<s>
Multiple	B-Application
Virtual	I-Application
Storage	I-Application
,	O
more	O
commonly	O
called	O
MVS	B-Application
,	O
is	O
the	O
most	O
commonly	O
used	O
operating	B-General_Concept
system	I-General_Concept
on	O
the	O
System/370	B-Device
,	O
System/390	B-Device
and	O
IBM	B-Device
Z	I-Device
IBM	B-Device
mainframe	I-Device
computers	I-Device
.	O
</s>
<s>
IBM	O
developed	O
MVS	B-Application
,	O
along	O
with	O
OS/VS1	B-Application
and	O
SVS	B-Application
,	O
as	O
a	O
successor	O
to	O
OS/360	B-Application
.	O
</s>
<s>
It	O
is	O
unrelated	O
to	O
IBM	O
's	O
other	O
mainframe	O
operating	B-General_Concept
system	I-General_Concept
lines	O
,	O
e.g.	O
,	O
VSE	B-Application
,	O
VM	B-Application
,	O
TPF	B-Operating_System
.	O
</s>
<s>
First	O
released	O
in	O
1974	O
,	O
MVS	B-Application
was	O
extended	O
by	O
program	O
products	O
with	O
new	O
names	O
multiple	O
times	O
,	O
retaining	O
the	O
term	O
MVS	B-Application
in	O
the	O
nomenclature	O
:	O
</s>
<s>
first	O
to	O
MVS/SE	O
(	O
MVS/System	O
Extensions	O
)	O
,	O
</s>
<s>
next	O
to	O
MVS/SP	O
(	O
MVS/System	O
Product	O
)	O
Version	O
1	O
,	O
</s>
<s>
next	O
to	O
MVS/XA	O
(	O
MVS/eXtended	O
Architecture	O
)	O
,	O
</s>
<s>
next	O
to	O
MVS/ESA	O
(	O
MVS/Enterprise	O
Systems	O
Architecture	O
)	O
,	O
</s>
<s>
finally	O
to	O
z/OS	B-Application
(	O
when	O
64-bit	B-Device
support	I-Device
was	O
added	O
with	O
the	O
zSeries	O
models	O
)	O
.	O
</s>
<s>
IBM	O
added	O
UNIX	B-Application
support	O
(	O
originally	O
called	O
OpenEdition	B-Operating_System
MVS	I-Operating_System
)	O
in	O
MVS/SP	O
V4.3	O
and	O
has	O
obtained	O
POSIX	O
and	O
UNIX™	B-Application
certifications	O
at	O
several	O
different	O
levels	O
from	O
IEEE	O
,	O
X/Open	O
and	O
The	B-Application
Open	I-Application
Group	I-Application
.	O
</s>
<s>
The	O
MVS	B-Application
core	O
remains	O
fundamentally	O
the	O
same	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
By	O
design	O
,	O
programs	O
written	O
for	O
MVS	B-Application
run	O
on	O
z/OS	B-Application
without	O
modification	O
.	O
</s>
<s>
At	O
first	O
IBM	O
described	O
MVS	B-Application
as	O
simply	O
a	O
new	O
release	O
of	O
OS/VS2	B-Application
,	O
but	O
it	O
is	O
,	O
in	O
fact	O
a	O
major	O
rewrite	O
.	O
</s>
<s>
OS/VS2	B-Application
release	O
1	O
is	O
an	O
upgrade	O
of	O
OS/360	B-Application
MVT	O
that	O
retained	O
most	O
of	O
the	O
original	O
code	O
and	O
,	O
like	O
MVT	O
,	O
is	O
mainly	O
written	O
in	O
assembly	B-Language
language	I-Language
.	O
</s>
<s>
The	O
MVS	B-Application
core	O
is	O
almost	O
entirely	O
written	O
in	O
Assembler	B-Language
XF	O
,	O
although	O
a	O
few	O
modules	O
were	O
written	O
in	O
PL/S	B-Language
,	O
but	O
not	O
the	O
performance-sensitive	O
ones	O
,	O
in	O
particular	O
not	O
the	O
Input/Output	B-Application
Supervisor	I-Application
(	O
IOS	B-Application
)	O
.	O
</s>
<s>
IBM	O
's	O
use	O
of	O
"	O
OS/VS2	B-Application
"	O
emphasized	O
upwards	O
compatibility	O
:	O
application	O
programs	O
that	O
ran	O
under	O
MVT	O
did	O
not	O
even	O
need	O
recompiling	O
to	O
run	O
under	O
MVS	B-Application
.	O
</s>
<s>
The	O
same	O
Job	B-Application
Control	I-Application
Language	I-Application
files	O
could	O
be	O
used	O
unchanged	O
;	O
utilities	O
and	O
other	O
non-core	O
facilities	O
like	O
TSO	B-Application
ran	O
unchanged	O
.	O
</s>
<s>
IBM	O
and	O
users	O
almost	O
unanimously	O
called	O
the	O
new	O
system	O
MVS	B-Application
from	O
the	O
start	O
,	O
and	O
IBM	O
continued	O
to	O
use	O
the	O
term	O
MVS	B-Application
in	O
the	O
naming	O
of	O
later	O
major	O
versions	O
such	O
as	O
MVS/XA	O
.	O
</s>
<s>
OS/360	B-Application
MFT	O
(	O
Multiprogramming	O
with	O
a	O
Fixed	O
number	O
of	O
Tasks	O
)	O
provides	O
multiprogramming	O
:	O
several	O
memory	O
partitions	B-Device
,	O
each	O
of	O
a	O
fixed	O
size	O
,	O
are	O
set	O
up	O
when	O
the	O
operating	B-General_Concept
system	I-General_Concept
is	O
installed	O
and	O
when	O
the	O
operator	O
redefines	O
them	O
.	O
</s>
<s>
For	O
example	O
,	O
there	O
could	O
be	O
a	O
small	O
partition	B-Device
,	O
two	O
medium	O
partitions	B-Device
,	O
and	O
a	O
large	O
partition	B-Device
.	O
</s>
<s>
If	O
there	O
were	O
two	O
large	O
programs	O
ready	O
to	O
run	O
,	O
one	O
would	O
have	O
to	O
wait	O
until	O
the	O
other	O
finished	O
and	O
vacated	O
the	O
large	O
partition	B-Device
.	O
</s>
<s>
OS/360	B-Application
R19	O
added	O
MFT	O
subtasking	O
(	O
multitasking	B-Operating_System
)	O
,	O
the	O
ability	O
for	O
a	O
job	O
to	O
dynamically	B-General_Concept
creat	O
new	O
tasks	O
with	O
the	O
ATTACH	O
macro	O
..	O
</s>
<s>
OS/360	B-Application
MVT	O
(	O
Multiprogramming	O
with	O
a	O
Variable	O
number	O
of	O
Tasks	O
)	O
was	O
an	O
enhancement	O
that	O
further	O
refined	O
memory	O
use	O
.	O
</s>
<s>
Instead	O
of	O
using	O
fixed-size	O
memory	O
partitions	B-Device
,	O
MVT	O
allocates	O
memory	O
to	O
regions	O
for	O
job	O
steps	O
as	O
needed	O
,	O
provided	O
enough	O
contiguous	O
physical	O
memory	O
is	O
available	O
.	O
</s>
<s>
This	O
is	O
a	O
significant	O
advance	O
over	O
MFT	O
's	O
