The instruction cycle is a fundamental process in the execution of a program by the central processing unit (CPU).
When interrupts occur, the CPU needs to handle them to ensure that important events are addressed promptly. The handling of interrupts typically involves temporarily suspending the normal instruction cycle, transferring control to an interrupt service routine (ISR), and then returning to the interrupted program.
<aside> 💡 An interrupt is like someone tapping you inbetween your work to do something urgent
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Here's a general overview of how the instruction cycle handles interrupts:
Normal Instruction Cycle:
The CPU is in the process of executing instructions from a program in memory, following the fetch-decode-execute-store cycle.
Interrupt Request (IRQ):
An external event or condition, such as input from a device or a timer reaching a specific value, triggers an interrupt request (IRQ). This request is often associated with a specific interrupt number or code.
Interrupt Handling:
The CPU acknowledges the interrupt request and enters the interrupt handling process. The current state of the CPU, including the program counter and register values, is saved to preserve the context of the interrupted program.
Interrupt Vector Table (IVT):
The CPU consults the interrupt vector table (IVT) to determine the memory address of the corresponding I**nterrupt Service Routine** (ISR) for the received interrupt number. The IVT serves as a mapping between interrupt numbers and their respective ISR addresses.
Transfer Control to ISR:
The CPU transfers control to the identified ISR by jumping to its memory address. The ISR is a piece of code specifically designed to handle the tasks associated with the interrupt, such as processing input from a device, updating system variables, or performing other necessary actions.
ISR Execution:
The ISR executes its instructions to handle the interrupt. This may involve interacting with hardware, updating data structures, or performing any other necessary operations.
Return from Interrupt (RTI):
After the ISR completes its tasks, it executes a "return from interrupt" (RTI) instruction, which restores the saved context from step 3. This includes restoring the program counter and register values to resume the interrupted program.
Resumption of Normal Instruction Cycle:
With the context restored, the CPU resumes the normal instruction cycle, continuing to execute instructions from the interrupted program.
Handling interrupts allows a computer system to respond efficiently to external events without constantly polling for them. The use of interrupt mechanisms enhances the overall responsiveness and multitasking capabilities of a computer system.
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