Foundry-Boot/doc/bios_interrupts.md

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:

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