returnfrominterrupt

Return from interrupt, often written as return_from_interrupt or returnFromInterrupt in source code, refers to the final sequence that an interrupt service routine (ISR) uses to hand control back to the code that was executing before the interrupt occurred. The purpose is to restore the processor’s state and resume normal execution after the interrupt has been serviced.

In a typical flow, an interrupt triggers the CPU to save the current execution context, jump to the ISR, and perform any necessary work such as acknowledging the interrupt, processing data, or signaling other parts of the system. When the ISR completes, it must restore the saved context, re-enable interrupts if appropriate, notify the interrupt controller that the work is finished (end-of-interrupt), and execute the architecture-specific return-from-interrupt instruction. This instruction ensures the program counter, processor flags, and stack pointer (as needed) are restored and execution resumes where it left off.

The exact mechanism for returning from an interrupt depends on the processor architecture. In x86 and x86-64, the return is typically performed by the IRET or IRETQ instruction, which pops the instruction pointer, code segment, and flags from the stack. In ARM, the return from an interrupt may involve special exception return instructions or a branch to a link register value, depending on the exact ARM version and mode. In MIPS, the return-from-interrupt uses the eret instruction after restoring necessary state.

Return-from-interrupt handling is a critical part of interrupt latency and system responsiveness. It must be carefully designed to preserve program correctness and support features like nested interrupts and concurrency.