Intel 8080 - B4_Page_42

Chapter 7. Interrupts
WRITING INTERRUPT SUBROUTINES
74
In general, any registers or condition bits changed by an interrupt subroutine must be restored before returning
to the interrupted program, or errors
will occur.
For example, suppose a program
is interrupted just prior to the instruction:
jC LOC .
and the carry bit equals
1. If the interrupt subroutine happens to reset the carry bit before returning to the
interrupted program, the jump to
LOC which should have occurred will not, causing the interrupted program
to produce erroneous results.
Like any other subroutine then, any interrupt subroutine should
save at least the condition bits and restore them
before performing a
RETURN operation. (The obvious and most convenient way to do this is to save the data
in the stack, using PUSH and POP operations.)
Further, the interrupt enable system
is automatically disabled whenever an interrupt is acknowledged. Except in
special cases, therefore, an interrupt subroutine should include an EI instruction somewhere to permit detection
and handling
of future interrupts. One instruction after an EI is executed, the interrupt subroutine may itself be
interrupted. This process may continue to any level, but as long as all pertinent data are saved and restored,
correct program execution
will continue automatically.
A typical interrupt subroutine, then, could appear
as follows:
Code
PUSH
EI
POP
RET
Operand
PSW
PSW
Comment
;SAVE CONDITION BITS AND ACCUMULATOR
;RE-ENABLE INTERRUPTS
;PERFORM NECESSARY ACTIONS
TO SERVICE
;THE INTERRUPT
;RESTORE MACHINE STATUS
;RETURN
TO INTERRUPTED PROGRAM