<s>
In	O
computing	O
job	B-Operating_System
control	I-Operating_System
refers	O
to	O
the	O
control	O
of	O
multiple	O
tasks	O
or	O
jobs	B-General_Concept
on	O
a	O
computer	O
system	O
,	O
ensuring	O
that	O
they	O
each	O
have	O
access	O
to	O
adequate	O
resources	O
to	O
perform	O
correctly	O
,	O
that	O
competition	O
for	O
limited	O
resources	O
does	O
not	O
cause	O
a	O
deadlock	B-Operating_System
where	O
two	O
or	O
more	O
jobs	B-General_Concept
are	O
unable	O
to	O
complete	O
,	O
resolving	O
such	O
situations	O
where	O
they	O
do	O
occur	O
,	O
and	O
terminating	O
jobs	B-General_Concept
that	O
,	O
for	O
any	O
reason	O
,	O
are	O
not	O
performing	O
as	O
expected	O
.	O
</s>
<s>
Job	B-Operating_System
control	I-Operating_System
has	O
developed	O
from	O
the	O
early	O
days	O
of	O
computers	O
where	O
human	O
operators	O
were	O
responsible	O
for	O
setting	O
up	O
,	O
monitoring	O
and	O
controlling	O
every	O
job	O
,	O
to	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
,	O
which	O
take	O
on	O
the	O
bulk	O
of	O
the	O
work	O
of	O
job	B-Operating_System
control	I-Operating_System
.	O
</s>
<s>
Modern	O
systems	O
permit	O
their	O
users	O
to	O
stop	O
and	O
resume	O
jobs	B-General_Concept
,	O
to	O
execute	O
them	O
in	O
the	O
foreground	O
(	O
with	O
the	O
ability	O
to	O
interact	O
with	O
the	O
user	O
)	O
or	O
in	O
the	O
background	O
.	O
</s>
<s>
Unix-like	B-Operating_System
systems	I-Operating_System
follow	I-Operating_System
this	I-Operating_System
pattern	I-Operating_System
.	O
</s>
<s>
It	O
became	O
obvious	O
to	O
the	O
early	O
computer	O
developers	O
that	O
their	O
fast	O
machines	O
spent	O
most	O
of	O
the	O
time	O
idle	O
because	O
the	O
single	O
program	O
they	O
were	O
executing	O
had	O
to	O
wait	O
while	O
a	O
slow	O
peripheral	O
device	O
completed	O
an	O
essential	O
operation	O
such	O
as	O
reading	O
or	O
writing	O
data	O
;	O
in	O
modern	O
terms	O
,	O
programs	O
were	O
I/O	B-General_Concept
-bound	I-General_Concept
,	O
not	O
compute-bound	O
.	O
</s>
<s>
Buffering	B-General_Concept
only	O
provided	O
a	O
partial	O
solution	O
;	O
eventually	O
an	O
output	O
buffer	B-General_Concept
would	O
occupy	O
all	O
available	O
memory	O
or	O
an	O
input	B-General_Concept
buffer	I-General_Concept
would	O
be	O
emptied	O
by	O
the	O
program	O
,	O
and	O
the	O
system	O
would	O
be	O
forced	O
to	O
wait	O
for	O
a	O
relatively	O
slow	O
device	O
to	O
complete	O
an	O
operation	O
.	O
</s>
<s>
A	O
more	O
general	O
solution	O
is	O
multitasking	B-Operating_System
.	O
</s>
<s>
More	O
than	O
one	O
running	O
program	O
,	O
or	O
process	B-Operating_System
,	O
is	O
present	O
in	O
the	O
computer	O
at	O
any	O
given	O
time	O
.	O
</s>
<s>
If	O
a	O
process	B-Operating_System
is	O
unable	O
to	O
continue	O
,	O
its	O
context	B-Operating_System
can	O
be	O
stored	O
and	O
the	O
computer	O
can	O
start	O
or	O
resume	O
the	O
execution	O
of	O
another	O
process	B-Operating_System
.	O
</s>
<s>
At	O
first	O
quite	O
unsophisticated	O
and	O
relying	O
on	O
special	O
programming	O
techniques	O
,	O
multitasking	B-Operating_System
soon	O
became	O
automated	O
,	O
and	O
was	O
usually	O
performed	O
by	O
a	O
special	O
process	B-Operating_System
called	O
the	O
scheduler	O
,	O
having	O
the	O
ability	O
to	O
interrupt	O
and	O
resume	O
the	O
execution	O
of	O
other	O
processes	O
.	O
</s>
<s>
Typically	O
a	O
driver	B-Application
for	O
a	O
peripheral	O
device	O
suspends	O
execution	O
of	O
the	O
