<s>
The	O
history	O
of	O
IBM	B-Device
mainframe	I-Device
operating	B-General_Concept
systems	I-General_Concept
is	O
significant	O
within	O
the	O
history	O
of	O
mainframe	B-Architecture
operating	B-General_Concept
systems	I-General_Concept
,	O
because	O
of	O
IBM	O
's	O
long-standing	O
position	O
as	O
the	O
world	O
's	O
largest	O
hardware	O
supplier	O
of	O
mainframe	B-Architecture
computers	I-Architecture
.	O
</s>
<s>
IBM	B-Device
mainframes	I-Device
run	O
operating	B-General_Concept
systems	I-General_Concept
supplied	O
by	O
IBM	O
and	O
by	O
third	O
parties	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
systems	I-General_Concept
on	O
early	O
IBM	B-Device
mainframes	I-Device
have	O
seldom	O
been	O
very	O
innovative	O
,	O
except	O
for	O
TSS/360	B-Application
and	O
the	O
virtual	B-Architecture
machine	I-Architecture
systems	O
beginning	O
with	O
CP-67	B-Application
.	O
</s>
<s>
IBM	O
's	O
current	O
mainframe	B-Architecture
operating	B-General_Concept
systems	I-General_Concept
,	O
z/OS	B-Application
,	O
z/VM	B-Application
,	O
z/VSE	B-Application
,	O
and	O
z/TPF	B-Operating_System
,	O
are	O
backward	B-General_Concept
compatible	I-General_Concept
successors	O
to	O
those	O
introduced	O
in	O
the	O
1960s	O
.	O
</s>
<s>
IBM	O
was	O
slow	O
to	O
introduce	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
General	O
Motors	O
produced	O
General	O
Motors	O
OS	O
in	O
1955	O
and	O
GM-NAA	B-Application
I/O	I-Application
in	O
1956	O
for	O
use	O
on	O
its	O
own	O
IBM	O
computers	O
;	O
and	O
in	O
1962	O
Burroughs	O
Corporation	O
released	O
MCP	B-Application
and	O
General	O
Electric	O
introduced	O
GECOS	B-Application
,	O
in	O
both	O
cases	O
for	O
use	O
by	O
their	O
customers	O
.	O
</s>
<s>
The	O
first	O
operating	B-General_Concept
systems	I-General_Concept
for	O
IBM	O
computers	O
were	O
written	O
in	O
the	O
mid-1950s	O
by	O
IBM	O
customers	O
with	O
very	O
expensive	O
machines	O
at	O
,	O
which	O
had	O
sat	O
idle	O
while	O
operators	O
set	O
up	O
jobs	O
manually	O
,	O
and	O
so	O
they	O
wanted	O
a	O
mechanism	O
for	O
maintaining	O
a	O
queue	B-Device
of	O
jobs	O
.	O
</s>
<s>
These	O
operating	B-General_Concept
systems	I-General_Concept
run	O
only	O
on	O
a	O
few	O
processor	O
models	O
and	O
are	O
suitable	O
only	O
for	O
scientific	O
and	O
engineering	O
calculations	O
.	O
</s>
<s>
Other	O
IBM	O
computers	O
or	O
other	O
applications	O
function	O
without	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
But	O
one	O
of	O
IBM	O
's	O
smaller	O
computers	O
,	O
the	O
IBM	B-Device
650	I-Device
,	O
introduced	O
a	O
feature	O
which	O
later	O
became	O
part	O
of	O
OS/360	B-Application
,	O
where	O
if	O
processing	O
is	O
interrupted	O
by	O
a	O
"	O
random	O
processing	O
error	O
"	O
(	O
hardware	O
glitch	O
)	O
,	O
the	O
machine	O
automatically	O
resumes	O
from	O
the	O
last	O
checkpoint	O
instead	O
of	O
requiring	O
the	O
operators	O
to	O
restart	O
the	O
job	O
manually	O
from	O
the	O
beginning	O
.	O
</s>
<s>
General	O
Motors	O
Research	O
division	O
produced	O
GM-NAA	B-Application
I/O	I-Application
for	O
its	O
IBM	B-Device
701	I-Device
in	O
1956	O
(	O
from	O
a	O
prototype	O
,	O
GM	O
Operating	B-General_Concept
System	I-General_Concept
,	O
developed	O
in	O
1955	O
)	O
,	O
and	O
updated	O
it	O
for	O
the	O
701	O
's	O
successor	O
.	O
</s>
<s>
In	O
1960	O
the	O
IBM	O
user	O
association	O
SHARE	O
took	O
it	O
over	O
and	O
produced	O
an	O
updated	O
version	O
,	O
SHARE	B-Application
Operating	I-Application
System	I-Application
.	O
</s>
<s>
Finally	O
IBM	O
took	O
over	O
the	O
project	O
and	O
supplied	O
an	O
enhanced	O
version	O
called	O
IBSYS	B-Operating_System
with	O
the	O
IBM	B-Device
7090	I-Device
and	O
IBM	B-Device
7094	I-Device
computers	O
.	O
</s>
<s>
IBSYS	B-Operating_System
required	O
8	O
tape	O
drivesfewer	O
if	O
one	O
or	O
more	O
disk	O
drives	O
are	O
present	O
.	O
</s>
<s>
Its	O
main	O
components	O
are	O
:	O
a	O
card-based	O
Job	B-Application
Control	I-Application
language	I-Application
,	O
which	O
is	O
the	O
main	O
user	O
interface	O
;	O
compilers	B-Language
for	O
FORTRAN	B-Application
and	O
COBOL	B-Application
;	O
an	O
assembler	B-Language
;	O
and	O
various	O
utilities	B-Application
including	O
a	O
sort	B-Algorithm
program	O
.	O
</s>
<s>
In	O
1958	O
,	O
the	O
University	B-Application
of	I-Application
Michigan	I-Application
Executive	I-Application
System	I-Application
adapted	O
GM-NAA	B-Application
I/O	I-Application
to	O
produce	O
UMES	B-Application
,	O
which	O
was	O
better	O
suited	O
to	O
the	O
large	O
number	O
of	O
small	O
jobs	O
created	O
by	O
students	O
.	O
</s>
<s>
UMES	B-Application
was	O
used	O
until	O
1967	O
when	O
it	O
was	O
replaced	O
by	O
the	O
MTS	B-Application
timesharing	B-General_Concept
system	O
.	O
</s>
<s>
Bell	O
Labs	O
produced	O
BESYS	B-Operating_System
(	O
sometimes	O
referred	O
to	O
as	O
BELLMON	O
)	O
and	O
used	O
it	O
until	O
the	O
mid-1960s	O
.	O
</s>
<s>
Before	O
IBSYS	B-Operating_System
,	O
IBM	O
produced	O
for	O
its	O
IBM	B-Device
709	I-Device
,	O
7090	B-Device
and	O
7094	B-Device
computers	O
a	O
tape-based	O
operating	B-General_Concept
system	I-General_Concept
whose	O
sole	O
purpose	O
was	O
to	O
compile	B-Language
FORTRAN	B-Application
programs	O
—	O
in	O
fact	O
FMS	O
and	O
the	O
FORTRAN	B-Application
compiler	B-Language
were	O
on	O
the	O
same	O
tape	O
.	O
</s>
<s>
MIT	O
's	O
Fernando	O
Corbató	O
produced	O
the	O
first	O
experimental	O
time-sharing	B-General_Concept
systems	I-General_Concept
,	O
such	O
as	O
CTSS	B-Operating_System