memory	B-General_Concept
management	I-General_Concept
,	O
but	O
has	O
some	O
weaknesses	O
:	O
if	O
a	O
job	O
allocates	O
memory	O
dynamically	B-General_Concept
(	O
as	O
most	O
sort	B-Algorithm
programs	O
and	O
database	B-Application
management	I-Application
systems	I-Application
do	O
)	O
,	O
the	O
programmers	O
has	O
to	O
estimate	O
the	O
job	O
's	O
maximum	O
memory	O
requirement	O
and	O
pre-define	O
it	O
for	O
MVT	O
.	O
</s>
<s>
Most	O
seriously	O
,	O
memory	O
can	O
become	O
fragmented	B-Architecture
,	O
i.e.	O
,	O
the	O
memory	O
not	O
used	O
by	O
current	O
jobs	O
could	O
be	O
divided	O
into	O
uselessly	O
small	O
chunks	O
between	O
the	O
areas	O
used	O
by	O
current	O
jobs	O
,	O
and	O
the	O
only	O
remedy	O
was	O
to	O
wait	O
until	O
some	O
current	O
jobs	O
finished	O
before	O
starting	O
any	O
new	O
ones	O
.	O
</s>
<s>
In	O
the	O
early	O
1970s	O
IBM	O
sought	O
to	O
mitigate	O
these	O
difficulties	O
by	O
introducing	O
virtual	B-Architecture
memory	I-Architecture
(	O
which	O
IBM	O
called	O
"	O
virtual	B-Architecture
storage	I-Architecture
"	O
)	O
,	O
which	O
allowed	O
programs	O
to	O
request	O
address	B-General_Concept
spaces	I-General_Concept
larger	O
than	O
physical	O
memory	O
.	O
</s>
<s>
The	O
original	O
implementations	O
had	O
a	O
single	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
,	O
shared	O
by	O
all	O
jobs	O
.	O
</s>
<s>
OS/VS1	B-Application
is	O
OS/360	B-Application
MFT	O
within	O
a	O
single	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
;	O
OS/VS2	B-Application
SVS	I-Application
was	O
OS/360	B-Application
MVT	O
within	O
a	O
single	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
.	O
</s>
<s>
So	O
OS/VS1	B-Application
and	O
SVS	B-Application
in	O
principle	O
had	O
the	O
same	O
disadvantages	O
as	O
MFT	O
and	O
MVT	O
,	O
but	O
the	O
impacts	O
are	O
less	O
severe	O
because	O
jobs	O
and	O
operators	O
could	O
request	O
much	O
larger	O
partitions	B-Device
with	O
a	O
2	O
KiB	O
granularity	O
(	O
for	O
OS/VS1	B-Application
)	O
or	O
regions	O
with	O
a	O
4	O
KiB	O
granularity	O
(	O
for	O
SVS	B-Application
)	O
,	O
and	O
the	O
requests	O
came	O
out	O
of	O
a	O
16MiB	O
address	B-General_Concept
space	I-General_Concept
even	O
if	O
physical	O
storage	O
was	O
smaller	O
.	O
</s>
<s>
As	O
in	O
OS/360	B-Application
MVT	O
,	O
TSO	B-Application
users	O
in	O
SVS	B-Application
are	O
assigned	O
to	O
a	O
TSO	B-Application
region	O
during	O
login	O
processing	O
and	O
competed	O
with	O
other	O
users	O
assigned	O
to	O
the	O
same	O
region	O
,	O
with	O
essentially	O
the	O
same	O
swap-in	O
and	O
swap-out	O
logic	O
as	O
TSO	B-Application
in	O
MVT	O
.	O
</s>
<s>
In	O
the	O
mid-1970s	O
IBM	O
introduced	O
MVS	B-Application
,	O
which	O
not	O
only	O
supported	O
virtual	B-Architecture
storage	I-Architecture
that	O
was	O
larger	O
than	O
the	O
available	O
real	O
storage	O
,	O
as	O
did	O
SVS	B-Application
,	O
but	O
also	O
allowed	O
an	O
indefinite	O
number	O
of	O
applications	O
to	O
run	O
in	O
different	O
address	B-General_Concept
spaces	I-General_Concept
.	O
</s>
<s>
Two	O
concurrent	O
programs	O
might	O
try	O
to	O
access	O
the	O
same	O
virtual	B-General_Concept
memory	I-General_Concept
address	I-General_Concept
,	O
but	O
the	O
virtual	B-Architecture
memory	I-Architecture
system	O
redirected	O
these	O
requests	O
to	O
different	O
areas	O
of	O
physical	O
memory	O
.	O
</s>
<s>
Each	O
of	O
these	O
address	B-General_Concept
spaces	I-General_Concept
consisted	O
of	O
three	O
areas	O
:	O
an	O
operating	B-General_Concept
system	I-General_Concept
(	O
one	O
instance	O
shared	O
by	O
all	O
jobs	O
)	O
,	O
an	O
application	O
area	O
unique	O
for	O
each	O
application	O
,	O
and	O
a	O
shared	O
virtual	O
area	O
used	O
for	O
various	O
purposes	O
,	O
including	O
inter-job	O
communication	O
.	O
</s>
<s>
This	O
made	O
MVS	B-Application
the	O
perfect	O
solution	O
for	O
business	O
problems	O
that	O
resulted	O
from	O
the	O
need	O
to	O
run	O
more	O
applications	O
.	O
</s>
<s>
MVS	B-Application
maximized	O
processing	O
potential	O
by	O
providing	O
multiprogramming	O
and	O
multiprocessing	B-Operating_System
capabilities	O
.	O
</s>
<s>
Like	O
its	O
MVT	O
and	O
OS/VS2	B-Application
SVS	I-Application
predecessors	O
,	O
MVS	B-Application
supported	O
multiprogramming	O
;	O
program	O
instructions	O
and	O
associated	O
data	O
are	O
scheduled	O
by	O
a	O
control	O
program	O
and	O
given	O
processing	O
cycles	O
.	O
</s>
<s>
Unlike	O
a	O
single-programming	O
operating	B-General_Concept
system	I-General_Concept
,	O
these	O
systems	O
maximize	O
the	O
use	O
of	O
the	O
processing	O
potential	O
by	O
dividing	O
processing	O
cycles	O
among	O
the	O
instructions	O
associated	O
with	O
several	O
different	O
concurrently	O
running	O
programs	O
.	O
</s>
<s>
Early	O
editions	O
of	O
MVS	B-Application
(	O
mid-1970s	O
)	O
are	O
among	O
the	O
first	O
of	O
the	O
IBM	O
OS	O
series	O
to	O
support	O
multiprocessor	B-Operating_System
configurations	O
,	O
though	O
the	O
M65MP	O
variant	O
of	O
OS/360	B-Application
running	O
on	O
360	O
Models	O
65	O
