<s>
PL/I	B-Language
(	O
Programming	O
Language	O
One	O
,	O
pronounced	O
and	O
sometimes	O
written	O
PL/1	B-Language
)	O
is	O
a	O
procedural	B-Application
,	O
imperative	B-Application
computer	O
programming	O
language	O
developed	O
and	O
published	O
by	O
IBM	O
.	O
</s>
<s>
It	O
is	O
designed	O
for	O
scientific	O
,	O
engineering	O
,	O
business	O
and	O
system	B-Application
programming	I-Application
.	O
</s>
<s>
PL/I	B-Language
'	O
s	O
main	O
domains	O
are	O
data	B-General_Concept
processing	I-General_Concept
,	O
numerical	B-General_Concept
computation	I-General_Concept
,	O
scientific	O
computing	O
,	O
and	O
system	B-Application
programming	I-Application
.	O
</s>
<s>
It	O
supports	O
recursion	O
,	O
structured	B-Language
programming	I-Language
,	O
linked	B-Application
data	I-Application
structure	I-Application
handling	O
,	O
fixed-point	O
,	O
floating-point	B-Algorithm
,	O
complex	O
,	O
character	O
string	O
handling	O
,	O
and	O
bit	B-Data_Structure
string	I-Data_Structure
handling	O
.	O
</s>
<s>
Business	O
users	O
were	O
moving	O
from	O
Autocoders	B-Device
via	O
COMTRAN	B-Language
to	O
COBOL	B-Application
,	O
while	O
scientific	O
users	O
programmed	O
in	O
Fortran	B-Application
,	O
ALGOL	B-Language
,	O
GEORGE	B-Language
,	O
and	O
others	O
.	O
</s>
<s>
The	O
IBM	B-Application
System/360	I-Application
(	O
announced	O
in	O
1964	O
and	O
delivered	O
in	O
1966	O
)	O
was	O
designed	O
as	O
a	O
common	O
machine	O
architecture	O
for	O
both	O
groups	O
of	O
users	O
,	O
superseding	O
all	O
existing	O
IBM	O
architectures	O
.	O
</s>
<s>
It	O
hoped	O
that	O
Fortran	B-Application
could	O
be	O
extended	O
to	O
include	O
the	O
features	O
needed	O
by	O
commercial	O
programmers	O
.	O
</s>
<s>
scientific	O
users	O
group	O
,	O
to	O
propose	O
these	O
extensions	O
to	O
Fortran	B-Application
.	O
</s>
<s>
Given	O
the	O
constraints	O
of	O
Fortran	B-Application
,	O
they	O
were	O
unable	O
to	O
do	O
this	O
and	O
embarked	O
on	O
the	O
design	O
of	O
a	O
new	B-Language
programming	I-Language
language	I-Language
based	O
loosely	O
on	O
ALGOL	B-Language
labeled	O
NPL	O
.	O
</s>
<s>
This	O
acronym	O
conflicted	O
with	O
that	O
of	O
the	O
UK	O
's	O
National	O
Physical	O
Laboratory	O
and	O
was	O
replaced	O
briefly	O
by	O
MPPL	O
(	O
MultiPurpose	O
Programming	O
Language	O
)	O
and	O
,	O
in	O
1965	O
,	O
with	O
PL/I	B-Language
(	O
with	O
a	O
Roman	O
numeral	O
"	O
I	O
"	O
)	O
.	O
</s>
<s>
IBM	O
took	O
NPL	O
as	O
a	O
starting	O
point	O
and	O
completed	O
the	O
design	O
to	O
a	O
level	O
that	O
the	O
first	O
compiler	B-Language
could	O
be	O
written	O
:	O
the	O
NPL	O
definition	O
was	O
incomplete	O
in	O
scope	O
and	O
in	O
detail	O
.	O
</s>
<s>
Control	O
of	O
the	O
