Interrupts – IDT and ISRs

This document describes how the Interrupt Descriptor Table (IDT) and Interrupt Service Routines (ISRs) are implemented in BenOS.

Overview

The IDT is a CPU structure used to handle exceptions, hardware interrupts, and software traps. Each entry in the IDT points to an interrupt or exception handler function (ISR). It is loaded during early kernel initialization.

Responsibilities

The IDT code is responsible for:

  • Defining and populating the IDT entries
  • Initializing exception handlers (0–31)
  • Initializing hardware interrupt handlers (32–47)
  • Loading the IDT

NOTE: Actually, the exception handlers and hardware interrupt handlers are not complete. The IDT can handle:

- General Protection Fault (#GP)

- Page Fault (#PF)

- Double Fault (#DF)

- Division Error (#DE)

- Invalid TSS (#TS)

- IRQ 0 (PIT)

- IRQ 1 (Keyboard)

Initialization flow

1. IDT structures

The IDT is defined as a static array of descriptors in kernel/cpu/include/idt.h. A pointer structure named idt_ptr_t is passed to lidt to load it in kernel/asm/idt_flush.asm.

// Source: kernel/cpu/include/idt.h

// The IDT descriptor structure
typedef struct
{
    uint16_t offset_low;
    uint16_t selector;
    uint8_t ist;
    uint8_t type_attribute;
    uint16_t offset_middle;
    uint32_t offset_high;
    uint32_t zero;
} __attribute__ ((packed)) idt_entry_t;

// The IDT pointer structure
typedef struct
{
    uint16_t limit;
    uint64_t base;
} __attribute__ ((packed)) idt_ptr_t;

2. Initialization function

NOTE: the functions described below depend of the idt array.

The IDT is initialized using the idt_init() function declared in kernel/cpu/include/idt.h. This function is defined in kernel/cpu/idt.c. First, it sets up the IDT pointer (defined as idt_ptr) and then, it sets up the IDT entries with the idt_set_entry() function defined in kernel/cpu/include/idt.h like the following:

// Source: kernel/cpu/include/idt.h

void idt_set_entry(int n, uint64_t base, uint16_t selector, uint8_t flags);

After setting up all the IDT entries, idt_init() flushes the IDT using idt_flush() defined in kernel/asm/idt_flush.asm.

; Source: kernel/asm/idt_flush.asm

idt_flush:
    mov rax, rdi
    lidt [rax]
    ret