and	O
67	B-Device
had	O
provided	O
limited	O
multiprocessor	B-Operating_System
support	O
.	O
</s>
<s>
The	O
360	O
Model	O
67	B-Device
had	O
also	O
hosted	O
the	O
multiprocessor	B-Operating_System
capable	O
TSS/360	B-Application
,	O
MTS	B-Application
and	O
CP-67	B-Application
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Because	O
multiprocessing	B-Operating_System
systems	O
can	O
execute	O
instructions	O
simultaneously	O
,	O
they	O
offer	O
greater	O
processing	O
power	O
than	O
single-processing	O
system	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
MVS	B-Application
was	O
able	O
to	O
address	O
the	O
business	O
problems	O
brought	O
on	O
by	O
the	O
need	O
to	O
process	O
large	O
amounts	O
of	O
data	O
.	O
</s>
<s>
Multiprocessing	B-Operating_System
systems	O
are	O
either	O
loosely	O
coupled	O
,	O
which	O
means	O
that	O
each	O
computer	O
has	O
access	O
to	O
a	O
common	O
workload	O
,	O
or	O
tightly	B-Operating_System
coupled	I-Operating_System
,	O
which	O
means	O
that	O
the	O
computers	O
share	O
the	O
same	O
real	O
storage	O
and	O
are	O
controlled	O
by	O
a	O
single	O
copy	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
MVS	B-Application
retained	O
both	O
the	O
loosely	O
coupled	O
multiprocessing	B-Operating_System
of	O
Attached	B-Operating_System
Support	I-Operating_System
Processor	I-Operating_System
(	O
ASP	O
)	O
and	O
the	O
tightly	B-Operating_System
coupled	I-Operating_System
multiprocessing	B-Operating_System
of	O
OS/360	B-Application
Model	O
65	O
Multiprocessing	B-Operating_System
.	O
</s>
<s>
In	O
tightly	B-Operating_System
coupled	I-Operating_System
systems	I-Operating_System
,	O
two	O
CPUs	O
shared	O
concurrent	O
access	O
to	O
the	O
same	O
memory	O
(	O
and	O
copy	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
)	O
and	O
peripherals	B-Device
,	O
providing	O
greater	O
processing	O
power	O
and	O
a	O
degree	O
of	O
graceful	B-General_Concept
degradation	I-General_Concept
if	O
one	O
CPU	O
failed	O
.	O
</s>
<s>
In	O
loosely	O
coupled	O
configurations	O
each	O
of	O
a	O
group	O
of	O
processors	O
(	O
single	O
and	O
/	O
or	O
tightly	B-Operating_System
coupled	I-Operating_System
)	O
had	O
its	O
own	O
memory	O
and	O
operating	B-General_Concept
system	I-General_Concept
but	O
shared	O
peripherals	B-Device
and	O
the	O
operating	B-General_Concept
system	I-General_Concept
component	O
JES3	B-Application
allowed	O
managing	O
the	O
whole	O
group	O
from	O
one	O
console	O
.	O
</s>
<s>
MVS	B-Application
JES3	B-Application
gave	O
users	O
the	O
opportunity	O
to	O
network	B-Architecture
together	O
two	O
or	O
more	O
data	O
processing	O
systems	O
via	O
shared	O
disks	O
and	O
Channel-to-Channel	O
Adapters	O
(	O
CTCA	O
's	O
)	O
.	O
</s>
<s>
This	O
capability	O
eventually	O
became	O
available	O
to	O
JES2	B-Application
users	O
as	O
Multi-Access	O
SPOOL	O
(	O
MAS	O
)	O
.	O
</s>
<s>
MVS	B-Application
originally	O
supported	O
24-bit	O
addressing	O
(	O
i.e.	O
,	O
up	O
to	O
16MB	O
)	O
.	O
</s>
<s>
As	O
the	O
underlying	O
hardware	O
progressed	O
,	O
it	O
supported	O
31-bit	O
(	O
XA	O
and	O
ESA	O
;	O
up	O
to	O
2048MB	O
)	O
and	O
now	O
(	O
as	O
z/OS	B-Application
)	O
64-bit	B-Device
addressing	O
.	O
</s>
<s>
The	O
most	O
significant	O
motives	O
for	O
the	O
rapid	O
upgrade	O
to	O
31-bit	O
addressing	O
were	O
the	O
growth	O
of	O
large	O
transaction-processing	O
networks	O
,	O
mostly	O
controlled	O
by	O
CICS	B-Application
,	O
which	O
ran	O
in	O
a	O
single	O
address	B-General_Concept
space	I-General_Concept
—	O
and	O
the	O
DB2	B-Application
relational	B-Application
database	I-Application
management	I-Application
system	I-Application
needed	O
more	O
than	O
8MB	O
of	O
application	O
address	B-General_Concept
space	I-General_Concept
to	O
run	O
efficiently	O
.	O
</s>
<s>
(	O
Early	O
versions	O
were	O
configured	O
into	O
two	O
address	B-General_Concept
spaces	I-General_Concept
that	O
communicated	O
via	O
the	O
shared	O
virtual	O
area	O
,	O
but	O
this	O
imposed	O
a	O
significant	O
overhead	O
since	O
all	O
such	O
communications	O
had	O
transmit	O
via	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
)	O
</s>
<s>
The	O
main	O
user	O
interfaces	O
to	O
MVS	B-Application
are	O
:	O
Job	B-Application
Control	I-Application
Language	I-Application
(	O
JCL	B-Application
)	O
,	O
which	O
was	O
originally	O
designed	O
for	O
batch	B-General_Concept
processing	I-General_Concept
but	O
from	O
the	O
1970s	O
onwards	O
was	O
also	O
used	O
to	O
start	O
and	O
allocate	O
resources	O
to	O
long-running	O
interactive	B-General_Concept
jobs	O
such	O
as	O
CICS	B-Application
;	O
and	O
TSO	B-Application
(	O
Time	B-Application
Sharing	I-Application
Option	I-Application
)	O
,	O
the	O
interactive	B-General_Concept
time-sharing	B-General_Concept
interface	O
,	O
which	O
was	O
mainly	O
used	O
to	O
run	O
development	O
tools	O
and	O
a	O
few	O
end-user	O
information	O
systems	O
.	O
</s>
<s>
ISPF	B-Operating_System
is	O
a	O
TSO	B-Application
application	O
for	O
users	O
on	O
3270-family	B-Operating_System
terminals	O
(	O