PL/I	B-Language
language	O
was	O
vested	O
initially	O
in	O
the	O
New	O
York	O
Programming	O
Center	O
and	O
later	O
at	O
the	O
IBM	O
UK	O
Laboratory	O
at	O
Hursley	O
.	O
</s>
<s>
The	O
SHARE	O
and	O
GUIDE	O
user	O
groups	O
were	O
involved	O
in	O
extending	O
the	O
language	O
and	O
had	O
a	O
role	O
in	O
IBM	O
's	O
process	B-Operating_System
for	O
controlling	O
the	O
language	O
through	O
their	O
PL/I	B-Language
Projects	O
.	O
</s>
<s>
The	O
experience	O
of	O
defining	O
such	O
a	O
large	O
language	O
showed	O
the	O
need	O
for	O
a	O
formal	O
definition	O
of	O
PL/I	B-Language
.	O
</s>
<s>
This	O
led	O
in	O
turn	O
to	O
one	O
of	O
the	O
first	O
large	O
scale	O
Formal	B-Architecture
Methods	I-Architecture
for	O
development	O
,	O
VDM	B-Application
.	O
</s>
<s>
Fred	O
Brooks	O
is	O
credited	O
with	O
ensuring	O
PL/I	B-Language
had	O
the	O
CHARACTER	O
data	O
type	O
.	O
</s>
<s>
The	O
language	O
was	O
first	O
specified	O
in	O
detail	O
in	O
the	O
manual	O
"	O
PL/I	B-Language
Language	O
Specifications	O
.	O
</s>
<s>
C28-6571	O
"	O
,	O
written	O
in	O
New	O
York	O
in	O
1965	O
,	O
and	O
superseded	O
by	O
"	O
PL/I	B-Language
Language	O
Specifications	O
.	O
</s>
<s>
IBM	O
continued	O
to	O
develop	O
PL/I	B-Language
in	O
the	O
late	O
sixties	O
and	O
early	O
seventies	O
,	O
publishing	O
it	O
in	O
the	O
GY33-6003	O
manual	O
.	O
</s>
<s>
These	O
manuals	O
were	O
used	O
by	O
the	O
Multics	B-Application
group	O
and	O
other	O
early	O
implementers	O
.	O
</s>
<s>
The	O
first	O
compiler	B-Language
was	O
delivered	O
in	O
1966	O
.	O
</s>
<s>
The	O
Standard	O
for	O
PL/I	B-Language
was	O
approved	O
in	O
1976	O
.	O
</s>
<s>
The	O
goals	O
for	O
PL/I	B-Language
evolved	O
during	O
the	O
early	O
development	O
of	O
the	O
language	O
.	O
</s>
<s>
Competitiveness	O
with	O
COBOL	B-Application
's	O
record	O
handling	O
and	O
report	O
writing	O
was	O
required	O
.	O
</s>
<s>
The	O
language	O
's	O
scope	O
of	O
usefulness	O
grew	O
to	O
include	O
system	B-Application
programming	I-Application
and	O
event-driven	B-Application
programming	I-Application
.	O
</s>
<s>
Additional	O
goals	O
for	O
PL/I	B-Language
were	O
:	O
</s>
<s>
To	O
achieve	O
these	O
goals	O
,	O
PL/I	B-Language
borrowed	O
ideas	O
from	O
contemporary	O
languages	O
while	O
adding	O
substantial	O
new	O
capabilities	O
and	O
casting	O
it	O
with	O
a	O
distinctive	O
concise	O
and	O
readable	O
syntax	O
.	O
</s>
<s>
Block	O
structure	O
,	O
with	O
underlying	O
semantics	O
(	O
including	O
recursion	O
)	O
,	O
similar	O
to	O
Algol	B-Language
60	I-Language
.	O
</s>
<s>
Suitable	O
for	O
a	O
character	O
set	O
of	O
60	O
glyphs	B-Application
and	O
sub-settable	O
to	O
48	O
.	O
</s>
<s>
Powerful	O
iterative	O
processing	O
