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wrote the supervisor program.
For
instance:
r""'-----T---T---'
1
OA
I
0
1
0
I
Supervisor
l _ _ _ _ _
.L.,; __
.1 ___
J
Call Instruction
t
I
I
Everthing
l _______
J
in Hex
t
I
I
I
I
This interruption code of
00 might be used to signal a supervisor
program that the problem program is
finished.
Given the following supervisor call
instruction (in hex), the binary bit
structure that would be placed in the
interruption code of the old PSW is
11010000 (bits 16-31 of the old PSW in
location 0032).
r----T----T-----'
1
OA
I
D
I
0
I
l -_ _ .1 ____ .1 _ _ _ J
MASKING INTERRUPTS
Supervisor
Call
Instruction
•
Interrupts are prevented from occur-
ring by mask bits in the current
psw.
•
The system mask is in bits 0 to 7 of
the PSW.
These are used to prevent
(mask) external and I/O interrupts
from interrupting faster than the
supervisor can handle them.
•
The machine check
~ask
is bit 13 of
the PSW.
It allows machine errors
to be ignored.
Machine checks are
not normally masked off except as a
diagnostic aid.
•
The program mask is bits 36 to 39 of
the PSW.
They are used to prevent 4
of the 16 program checks from caus-
ing interrupts.
•
11 program check interrupts, and the
supervisor call interrupt, cannot be
masked.
Sometimes in a program, it is not desir-
able to allow an interrupt.
This is
most apparent when we consider the I/O
interrupt.
In the System/360 it is
possible to have simUltaneous
1/0
opera-
tions on two or more channels.
When one
operation is completed, an I/O i.nterrupt
will usually occur.
The PSW will be
stored to give the supervisor program
the reason (which
1/0
unit) for the
interrupt.
This old PSW also gives the
supervisor program a way in which to
return to the interrupted problem pro-
gram.
If we were to allow another
1/0
interrupt before the first one had been
completely handled, the old PSW (from
the problem program) would
be
lost.
The
supervisor program would not be able to
return to the problem program via the
Load PSW instruction as shown below.
r---------------,
1st I/O Interrupt--
T
>
1
J
I
I
I
I
I
I
I
I
I
Location
I
Problem I
0056
I
Program I
r------, .• __________
J
I
10Id PSWI
I
l ______ J - - - - - ,
I
I
I
I
I
I
I
I
I
1
I
________________
J
r-.1----,
Current PSWI
1
L _____ J
2
t
I
I
I
0120
I
r--------------,
1/0
Interrupt
Routine In
r-----~-----J
I
I
I
I
I
INew PSWI
L ______
~
I
I
I
I
I
l -__________
> I
I
r----------------->I
I
I
Supervisor
Program
I
I
Load PSW
I
I
from 0056
I
L... ______________ J
I
If a 2nd
1/0
interrupt were allowed to
occur he.re, the current PSW at this
point would be stored in location 0056.
This would cause the Old PSW from the
problem program to be destroyed.
1-61
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