and	O
later	O
,	O
on	O
VM	B-Application
as	O
well	O
)	O
,	O
which	O
allows	O
the	O
user	O
to	O
accomplish	O
the	O
same	O
tasks	O
as	O
TSO	B-Application
's	O
command	B-Application
line	I-Application
but	O
in	O
a	O
menu	O
and	O
form	O
oriented	O
manner	O
,	O
and	O
with	O
a	O
full	O
screen	O
editor	O
and	O
file	O
browser	O
.	O
</s>
<s>
TSO	B-Application
's	O
basic	B-Language
interface	O
is	O
command	B-Application
line	I-Application
,	O
</s>
<s>
MVS	B-Application
took	O
a	O
major	O
step	O
forward	O
in	O
fault-tolerance	B-General_Concept
,	O
built	O
on	O
the	O
earlier	O
STAE	O
facility	O
,	O
that	O
IBM	O
called	O
software	O
recovery	O
.	O
</s>
<s>
This	O
profound	O
assumption	O
was	O
pivotal	O
in	O
adding	O
great	O
percentages	O
of	O
fault-tolerance	B-General_Concept
code	O
to	O
the	O
system	O
and	O
likely	O
contributed	O
to	O
the	O
system	O
's	O
success	O
in	O
tolerating	O
software	O
and	O
hardware	O
failures	O
.	O
</s>
<s>
)	O
,	O
but	O
IBM	O
has	O
,	O
in	O
many	O
dimensions	O
,	O
enhanced	O
these	O
fault-tolerant	B-General_Concept
software	O
recovery	O
and	O
rapid	O
problem	O
resolution	O
features	O
,	O
over	O
time	O
.	O
</s>
<s>
The	O
worst	O
thing	O
possible	O
was	O
to	O
take	O
down	O
a	O
user	O
address	B-General_Concept
space	I-General_Concept
(	O
a	O
'	O
job	O
 '	O
)	O
in	O
the	O
case	O
of	O
unrepaired	O
errors	O
.	O
</s>
<s>
Though	O
it	O
was	O
an	O
initial	O
design	O
point	O
,	O
it	O
was	O
not	O
until	O
the	O
most	O
recent	O
MVS	B-Application
version	O
(	O
z/OS	B-Application
)	O
,	O
that	O
recovery	O
program	O
was	O
not	O
only	O
guaranteed	O
its	O
own	O
recovery	O
routine	O
,	O
but	O
each	O
recovery	O
routine	O
now	O
has	O
its	O
own	O
recovery	O
routine	O
.	O
</s>
<s>
This	O
recovery	O
structure	O
was	O
embedded	O
in	O
the	O
basic	B-Language
MVS	B-Application
control	O
program	O
,	O
and	O
programming	O
facilities	O
are	O
available	O
and	O
used	O
by	O
application	O
program	O
developers	O
and	O
3rd	O
party	O
developers	O
.	O
</s>
<s>
Practically	O
,	O
the	O
MVS	B-Application
software	O
recovery	O
made	O
problem	O
debugging	O
both	O
easier	O
and	O
more	O
difficult	O
.	O
</s>
<s>
IBM	O
continued	O
to	O
support	O
the	O
major	O
serviceability	O
tool	O
Dynamic	O
Support	O
System	O
(	O
DSS	O
)	O
that	O
it	O
had	O
introduced	O
in	O
OS/VS1	B-Application
and	O
OS/VS2	B-Application
Release	O
1	O
.	O
</s>
<s>
This	O
interactive	B-General_Concept
facility	O
could	O
be	O
invoked	O
to	O
initiate	O
a	O
session	O
to	O
create	O
diagnostic	O
procedures	O
,	O
or	O
invoke	O
already-stored	O
procedures	O
.	O
</s>
<s>
IBM	O
dropped	O
support	O
for	O
DSS	O
with	O
Selectable	O
Unit	O
7	O
(	O
SU7	O
)	O
,	O
an	O
update	O
to	O
OS/VS2	B-Application
Release	O
3.7	O
required	O
by	O
the	O
program	O
product	O
OS/VS2	B-Application
MVS/System	O
Extensions	O
(	O
MVS/SE	O
)	O
,	O
Program	O
Number	O
5740-XEl	O
.	O
</s>
<s>
The	O
User	O
group	O
SHARE	O
passed	O
a	O
requirement	O
that	O
IBM	O
reinstate	O
DSS	O
,	O
and	O
IBM	O
provided	O
a	O
PTF	B-Application
to	O
allow	O
use	O
of	O
DSS	O
after	O
MVS/SE	O
was	O
installed	O
.	O
</s>
<s>
IBM	O
again	O
dropped	O
support	O
for	O
DSS	O
with	O
SU64	O
,	O
an	O
update	O
to	O
OS/VS2	B-Application
Release	O
3.8	O
required	O
by	O
Release	O
2	O
of	O
MVS/SE	O
.	O
</s>
<s>
same	O
machine	O
if	O
that	O
machine	O
was	O
controlled	O
by	O
VM/370	B-Application
.	O
</s>
<s>
In	O
this	O
case	O
VM/370	B-Application
was	O
the	O
real	O
operating	B-General_Concept
system	I-General_Concept
,	O
and	O
regarded	O
the	O
"	O
guest	O
"	O
operating	B-General_Concept
systems	I-General_Concept
as	O
applications	O
with	O
unusually	O
high	O
privileges	O
.	O
</s>
<s>
As	O
a	O
result	O
of	O
later	O
hardware	O
enhancements	O
one	O
instance	O
of	O
an	O
operating	B-General_Concept
system	I-General_Concept
(	O
either	O
MVS	B-Application
,	O
or	O
VM	B-Application
with	O
guests	O
,	O
or	O
other	O
)	O
could	O
also	O
occupy	O
a	O
Logical	B-Device
Partition	I-Device
(	O
LPAR	B-Device
)	O
instead	O
of	O
an	O
entire	O
physical	O
system	O
.	O
</s>
<s>
Multiple	O
MVS	B-Application
instances	O
can	O
be	O
organized	O
and	O
collectively	O
administered	O
in	O
a	O
structure	O
called	O
a	O
systems	O
complex	O
or	O
sysplex	B-Operating_System
,	O
introduced	O
in	O
September	O
,	O
1990	O
.	O
</s>
<s>
Instances	O
interoperate	O
through	O
a	O
software	O
component	O
called	O
a	O
Cross-system	B-Application
Coupling	I-Application
Facility	I-Application
(	O
XCF	O
)	O
and	O
a	O
hardware	O
component	O
called	O
a	O
Hardware	O
Coupling	O
Facility	O
(	O
CF	O
or	O
Integrated	O
Coupling	O
Facility	O
,	O
ICF	O
,	O
if	O
co-located	O
on	O
the	O
same	O
mainframe	O
hardware	O
)	O
.	O
</s>
<s>
Multiple	O
sysplexes	O
can	O
be	O
joined	O
via	O
standard	O
network	B-Architecture
protocols	O
such	O
as	O
IBM	O
's	O
proprietary	B-Application
Systems	B-Architecture
Network	I-Architecture
Architecture	I-Architecture
(	O
SNA	O
)	O
or	O
,	O
more	O
recently	O
,	O
via	O
TCP/IP	B-Protocol
.	O
</s>
<s>
The	O
z/OS	B-Application
operating	B-General_Concept