,	O
from	O
1957	O
to	O
the	O
early	O
1960s	O
,	O
using	O
slightly	O
modified	O
IBM	B-Device
709	I-Device
,	O
IBM	B-Device
7090	I-Device
,	O
and	O
IBM	B-Device
7094	I-Device
mainframes	B-Architecture
;	O
these	O
systems	O
were	O
based	O
on	O
a	O
proposal	O
by	O
John	O
McCarthy	O
.	O
</s>
<s>
In	O
the	O
1960s	O
IBM	O
's	O
own	O
laboratories	O
created	O
experimental	O
time-sharing	B-General_Concept
systems	I-General_Concept
,	O
using	O
standard	O
mainframes	B-Architecture
with	O
hardware	O
and	O
microcode	B-Device
modifications	O
to	O
support	O
virtual	B-Architecture
memory	I-Architecture
:	O
IBM	B-Device
M44/44X	I-Device
in	O
the	O
early	O
1960s	O
;	O
CP-40	B-Application
from	O
1964	O
to	O
1967	O
;	O
CP-67	B-Application
from	O
1967	O
to	O
1972	O
.	O
</s>
<s>
The	O
company	O
even	O
released	O
CP-67	B-Application
without	O
warranty	O
or	O
technical	O
support	O
to	O
several	O
large	O
customers	O
from	O
1968	O
to	O
1972	O
.	O
</s>
<s>
CP-40	B-Application
and	O
CP-67	B-Application
used	O
modified	O
System/360	B-Application
CPUs	B-Device
,	O
but	O
the	O
M44/44X	B-Device
was	O
based	O
on	O
the	O
IBM	B-Device
7044	I-Device
,	O
an	O
earlier	O
generation	O
of	O
CPU	B-Device
which	O
was	O
very	O
different	O
internally	O
.	O
</s>
<s>
These	O
experimental	O
systems	O
were	O
too	O
late	O
to	O
be	O
incorporated	O
into	O
the	O
System/360	B-Application
series	O
which	O
IBM	O
announced	O
in	O
1964	O
but	O
encouraged	O
the	O
company	O
to	O
add	O
virtual	B-Architecture
memory	I-Architecture
and	O
virtual	B-Architecture
machine	I-Architecture
capabilities	O
to	O
its	O
System/370	B-Device
mainframes	B-Architecture
and	O
their	O
operating	B-General_Concept
systems	I-General_Concept
in	O
1972	O
:	O
</s>
<s>
The	O
M44/44X	B-Device
showed	O
that	O
a	O
partial	O
approach	O
to	O
virtual	B-Architecture
machines	I-Architecture
was	O
not	O
good	O
enough	O
and	O
that	O
thrashing	B-General_Concept
could	O
severely	O
reduce	O
the	O
speed	O
of	O
virtual	B-Architecture
memory	I-Architecture
systems	O
.	O
</s>
<s>
Thrashing	B-General_Concept
is	O
a	O
condition	O
in	O
which	O
the	O
system	O
runs	O
very	O
slowly	O
because	O
it	O
spends	O
a	O
lot	O
of	O
its	O
time	O
shuffling	O
virtual	B-Architecture
memory	I-Architecture
pages	O
between	O
physical	O
memory	O
and	O
disk	O
files	O
.	O
</s>
<s>
IBM	O
learned	O
from	O
CP-40	B-Application
and	O
CP-67	B-Application
:	O
how	O
to	O
make	O
the	O
thrashing	B-General_Concept
problem	O
manageable	O
;	O
that	O
its	O
other	O
virtual	B-Architecture
memory	I-Architecture
and	O
virtual	B-Architecture
machine	I-Architecture
technologies	O
were	O
sufficiently	O
fast	O
and	O
reliable	O
to	O
be	O
used	O
in	O
the	O
high-volume	O
commercial	O
systems	O
which	O
were	O
its	O
core	O
business	O
.	O
</s>
<s>
In	O
particular	O
,	O
IBM	O
's	O
David	O
Sayre	O
convinced	O
the	O
company	O
that	O
automated	O
virtual	B-Architecture
memory	I-Architecture
management	O
could	O
consistently	O
perform	O
at	O
least	O
as	O
well	O
as	O
the	O
best	O
programmer-designed	O
overlay	B-General_Concept
schemes	O
.	O
</s>
<s>
In	O
1968	O
a	O
consulting	O
firm	O
called	O
Computer	O
Software	O
Systems	O
used	O
the	O
released	O
version	O
of	O
CP-67	B-Application
to	O
set	O
up	O
a	O
commercial	O
time-sharing	B-General_Concept
service	O
.	O
</s>
<s>
The	O
company	O
's	O
technical	O
team	O
included	O
2	O
recruits	O
from	O
MIT	O
(	O
see	O
CTSS	B-Operating_System
above	O
)	O
,	O
Dick	O
Orenstein	O
and	O
Harold	O
Feinleib	O
.	O
</s>
<s>
As	O
it	O
grew	O
,	O
the	O
company	O
renamed	O
itself	O
National	O
CSS	O
and	O
modified	O
the	O
software	O
to	O
increase	O
the	O
number	O
of	O
paying	O
users	O
it	O
could	O
support	O
until	O
the	O
system	O
was	O
sufficiently	O
different	O
that	O
it	O
warranted	O
a	O
new	O
name	O
,	O
VP/CSS	B-Application
.	O
</s>
<s>
VP/CSS	B-Application
was	O
the	O
delivery	O
mechanism	O
for	O
National	O
CSS	O
 '	O
services	O
until	O
the	O
early	O
1980s	O
,	O
when	O
it	O
switched	O
to	O
IBM	O
's	O
VM/370	B-Application
(	O
see	O
below	O
)	O
.	O
</s>
<s>
Universities	O
produced	O
three	O
other	O
S/360	B-Application
time-sharing	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
in	O
the	O
late	O
1960s	O
:	O
</s>
<s>
The	O
Michigan	B-Application
Terminal	I-Application
System	I-Application
(	O
MTS	B-Application
)	O
was	O
developed	O
in	O
1967	O
by	O
a	O
consortium	O
of	O
universities	O
led	O
by	O
the	O
University	O
of	O
Michigan	O
.	O
</s>
<s>
All	O
versions	O
ran	O
on	O
IBM	O
's	O
mainframes	B-Architecture
which	O
had	O
virtual	B-Architecture
memory	I-Architecture
capability	O
,	O
starting	O
with	O
the	O
S/360	B-Device
-67	I-Device
.	O
</s>
<s>
MTS	B-Application
remained	O
in	O
use	O
until	O
1999	O
.	O
</s>
<s>
McGill	O
University	O
in	O
Montreal	O
started	O
developing	O
MUSIC	B-Application
(	O
McGill	O
University	O
System	O
for	O
Interactive	O
Computing	O
)	O
in	O
1969	O
.	O
</s>
<s>
MUSIC	B-Application
was	O
enhanced	O
several	O
times	O
and	O
eventually	O
supported	O
text	O
searches	O
,	O
web	O
publishing	O
and	O
email	O
as	O
well	O
as	O
software	O
development	O
.	O
</s>
<s>
MUSIC	B-Application
was	O
marketed	O
by	O
IBM	O
mainly	O
to	O
educational	O
institutions	O
as	O
a	O
cost-effective	O
operating	B-General_Concept
system	I-General_Concept
for	O
its	O
hardware	O
,	O
and	O
eventually	O
became	O
an	O
IBM	O
Systems	O
Product	O
(	O
MUSIC/SP	B-Application
or	O
Multi-User	O
System	O
for	O
Interactive	O
Computing	O
/	O
System	O
Product	O
)	O
in	O
1985	O
.	O