with	O
good	O
support	O
for	O
structured	B-Language
programming	I-Language
.	O
</s>
<s>
New	O
attributes	O
,	O
statements	O
and	O
statement	O
options	O
could	O
be	O
added	O
to	O
PL/I	B-Language
without	O
invalidating	O
existing	O
programs	O
.	O
</s>
<s>
Exception	B-General_Concept
handling	I-General_Concept
capabilities	O
for	O
controlling	O
and	O
intercepting	O
exceptional	O
conditions	O
at	O
run	O
time	O
.	O
</s>
<s>
Programs	O
divided	O
into	O
separately	O
compilable	O
sections	O
,	O
with	O
extensive	O
compile-time	O
facilities	O
(	O
a.k.a.	O
</s>
<s>
External	O
names	O
to	O
bind	O
separately	O
compiled	B-Language
procedures	O
into	O
a	O
single	O
program	O
.	O
</s>
<s>
and	O
the	O
ANSI	O
PL/I	B-Language
General-Purpose	O
Subset	O
Standard	O
.	O
</s>
<s>
A	O
PL/I	B-Language
program	O
consists	O
of	O
a	O
set	O
of	O
procedures	O
,	O
each	O
of	O
which	O
is	O
written	O
as	O
a	O
sequence	O
of	O
statements	O
.	O
</s>
<s>
(	O
Features	O
such	O
as	O
multi-tasking	B-Operating_System
and	O
the	O
PL/I	B-Language
preprocessor	I-Language
are	O
not	O
in	O
the	O
Standard	O
but	O
are	O
supported	O
in	O
the	O
PL/I	B-Language
F	O
compiler	B-Language
and	O
some	O
other	O
implementations	O
are	O
discussed	O
in	O
the	O
Language	O
evolution	O
section	O
.	O
)	O
</s>
<s>
The	O
base	O
,	O
scale	O
,	O
precision	O
and	O
scale	B-Algorithm
factor	I-Algorithm
of	O
the	O
Picture-for-arithmetic	O
type	O
is	O
encoded	O
within	O
the	O
picture-specification	O
.	O
</s>
<s>
There	O
are	O
no	O
reserved	O
words	O
in	O
PL/I	B-Language
.	O
</s>
<s>
FIXED	O
[( 	O
precision	O
)	O
]	O
INTERNAL	O
or	O
INT	O
FLOAT	B-Algorithm
[( 	O
number	O
of	O
digits	O
)	O
]	O
LIKE	O
unsubscripted	O
reference	O
FORMAT	O
LOCAL	O
LABEL	O
OPTIONS(options )	O
MEMBER	O
PARAMETER	O
or	O
PARM	O
NONVARYING	O
or	O
NONVAR	O
POSITION	O
[( 	O
expression	O
)	O
]	O
or	O
POS	O
...	O
</s>
<s>
Current	O
compilers	B-Language
from	O
Micro	O
Focus	O
,	O
and	O
particularly	O
that	O
from	O
IBM	O
implement	O
many	O
extensions	O
over	O
the	O
standardized	O
version	O
of	O
the	O
language	O
.	O
</s>
<s>
The	O
IBM	O
extensions	O
are	O
summarised	O
in	O
the	O
Implementation	O
sub-section	O
for	O
the	O
compiler	B-Language
later	O
.	O
</s>
<s>
Although	O
there	O
are	O
some	O
extensions	O
common	O
to	O
these	O
compilers	B-Language
the	O
lack	O
of	O
a	O
current	O
standard	O
means	O
that	O
compatibility	O
is	O
not	O
guaranteed	O
.	O
</s>
<s>
In	O
1969	O
ANSI	O
established	O
a	O
"	O
Composite	O
Language	O
Development	O
Committee	O
"	O
,	O
nicknamed	O
"	O
Kludge	O
"	O
,	O
later	O
renamed	O
X3J1	B-Language
PL/I	B-Language
.	O
</s>
<s>
Standardization	O
became	O
a	O
joint	O
effort	O
of	O
ECMA	O
TC/10	O
and	O