system	I-General_Concept
(	O
MVS	B-Application
 '	O
most	O
recent	O
descendant	O
)	O
also	O
has	O
native	O
support	O
to	O
execute	O
POSIX	O
and	O
Single	O
UNIX	B-Application
Specification	O
applications	O
.	O
</s>
<s>
The	O
support	O
began	O
with	O
MVS/SP	O
V4R3	O
,	O
and	O
IBM	O
has	O
obtained	O
UNIX	B-Application
95	O
certification	O
for	O
z/OS	B-Application
V1R2	O
and	O
later	O
.	O
</s>
<s>
The	O
system	O
is	O
typically	O
used	O
in	O
business	O
and	O
banking	O
,	O
and	O
applications	O
are	O
often	O
written	O
in	O
COBOL	B-Application
.	O
</s>
<s>
COBOL	B-Application
programs	O
were	O
traditionally	O
used	O
with	O
transaction	O
processing	O
systems	O
like	O
IMS	B-Application
and	O
CICS	B-Application
.	O
</s>
<s>
For	O
a	O
program	O
running	O
in	O
CICS	B-Application
,	O
special	O
EXEC	O
CICS	B-Application
statements	O
are	O
inserted	O
in	O
the	O
COBOL	B-Application
source	O
code	O
.	O
</s>
<s>
A	O
preprocessor	O
(	O
translator	O
)	O
replaces	O
those	O
EXEC	O
CICS	B-Application
statements	O
with	O
the	O
appropriate	O
COBOL	B-Application
code	O
to	O
call	O
CICS	B-Application
before	O
the	O
program	O
is	O
compiled	O
—	O
not	O
altogether	O
unlike	O
SQL	B-Language
used	O
to	O
call	O
DB2	B-Application
.	O
</s>
<s>
Applications	O
can	O
also	O
be	O
written	O
in	O
other	O
languages	O
such	O
as	O
C	B-Language
,	O
C++	B-Language
,	O
Java	B-Language
,	O
assembly	B-Language
language	I-Language
,	O
FORTRAN	B-Application
,	O
BASIC	B-Language
,	O
RPG	B-Language
,	O
and	O
REXX	B-Language
.	O
</s>
<s>
MVS	B-Application
systems	O
are	O
traditionally	O
accessed	O
by	O
3270	B-Operating_System
terminals	I-Operating_System
or	O
by	O
PCs	O
running	O
3270	B-Operating_System
emulators	O
.	O
</s>
<s>
However	O
,	O
many	O
mainframe	O
applications	O
these	O
days	O
have	O
custom	O
web	O
or	O
GUI	B-Application
interfaces	O
.	O
</s>
<s>
The	O
z/OS	B-Application
operating	B-General_Concept
system	I-General_Concept
has	O
built-in	O
support	O
for	O
TCP/IP	B-Protocol
.	O
</s>
<s>
System	O
management	O
,	O
done	O
in	O
the	O
past	O
with	O
a	O
3270	B-Operating_System
terminal	I-Operating_System
,	O
is	O
now	O
done	O
through	O
the	O
Hardware	O
Management	O
Console	O
(	O
HMC	O
)	O
and	O
,	O
increasingly	O
,	O
Web	O
interfaces	O
.	O
</s>
<s>
Operator	O
consoles	O
are	O
provided	O
through	O
2074	O
emulators	O
,	O
so	O
you	O
are	O
unlikely	O
to	O
see	O
any	O
S/390	B-Device
or	O
zSeries	O
processor	O
with	O
a	O
real	O
3270	B-Operating_System
connected	O
to	O
it	O
.	O
</s>
<s>
The	O
native	O
character	O
encoding	O
scheme	O
of	O
MVS	B-Application
and	O
its	O
peripherals	B-Device
is	O
EBCDIC	B-Language
,	O
but	O
the	O
TR	O
instruction	O
made	O
it	O
easy	O
to	O
translate	O
to	O
other	O
7	O
-	O
and	O
8-bit	O
codes	O
.	O
</s>
<s>
Over	O
time	O
IBM	O
added	O
hardware-accelerated	O
services	O
to	O
perform	O
translation	O
to	O
and	O
between	O
larger	O
codes	O
,	O
hardware-specific	O
service	O
for	O
Unicode	O
transforms	O
and	O
software	O
support	O
of	O
,	O
e.g.	O
,	O
ASCII	B-Protocol
,	O
ISO/IEC	O
8859	O
,	O
UTF-8	O
,	O
UTF-16	O
,	O
and	O
UTF-32	O
.	O
</s>
<s>
Files	O
,	O
other	O
than	O
Unix	B-Application
files	O
,	O
are	O
properly	O
called	O
data	B-Application
sets	I-Application
in	O
MVS	B-Application
.	O
</s>
<s>
Data	B-Application
set	I-Application
names	O
(	O
DSNs	O
,	O
mainframe	O
term	O
for	O
filenames	O
)	O
are	O
organized	O
in	O
a	O
hierarchy	O
whose	O
levels	O
are	O
separated	O
with	O
dots	O
,	O
e.g.	O
</s>
<s>
By	O
convention	O
,	O
the	O
components	O
separated	O
by	O
the	O
dots	O
are	O
used	O
to	O
organize	O
files	O
similarly	O
to	O
directories	B-Application
in	O
other	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
there	O
are	O
utility	B-Application
programs	I-Application
that	O
performed	O
similar	O
functions	O
to	O
those	O
of	O
Windows	B-Protocol
Explorer	I-Protocol
(	O
but	O
without	O
the	O
GUI	B-Application
and	O
usually	O
in	O
batch	B-General_Concept
processing	I-General_Concept
mode	O
)	O
-	O
adding	O
,	O
renaming	O
or	O
deleting	O
new	O
elements	O
and	O
reporting	O
all	O
the	O
contents	O
of	O
a	O
specified	O
element	O
.	O
</s>
<s>
However	O
,	O
unlike	O
in	O
many	O
other	O
systems	O
,	O
these	O
levels	O
are	O
not	O
usually	O
actual	O
directories	B-Application
but	O
just	O
a	O
naming	O
convention	O
(	O
like	O
the	O
original	O
Macintosh	B-Application
File	I-Application
System	I-Application
,	O
where	O
folder	B-Application
hierarchy	O
was	O
an	O
illusion	O
maintained	O
by	O
the	O
Finder	O
)	O
.	O
</s>
<s>
TSO	B-Application
supports	O
a	O
default	O
prefix	O
for	O
files	O
(	O
similar	O
to	O
a	O
"	O
current	O
directory	B-Application
"	O
concept	O
)	O
,	O
and	O
RACF	B-Application
supports	O
setting	O
up	O
access	O
controls	O
based	O
on	O
filename	O
patterns	O
,	O
analogous	O
to	O
access	O
controls	O
on	O
directories	B-Application
on	O
other	O
platforms	O
.	O
</s>
<s>
As	O
with	O
other	O
members	O
of	O
the	O
OS	O
family	O
,	O
MVS	B-Application
 '	O
data	B-Application
sets	I-Application
are	O