current	O
process	B-Operating_System
if	O
the	O
device	O
is	O
unable	O
to	O
complete	O
an	O
operation	O
immediately	O
,	O
and	O
the	O
scheduler	O
places	O
the	O
process	B-Operating_System
on	O
its	O
queue	B-General_Concept
of	O
sleeping	O
jobs	B-General_Concept
.	O
</s>
<s>
When	O
the	O
peripheral	O
completed	O
the	O
operation	O
the	O
process	B-Operating_System
is	O
re-awakened	O
.	O
</s>
<s>
Similar	O
suspension	O
and	O
resumption	O
may	O
also	O
apply	O
to	O
inter-process	B-Operating_System
communication	I-Operating_System
,	O
where	O
processes	O
have	O
to	O
communicate	O
with	O
one	O
another	O
in	O
an	O
asynchronous	O
manner	O
but	O
may	O
sometimes	O
have	O
to	O
wait	O
for	O
a	O
reply	O
.	O
</s>
<s>
A	O
process	B-Operating_System
that	O
seldom	O
needs	O
to	O
interact	O
with	O
peripherals	O
or	O
other	O
processes	O
would	O
simply	O
hog	O
processor	O
resource	O
until	O
it	O
completed	O
or	O
was	O
halted	O
by	O
manual	O
intervention	O
.	O
</s>
<s>
The	O
result	O
,	O
particularly	O
for	O
interactive	B-General_Concept
systems	O
running	O
tasks	O
that	O
frequently	O
interact	O
with	O
the	O
outside	O
world	O
,	O
is	O
that	O
the	O
system	O
is	O
sluggish	O
and	O
slow	O
to	O
react	O
in	O
a	O
timely	O
manner	O
.	O
</s>
<s>
This	O
problem	O
is	O
resolved	O
by	O
allocating	O
a	O
"	O
timeslice	O
"	O
to	O
each	O
process	B-Operating_System
,	O
a	O
period	O
of	O
uninterrupted	O
execution	O
after	O
which	O
the	O
scheduler	O
automatically	O
puts	O
it	O
on	O
the	O
sleep	O
queue	B-General_Concept
.	O
</s>
<s>
Process	B-Operating_System
could	O
be	O
given	O
different	O
priorities	O
,	O
and	O
the	O
scheduler	O
could	O
then	O
allocate	O
varying	O
shares	O
of	O
available	O
execution	O
time	O
to	O
each	O
process	B-Operating_System
on	O
the	O
basis	O
of	O
the	O
assigned	O
priorities	O
.	O
</s>
<s>
This	O
system	O
of	O
pre-emptive	B-Operating_System
multitasking	B-Operating_System
forms	O
the	O
basis	O
of	O
most	O
modern	O
job	B-Operating_System
control	I-Operating_System
systems	O
.	O
</s>
<s>
While	O
batch	B-General_Concept
processing	I-General_Concept
can	O
run	O
around	O
the	O
clock	O
,	O
with	O
or	O
without	O
computer	O
operators	O
,	O
since	O
the	O
computer	O
is	O
much	O
faster	O
than	O
a	O
person	O
,	O
most	O
decision-making	O
occurs	O
before	O
the	O
job	O
even	O
begins	O
to	O
run	O
,	O
and	O
requires	O
planning	O
by	O
the	O
"	O
programmer.	O
"	O
</s>
<s>
Although	O
a	O
computer	O
operator	O
may	O
be	O
present	O
,	O
batch	B-General_Concept
processing	I-General_Concept
is	O
intended	O
to	O
mostly	O
operate	O
without	O
human	O
intervention	O
.	O
</s>
<s>
Early	O
computer	O
resident	B-Application
monitors	I-Application
and	O
operating	B-General_Concept
systems	I-General_Concept
were	O
relatively	O
primitive	O
and	O
were	O
not	O
capable	O
of	O
sophisticated	O
resource	O
allocation	O
.	O
</s>
<s>
Batch	B-General_Concept
processing	I-General_Concept
was	O
common	O
,	O
and	O
interactive	B-General_Concept
computer	O
systems	O
rare	O
and	O
expensive	O
.	O
</s>
<s>
Job	B-Application
control	I-Application
languages	I-Application
(	O
JCLs	O
)	O
developed	O
as	O
primitive	O
instructions	O
,	O
typically	O
punched	O
on	O
cards	O
at	O
the	O
head	O
of	O
a	O
deck	O
containing	O
input	O
data	O
,	O
requesting	O
resources	O
such	O
as	O
memory	O
allocation	O
,	O
serial	O
numbers	O
or	O
names	O
of	O
magnetic	O
tape	O
spools	O
to	O
be	O
made	O
