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Binary
0000000000001000
Hexadecimal
0008
There are five old PSW's in main
storage.
How does the supervisor know
which one to use?
The answer is of
course, that each of five new PSW's
point to different routines in the
supervisor.
These routines in turn use
the old PSW that corresponds to the
particular class of interrupt.
For
instance; the program check routine in
the supervisor uses the old PSW at loca-
tion 0040 while the supervisor call
routine will use the old PSW at location
0032.
InterruEt
Old PSW
New PSW
External
0024
0088
Supervisor
0032
0096
Program
0040
0104
Machine
0048
0112
I/O
0056
0120
In the case of an interrupt caused by
a machine check, the PSW that was con-
trolling the program prior to the inter-
rupt is stored automatically in location
0048.
Then the doubleword at location
0112
is brought out and becomes the
current PSW.
This PSW directs the sys-
tem to that area of the supervisor pro-
gram that handles machine checks.
The
machine check handling routine of the
supervisor is written so that the dou-
bleword at location 0048 is processed as
the old PSW.
In the case of an interrupt caused by
a program check, the current PSW at the
time the interrupt occurs is stored
automatically as the old PSW at location
0040.
Then the doubleword at location
0140 is brought out and becomes the
current PSW.
This PSW directs the sys-
temto that area of the supervisor that
handles program checks.
The program
check handling routine of the supervisor
is written so that the doubleword at
location 0040 will
be
processed as the
old PSW.
In the case of an interrupt caused by
the instruction supervisor call, the
current PSW (prior to the interrupt) is
stored in location 0032.
Then the dou-
bleword at location 0096 is brought out
and becomes the current PSW.
This PSW
directs the system to that portion of
the supervisor which handles supervisor
calls.
One way a problem program could
notify the supervisor that the program
is finished is to issue a supervisor
1-58
call instruction.
Thus the last
instruction of a'problem program would
probably be a supervisor call instruc-
tion.
If the interrupt key on the
operator's console is pressed, an exter-
nal interrupt occurs.
In this case, the
current PSW is automatically stored at
location 0024.
On an e.xternal
interrupt, the doubleword at location
0088 is brought out and becomes the new
current PSW.
An interrupt may also be caused by
the end of an I/O operation.
I/O inter-
rupts generally occur at the end of an
I/O operation.
Most I/O operations are
overlapped with processing.
The I/O
interrupt is an efficient way of sig-
nalling the supervisor that the I/O
operation is finished.
An I/O interrupt
causes the psw to be sto.red at location
0056.
The new PSW at location 0120 is
brought out and becomes the current PSW.
This PSW directs the system to that
section of the supervisor program that
handles I/O interrupts.
Problem Prog.
Supervisor
Prog. I/O
Unit
t
I
I
t
t
t
t
I --------- I
I
I
Supervisor
_~
_____
J
I
tlSupervisor
Call Inter-
rupt
I
IHandles I/O
I
!Interrupt
I
I
Supervisor
I/O Operation
Starts I/O
OVerlapped
Operation
with
Processing
At the end of the I/O operation, the
I/O interrupt causes the PSW used in the
processing of the problem program to be
stored in location 0056.
The doubleword
at location 0120 is brought out of main
storage and used as the PSW to control
the processing of the I/O interrupt
routine in the supervisor program.
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