record-oriented	B-Application
.	O
</s>
<s>
MVS	B-Application
inherited	O
three	O
main	O
types	O
from	O
its	O
predecessors	O
:	O
</s>
<s>
Sequential	O
data	B-Application
sets	I-Application
were	O
normally	O
read	O
one	O
record	O
at	O
a	O
time	O
from	O
beginning	O
to	O
end	O
.	O
</s>
<s>
In	O
BDAM	B-Application
(	O
direct	O
access	O
)	O
data	B-Application
sets	I-Application
,	O
the	O
application	O
program	O
had	O
to	O
specify	O
the	O
physical	O
location	O
of	O
the	O
data	O
it	O
wanted	O
to	O
access	O
(	O
usually	O
by	O
specifying	O
the	O
offset	O
from	O
the	O
start	O
of	O
the	O
data	B-Application
set	I-Application
)	O
.	O
</s>
<s>
In	O
ISAM	B-Application
data	B-Application
sets	I-Application
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
as	O
a	O
key	O
to	O
look	O
up	O
specific	O
records	O
.	O
</s>
<s>
The	O
key	O
quite	O
often	O
consisted	O
of	O
multiple	O
fields	B-Application
but	O
these	O
had	O
to	O
be	O
contiguous	O
and	O
in	O
the	O
right	O
order	O
;	O
and	O
key	O
values	O
had	O
to	O
be	O
unique	O
.	O
</s>
<s>
Hence	O
an	O
IBM	O
ISAM	B-Application
file	O
could	O
have	O
only	O
one	O
key	O
,	O
equivalent	O
to	O
the	O
primary	B-Application
key	I-Application
of	O
a	O
relational	B-Application
database	I-Application
table	O
;	O
ISAM	B-Application
could	O
not	O
support	O
foreign	B-Application
keys	I-Application
.	O
</s>
<s>
Sequential	O
and	O
ISAM	B-Application
datasets	O
could	O
store	O
either	O
fixed-length	O
or	O
variable	O
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>
All	O
of	O
these	O
are	O
based	O
on	O
the	O
VTOC	B-Application
disk	O
structure	O
.	O
</s>
<s>
Early	O
IBM	O
database	B-Application
management	I-Application
systems	I-Application
used	O
various	O
combinations	O
of	O
ISAM	B-Application
and	O
BDAM	B-Application
datasets	O
-	O
usually	O
BDAM	B-Application
for	O
the	O
actual	O
data	O
storage	O
and	O
ISAM	B-Application
for	O
indexes	O
.	O
</s>
<s>
In	O
the	O
early	O
1970s	O
IBM	O
's	O
virtual	B-Architecture
memory	I-Architecture
operating	B-General_Concept
systems	I-General_Concept
introduced	O
a	O
new	O
file	O
management	O
component	O
,	O
VSAM	O
,	O
which	O
provided	O
similar	O
facilities	O
:	O
</s>
<s>
Entry-Sequenced	O
Datasets	O
(	O
ESDS	O
)	O
provided	O
facilities	O
similar	O
to	O
those	O
of	O
both	O
sequential	O
and	O
BDAM	B-Application
datasets	O
,	O
since	O
they	O
could	O
be	O
read	O
either	O
from	O
start	O
to	O
finish	O
or	O
directly	O
by	O
specifying	O
an	O
offset	O
from	O
the	O
start	O
.	O
</s>
<s>
Key-Sequenced	O
Datasets	O
(	O
KSDS	O
)	O
are	O
a	O
major	O
upgrade	O
from	O
ISAM	B-Application
:	O
they	O
allow	O
secondary	O
keys	O
with	O
non-unique	O
values	O
and	O
keys	O
formed	O
by	O
concatenating	O
non-contiguous	O
fields	B-Application
in	O
any	O
order	O
;	O
they	O
greatly	O
reduced	O
the	O
performance	O
problems	O
caused	O
by	O
overflow	O
records	O
in	O
ISAM	B-Application
;	O
and	O
they	O
greatly	O
reduced	O
the	O
risk	O
that	O
a	O
software	O
or	O
hardware	O
failure	O
in	O
the	O
middle	O
of	O
an	O
index	O
update	O
might	O
corrupt	O
the	O
index	O
.	O
</s>
<s>
These	O
VSAM	O
formats	O
became	O
the	O
basis	O
of	O
IBM	O
's	O
database	B-Application
management	I-Application
systems	I-Application
,	O
IMS/VS	B-Application
and	O
DB2	B-Application
-	O
usually	O
ESDS	O
for	O
the	O
actual	O
data	O
storage	O
and	O
KSDS	O
for	O
indexes	O
.	O
</s>
<s>
VSAM	O
also	O
included	O
a	O
catalog	O
component	O
used	O
for	O
user	O
catalogs	O
and	O
MVS	B-Application
 '	O
master	O
catalog	O
.	O
</s>
<s>
Partitioned	O
data	B-Application
sets	I-Application
(	O
PDS	O
)	O
are	O
sequential	O
data	B-Application
sets	I-Application
subdivided	O
into	O
"	O
members	O
"	O
that	O
could	O
each	O
be	O
processed	O
as	O
sequential	O
files	O
in	O
their	O
own	O
right	O
(	O
like	O
a	O
folder	B-Application
in	O
a	O
file	O
system	O
)	O
.	O
</s>
<s>
Generation	O
Data	O
Groups	O
(	O
GDGs	O
)	O
are	O
groups	O
of	O
like	O
named	O
data	B-Application
sets	I-Application
,	O
which	O
can	O
be	O
referenced	O
by	O
absolute	O
generation	O
number	O
,	O
or	O
by	O
an	O
offset	O
from	O
the	O
most	O
recent	O
generation	O
.	O
</s>
<s>
But	O
one	O
could	O
set	O
up	O
GDGs	O
with	O
more	O
than	O
3	O
generations	O
and	O
some	O
applications	O
used	O
GDGs	O
to	O
collect	O
data	O
from	O
several	O
sources	O
and	O
feed	O
the	O
information	O
to	O
one	O
program	O
-	O
each	O
collecting	O
program	O
created	O
a	O
new	O
generation	O
of	O
the	O
file	O
and	O
the	O
final	O
program	O
read	O
the	O
whole	O
group	O
as	O
a	O
single	O
sequential	O
file	O
(	O
by	O
not	O
specifying	O
a	O
generation	O
in	O
the	O
JCL	B-Application
)	O
.	O
</s>
<s>
Modern	O
versions	O
of	O
MVS	B-Application
(	O
e.g.	O
,	O
z/OS	B-Application
)	O
use	O
datasets	O
as	O
containers	O
for	O
Unix	B-Application
filesystems	O
along	O
with	O
facilities	O
for	O
partially	O
integrating	O
them	O
.	O
</s>
<s>
That	O
is	O
,	O
Unix	B-Application
programs	O
using	O
fopen( )	O
can	O
access	O
an	O
MVS	B-Application
dataset	O
and	O
a	O
user	O
can	O
allocate	O
a	O
