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Interrupts & Exceptions

The Interrupt system is the backbone of TilekarOS, handling hardware events, CPU faults, and system calls.

1. The IDT (Interrupt Descriptor Table)

The IDT is an array of 256 gates. Each gate points to an assembly "stub" that handles a specific interrupt.

IDT Structures

  • IDTEntry: Each entry is 8 bytes. It contains the 32-bit address of the ISR (split into high/low words), the kernel code segment selector (0x08), and attributes.
  • Attributes: Bits 0-3 (Gate Type), Bit 4 (S), Bits 5-6 (DPL), Bit 7 (P).
  • IDTR: A 6-byte structure used by the lidt instruction, containing the limit (size - 1) and the linear address of the IDT array.

Vector Layout

  • 0 - 31: CPU Exceptions (Faults/Traps).
  • 0: Divide-by-Zero
  • 13: General Protection Fault
  • 14: Page Fault (presents faulting address in CR2).
  • 32 - 47: Hardware Interrupts (IRQs).
  • 32: Timer (PIT)
  • 33: Keyboard
  • 128 (0x80): System Call Entry.

2. Programmable Interrupt Controller (PIC)

TilekarOS remaps the 8259 PIC so that IRQs start at index 32, avoiding conflicts with CPU exceptions.

PIC Mapping

  • Master PIC: Remapped to 0x20 (32). Handles IRQs 0-7.
  • Slave PIC: Remapped to 0x28 (40). Handles IRQs 8-15.

End of Interrupt (EOI)

After handling an IRQ, the kernel MUST send an EOI signal (0x20) to the PIC.

  • If the IRQ was on the Slave PIC (>= 40), an EOI must be sent to both the Slave and the Master.
  • If the IRQ was only on the Master PIC, an EOI is only sent to the Master.

3. Interrupt Lifecycle

  1. Hardware/CPU Triggers: CPU pushes EFLAGS, CS, and EIP. If a privilege switch occurred, SS and ESP are also pushed.
  2. Assembly Stub (idt.asm):
    • Pushes an error code (or 0 if none).
    • Pushes the interrupt number.
    • Saves all registers (pushad).
    • Saves segment registers (gs, fs, es, ds).
    • Sets DS and ES to the kernel data segment.
  3. C Handler: irq_handler or isr_handler is called.
    • All registers are accessible via the InteruptReg pointer.
  4. Restore: Registers are restored (popad), and iret returns to the previous execution flow.

4. Handler Registration

Kernel modules can register handlers for specific IRQs using irq_install_handler(irq_no, handler).

5. Register Context (InteruptReg)

All handlers receive a pointer to the saved registers on the stack: 

typedef struct {
    uint32_t gs, fs, es, ds;
    uint32_t edi, esi, ebp, esp, ebx, edx, ecx, eax;
    uint32_t intr_num, err_cod;
    uint32_t eip, csm, eflags, useresp, ss;
} InteruptReg;