</s>
<s>
ORVYL	B-Application
and	I-Application
WYLBUR	I-Application
were	O
developed	O
by	O
Stanford	O
University	O
in	O
1967-68	O
for	O
the	O
IBM	B-Application
S/360	I-Application
-67	O
.	O
</s>
<s>
They	O
provided	O
some	O
of	O
the	O
first	O
time-sharing	B-General_Concept
capabilities	O
on	O
IBM	B-Application
S/360	I-Application
computers	O
.	O
</s>
<s>
So	O
in	O
1964	O
,	O
the	O
company	O
announced	O
System/360	B-Application
,	O
a	O
new	O
range	O
of	O
computers	O
which	O
all	O
used	O
the	O
same	O
peripherals	O
and	O
most	O
of	O
which	O
could	O
run	O
the	O
same	O
programs	O
.	O
</s>
<s>
IBM	O
originally	O
intended	O
that	O
System/360	B-Application
should	O
have	O
only	O
one	O
batch-oriented	B-General_Concept
operating	B-General_Concept
system	I-General_Concept
,	O
OS/360	B-Application
.	O
</s>
<s>
There	O
are	O
at	O
least	O
two	O
accounts	O
of	O
why	O
IBM	O
later	O
decided	O
it	O
should	O
also	O
produce	O
a	O
simpler	O
batch-oriented	B-General_Concept
operating	B-General_Concept
system	I-General_Concept
,	O
DOS/360	B-Application
:	O
</s>
<s>
because	O
it	O
found	O
that	O
OS/360	B-Application
would	O
not	O
fit	O
into	O
the	O
limited	O
memory	O
available	O
on	O
the	O
smaller	O
System/360	B-Application
models	O
;	O
</s>
<s>
or	O
because	O
it	O
realized	O
that	O
the	O
development	O
of	O
OS/360	B-Application
would	O
take	O
much	O
longer	O
than	O
expected	O
,	O
and	O
introduced	O
DOS/360	B-Application
as	O
one	O
of	O
a	O
series	O
of	O
stop-gaps	O
to	O
prevent	O
System/360	B-Application
hardware	O
sales	O
from	O
collapsingthe	O
others	O
were	O
BOS/360	B-Application
(	O
Basic	B-Application
Operating	I-Application
System	I-Application
,	O
for	O
the	O
smallest	O
machines	O
)	O
and	O
TOS/360	B-Application
(	O
Tape	O
Operating	B-General_Concept
System	I-General_Concept
,	O
for	O
machines	O
with	O
only	O
tape	O
drives	O
)	O
.	O
</s>
<s>
System/360	B-Application
'	O
s	O
operating	B-General_Concept
systems	I-General_Concept
were	O
more	O
complex	O
than	O
previous	O
IBM	O
operating	B-General_Concept
systems	I-General_Concept
for	O
several	O
reasons	O
,	O
including	O
:	O
</s>
<s>
They	O
had	O
to	O
support	O
multiprogrammingswitching	O
to	O
run	O
another	O
in-progress	O
application	O
when	O
the	O
current	O
application	O
was	O
blocked	O
waiting	O
for	O
I/O	B-General_Concept
operations	I-General_Concept
(	O
such	O
as	O
disk	O
reads	O
)	O
to	O
complete	O
.	O
</s>
<s>
Without	O
multiprogramming	O
,	O
the	O
faster	O
CPUs	B-Device
in	O
the	O
range	O
would	O
have	O
spent	O
most	O
of	O
their	O
time	O
idle	O
,	O
waiting	O
for	O
slow	O
I/O	B-General_Concept
operations	I-General_Concept
.	O
</s>
<s>
Hence	O
,	O
the	O
operating	B-General_Concept
systems	I-General_Concept
had	O
to	O
be	O
the	O
real	O
masters	O
of	O
the	O
systems	O
,	O
to	O
provide	O
whatever	O
services	O
the	O
applications	O
validly	O
requested	O
,	O
and	O
to	O
handle	O
crashes	O
or	O
misbehavior	O
in	O
one	O
application	O
without	O
stopping	O
others	O
that	O
were	O
running	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
This	O
made	O
the	O
development	O
of	O
OS/360	B-Application
and	O
other	O
System/360	B-Application
software	O
one	O
of	O
the	O
largest	O
software	O
projects	O
anyone	O
had	O
attempted	O
,	O
and	O
IBM	O
soon	O
ran	O
into	O
trouble	O
,	O
with	O
huge	O
time	O
and	O
cost	O
overruns	O
and	O
large	O
numbers	O
of	O
bugs	B-Error_Name
.	O
</s>
<s>
These	O
problems	O
were	O
only	O
magnified	O
because	O
to	O
develop	O
and	O
test	O
System/360	B-Application
operating	B-General_Concept
systems	I-General_Concept
on	O
real	O
hardware	O
,	O
IBM	O
first	O
had	O
to	O
develop	O
Basic	B-Application
Programming	I-Application
Support/360	I-Application
(	O
BPS/360	B-Application
)	O
.	O
</s>
<s>
BPS	O
was	O
used	O
to	O
develop	O
the	O
tools	O
needed	O
to	O
develop	O
DOS/360	B-Application
and	O
OS/360	B-Application
,	O
as	O
well	O
as	O
the	O
first	O
versions	O
of	O
tools	O
it	O
would	O
supply	O
with	O
these	O
operating	O
systemscompilers	O
for	O
FORTRAN	B-Application
and	O
COBOL	B-Application
,	O
utilities	B-Application
including	O
Sort	B-Algorithm
,	O
and	O
above	O
all	O
the	O
assembler	B-Language
it	O
needed	O
to	O
build	O
all	O
the	O
other	O
software	O
.	O
</s>
<s>
IBM	O
's	O
competitors	O
took	O
advantage	O
of	O
the	O
delays	O
in	O
OS/360	B-Application
and	O
the	O
System/360	B-Application
to	O
announce	O
systems	O
aimed	O
at	O
what	O
they	O
thought	O
were	O
the	O
most	O
vulnerable	O
parts	O
of	O
IBM	O
's	O
market	O
.	O
</s>
<s>
To	O
prevent	O
sales	O
of	O
System/360	B-Application
from	O
collapsing	O
,	O
IBM	O
released	O
four	O
stop-gap	O
operating	B-General_Concept
systems	I-General_Concept
:	O
</s>
<s>
Basic	B-Application
Operating	I-Application
System/360	I-Application
(	O
BOS/360	B-Application
)	O
,	O
which	O
loaded	O
from	O
a	O
disk	O
drive	O
or	O
tape	O
drive	O
and	O
supported	O
tape	O
drives	O
and	O
a	O
few	O
disks	O
.	O
</s>
<s>
This	O
system	O
was	O
supplied	O
to	O
beta	O
test	O
customers	O
and	O
may	O
have	O
been	O
an	O
early	O
version	O
of	O
DOS/360	B-Application
.	O
</s>
<s>
TOS/360	B-Application
,	O
which	O
was	O
designed	O
to	O
provide	O
an	O
upgrade	O
path	O
for	O
customers	O
who	O
had	O
IBM	B-Device
1401	I-Device
computers	O
with	O
tape	O
drives	O
and	O
no	O
disks	O
.	O
</s>
<s>
DOS/360	B-Application
,	O
which	O
was	O
built	O
by	O
the	O
developers	O
of	O
BOS/360	B-Application
and	O
TOS/360	B-Application
(	O
IBM	O
's	O
small	O
business	O
computers	O
division	O
)	O
and	O
