# BIOS Interrupt Vector Table (IVT)

## Overview
The BIOS Interrupt Vector Table (IVT) is a crucial data structure in real mode that maps interrupt numbers to their handler routines. It is located at the very beginning of memory, starting at physical address 0x0000:0x0000.

## Structure
- The IVT contains 256 entries (0x00 to 0xFF)
- Each entry is 4 bytes long:
  - 2 bytes for the offset
  - 2 bytes for the segment
- Total size: 256 * 4 = 1024 bytes (0x400)

```
Memory Layout:
0x0000:0x0000 - Int 0x00 vector (Divide by zero)
0x0000:0x0004 - Int 0x01 vector (Single step)
0x0000:0x0008 - Int 0x02 vector (NMI)
...
0x0000:0x0040 - Int 0x10 vector (Video services)
...
0x0000:0x03FC - Int 0xFF vector (Last vector)
```

## Common BIOS Interrupts
- **Int 0x10**: Video Services
  - AH=0x00: Set video mode
  - AH=0x0E: Write character in TTY mode
  - AH=0x13: Write string

- **Int 0x13**: Disk Services
  - AH=0x00: Reset disk system
  - AH=0x02: Read sectors
  - AH=0x03: Write sectors
  - AH=0x41: Check extensions present
  - AH=0x42: Extended read sectors
  - AH=0x43: Extended write sectors

- **Int 0x16**: Keyboard Services
  - AH=0x00: Read keystroke
  - AH=0x01: Check for keystroke

## How It Works
1. When an interrupt occurs (via hardware or `int` instruction):
   - CPU pushes FLAGS, CS, and IP onto stack
   - CPU disables further interrupts
   - CPU looks up handler address in IVT
   - CPU jumps to handler address

2. Example of int 0x10 call:
```nasm
mov ah, 0x0E    ; TTY output function
mov al, 'A'     ; Character to print
int 0x10        ; Call BIOS video interrupt
```

3. The BIOS handler:
   - Receives control
   - Reads parameters from registers
   - Performs requested operation
   - Returns via IRET instruction

## Important Notes
1. BIOS interrupts are only available in real mode
2. When transitioning to protected mode:
   - BIOS interrupts become unavailable
   - Must set up new Interrupt Descriptor Table (IDT)
   - Must provide own interrupt handlers

3. Some BIOS operations require interrupts to be enabled:
   - Disk I/O (int 0x13)
   - Keyboard input (int 0x16)
   - Some video operations (int 0x10)

4. Memory Map Considerations:
   - IVT: 0x0000 - 0x03FF
   - BIOS Data Area: 0x0400 - 0x04FF
   - Your code should not overwrite these areas