Advertisement
The control program can be given
other functions as well.
In fact, some
control programs have reached a very
high degree of sophistication.
Of
course, the more functions that a con-
trol program has, the more main storage
space it requires.
This leaves less
storage for problem programs.
This
problem is somewhat solved by placing
those sections of the control program
that have infrequent usage on a high
speed fast access I/O device such as a
disk storage unit.
Only those sections
that are necessary to supervise the
running of problem programs are kept in
main storage.
The portion of the con-
trol program that resides in main stor-
age is known as the supervisor.
The
supervisor program calls in other sec-
tions of the control program when neces-
sary.
In the simplest use of the control
program, it was used only to bring in
the next problem program.
The problem
programs handled their own input-output
operations (Figure 1-27).
In review then, control programs have
come in general acceptance because of
the need to reduce machine idle time and
manual intervention and to increase the
overall efficiency of a data processing
installation.
INTERRUPTS AND THE PSW
•
An interrupt terminates the current
sequence of instructions and causes
a machine forced "branch" to the
supervisor program.
•
•
An interrupt stores the current PSW
in main storage, and fetches a new
PSW from main storage.
Processing resumes at the instruc-
tion address specified by the
instruction address portion of the
current PSW just loaded.
•
There are five classes of
interrupts.
Each has an old and new
PSW locations in main storage.
So far we have discussed the use of a
control program to bring in new problem
programs when the old ones are finished.
Because there is no halt instruction in
System/360, a problem program when fin-
ished must be able to somehow branch
into the supervisor.
Also when a
machine or program check occurs, an
automatic branch to the supervisE>r usu-
ally occur.
These automatic branches into the
supervisor are called interrupts.
That
is, the current sequence of instructions
is interrupted and an automatic branch
is taken to a new sequence of instruc-
tions.
Both machine checks and program
checks can cause automatic branches or
interrupts.
When a problem program is
finished, it signals the supervisor via
an interrupt.
An interrupt is quite similar to a
branch.
However, it does much more than
a simple branch instruction.
A branch
instruction only replaces the instruc-
tion address portion of the current PSW.
r------T--~-T----T----l
Instruction
I
Branch
I
X2
I
B2
I
D2
I
L ______
~
___
~
___
~
___ J
I
I
L-----------T--J
Effective
I
Address
I
I
I
r--------J.--,
I
I
I
40
631
r-----------------T--------------,
PSW
I
I
Instruction
I
,
I
Address
I
L __________
-------.L------------J
When an interrupt occurs:
(1) the
current PSW is placed in main storage
where it's called old PSW and (2) a new
PSW is brought out of main storage and
becomes the current PSW.
1-53
Advertisement
