an interrupt is the ability of the CPU to pause the execution of a program, execute another routine (the interrupt handler), and then return to the original program to resume execution.

Types of Interrupts

Interrupts can be broadly classified into two types:

Software Interrupts

These are generated by software programs to indicate error conditions, such as:

  • Attempting to divide by zero
  • Unauthorized memory access
  • Attempting to execute an erroneous instruction

Hardware Interrupts

These are triggered by hardware events, such as:

  • Completion of I/O operations
  • Hardware malfunctions

CPU Response to an Interrupt Signal

When the CPU receives an interrupt signal, it:

  1. Stops executing the current program.
  2. Begins processing the interrupt.
  3. After the interrupt is handled, it resumes execution of the original program from where it left off.

Interrupt Handlers

An interrupt handler is a specific routine or set of instructions designed to service a particular interrupt.

Handling Multiple Interrupts

If an interrupt occurs while another interrupt is being processed, the current process is stored in memory, and the new interrupt is handled. This process can be nested, with interrupts being stacked and processed in order of priority or arrival.

For example:

  1. Program A is running.
  2. Interrupt B occurs; Program A is stored, and Program B runs.
  3. Interrupt C occurs; Program B is stored, and Program C runs.
  4. Program C finishes, and Program B is restored and continues.
  5. Program B finishes, and Program A is restored and continues.

Instruction Cycle States

The instruction cycle involves various states within the CPU and memory access. These include:

  • Fetch: Read operations fetch instructions and data from memory.
  • Decode: Obtain and decode the instruction.
  • Execute: Perform the operation using the ALU (Arithmetic Logic Unit).

Impact of Addressing on Program Execution

The number of addresses in the instruction format influences the program’s execution speed and the space it occupies in memory. The execution speed depends on the number of memory accesses required by the CPU.

Addressing Modes

An addressing mode specifies how to determine the address of operands within an instruction.

Types of Addressing Modes

  1. Direct Addressing:
    • Advantages: Simple implementation by the CPU.
    • Disadvantages: Memory space that can be addressed is restricted by the length of the address field in the instruction format.
  2. Indirect Addressing:
    • Advantages: Large memory address space to reference operands.
    • Disadvantages: Greater complexity in implementation and the need for two memory accesses for each operand.
  3. Immediate Addressing:
    • Advantages: Reading operands does not require access to memory.
    • Disadvantages: The range of integer values is restricted to the length of the address field.
  4. Register Addressing:
    • Advantages: Requires few bits within the instruction format for routing registers.
    • Disadvantages: It is necessary to include in curricula copy of operating instructions from memory to CPU.
  5. Register Indirect Addressing:
    • Advantages: It requires only one memory access for each operand.
    • Disadvantages: Hardly applied in practice as it is defined as one can not know in advance which addresses are loaded into the registers.

Determining Addressing Mode

The CPU determines which addressing mode to use in an instruction through various methods:

Method 1

Each instruction can use a specific addressing mode. For example:

  • LOAD – Indirection
  • ADD – Indirection

Method 2

Each instruction can use any addressing mode, but each mode is assigned a specific number. A field within the instruction format specifies the digit of this addressing mode, which is decoded before execution. For example:

  • ADD direct md 00
  • ADD direct md 01
  • ADD direct md 10