Interrupt Vector Table

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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.

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

See also