Intel 8080 - B4_Page_41

Chapter 7. Interrupts
Transfers
}
Beginning of
Device 'x'
control to
0038
..
subroutine for
supplies RST 7H
003F
device 'x'
Note that any of these 8-byte subroutines may in turn call longer subroutines to process the interrupt, if
necessary.
Any device
may supply an RST instruction (and indeed may supply anyone-byte 8080 instruction).
The following is an example of an Interrupt sequence:
ARBITRARY
MEMORY ADDRESS
3COB
3COC
0000
INSTRUCTION
:~~ ~:! ~ {,n"""Pt '<om 0";,, ,
Device 1 supplies
In","";on ,/
RST OH
Program Counter :::
3COC pushed onto
the stack.
Control transferred to
to 0000
Instruction 2
RET-----------------------.,
Stack popped into
program counter
A
B
C
Device 1
signals an interrupt as the CPU is executing the instruction at 3COB. This imtruction is completed.
The program counter remains set to 3COC, and the instruction RST OH supplied by device 1 is executed.
Since this is a call to location zero, 3COC is pushed onto the stack and program control is transferred to
location ooOOH. (This subroutine may perform jumps, calls, or any other operation.) When the RETURN is
executed, address 3COC is popped off the stack and replaces the contents of the program counter, causing
execution to contir.ue at this point.
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