What is idt?

IDT generally refers to the Interrupt Descriptor Table. It's a crucial data structure in x86-based computer architectures, specifically used by the operating system or hypervisor to handle hardware and software interrupts (also known as exceptions).

• Purpose: The IDT maps interrupt vector numbers to interrupt handlers (also known as Interrupt Service Routines or ISRs). When an interrupt occurs, the processor uses the interrupt vector to index into the IDT and locate the appropriate handler to execute.

• Structure: Each entry in the IDT is an Interrupt Descriptor, typically 8 bytes in size. This descriptor contains information such as the segment selector for the interrupt handler's code segment, the offset of the handler within that segment, and various flags (e.g., present bit, privilege level).

• Interrupt Vectors: These are small integer numbers (0-255) that identify specific interrupts or exceptions. Some interrupt vectors are reserved for hardware interrupts (e.g., keyboard, timer), while others are reserved for software exceptions (e.g., divide by zero, page fault) and system calls.

• Loading the IDT: The LIDT instruction (Load Interrupt Descriptor Table) is used to load the base address and limit of the IDT into a special processor register (IDTR). This allows the processor to locate the IDT in memory. The IDT is typically set up by the operating system during system initialization.

• Interrupt Handling Process: When an interrupt occurs:

  1. The processor saves the current state (e.g., instruction pointer, code segment) onto the stack.
  2. The processor retrieves the interrupt handler's address from the IDT based on the interrupt vector.
  3. The processor jumps to the interrupt handler, which executes the code to handle the interrupt.
  4. After the handler finishes, it executes a special instruction (e.g., IRET - Interrupt Return) to restore the saved state and return control to the interrupted program.

• Protection: The IDT is protected by the operating system to prevent unauthorized modification of interrupt handlers. This helps to maintain system stability and security. Privilege levels also control which processes can trigger certain interrupts. System calls, for instance, use interrupts to safely transition from user mode to kernel mode.