Specifications

Processor Description (V40)
The purpose of an interrupt is to redirect the CPU from its current
activity to an interrupt service routine designed to handle the needs of
the interrupting source. Every interrupting source is associated with a
number that points the CPU to a location in memory that contains the
address of the interrupt service routine. This number is called an
interrupt vector. The area in memory where the addresses of the
interrupt service routines are stored is called the interrupt vector table.
During an interrupt cycle, the CPU multiplies the vector by four to
obtain the location of the service routine address in the vector table.
The CPU then transfers control to the service routine at the address
read from the vector table. (See Figure 5-8.) It is the programmer’s
responsibility to load the address of the service routine into the vector
table. The address includes a segment and offset value in the format
shown.
Programmed To
Provide Unique
Vector (N) For
Each Input
Multiplies Vector
(N) By Four
Offset
PFP (IP)
Segment
PS (CS)
Interrupt
Service
Routine
4N
4N+1
4N+2
4N+3
0
7
(N=2)
NMI
N
4
4N
x
•
•
•
ICU CPU MEMORY
Figure 5–8. Interrupt Processing.
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