Unix	B-Application
file	O
as	O
though	O
it	O
were	O
a	O
dataset	O
,	O
with	O
some	O
restrictions	O
.	O
</s>
<s>
The	O
Hierarchical	B-Application
File	I-Application
System	I-Application
(	O
HFS	B-Application
)	O
(	O
not	O
to	O
be	O
confused	O
with	O
Apple	O
's	O
Hierarchical	B-Application
File	I-Application
System	I-Application
)	O
uses	O
a	O
unique	O
type	O
of	O
dataset	O
,	O
while	O
the	O
newer	O
z/OS	B-Application
File	O
System	O
(	O
zFS	B-Application
)	O
(	O
not	O
to	O
be	O
confused	O
with	O
Sun	O
's	O
ZFS	B-Application
)	O
uses	O
a	O
VSAM	O
Linear	O
Data	B-Application
Set	I-Application
(	O
LDS	O
)	O
.	O
</s>
<s>
Programs	O
running	O
on	O
network-connected	O
computers	O
(	O
such	O
as	O
the	O
IBM	B-Device
AS/400	I-Device
)	O
can	O
use	O
local	O
data	O
management	O
interfaces	O
to	O
transparently	O
create	O
,	O
manage	O
,	O
and	O
access	O
VSAM	O
record-oriented	B-Application
files	I-Application
by	O
using	O
client-server	O
products	O
implemented	O
according	O
to	O
Distributed	B-Operating_System
Data	I-Operating_System
Management	I-Operating_System
Architecture	I-Operating_System
(	O
DDM	O
)	O
.	O
</s>
<s>
DDM	O
is	O
also	O
the	O
base	O
architecture	O
for	O
the	O
MVS	B-Application
DB2	B-Application
server	O
that	O
implements	O
Distributed	B-Language
Relational	I-Language
Database	I-Language
Architecture	I-Language
(	O
DRDA	B-Language
)	O
.	O
</s>
<s>
In	O
addition	O
to	O
new	O
functionality	O
that	O
IBM	O
added	O
with	O
releases	O
and	O
sub-releases	O
of	O
OS/VS2	B-Application
,	O
IBM	O
provided	O
a	O
number	O
of	O
free	O
Incremental	O
Change	O
Releases	O
(	O
ICRs	O
)	O
and	O
Selectable	O
Units	O
(	O
SUs	O
)	O
and	O
chargeable	O
program	O
products	O
and	O
field	O
developed	O
programs	O
that	O
IBM	O
eventually	O
bundled	O
as	O
part	O
of	O
z/OS	B-Application
.	O
</s>
<s>
MVS/XA	O
Data	B-Application
Facility	I-Application
Product	I-Application
Version	O
2	O
Release	O
1	O
,	O
</s>
<s>
TSO	B-Application
Programming	O
Control	O
Facility	O
-	O
II	O
(	O
PCF	O
II	O
)	O
,	O
FDP	O
5798-CLW	O
,	O
</s>
<s>
MVS/System	O
Product-JES2	O
Version	O
3	O
,	O
</s>
<s>
MVS/System	O
Product-JES3	O
Version	O
3	O
,	O
</s>
<s>
When	O
IBM	O
announced	O
MVS/SP	O
Version	O
2	O
(	O
MVS/XA	O
)	O
,	O
it	O
also	O
announced	O
Data	O
Facility	O
Product™	O
(	O
DFP™	O
)	O
as	O
a	O
replacement	O
for	O
and	O
upgrade	O
to	O
the	O
other	O
five	O
products	O
above	O
,	O
which	O
it	O
said	O
would	O
be	O
withdrawn	O
from	O
marketing	O
,	O
effective	O
December	O
1	O
,	O
1984	O
.	O
</s>
<s>
DFP/370	O
Release	O
1	O
(	O
program	O
number	O
5665-295	O
)	O
,	O
announced	O
June	O
7	O
,	O
1983	O
,	O
was	O
for	O
MVS/SP	O
Version	O
1	O
,	O
MVS/SE	O
and	O
OS/VS2	B-Application
R3.8	O
,	O
and	O
was	O
optional	O
,	O
but	O
MVS/Extended	O
Architecture	O
Data	B-Application
Facility	I-Application
Product	I-Application
(	O
5665-284	O
)	O
was	O
a	O
corequisite	O
for	O
MVS/SP	O
Version	O
2	O
(	O
MVS/XA	O
)	O
.	O
</s>
<s>
DFP	O
is	O
no	O
longer	O
available	O
as	O
a	O
separate	O
product	O
,	O
but	O
has	O
become	O
part	O
of	O
Data	B-Operating_System
Facility	I-Operating_System
Storage	I-Operating_System
Management	I-Operating_System
Subsystem	I-Operating_System
,	O
under	O
the	O
name	O
DFSMSdfp	O
.	O
</s>
<s>
MVS	B-Application
has	O
now	O
evolved	O
into	O
z/OS	B-Application
;	O
older	O
MVS	B-Application
releases	O
are	O
no	O
longer	O
supported	O
by	O
IBM	O
and	O
,	O
since	O
2007	O
,	O
only	O
64-bit	B-Device
z/OS	B-Application
releases	O
are	O
supported	O
.	O
</s>
<s>
z/OS	B-Application
supports	O
running	O
older	O
24-bit	O
and	O
31-bit	O
MVS	B-Application
applications	O
alongside	O
newer	O
64-bit	B-Device
applications	O
.	O
</s>
<s>
MVS	B-Application
releases	O
up	O
to	O
3.8j	O
(	O
24-bit	O
,	O
released	O
in	O
1981	O
)	O
were	O
freely	O
available	O
and	O
it	O
is	O
now	O
possible	O
to	O
run	O
the	O
MVS	B-Application
3.8j	O
release	O
in	O
mainframe	O
emulators	O
for	O
free	O
.	O
</s>
<s>
MVS/370	O
is	O
a	O
generic	O
term	O
for	O
all	O
versions	O
of	O
the	O
MVS	B-Application
operating	I-Application
system	I-Application
prior	O
to	O
MVS/XA	O
.	O
</s>
<s>
The	O
System/370	B-Device
architecture	O
,	O
at	O
the	O
time	O
MVS	B-Application
was	O
released	O
,	O
supported	O
only	O
24-bit	O
virtual	O
addresses	O
,	O
so	O
the	O
MVS/370	O
operating	B-General_Concept
system	I-General_Concept
architecture	O
is	O
based	O
on	O
a	O
24-bit	O
address	O
.	O
</s>
<s>
Because	O
of	O
this	O
24-bit	O
address	O
length	O
,	O
programs	O
running	O
under	O
MVS/370	O
are	O
each	O
given	O
16MB	O
of	O
contiguous	O
virtual	B-Architecture
storage	I-Architecture
.	O
</s>
<s>
MVS/XA	O
,	O
or	O
Multiple	O
Virtual	O
Storage/Extended	O
Architecture	O
,	O
was	O
a	O
version	O
of	O
MVS	B-Application
that	O
supported	O
the	O
370-XA	O
architecture	O
,	O
which	O
had	O
a	O
new	O
I/O	O
architecture	O
and	O
also	O
expanded	O
addresses	O
from	O
24bits	O
to	O
31bits	O
,	O
providing	O
a	O
2gigabyte	O
addressable	O
memory	O
area	O
.	O
</s>
<s>
MVS/XA	O
supported	O
a	O
24-bit	O
legacy	O