went	O
on	O
to	O
become	O
a	O
mainstream	O
operating	B-General_Concept
system	I-General_Concept
whose	O
descendant	O
z/VSE	B-Application
is	O
still	O
widely	O
used	O
.	O
</s>
<s>
Operating	B-Application
System/360	I-Application
(	O
OS/360	B-Application
)	O
with	O
only	O
the	O
Primary	O
Control	O
Program	O
(	O
PCP	O
)	O
option	O
,	O
which	O
did	O
n't	O
support	O
multiprogramming	O
.	O
</s>
<s>
When	O
IBM	O
announced	O
the	O
S/360	B-Device
-67	I-Device
it	O
also	O
announced	O
a	O
timesharing	B-General_Concept
operating	B-General_Concept
system	I-General_Concept
,	O
TSS/360	B-Application
,	O
that	O
would	O
use	O
the	O
new	O
virtual	B-Architecture
memory	I-Architecture
capabilities	O
of	O
the	O
360/67	B-Device
.	O
</s>
<s>
TSS/360	B-Application
was	O
late	O
and	O
early	O
releases	O
were	O
slow	O
and	O
unreliable	O
.	O
</s>
<s>
By	O
this	O
time	O
the	O
alternative	O
operating	B-General_Concept
system	I-General_Concept
CP-67	B-Application
,	O
developed	O
by	O
IBM	O
's	O
Cambridge	O
Scientific	O
Center	O
,	O
was	O
running	O
well	O
enough	O
for	O
IBM	O
to	O
offer	O
it	O
"	O
without	O
warranty	O
"	O
as	O
a	O
timesharing	B-General_Concept
facility	O
for	O
a	O
few	O
large	O
customers	O
.	O
</s>
<s>
CP-67	B-Application
would	O
go	O
on	O
to	O
become	O
VM/370	B-Application
and	O
eventually	O
z/VM	B-Application
.	O
</s>
<s>
IBM	O
ultimately	O
offered	O
three	O
releases	O
of	O
a	O
TSS/370	B-Application
PRPQ	O
as	O
a	O
migration	O
path	O
for	O
its	O
TSS/360	B-Application
customers	O
,	O
and	O
then	O
dropped	O
it	O
.	O
</s>
<s>
The	O
traumas	O
of	O
producing	O
the	O
System/360	B-Application
operating	B-General_Concept
systems	I-General_Concept
gave	O
a	O
boost	O
to	O
the	O
emerging	O
discipline	O
of	O
software	B-General_Concept
engineering	I-General_Concept
,	O
the	O
attempt	O
to	O
apply	O
scientific	O
principles	O
to	O
the	O
development	O
of	O
software	O
,	O
and	O
the	O
management	O
of	O
software	O
projects	O
.	O
</s>
<s>
Frederick	O
P	O
.	O
Brooks	O
,	O
who	O
was	O
a	O
senior	O
project	O
manager	O
for	O
the	O
whole	O
System/360	B-Application
project	O
and	O
then	O
was	O
given	O
specific	O
responsibility	O
for	O
OS/360	B-Application
(	O
which	O
was	O
already	O
long	O
overdue	O
)	O
,	O
wrote	O
an	O
acclaimed	O
book	O
,	O
The	O
Mythical	O
Man-Month	O
,	O
based	O
on	O
the	O
problems	O
encountered	O
and	O
lessons	O
learned	O
during	O
the	O
project	O
,	O
two	O
of	O
which	O
were	O
:	O
</s>
<s>
Brooks	O
called	O
this	O
the	O
"	O
second-system	B-General_Concept
effect	I-General_Concept
"	O
,	O
and	O
cited	O
OS/360	B-Application
as	O
a	O
very	O
comprehensive	O
example	O
.	O
</s>
<s>
While	O
OS/360	B-Application
was	O
the	O
preferred	O
operating	B-General_Concept
system	I-General_Concept
for	O
the	O
higher-end	O
System/360	B-Application
machines	O
,	O
DOS/360	B-Application
was	O
the	O
usual	O
operating	B-General_Concept
system	I-General_Concept
for	O
the	O
less	O
powerful	O
machines	O
.	O
</s>
<s>
It	O
provided	O
a	O
set	O
of	O
utility	B-Application
programs	I-Application
,	O
a	O
macro	O
assembler	B-Language
,	O
and	O
compilers	B-Language
for	O
FORTRAN	B-Application
and	O
COBOL	B-Application
.	O
</s>
<s>
Support	O
for	O
RPG	B-Language
came	O
later	O
,	O
and	O
eventually	O
a	O
PL/I	B-Language
subset	O
.	O
</s>
<s>
And	O
it	O
supported	O
a	O
useful	O
range	O
of	O
file	O
organizations	O
with	O
access	B-Application
methods	I-Application
to	O
help	O
in	O
using	O
them	O
:	O
</s>
<s>
In	O
indexed	O
(	O
ISAM	B-Application
)	O
files	O
a	O
specified	O
section	O
of	O
each	O
record	O
was	O
defined	O
as	O
a	O
key	O
that	O
could	O
be	O
used	O
to	O
look	O
up	O
specific	O
records	O
.	O
</s>
<s>
In	O
direct	O
access	O
(	O
BDAM	B-Application
)	O
files	O
,	O
the	O
application	O
program	O
had	O
to	O
specify	O
the	O
physical	O
location	O
on	O
the	O
disk	O
of	O
the	O
data	O
it	O
wanted	O
to	O
access	O
.	O
</s>
<s>
BDAM	B-Application
programming	O
was	O
not	O
easy	O
and	O
most	O
customers	O
never	O
used	O
it	O
themselves	O
,	O
but	O
it	O
was	O
the	O
fastest	O
way	O
to	O
access	O
data	O
on	O
disks	O
and	O
many	O
software	O
companies	O
used	O
it	O
in	O
their	O
products	O
,	O
especially	O
database	B-Application
management	I-Application
systems	I-Application
such	O
as	O
ADABAS	B-Application
,	O
IDMS	B-Application
and	O
IBM	O
's	O
DL/I	B-Language
.	O
</s>
<s>
Sequential	O
and	O
ISAM	B-Application
files	O
could	O
store	O
either	O
fixed-length	O
or	O
variable-length	O
records	O
,	O
and	O
all	O
types	O
could	O
occupy	O
more	O
than	O
one	O
disk	O
volume	O
.	O
</s>
<s>
DOS/360	B-Application
also	O
offered	O
BTAM	B-Application
,	O
a	O
data	O
communications	O
facility	O
that	O
was	O
primitive	O
and	O
hard	O
to	O
use	O
by	O
today	O
's	O
standards	O
.	O
</s>
<s>
But	O
BTAM	B-Application
could	O
communicate	O
with	O
almost	O
any	O
type	O
of	O
terminal	O
,	O
which	O
was	O
a	O
big	O
advantage	O
at	O
a	O
time	O
when	O
there	O
was	O
hardly	O
any	O
standardization	O
of	O
communications	O
protocols	O
.	O
</s>
<s>
But	O
DOS/360	B-Application
had	O
significant	O
limitations	O
compared	O
with	O
OS/360	B-Application
,	O
which	O
was	O
used	O
to	O
control	O
most	O
larger	O
System/360	B-Application
machines	O
:	O
</s>
<s>
A	O
later	O
enhancement	O
allowed	O
3	O
at	O
the	O
same	O
time	O
,	O
in	O
one	O
of	O
3	O
"	O
partitions	O
"	O
whose	O
size	O
was	O
set	O
by	O
each	O
customer	O
when	O
DOS/360	B-Application
was	O
installed	O
.	O
</s>
<s>
The	O
JCL	B-Application
it	O
used	O
for	O
submitting	O
jobs	O
