User's Manual
CHAPTER 9
INTERRUPTS
AND
EXCEPTIONS
Interrupts and exceptions are special cases of control transfer within a program. An interrupt occurs
as
a result
of
an event that
is
independent of the currently executing program, while exceptions are a
direct result of the program currently being executed, Interrupts may be external or internal. External
interrupts are generated by either the
INTR
or
NMI
input pins. Internal interrupts are caused by the
INT
instruction. Exceptions occur when an instruction cannot be completed normally. Although their
causes differ, interrupts and exceptions use the same control transfer techniques and privilege rules;
therefore, in the following discussions the term interrupt
will
also apply to exceptions.
The program used to service an interrupt may execute in the context of
.the task that caused the
interrupt (i.e., used the same
TSS, LDT, stacks, etc.) or may be a separate task. The choice depends
on
the function to be performed and the level of isolation required.
9.1 INTERRUPT DESCRIPTOR TABLE
Many different events may cause an interrupt.
To
allow the reason for an interrupt to be easily identi-
fied, each interrupt source
is
given a number called the interrupt vector. Up
to
256 different interrupt
vectors (numbers) are possible.
See figure
9-1.
A table
is
used to define the handler for each interrupt vector. The Interrupt Descriptor Table (IDT)
defines the interrupt handlers for up
to
256 different interrupts. The IDT
is
in
physical memory, pointed
to by the contents of the on-chip IDT register that contains a 24-bit base and a 16-bit limit. The
IDTR
is
normally loaded with the LIDT instruction
by
code
that
executes at privilege level 0 during system
initialization. The
IDT
may be located anywhere in the physical address space of the 80286.
IOTR
l
23
'I'
cPU
r+-
15
0
J
lOT
LIMIT
1-1-
lOT
BASE
0
,>,
MEMORY
GATE
FOR
INTERRUPT
#n
GATE
FOR
INTERRUPT
#n-1
·
·
·
GATE
FOR
INTERRUPT
# 1
GATE
FOR
INTERRUPT
#0
,
"
INTERRUPT
DESCRIPTOR
TABLE
(lOT)
Figure 9-1.
Interrupt
Descriptor
Table Definition
9-1
THE
lOT
MAY
CONTAIN
INTERRUPT
GATES,
TRAPS
OR
TASK
GATES
ONLY.
G30108