Interrupt Handlers

Interrupt Vector Table

This is a table that maps id of the interrupt to the address of the handler which must handle the interrupt.

why is it called vector

It's called a vector table because this table shows the direction where to go.

Hardware Interrupts

In case of hardware interrupts, the table contains the list of device drivers.

1. During boot, the BIOS reads the configuration requirements of each device and assigns IRQ ID for each device and stores in its configuration space.
2. During the kernel initialization, the driver of each device then reads the IRQ ID from it's own device's configuration space and updates the IVT table with its handler address for that specific IRQ ID.

Polling Interrupts

In case of hardware interrupts, the CPU polls the interrupt flag at specific intervals.

This is why interrupts are always asynchronous.

Software Interrupts

This interrupt comes only from userspace applications to the kernel for any service that it needs from the operating system.

During boot, the kernel registers itself as one of the handlers in the IVT table.

These software interrupts are called as system calls.

ID of the software interrupt handler

In case of Linux, the ID of the software interrupt handler in the IVT is always 128 (0x80)

IRET is the instruction that switches the CPU mode back to user mode.