was	O
designed	O
to	O
be	O
easy	O
for	O
the	O
low-end	O
machines	O
to	O
process	O
,	O
and	O
as	O
a	O
result	O
,	O
programmers	O
did	O
not	O
find	O
it	O
easy	O
to	O
read	O
or	O
write	O
.	O
</s>
<s>
There	O
was	O
no	O
spooling	B-Device
sub-system	O
to	O
improve	O
the	O
efficiency	O
of	O
punched	B-Architecture
card	I-Architecture
and	O
printer	O
use	O
.	O
</s>
<s>
In	O
the	O
late	O
1960s	O
,	O
an	O
independent	O
software	O
company	O
started	O
selling	O
a	O
spooler	B-Device
called	O
GRASP	O
.	O
</s>
<s>
DOS/360	B-Application
had	O
no	O
relocating	B-Library
loader	I-Library
,	O
so	O
users	O
had	O
to	O
link	B-Application
edit	I-Application
a	O
separate	O
executable	O
version	O
of	O
each	O
program	O
for	O
each	O
partition	O
in	O
which	O
the	O
program	O
was	O
likely	O
to	O
be	O
run	O
.	O
</s>
<s>
When	O
the	O
Core	O
Image	O
Library	O
became	O
full	O
,	O
it	O
had	O
to	O
be	O
compressed	O
by	O
one	O
of	O
the	O
utility	B-Application
programs	I-Application
,	O
and	O
this	O
could	O
halt	O
development	O
work	O
for	O
as	O
much	O
as	O
half	O
a	O
day	O
.	O
</s>
<s>
Its	O
application	B-Application
programming	I-Application
interface	I-Application
was	O
different	O
from	O
that	O
of	O
OS/360	B-Application
.	O
</s>
<s>
DOS/360	B-Application
programs	O
written	O
in	O
high	B-Language
level	I-Language
languages	I-Language
such	O
as	O
COBOL	B-Application
needed	O
small	O
modifications	O
before	O
they	O
could	O
be	O
used	O
with	O
OS/360	B-Application
and	O
assembly	B-Language
language	I-Language
programs	O
needed	O
larger	O
changes	O
.	O
</s>
<s>
IBM	O
expected	O
that	O
DOS/360	B-Application
users	O
would	O
soon	O
upgrade	O
to	O
OS/360	B-Application
,	O
but	O
despite	O
its	O
limitations	O
,	O
DOS/360	B-Application
became	O
the	O
world	O
's	O
most	O
widely	O
used	O
operating	B-General_Concept
system	I-General_Concept
because	O
:	O
</s>
<s>
Most	O
of	O
these	O
cheaper	O
models	O
had	O
far	O
less	O
core	O
memory	O
than	O
required	O
by	O
OS/360	B-Application
.	O
</s>
<s>
DOS/360	B-Application
ran	O
well	O
on	O
the	O
System/360	B-Application
processors	O
which	O
medium-sized	O
organizations	O
could	O
afford	O
,	O
and	O
it	O
was	O
better	O
than	O
the	O
"	O
operating	B-General_Concept
systems	I-General_Concept
"	O
these	O
customers	O
had	O
before	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
its	O
descendant	O
z/VSE	B-Application
is	O
still	O
widely	O
used	O
today	O
,	O
as	O
of	O
2005	O
.	O
</s>
<s>
OS/360	B-Application
included	O
multiple	O
levels	O
of	O
support	O
,	O
a	O
single	O
API	B-Application
,	O
and	O
much	O
shared	O
code	O
.	O
</s>
<s>
The	O
same	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
,	O
so	O
application	O
programs	O
could	O
be	O
transferred	O
among	O
PCP	O
,	O
MFT	O
,	O
and	O
MVT	O
without	O
even	O
needing	O
re-compilation	B-Language
.	O
</s>
<s>
The	O
same	O
JCL	B-Application
,	O
which	O
was	O
more	O
flexible	O
and	O
easier	O
to	O
use	O
than	O
that	O
of	O
DOS/360	B-Application
.	O
</s>
<s>
The	O
same	O
facilities	O
(	O
access	B-Application
methods	I-Application
)	O
as	O
DOS/360	B-Application
for	O
reading	O
and	O
writing	O
files	O
(	O
sequential	O
,	O
indexed	O
,	O
and	O
direct	O
)	O
and	O
for	O
data	O
communications	O
(	O
BTAM	B-Application
)	O
.	O
</s>
<s>
An	O
additional	O
file	O
structure	O
,	O
partitioned	O
,	O
and	O
access	B-Application
method	I-Application
(	O
BPAM	B-Application
)	O
,	O
which	O
was	O
mainly	O
used	O
for	O
managing	O
program	O
libraries	O
.	O
</s>
<s>
Although	O
partitioned	O
files	O
needed	O
to	O
be	O
compressed	O
to	O
reclaim	O
free	O
space	O
,	O
this	O
seldom	O
halted	O
development	O
work	O
as	O
it	O
did	O
with	O
DOS/360	B-Application
'	O
s	O
Core	O
Image	O
Library	O
,	O
because	O
PCP	O
,	O
MFT	O
,	O
and	O
MVT	O
allowed	O
an	O
indefinite	O
number	O
of	O
partitioned	O
files	O
,	O
and	O
each	O
project	O
generally	O
had	O
at	O
least	O
one	O
.	O
</s>
<s>
A	O
spooling	B-Device
facility	O
(	O
which	O
DOS/360	B-Application
lacked	O
)	O
.	O
</s>
<s>
Experience	O
indicated	O
that	O
it	O
was	O
not	O
advisable	O
to	O
install	O
OS/360	B-Application
on	O
systems	O
with	O
less	O
than	O
256KB	O
of	O
memory	O
,	O
which	O
was	O
a	O
common	O
limitation	O
in	O
the	O
1960s	O
.	O
</s>
<s>
MVT	O
was	O
considerably	O
larger	O
and	O
more	O
complex	O
than	O
MFT	O
and	O
therefore	O
was	O
used	O
on	O
the	O
most	O
powerful	O
System/360	B-Application
CPUs	B-Device
.	O
</s>
<s>
It	O
treated	O
all	O
memory	O
not	O
used	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
as	O
a	O
single	O
pool	O
from	O
which	O
contiguous	O
"	O
regions	O
"	O
could	O
be	O
allocated	O
as	O
required	O
by	O
an	O
indefinite	O
number	O
of	O
simultaneous	O
application	O
programs	O
.	O
</s>
<s>
In	O
1971	O
the	O
Time	B-Application
Sharing	I-Application
Option	I-Application
(	O
TSO	O
)	O
for	O
use	O
with	O
MVT	O
was	O
added	O
.	O
</s>
<s>
TSO	O
became	O
widely	O
used	O
for	O
program	O
development	O
because	O
it	O
provided	O
:	O
an	O
editor	O
,	O
debuggers	B-Application
for	O
some	O
of	O
the	O
programming	O
languages	O
used	O
on	O
System/360	B-Application
,	O
and	O
the	O
ability	O
to	O
submit	O
batch	B-General_Concept
jobs	I-General_Concept
,	O
be	O
notified	O
of	O
their	O
completion	O
,	O
and	O
view	O
the	O
results	O
without	O
waiting	O
for	O
printed	O
reports	O
.	O
</s>
<s>
TSO	O
communicated	O
with	O
terminals	O
by	O
using	O
TCAM	O
(	O
Telecommunications	B-Application
Access	I-Application
Method	I-Application
)	O
,	O
which	O
eventually	O
replaced	O
