7.
INTERRUPTS
INTERRUPT CONCEPTS
The following
is
a general description
of
interrupt handling and applies to both the
8080
and 8085 processors.
However, the
8085 processor has some additional hardware features for interrupt handling. For more infor-
mation on these features,
see the description
of
the 8085 processor
in
Chapter 1 and the descriptions
of
the
RIM,
SIM, and RST instructions
in
Chapter 3.
Often, events occur external to the central processing unit which require immediate action by the CPU. For
example, suppose a device
is
sending a string
of
80
characters to the CPU, one
at
a time,
at
fixed intervals.
There dre two ways to handle such a situation:
A.
A program could
be
written which accepts the first character, waits until the next character
is
ready (e.g., executes a timeout
by
incrementing a sufficiently large counter), then accepts the
next character, and proceeds
in
this fashion until the entire 80 character string has been received.
This method
is
referred to as programmed Input/Output.
B.
The device controller could interrupt the CPU when a character
is
ready to
be
input, forcing a
branch from the executing program to a special interrupt service routine.
The interrupt sequence may
be
illustrated
as
follows:
Normal
Program
Execution
INTERRUPT
Interrupt Service
Routine
Program
Execution
Continues
7·1
To Next Page
Loading...
