Hardware Interrupts

IRQ

Interrupt Requests (IRQs) are used to identify hardware events. They are represented as uint8_t values. Userspace programs (such as a driver) can use syscalls to wait for an IRQ.

Before waiting for an IRQ, a userspace task must unmask it by calling irq_unmask(irq). Afterwards, irq_wait(irq) blocks the task until the hardware raises the interrupt. See syscalls.md for the full syscall reference.

x86_64

Since APIC is exclusively in use (meaning no legacy PIC), there have been set some conventions for IRQ numbers. For each IOAPIC pin, there is a corresponding IRQ number. The pin’s number is always identical to the IRQ number.

The interrupt vectors in range 0x00-0x1F are used for CPU exceptions. Due to legacy PIC and its conventions, 0x20-0x2F are not used, as it could lead to some undesired behavior. Therefore, 0x30 has been chosen as the vector offset for the IRQs. That means for every IRQ, its interrupt vector is 0x30 + the IRQ number.

aarch64

On aarch64 (that is, 64-bit ARM architecture), IRQs are used to represent shared peripheral interrupts (SPIs). It has been decided that the IRQ numbers correspond to the SPI numbers. Because interrupt ids (INTIDs) of SPIs start from 0x20, as the range 0x00-0x0F is used for software generated interrupts (SGIs), and 0x10-0x1F is used for private peripheral interrupts (PPIs), the IRQ number of an SPI is 0x20 + INTID.