the	O
earlier	O
Queued	B-Application
Telecommunications	I-Application
Access	I-Application
Method	I-Application
(	O
QTAM	B-Application
)	O
.	O
</s>
<s>
TCAM	O
's	O
name	O
suggests	O
that	O
IBM	O
hoped	O
it	O
would	O
become	O
the	O
standard	O
access	B-Application
method	I-Application
for	O
data	O
communications	O
,	O
but	O
in	O
fact	O
,	O
TCAM	O
was	O
used	O
almost	O
entirely	O
for	O
TSO	O
and	O
was	O
largely	O
superseded	O
by	O
VTAM	B-Application
from	O
the	O
late	O
1970s	O
onwards	O
.	O
</s>
<s>
System/360	B-Application
'	O
s	O
hardware	O
and	O
operating	B-General_Concept
systems	I-General_Concept
were	O
designed	O
for	O
processing	O
batch	B-General_Concept
jobs	I-General_Concept
which	O
in	O
extreme	O
cases	O
might	O
run	O
for	O
hours	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
they	O
were	O
unsuitable	O
for	O
transaction	B-General_Concept
processing	I-General_Concept
,	O
in	O
which	O
there	O
are	O
thousands	O
of	O
units	O
of	O
work	O
per	O
day	O
and	O
each	O
takes	O
between	O
30	O
seconds	O
and	O
a	O
very	O
few	O
minutes	O
.	O
</s>
<s>
In	O
1968	O
IBM	O
released	O
IMS	B-Application
to	O
handle	O
transaction	B-General_Concept
processing	I-General_Concept
,	O
and	O
in	O
1969	O
it	O
released	O
CICS	B-Application
,	O
a	O
simpler	O
transaction	B-Application
processing	I-Application
system	I-Application
which	O
a	O
group	O
of	O
IBM	O
's	O
staff	O
had	O
developed	O
for	O
a	O
customer	O
.	O
</s>
<s>
IMS	B-Application
was	O
only	O
available	O
for	O
OS/360	B-Application
and	O
its	O
successors	O
,	O
but	O
CICS	B-Application
was	O
also	O
available	O
for	O
DOS/360	B-Application
and	O
its	O
successors	O
.	O
</s>
<s>
Strictly	O
speaking	O
TP	O
monitors	O
were	O
not	O
operating	B-General_Concept
system	I-General_Concept
components	O
but	O
application	O
programs	O
which	O
managed	O
other	O
application	O
programs	O
.	O
</s>
<s>
In	O
the	O
1970s	O
and	O
1980s	O
,	O
several	O
third-party	O
TP	O
monitors	O
competed	O
with	O
CICS	B-Application
(	O
notably	O
COM-PLETE	O
,	O
DATACOM/DC	O
,	O
ENVIRON/1	O
,	O
INTERCOMM	O
,	O
SHADOW	O
II	O
,	O
TASK/MASTER	O
and	O
WESTI	O
)	O
,	O
but	O
IBM	O
gradually	O
improved	O
CICS	B-Application
to	O
the	O
point	O
where	O
most	O
customers	O
abandoned	O
the	O
alternatives	O
.	O
</s>
<s>
In	O
the	O
1950s	O
airlines	O
were	O
expanding	O
rapidly	O
but	O
this	O
growth	O
was	O
held	O
back	O
by	O
the	O
difficulty	O
of	O
handling	O
thousands	O
of	O
bookings	O
manually	O
(	O
using	O
card	B-Architecture
files	O
)	O
.	O
</s>
<s>
In	O
1957	O
IBM	O
signed	O
a	O
development	O
contract	O
with	O
American	O
Airlines	O
for	O
the	O
development	O
of	O
a	O
computerized	O
reservations	O
system	O
,	O
which	O
became	O
known	O
as	O
SABRE	B-Application
.	O
</s>
<s>
The	O
first	O
experimental	O
system	O
went	O
live	O
in	O
1960	O
and	O
the	O
system	O
took	O
over	O
all	O
booking	O
functions	O
in	O
1964in	O
both	O
cases	O
using	O
IBM	B-Device
7090	I-Device
mainframes	B-Architecture
.	O
</s>
<s>
In	O
the	O
early	O
1960s	O
IBM	O
undertook	O
similar	O
projects	O
for	O
other	O
airlines	O
and	O
soon	O
decided	O
to	O
produce	O
a	O
single	O
standard	O
booking	O
system	O
,	O
PARS	B-Application
,	O
to	O
run	O
on	O
System/360	B-Application
computers	O
.	O
</s>
<s>
In	O
SABRE	B-Application
and	O
early	O
versions	O
of	O
PARS	B-Application
there	O
was	O
no	O
separation	O
between	O
the	O
application	O
and	O
operating	B-General_Concept
system	I-General_Concept
components	O
of	O
the	O
software	O
,	O
but	O
in	O
1968	O
IBM	O
divided	O
it	O
into	O
PARS	B-Application
(	O
application	O
)	O
and	O
ACP	B-Application
(	O
operating	B-General_Concept
system	I-General_Concept
)	O
.	O
</s>
<s>
Later	O
versions	O
of	O
ACP	B-Application
were	O
named	O
ACP	B-Application
/	O
TPF	B-Operating_System
and	O
then	O
TPF	B-Operating_System
(	O
Transaction	B-Operating_System
Processing	I-Operating_System
Facility	I-Operating_System
)	O
as	O
non-airline	O
businesses	O
adopted	O
this	O
operating	B-General_Concept
system	I-General_Concept
for	O
handling	O
large	O
volumes	O
of	O
online	O
transactions	O
.	O
</s>
<s>
The	O
latest	O
version	O
is	O
z/TPF	B-Operating_System
.	O
</s>
<s>
IBM	O
developed	O
ACP	B-Application
and	O
its	O
successors	O
because	O
:	O
in	O
the	O
mid-1960s	O
IBM	O
's	O
standard	O
operating	B-General_Concept
systems	I-General_Concept
(	O
DOS/360	B-Application
and	O
OS/360	B-Application
)	O
were	O
batch-oriented	B-General_Concept
and	O
could	O
not	O
handle	O
large	O
numbers	O
of	O
short	O
transactions	O
quickly	O
enough	O
;	O
even	O
its	O
transaction	O
monitors	O
IMS	B-Application
and	O
CICS	B-Application
,	O
which	O
run	O
under	O
the	O
control	O
of	O
standard	O
general-purpose	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
,	O
are	O
not	O
fast	O
enough	O
for	O
handling	O
reservations	O
on	O
hundreds	O
of	O
flights	O
from	O
thousands	O
of	O
travel	O
agents	O
.	O
</s>
<s>
The	O
last	O
"	O
public	O
domain	O
"	O
version	O
of	O
ACP	B-Application
,	O
hence	O
its	O
last	O
"	O
free	O
"	O
version	O
,	O
was	O
ACP	B-Application
9.2	O
,	O
which	O
was	O
distributed	O
on	O
a	O
single	O
mini-reel	O
with	O
an	O
accompanying	O
manual	O
set	O
(	O
about	O
two	O
dozen	O
manuals	O
,	O
which	O
occupied	O
perhaps	O
48	O
lineal	O
inches	O
of	O
shelf	O
space	O
)	O
and	O
which	O
could	O
be	O
restored	O
to	O
IBM	O
3340	O
disk	O
drives	O
and	O
which	O
would	O
,	O
thereby	O
,	O
provide	O
a	O
fully	O
functional	O
ACP	B-Application
system	O
.	O
</s>
<s>
ACP	B-Application
9.2	O
