softwareinterrupt

A software interrupt, in computing, is an interrupt triggered by software rather than by an external hardware device. It provides a controlled mechanism for a user-level program to request services from the operating system, such as I/O, process management, or memory management. Software interrupts are distinct from hardware interrupts, which arise from peripherals or internal timing events.

Mechanism and flow: a software interrupt is typically invoked by a privileged instruction or trap that transfers execution from user mode to kernel mode. The operating system uses a dispatch table or a vector to identify the requested service, often indicated by a system call number and arguments passed through registers or a stack frame. The CPU saves the program state, switches to a higher privilege level, and executes the appropriate kernel routine. Upon completion, control returns to user space, usually through a specific instruction that restores state and delivers a return value.

Architectural variants: different architectures implement software interrupts with different instructions. For example, x86 historically used the INT instruction (and more recently SYSENTER/SYSCALL in many environments). ARM uses software interrupt instructions such as SWI or SVC. RISC-V employs the ECALL instruction for environment calls. While the exact mechanism varies, the underlying concept remains the same: a software-triggered transition to the operating system to perform privileged operations.

Purpose and considerations: software interrupts enable protected, controlled access to system resources, enforcing isolation and security boundaries. They introduce overhead compared to in-process calls, so operating systems optimize common paths but still validate parameters to prevent misuse or security breaches.