ANSI	O
X3J1	B-Language
.	O
</s>
<s>
The	O
major	O
features	O
omitted	O
from	O
the	O
base	O
document	O
were	O
multitasking	B-Operating_System
and	O
the	O
attributes	O
for	O
program	O
optimization	O
(	O
e.g.	O
</s>
<s>
In	O
the	O
event	O
that	O
the	O
committees	O
disagreed	O
,	O
the	O
chairs	O
,	O
initially	O
Michael	O
Marcotty	O
of	O
General	O
Motors	O
and	O
C.A.R.	O
</s>
<s>
In	O
addition	O
to	O
IBM	O
,	O
Honeywell	O
,	O
CDC	O
,	O
Data	O
General	O
,	O
Digital	O
Equipment	O
Corporation	O
,	O
Prime	O
Computer	O
,	O
Burroughs	O
,	O
RCA	O
,	O
and	O
Univac	O
served	O
on	O
X3J1	B-Language
along	O
with	O
major	O
users	O
Eastman	O
Kodak	O
,	O
MITRE	O
,	O
Union	O
Carbide	O
,	O
Bell	O
Laboratories	O
,	O
and	O
various	O
government	O
and	O
university	O
representatives	O
.	O
</s>
<s>
Further	O
development	O
of	O
the	O
language	O
occurred	O
in	O
the	O
standards	O
bodies	O
,	O
with	O
continuing	O
improvements	O
in	O
structured	B-Language
programming	I-Language
and	O
internal	O
consistency	O
,	O
and	O
with	O
the	O
omission	O
of	O
the	O
more	O
obscure	O
or	O
contentious	O
features	O
.	O
</s>
<s>
As	O
language	O
development	O
neared	O
an	O
end	O
,	O
X3J1/TC10	O
realized	O
that	O
there	O
were	O
a	O
number	O
of	O
problems	O
with	O
a	O
document	O
written	O
in	O
English	O
text	O
.	O
</s>
<s>
The	O
standard	O
is	O
couched	O
as	O
a	O
formal	O
definition	O
using	O
a	O
"	O
PL/I	B-Language
Machine	O
"	O
to	O
specify	O
the	O
semantics	O
.	O
</s>
<s>
A	O
"	O
PL/I	B-Language
General-Purpose	O
Subset	O
"	O
(	O
"	O
Subset-G	O
"	O
)	O
standard	O
was	O
issued	O
by	O
ANSI	O
in	O
1981	O
and	O
a	O
revision	O
published	O
in	O
1987	O
.	O
</s>
<s>
The	O
General	O
Purpose	O
subset	O
was	O
widely	O
adopted	O
as	O
the	O
kernel	O
for	O
PL/I	B-Language
implementations	O
.	O
</s>
<s>
PL/I	B-Language
was	O
first	O
implemented	O
by	O
IBM	O
,	O
at	O
its	O
Hursley	O
Laboratories	O
in	O
the	O
United	O
Kingdom	O
,	O
as	O
part	O
of	O
the	O
development	O
of	O
System/360	B-Application
.	O
</s>
<s>
The	O
first	O
production	O
PL/I	B-Language
compiler	B-Language
was	O
the	O
PL/I	B-Language
F	O
compiler	B-Language
for	O
the	O
OS/360	B-Application
Operating	B-General_Concept
System	I-General_Concept
,	O
built	O
by	O
John	O
Nash	O
's	O
team	O
at	O
Hursley	O
in	O
the	O
UK	O
:	O
the	O
runtime	O
library	O
team	O
was	O
managed	O
by	O
I.M.	O
</s>
<s>
The	O
PL/I	B-Language
F	O
compiler	B-Language
was	O
written	O
entirely	O
in	O
System/360	B-Application
assembly	O
language	O
.	O
</s>
<s>
OS/360	B-Application
is	O
a	O
real-memory	O
environment	O
and	O
the	O
compiler	B-Language
was	O
designed	O
for	O
systems	O
with	O
as	O
little	O
as	O
64	O
kilobytes	O
of	O
real	O
storage	O
–	O
F	O