was	O
intended	O
,	O
primarily	O
,	O
for	O
bank	O
cards	O
like	O
MasterCard	O
and	O
other	O
financial	O
applications	O
,	O
but	O
it	O
could	O
also	O
be	O
utilized	O
for	O
airline	O
reservation	O
systems	O
,	O
too	O
,	O
as	O
by	O
this	O
time	O
ACP	B-Application
had	O
become	O
a	O
more	O
general-purpose	O
OS	O
.	O
</s>
<s>
ACP	B-Application
had	O
by	O
then	O
incorporated	O
a	O
hypervisor	B-Operating_System
module	O
(	O
CHYR	O
)	O
which	O
supported	O
a	O
virtual	O
OS	O
(	O
usually	O
VS1	B-Application
,	O
but	O
possibly	O
also	O
VS2	B-Application
)	O
as	O
a	O
guest	O
,	O
with	O
which	O
program	O
development	O
or	O
file	O
maintenance	O
could	O
be	O
accomplished	O
concurrently	O
with	O
the	O
online	O
functions	O
.	O
</s>
<s>
In	O
some	O
instances	O
,	O
production	O
work	O
was	O
run	O
under	O
VS2	B-Application
under	O
the	O
hypervisor	B-Operating_System
,	O
including	O
,	O
possibly	O
,	O
IMS	B-Application
DB	O
.	O
</s>
<s>
The	O
Model	B-Device
20	I-Device
was	O
labeled	O
as	O
part	O
of	O
the	O
System/360	B-Application
range	O
because	O
it	O
could	O
be	O
connected	O
to	O
some	O
of	O
the	O
same	O
peripherals	O
,	O
but	O
it	O
was	O
a	O
16-bit	B-Device
machine	O
and	O
not	O
entirely	O
program-compatible	O
with	O
other	O
members	O
of	O
the	O
System/360	B-Application
range	O
.	O
</s>
<s>
Three	O
operating	B-General_Concept
systems	I-General_Concept
were	O
developed	O
by	O
IBM	O
's	O
labs	O
in	O
Germany	O
,	O
for	O
different	O
360/20	B-Device
configurations	O
;	O
DPSwith	O
disks	O
(	O
minimum	O
memory	O
required	O
:	O
12KB	O
)	O
;	O
TPSno	O
disk	O
but	O
with	O
tapes	O
(	O
minimum	O
memory	O
required	O
:	O
8KB	O
)	O
;	O
and	O
CPSpunched-card-based	O
(	O
minimum	O
memory	O
required	O
:	O
4KB	O
)	O
.	O
</s>
<s>
These	O
had	O
no	O
direct	O
successors	O
since	O
IBM	O
introduced	O
the	O
System/3	B-Device
range	O
of	O
small	O
business	O
computers	O
in	O
1969	O
and	O
System/3	B-Device
had	O
a	O
different	O
internal	O
design	O
from	O
the	O
360/20	B-Device
and	O
different	O
peripherals	O
from	O
IBM	O
's	O
mainframes	B-Architecture
.	O
</s>
<s>
The	O
360/44	B-Device
is	O
another	O
processor	O
that	O
uses	O
the	O
System/360	B-Application
peripherals	O
but	O
has	O
a	O
modified	O
instruction	O
set	O
.	O
</s>
<s>
It	O
was	O
designed	O
for	O
scientific	O
computation	O
using	O
floating	B-Algorithm
point	I-Algorithm
numbers	I-Algorithm
,	O
such	O
as	O
geological	O
or	O
meteorological	O
analyses	O
.	O
</s>
<s>
Because	O
of	O
the	O
internal	O
differences	O
and	O
the	O
specialized	O
type	O
of	O
work	O
for	O
which	O
it	O
was	O
designed	O
,	O
the	O
360/44	B-Device
has	O
its	O
own	O
operating	B-General_Concept
system	I-General_Concept
,	O
PS/44	O
.	O
</s>
<s>
An	O
optional	O
feature	O
allows	O
a	O
System/360	B-Application
emulator	B-Application
to	O
run	O
in	O
hidden	O
storage	O
and	O
implement	O
the	O
missing	O
instructions	O
in	O
order	O
to	O
run	O
OS/360	B-Application
.	O
</s>
<s>
The	O
360/44	B-Device
and	O
PS/44	O
have	O
no	O
direct	O
successors	O
.	O
</s>
<s>
System/370	B-Device
was	O
announced	O
in	O
1970	O
with	O
essentially	O
the	O
same	O
facilities	O
as	O
System/360	B-Application
but	O
with	O
about	O
4	O
times	O
the	O
processor	O
speeds	O
of	O
similarly-priced	O
System/360	B-Application
CPUs	B-Device
.	O
</s>
<s>
Then	O
in	O
1972	O
IBM	O
announced	O
"	O
System/370	B-Device
Advanced	O
Functions	O
"	O
,	O
of	O
which	O
the	O
main	O
item	O
was	O
that	O
future	O
sales	O
of	O
System/370	B-Device
would	O
include	O
virtual	B-Architecture
memory	I-Architecture
capability	O
and	O
this	O
could	O
also	O
be	O
retro-fitted	O
to	O
existing	O
System/370	B-Device
CPUs	B-Device
.	O
</s>
<s>
Hence	O
IBM	O
also	O
committed	O
to	O
delivering	O
enhanced	O
operating	B-General_Concept
systems	I-General_Concept
which	O
could	O
support	O
the	O
use	O
of	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
Most	O
of	O
the	O
new	O
operating	B-General_Concept
systems	I-General_Concept
are	O
distinguished	O
from	O
their	O
predecessors	O
by	O
the	O
presence	O
of	O
"	O
/VS	O
"	O
in	O
their	O
names	O
.	O
</s>
<s>
"	O
VS	O
"	O
stands	O
for	O
"	O
Virtual	B-Architecture
Storage	I-Architecture
"	O
.	O
</s>
<s>
IBM	O
avoided	O
the	O
term	O
"	O
virtual	B-Architecture
memory	I-Architecture
"	O
,	O
allegedly	O
because	O
the	O
word	O
"	O
memory	O
"	O
might	O
be	O
interpreted	O
to	O
imply	O
that	O
IBM	O
computers	O
could	O
forget	O
things	O
.	O
</s>
<s>
All	O
modern	O
IBM	B-Device
mainframe	I-Device
operating	B-General_Concept
systems	I-General_Concept
except	O
z/TPF	B-Operating_System
are	O
descendants	O
of	O
those	O
included	O
in	O
the	O
"	O
System/370	B-Device
Advanced	O
Functions	O
"	O
announcementz/TPF	O
is	O
a	O
descendant	O
of	O
ACP	B-Application
,	O
the	O
system	O
which	O
IBM	O
initially	O
developed	O
to	O
support	O
high-volume	O
airline	O
reservations	O
applications	O
.	O
</s>
<s>
DOS/VS	B-Application
is	O
the	O
successor	O
to	O
DOS/360	B-Application
,	O
and	O
offers	O
similar	O
facilities	O
,	O
with	O
the	O
addition	O
of	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
In	O
addition	O
to	O
virtual	B-Architecture
memory	I-Architecture
DOS/VS	B-Application
provided	O
other	O
enhancements	O
:	O
</s>
<s>
A	O
relocating	B-Library
loader	I-Library
,	O
so	O
that	O
it	O
is	O
no	O
longer	O
necessary	O
to	O
link-edit	O
a	O
separate	O
copy	O
of	O
each	O
program	O
for	O
each	O
partition	O
in	O
which	O
it	O
is	O
to	O
run	O
.	O
</s>
<s>
An	O
improved	O
spooling	B-Device
component	O
,	O
