How to Trigger Software Interrupts in C Inline Assembly

This article explains how to manually generate software interrupts using inline assembly within the C programming language. You will learn the underlying mechanics of software interrupts, see architecture-specific implementations for both x86 and ARM processors using standard GCC inline assembly syntax, and understand the necessary precautions regarding privilege levels and operating system handling.

Understanding Software Interrupts

A software interrupt is an instruction executed by a program that forces the CPU to pause execution, save its current state, and transfer control to an interrupt service routine (ISR) mapped in an interrupt vector table. Unlike hardware interrupts triggered by external peripherals, software interrupts are synchronous and explicitly invoked by running code. Common uses include transitioning between user mode and kernel mode (system calls), debugging (breakpoints), and triggering specific firmware routines.

Triggering Interrupts on x86 and x86-64

On x86 and x86-64 platforms, software interrupts are triggered using the INT instruction, followed by an immediate 8-bit vector number (from 0 to 255).

In GCC or Clang, you can insert this instruction using extended inline assembly:

#include <stdio.h>

void trigger_interrupt(void) {
    // Triggers interrupt vector 0x80 (traditionally used for 32-bit Linux syscalls)
    __asm__ volatile ("int $0x80" : : : "memory");
}

void trigger_breakpoint(void) {
    // Triggers interrupt vector 3 (standard x86 debug breakpoint)
    __asm__ volatile ("int $3");
}

int main(void) {
    printf("Triggering breakpoint...\n");
    trigger_breakpoint();
    return 0;
}
  • The volatile qualifier prevents the compiler from optimizing out, reordering, or caching the assembly block.
  • The $0x80 syntax indicates an immediate value in AT&T assembly syntax.
  • The "memory" clobber informs the compiler that the instruction might modify memory, forcing memory caches to flush.

Triggering Interrupts on ARM Architectures

ARM processors do not use the INT instruction. Instead, they implement the SVC (Supervisor Call) instruction, previously known as SWI (Software Interrupt).

#include <stdio.h>

void trigger_arm_svc(void) {
    // Triggers a supervisor call with an immediate value of 0
    __asm__ volatile ("svc #0" : : : "memory");
}

void trigger_arm_bkpt(void) {
    // Triggers a hardware-assisted breakpoint
    __asm__ volatile ("bkpt #0");
}

On ARM Cortex-M microcontrollers, software interrupts can also be triggered at the hardware level by writing directly to the Interrupt Control and State Register (ICSR) or the Software Triggered Interrupt Register (STIR) in addition to using svc.

Compiler Portability: GCC vs. MSVC

  • GCC/Clang: Supports the __asm__ volatile (...) syntax on all supported targets and architectures.
  • MSVC (Microsoft Visual C++): Only supports the legacy __asm { ... } block on 32-bit x86 targets. On 64-bit Windows, MSVC disables inline assembly entirely. Instead, you must use compiler intrinsics such as __debugbreak() to raise an interrupt 3, or implement the assembly routine in a separate .asm file assembled via MASM.

Execution Permissions and Operating System Constraints

Executing arbitrary software interrupts in modern, protected-mode operating systems (such as Linux or Windows) requires proper permissions:

  1. Privilege Violations: In user space, calling an interrupt vector that has a descriptor privilege level (DPL) set to kernel mode without proper gating triggers a General Protection Fault (#GP), typically terminating the program with a segmentation fault.
  2. Safe Vectors: Vector 3 (int $3 / bkpt) is safe in user space because the OS configures its descriptor to allow user-mode execution for debuggers.
  3. Register Context: When using software interrupts for system calls, arguments must be placed into specific general-purpose registers before calling the interrupt, adhering to the platform's Application Binary Interface (ABI).