being	O
64	O
kB	O
in	O
S/360	B-Application
parlance	O
.	O
</s>
<s>
To	O
fit	O
a	O
large	O
compiler	B-Language
into	O
the	O
44	O
kilobytes	O
of	O
memory	O
available	O
on	O
a	O
64-kilobyte	O
machine	O
,	O
the	O
compiler	B-Language
consists	O
of	O
a	O
control	O
phase	O
and	O
a	O
large	O
number	O
of	O
compiler	B-Language
phases	O
(	O
approaching	O
100	O
)	O
.	O
</s>
<s>
The	O
phases	O
are	O
brought	O
into	O
memory	O
from	O
disk	O
,	O
one	O
at	O
a	O
time	O
,	O
to	O
handle	O
particular	O
language	O
features	O
and	O
aspects	O
of	O
compilation	B-Language
.	O
</s>
<s>
Each	O
phase	O
makes	O
a	O
single	O
pass	O
over	O
the	O
partially-compiled	O
program	O
,	O
usually	O
held	O
in	O
memory	O
.	O
</s>
<s>
Aspects	O
of	O
the	O
language	O
were	O
still	O
being	O
designed	O
as	O
PL/I	B-Language
F	O
was	O
implemented	O
,	O
so	O
some	O
were	O
omitted	O
until	O
later	O
releases	O
.	O
</s>
<s>
PL/I	B-Language
RECORD	O
I/O	O
was	O
shipped	O
with	O
PL/I	B-Language
F	O
Release	O
2	O
.	O
</s>
<s>
The	O
list	B-Language
processing	I-Language
functions	O
Based	O
Variables	O
,	O
Pointers	O
,	O
Areas	O
and	O
Offsets	O
and	O
LOCATE-mode	O
I/O	O
were	O
first	O
shipped	O
in	O
Release	O
4	O
.	O
</s>
<s>
In	O
a	O
major	O
attempt	O
to	O
speed	O
up	O
PL/I	B-Language
code	O
to	O
compete	O
with	O
Fortran	B-Application
object	O
code	O
,	O
PL/I	B-Language
F	O
Release	O
5	O
does	O
substantial	O
program	O
optimization	O
of	O
DO-loops	O
facilitated	O
by	O
the	O
REORDER	O
option	O
on	O
procedures	O
.	O
</s>
<s>
A	O
version	O
of	O
PL/I	B-Language
F	O
was	O
released	O
on	O
the	O
TSS/360	B-Application
timesharing	O
operating	B-General_Concept
system	I-General_Concept
for	O
the	O
System/360	B-Device
Model	I-Device
67	I-Device
,	O
adapted	O
at	O
the	O
IBM	O
Mohansic	O
Lab	O
.	O
</s>
<s>
The	O
IBM	O
La	O
Gaude	O
Lab	O
in	O
France	O
developed	O
"	O
Language	O
Conversion	O
Programs	O
"	O
to	O
convert	O
Fortran	B-Application
,	O
Cobol	B-Application
,	O
and	O
Algol	B-Language
programs	O
to	O
the	O
PL/I	B-Language
F	O
level	O
of	O
PL/I	B-Language
.	O
</s>
<s>
The	O
PL/I	B-Language
D	O
compiler	B-Language
,	O
using	O
16	O
kilobytes	O
of	O
memory	O
,	O
was	O
developed	O
by	O
IBM	O
Germany	O
for	O
the	O
DOS/360	O
low	O
end	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
It	O
implements	O
a	O
subset	O
of	O
the	O
PL/I	B-Language
language	O
requiring	O
all	O
strings	O
and	O
arrays	O
to	O
have	O
fixed	O
extents	O
,	O
thus	O
simplifying	O
the	O
run-time	O
environment	O
.	O
</s>
<s>
Reflecting	O
the	O
underlying	O
operating	B-General_Concept
system	I-General_Concept
,	O
it	O
lacks	O
dynamic	O
storage	O
allocation	O
and	O
the	O
controlled	O
storage	O
class	O
.	O
</s>
<s>