POWER/VS	O
.	O
</s>
<s>
DOS/VS	B-Application
was	O
followed	O
by	O
significant	O
upgrades	O
:	O
DOS/VSE	B-Application
and	O
VSE/SP	O
(	O
1980s	O
)	O
,	O
VSE/ESA	O
(	O
1991	O
)	O
,	O
and	O
z/VSE	B-Application
(	O
2005	O
)	O
.	O
</s>
<s>
OS/VS1	B-Application
succeeded	O
MFT	O
,	O
with	O
similar	O
facilities	O
,	O
and	O
adding	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
IBM	O
released	O
fairly	O
minor	O
enhancements	O
of	O
OS/VS1	B-Application
until	O
1983	O
,	O
and	O
in	O
1984	O
announced	O
that	O
there	O
would	O
be	O
no	O
more	O
.	O
</s>
<s>
OS/VS1	B-Application
and	O
TSS/370	B-Application
are	O
the	O
only	O
IBM	B-Device
System/370	I-Device
operating	B-General_Concept
systems	I-General_Concept
that	O
do	O
not	O
have	O
modern	O
descendants	O
.	O
</s>
<s>
The	O
Special	B-Application
Real	I-Application
Time	I-Application
Operating	I-Application
System	I-Application
(	O
SRTOS	B-Application
)	O
,	O
Programming	O
RPQ	O
Z06751	O
,	O
is	O
a	O
variant	O
of	O
OS/VS1	B-Application
extended	O
to	O
support	O
real-time	B-General_Concept
computing	I-General_Concept
.	O
</s>
<s>
OS/VS2	B-Application
Release	O
1	O
(	O
SVS	B-Application
)	O
is	O
a	O
replacement	O
for	O
MVT	O
with	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
In	O
1974	O
IBM	O
released	O
what	O
it	O
described	O
as	O
OS/VS2	B-Application
Release	O
2	O
but	O
which	O
is	O
a	O
major	O
rewrite	O
that	O
was	O
upwards-compatible	O
with	O
the	O
earlier	O
OS/VS2	B-Application
SVS	B-Application
.	O
</s>
<s>
Different	O
applications	O
thought	O
they	O
were	O
using	O
the	O
same	O
range	O
of	O
virtual	O
addresses	O
,	O
but	O
the	O
new	O
system	O
's	O
virtual	B-Architecture
memory	I-Architecture
facilities	O
mapped	O
these	O
to	O
different	O
ranges	O
of	O
real	O
memory	O
addresses	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
the	O
new	O
system	O
rapidly	O
became	O
known	O
as	O
"	O
MVS	B-Application
"	O
(	O
Multiple	B-Application
Virtual	I-Application
Storages	I-Application
)	O
,	O
the	O
original	O
OS/VS2	B-Application
became	O
known	O
as	O
"	O
SVS	B-Application
"	O
(	O
Single	O
Virtual	B-Architecture
Storage	I-Architecture
)	O
.	O
</s>
<s>
IBM	O
itself	O
accepted	O
this	O
terminology	O
and	O
labelled	O
MVS	B-Application
's	O
successors	O
"	O
MVS/...	O
"	O
.	O
</s>
<s>
The	O
other	O
distinctive	O
features	O
of	O
MVS	B-Application
are	O
:	O
its	O
main	O
catalog	O
must	O
be	O
a	O
VSAM	O
catalog	O
;	O
it	O
supports	O
"	B-Operating_System
tightly-coupled	I-Operating_System
multiprocessing	I-Operating_System
"	I-Operating_System
(	O
2	O
or	O
more	O
CPUs	B-Device
share	O
the	O
same	O
memory	O
and	O
copy	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
)	O
;	O
it	O
includes	O
a	O
System	O
Resources	O
Manager	O
(	O
renamed	O
Workload	O
Manager	O
in	O
later	O
versions	O
)	O
which	O
allows	O
users	O
to	O
load	O
additional	O
work	O
on	O
to	O
the	O
system	O
without	O
reducing	O
the	O
performance	O
of	O
high-priority	O
jobs	O
.	O
</s>
<s>
IBM	O
has	O
released	O
several	O
MVS	B-Application
upgrades	O
:	O
MVS/SE	O
,	O
MVS/SP	O
Version	O
1	O
,	O
MVS/XA	O
(	O
1981	O
)	O
,	O
MVS/ESA	O
(	O
1985	O
)	O
,	O
OS/390	B-Application
(	O
1996	O
)	O
and	O
currently	O
z/OS	B-Application
(	O
2001	O
)	O
.	O
</s>
<s>
VM/370	B-Application
combines	O
a	O
virtual	B-Architecture
machine	I-Architecture
facility	O
with	O
a	O
single-user	O
system	O
called	O
Conversational	B-Operating_System
Monitor	I-Operating_System
System	I-Operating_System
(	O
CMS	O
)	O
;	O
this	O
combination	O
provides	O
time-sharing	B-General_Concept
by	O
allowing	O
each	O
user	O
to	O
run	O
a	O
copy	O
of	O
CMS	O
on	O
a	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
This	O
combination	O
was	O
a	O
direct	O
descendant	O
of	O
CP/CMS	B-Application
.	O
</s>
<s>
The	O
virtual	B-Architecture
machine	I-Architecture
facility	O
was	O
often	O
used	O
for	O
testing	O
new	O
software	O
while	O
normal	O
production	O
work	O
continues	O
on	O
another	O
virtual	B-Architecture
machine	I-Architecture
,	O
and	O
the	O
CMS	O
timesharing	B-General_Concept
system	O
was	O
widely	O
used	O
for	O
program	O
development	O
.	O
</s>
<s>
VM/370	B-Application
was	O
followed	O
by	O
a	O
series	O
of	O
upgrades	O
:	O
VM/SEPP	O
(	O
"	O
Systems	O
Extensions	O
Program	O
Product	O
"	O
)	O
,	O
VM/BSEPP	O
(	O
"	O
Basic	O
Systems	O
Extensions	O
Program	O
Product	O
"	O
)	O
,	O
VM/SP	B-Application
(	O
System	O
Product	O
)	O
,	O
VM/SP	B-Application
HPO	O
(	O
"	O
High	O
Performance	O
Option	O
"	O
)	O
,	O
VM/XA	O
MA	O
(	O
"	O
Extended	O
Architecture	O
Migration	O
Aid	O
"	O
)	O
,	O
VM/XA	O
SF	O
(	O
"	O
Extended	O
Architecture	O
System	O
Facility	O
"	O
)	O
,	O
VM/XA	O
SP	O
(	O
"	O
Extended	O
Architecture	O
System	O
Product	O
"	O
)	O
,	O
VM/ESA	B-Application
(	O
"	O
Enterprise	O
Systems	O
Architecture	O
"	O
)	O
,	O
and	O
z/VM	B-Application
.	O
</s>
<s>
IBM	O
also	O
produced	O
optional	O
microcode	B-Device
assists	O
for	O
VM	B-Application
and	O
successors	O
,	O
to	O
speed	O
up	O
the	O
hypervisor	B-Operating_System
's	O
emulation	B-Application
of	O
privileged	O
instructions	O
(	O
those	O
which	O
only	O
operating	B-General_Concept
systems	I-General_Concept
can	O
use	O
)	O
on	O
behalf	O
of	O
"	O
guest	O
"	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
As	O
part	O
of	O
370/Extended	O
Architecture	O
,	O
IBM	O
added	O
the	O
Start	O
Interpretive	O
Execution	O
(	O
SIE	O
)	O
instruction	O
to	O
allow	O
a	O
further	O
speedup	O
of	O
the	O
CP	O
hypervisor	B-Operating_System
.	O
</s>