addressing	O
mode	O
for	O
older	O
24-bit	O
applications	O
(	O
i.e.	O
</s>
<s>
MVS	B-Application
Enterprise	O
System	O
Architecture	O
(	O
MVS/ESA	O
)	O
is	O
any	O
version	O
of	O
MVS	B-Application
prior	O
to	O
OS/390	B-Application
that	O
supports	O
S/370	B-Device
Enterprise	O
System	O
Architecture	O
(	O
S/370	B-Device
-ESA	O
)	O
.	O
</s>
<s>
MVS/ESA	O
extends	O
the	O
24-bit	O
and	O
31-bit	O
addressing	O
modes	O
of	O
MVS/XA	O
by	O
adding	O
an	O
access	O
register	O
(	O
AR	O
)	O
mode	O
for	O
references	O
between	O
address	B-General_Concept
spaces	I-General_Concept
.	O
</s>
<s>
IBM	O
introduced	O
MVS/ESA	O
as	O
MVS/SP	O
Version	O
3	O
in	O
February	O
1988	O
,	O
then	O
MVS/ESA	O
SP	O
Version	O
4	O
and	O
MVS/ESA	O
SP	O
Version	O
5	O
.	O
</s>
<s>
IBM	O
replaced	O
it	O
with	O
OS/390	B-Application
in	O
late	O
1995	O
and	O
subsequently	O
with	O
z/OS	B-Application
.	O
</s>
<s>
MVS/ESA	O
OpenEdition	O
:	O
upgrade	O
to	O
Version	O
4	O
Release	O
3	O
of	O
MVS/ESA	O
announced	O
February	O
1993	O
with	O
support	O
for	O
POSIX	O
and	O
other	O
standards	O
.	O
</s>
<s>
X/Open	O
and	O
its	O
successor	O
,	O
The	B-Application
Open	I-Application
Group	I-Application
.	O
</s>
<s>
Works	O
with	O
a	O
hierarchical	B-Application
file	I-Application
system	I-Application
provided	O
by	O
DFSMS	B-Operating_System
(	O
Data	O
Facility	O
System	O
Managed	O
Storage	O
)	O
.	O
</s>
<s>
Independent	O
specialists	O
estimate	O
that	O
it	O
was	O
over	O
80%	O
open	O
systems-compliant	O
—	O
more	O
than	O
most	O
Unix	B-Application
systems	I-Application
.	O
</s>
<s>
From	O
mid	O
1995	O
,	O
as	O
all	O
of	O
the	O
open	O
features	O
became	O
a	O
standard	O
part	O
of	O
vanilla	B-General_Concept
MVS/ESA	O
SP	O
Version	O
5	O
Release	O
1	O
,	O
IBM	O
stopped	O
distinguishing	O
OpenEdition	O
from	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Under	O
OS/390	B-Application
V2R6	O
it	O
became	O
UNIX	B-Operating_System
System	I-Operating_System
Services	I-Operating_System
,	O
and	O
has	O
kept	O
that	O
name	O
under	O
z/OS	B-Application
.	O
</s>
<s>
In	O
late	O
1995	O
IBM	O
bundled	O
MVS	B-Application
with	O
several	O
program	O
products	O
and	O
changed	O
the	O
name	O
from	O
MVS/ESA	O
to	O
OS/390	B-Application
.	O
</s>
<s>
The	O
current	O
level	O
of	O
MVS	B-Application
is	O
marketed	O
as	O
z/OS	B-Application
.	O
</s>
<s>
Japanese	O
mainframe	O
manufacturers	O
Fujitsu	O
and	O
Hitachi	O
both	O
repeatedly	O
and	O
illegally	O
obtained	O
IBM	O
's	O
MVS	B-Application
source	O
code	O
and	O
internal	O
documentation	O
in	O
one	O
of	O
the	O
20th	O
century	O
's	O
most	O
famous	O
cases	O
of	O
industrial	O
espionage	O
.	O
</s>
<s>
Fujitsu	O
relied	O
heavily	O
on	O
IBM	O
's	O
code	O
in	O
its	O
MSP	O
mainframe	O
operating	B-General_Concept
system	I-General_Concept
,	O
and	O
likewise	O
Hitachi	O
did	O
the	O
same	O
for	O
its	O
VOS3	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
IBM	O
cooperated	O
with	O
the	O
U.S.	O
Federal	O
Bureau	O
of	O
Investigation	O
in	O
a	O
sting	O
operation	O
,	O
reluctantly	O
supplying	O
Fujitsu	O
and	O
Hitachi	O
with	O
proprietary	B-Application
MVS	B-Application
and	O
mainframe	O
hardware	O
technologies	O
during	O
the	O
course	O
of	O
multi-year	O
investigations	O
culminating	O
in	O
the	O
early	O
1980s	O
—	O
investigations	O
which	O
implicated	O
senior	O
company	O
managers	O
and	O
even	O
some	O
Japanese	O
government	O
officials	O
.	O
</s>
<s>
Because	O
of	O
this	O
historical	O
copying	O
,	O
MSP	O
and	O
VOS3	O
are	O
properly	O
classified	O
as	O
"	B-Application
forks	I-Application
"	I-Application
of	O
MVS	B-Application
,	O
and	O
many	O
third-party	O
software	O
vendors	O
with	O
MVS-compatible	O
products	O
were	O
able	O
to	O
produce	O
MSP	O
-	O
and	O
VOS3-compatible	O
versions	O
with	O
little	O
or	O
no	O
modification	O
.	O
</s>
<s>
When	O
IBM	O
introduced	O
its	O
64-bit	B-Device
z/Architecture	B-Device
mainframes	O
in	O
the	O
year	O
2000	O
,	O
IBM	O
also	O
introduced	O
the	O
64-bit	B-Device
z/OS	B-Application
operating	B-General_Concept
system	I-General_Concept
,	O
the	O
direct	O
successor	O
to	O
OS/390	B-Application
and	O
MVS	B-Application
.	O
</s>
<s>
Fujitsu	O
and	O
Hitachi	O
opted	O
not	O
to	O
license	O
IBM	O
's	O
z/Architecture	B-Device
for	O
their	O
quasi-MVS	O
operating	O
systems	O
and	O
hardware	O
systems	O
,	O
and	O
so	O
MSP	O
and	O
VOS3	O
,	O
while	O
still	O
nominally	O
supported	O
by	O
their	O
vendors	O
,	O
maintain	O
most	O
of	O
MVS	B-Application
's	O
1980s	O
architectural	O
limitations	O
to	O
the	O
present	O
day	O
.	O
</s>
<s>
Since	O
z/OS	B-Application
still	O
supports	O
MVS-era	O
applications	O
and	O
technologies	O
—	O
z/OS	B-Application
still	O
contains	O
most	O
of	O
MVS	B-Application
's	O
code	O
,	O
albeit	O
greatly	O
enhanced	O
and	O
improved	O
over	O
decades	O
of	O
evolution	O
—	O
applications	O
(	O
and	O
operational	O
procedures	O
)	O
running	O
on	O
MSP	O
and	O
VOS3	O
can	O
move	O
to	O
z/OS	B-Application
much	O
more	O
easily	O
than	O
to	O
other	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