available	O
during	O
execution	O
,	O
or	O
assignment	O
of	O
filenames	O
or	O
devices	O
to	O
device	O
numbers	O
referenced	O
by	O
the	O
job	O
.	O
</s>
<s>
A	O
typical	O
example	O
of	O
this	O
kind	O
of	O
language	O
,	O
still	O
in	O
use	O
on	O
mainframes	O
,	O
is	O
IBM	O
's	O
Job	B-Application
Control	I-Application
Language	I-Application
(	O
also	O
known	O
as	O
JCL	O
)	O
.	O
</s>
<s>
Though	O
the	O
format	O
of	O
early	O
JCLs	O
was	O
intended	O
for	O
punched	B-Architecture
card	I-Architecture
use	O
,	O
the	O
format	O
survived	O
the	O
transition	O
to	O
storage	O
in	O
computer	O
files	O
on	O
disk	O
.	O
</s>
<s>
Non-IBM	O
mainframe	O
batch	B-General_Concept
systems	I-General_Concept
had	O
some	O
form	O
of	O
job	B-Application
control	I-Application
language	I-Application
,	O
whether	O
called	O
that	O
or	O
not	O
;	O
their	O
syntax	O
was	O
completely	O
different	O
from	O
IBM	O
versions	O
,	O
but	O
they	O
usually	O
provided	O
similar	O
capabilities	O
.	O
</s>
<s>
Interactive	B-General_Concept
systems	O
include	O
"	O
command	B-Language
languages	I-Language
"	O
command	O
files	O
(	O
such	O
as	O
PCDOS	O
"	O
.bat	O
"	O
files	O
)	O
can	O
be	O
run	O
non-interactively	O
,	O
but	O
these	O
usually	O
do	O
not	O
provide	O
as	O
robust	O
an	O
environment	O
for	O
running	O
unattended	O
jobs	B-General_Concept
as	O
JCL	O
.	O
</s>
<s>
On	O
some	O
computer	O
systems	O
the	O
job	B-Application
control	I-Application
language	I-Application
and	O
the	O
interactive	B-General_Concept
command	B-Language
language	I-Language
may	O
be	O
different	O
.	O
</s>
<s>
For	O
example	O
,	O
TSO	B-Application
on	O
z/OS	O
systems	O
uses	O
CLIST	B-Operating_System
or	O
Rexx	B-Language
as	O
command	B-Language
languages	I-Language
along	O
with	O
JCL	O
for	O
batch	B-General_Concept
work	O
.	O
</s>
<s>
As	O
time	O
sharing	O
systems	O
developed	O
,	O
interactive	B-General_Concept
job	B-Operating_System
control	I-Operating_System
emerged	O
.	O
</s>
<s>
An	O
end-user	O
in	O
a	O
time	O
sharing	O
system	O
could	O
submit	O
a	O
job	O
interactively	O
from	O
his	O
remote	O
terminal	B-General_Concept
(	O
remote	B-Protocol
job	I-Protocol
entry	I-Protocol
)	O
,	O
communicate	O
with	O
the	O
operators	O
to	O
warn	O
them	O
of	O
special	O
requirements	O
,	O
and	O
query	O
the	O
system	O
as	O
to	O
its	O
progress	O
.	O
</s>
<s>
During	O
interactive	B-General_Concept
execution	O
he	O
could	O
interrupt	O
the	O
job	O
and	O
let	O
it	O
continue	O
in	O
the	O
background	O
or	O
kill	O
it	O
.	O
</s>
<s>
This	O
development	O
of	O
interactive	B-General_Concept
computing	I-General_Concept
in	O
a	O
multitasking	B-Operating_System
environment	O
led	O
to	O
the	O
development	O
of	O
the	O
modern	O
shell	B-Operating_System
.	O
</s>
<s>
Pre-emptive	B-Operating_System
multitasking	B-Operating_System
with	O
job	B-Operating_System
control	I-Operating_System
assures	O
that	O
a	O
system	O
operates	O
in	O
a	O
timely	O
manner	O
most	O
of	O
the	O
time	O
.	O
</s>
<s>
In	O
such	O
circumstances	O
,	O
job	B-Operating_System
control	I-Operating_System
is	O
more	O
complex	O
and	O
the	O
role	O
of	O
scheduling	O
is	O
more	O
important	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
the	O
Xerox	O
RBM	O
(	O
Real-time/Batch	O
Monitor	O
)	O
systems	O
,	O
for	O
example	O
,	O
two	O
other	O
capabilities	O
existed	O
:	O
</s>
<s>
background	O
job	O
streams	O
(	O
batch	B-General_Concept
jobs	I-General_Concept
)	O
.	O
</s>
