Hardware Reference
In-Depth Information
bottom 1 KB of memory (addresses 0000H to 03FFH). Each of the locations in
the Interrupt Pointer Table can be loaded with a pointer to a different interrupt
service routine. Each pointer contains 2 bytes for loading into the Code Segment
(CS) Register and 2 bytes for loading into the Instruction Pointer (IP). This
allows the programmer to place interrupt service routines in any appropriate
place within the 1 MB physical address space.
Each of the 256
Interrupt Pointers
is allocated a different type number. A
Type 0 interrupt has its associated Interrupt Pointer in the lowest 4 bytes of
memory (0000H to 0003H). A Type 1 interrupt will have its pointer located in
the next 4 bytes of memory (0004H to 0007H), and so on.
The structure of the 8086 Interrupt Pointer Table is shown in Figure 5.3.
Interrupt types 0 to 4 have dedicated functions while Types 5 to 31 are reserved.
Hence there are 224 remaining locations in which Interrupt Pointers may be
stored. The interrupting device places a byte on the data bus in response to an
Figure 5.3
8086 Interrupt Pointer table
