Interrupt Vector Table
On the x86 architecture, the Interrupt Vector Table (IVT) is a table that specifies the addresses of all the 256 interrupt handlers used in real mode.
The IVT is typically located at 0000:0000H, and is 400H bytes in size (4 bytes for each interrupt). Although the default address can be changed using the LIDT instruction on newer CPUs, this is usually not done because it is both inconvenient and incompatible with other implementations and/or older software (e.g. MS-DOS programs). However, note that the code must remain in the first MiB of RAM.
Contents |
Structure
The entries are consecutive, meaning the first entry pointed by the IDTR is interrupt handler 0, and the others follow in succession. The format of an entry is:
+-----------+-----------+ | Segment | Offset | +-----------+-----------+ 4 2 0
We can therefore see that it's really easy to get the address of the interrupt handler we're looking for: IDTR * 4. In order to change an interrupt handler, all that needs to be done is to change its address in the table.
CPU Interrupt Layout
IVT Offset | INT # | Description -----------+-----------+----------------------------------- 0x0000 | 0x00 | Divide by 0 0x0004 | 0x01 | Reserved 0x0008 | 0x02 | NMI Interrupt 0x000C | 0x03 | Breakpoint (INT3) 0x0010 | 0x04 | Overflow (INTO) 0x0014 | 0x05 | Bounds range exceeded (BOUND) 0x0018 | 0x06 | Invalid opcode (UD2) 0x001C | 0x07 | Device not available (WAIT/FWAIT) 0x0020 | 0x08 | Double fault 0x0024 | 0x09 | Coprocessor segment overrun 0x0028 | 0x0A | Invalid TSS 0x002C | 0x0B | Segment not present 0x0030 | 0x0C | Stack-segment fault 0x0034 | 0x0D | General protection fault 0x0038 | 0x0E | Page fault 0x003C | 0x0F | Reserved 0x0040 | 0x10 | x87 FPU error 0x0044 | 0x11 | Alignment check 0x0048 | 0x12 | Machine check 0x004C | 0x13 | SIMD Floating-Point Exception 0x00xx | 0x14-0x1F | Reserved 0x0xxx | 0x20-0xFF | User definable
Default Hardware Interrupt Layout
Master 8259:
Some interrupts mapped by the 8259 by default overlap with some of the processor's exception handlers. These can be remapped via the 8259's IO ports.
IVT Offset | INT # | IRQ # | Description -----------+-------+-------+------------------------------ 0x0020 | 0x08 | 0 | PIT 0x0024 | 0x09 | 1 | Keyboard 0x0028 | 0x0A | 2 | 8259A slave controller 0x002C | 0x0B | 3 | COM2 / COM4 0x0030 | 0x0C | 4 | COM1 / COM3 0x0034 | 0x0D | 5 | LPT2 0x0038 | 0x0E | 6 | Floppy controller 0x003C | 0x0F | 7 | LPT1
Slave 8259:
IVT Offset | INT # | IRQ # | Description -----------+-------+-------+------------------------------ 0x01C0 | 0x70 | 8 | RTC 0x01C4 | 0x71 | 9 | Unassigned 0x01C8 | 0x72 | 10 | Unassigned 0x01CC | 0x73 | 11 | Unassigned 0x01D0 | 0x74 | 12 | Mouse controller 0x01D4 | 0x75 | 13 | Math coprocessor 0x01D8 | 0x76 | 14 | Hard disk controller 1 0x01DC | 0x77 | 15 | Hard disk controller 2