It	O
was	O
shipped	O
within	O
a	O
year	O
of	O
PL/I	B-Language
F	O
.	O
</s>
<s>
Compilers	B-Language
were	O
implemented	O
by	O
several	O
groups	O
in	O
the	O
early	O
1960s	O
.	O
</s>
<s>
The	O
Multics	B-Application
project	O
at	O
MIT	O
,	O
one	O
of	O
the	O
first	O
to	O
develop	O
an	O
operating	B-General_Concept
system	I-General_Concept
in	O
a	O
high-level	O
language	O
,	O
used	O
Early	O
PL/I	B-Language
(	O
EPL	O
)	O
,	O
a	O
subset	O
dialect	O
of	O
PL/I	B-Language
,	O
as	O
their	O
implementation	O
language	O
in	O
1964	O
.	O
</s>
<s>
Initially	O
,	O
it	O
was	O
developed	O
using	O
the	O
TMG	B-Application
compiler-compiler	O
.	O
</s>
<s>
The	O
influential	O
Multics	B-Application
PL/I	B-Language
compiler	B-Language
was	O
the	O
source	O
of	O
compiler	B-Language
technology	O
used	O
by	O
a	O
number	O
of	O
manufacturers	O
and	O
software	O
groups	O
.	O
</s>
<s>
EPL	O
was	O
a	O
system	B-Language
programming	I-Language
language	I-Language
and	O
a	O
dialect	O
of	O
PL/I	B-Language
that	O
had	O
some	O
capabilities	O
absent	O
in	O
the	O
original	O
PL/I	B-Language
.	O
</s>
<s>
The	O
Honeywell	O
PL/I	B-Language
compiler	B-Language
(	O
for	O
Series	O
60	O
)	O
is	O
an	O
implementation	O
of	O
the	O
full	O
ANSI	O
X3J1	B-Language
standard	O
.	O
</s>
<s>
The	O
PL/I	B-Language
Optimizer	O
and	O
Checkout	O
compilers	B-Language
produced	O
in	O
Hursley	O
support	O
a	O
common	O
level	O
of	O
PL/I	B-Language
language	O
and	O
aimed	O
to	O
replace	O
the	O
PL/I	B-Language
F	O
compiler	B-Language
.	O
</s>
<s>
The	O
checkout	O
compiler	B-Language
is	O
a	O
rewrite	O
of	O
PL/I	B-Language
F	O
in	O
BSL	O
,	O
IBM	O
's	O
PL/I	B-Language
-like	O
proprietary	O
implementation	O
language	O
(	O
later	O
PL/S	B-Language
)	O
.	O
</s>
<s>
The	O
performance	O
objectives	O
set	O
for	O
the	O
compilers	B-Language
are	O
shown	O
in	O
an	O
IBM	O
presentation	O
to	O
the	O
BCS	O
.	O
</s>
<s>
The	O
compilers	B-Language
had	O
to	O
produce	O
identical	O
results	O
the	O
Checkout	O
Compiler	B-Language
is	O
used	O
to	O
debug	O
programs	O
that	O
would	O
then	O
be	O
submitted	O
to	O
the	O
Optimizer	O
.	O
</s>
<s>
Given	O
that	O
the	O
compilers	B-Language
had	O
entirely	O
different	O
designs	O
and	O
were	O
handling	O
the	O
full	O
PL/I	B-Language
language	O
this	O
goal	O
was	O
challenging	O
:	O
it	O
was	O
achieved	O
.	O
</s>
<s>
The	O
PL/I	B-Language
optimizing	O
compiler	B-Language
took	O
over	O
from	O
the	O
PL/I	B-Language
F	O
compiler	B-Language
and	O
was	O
IBM	O
's	O
workhorse	O
compiler	B-Language
from	O
the	O
1970s	O
to	O
the	O
1990s	O
.	O
</s>
<s>
Like	O
PL/I	B-Language
F	O
,	O
it	O
is	O
a	O
multiple	O
pass	O
compiler	B-Language
with	O
a	O
44	O
kilobyte	O
design	O
point	O
,	O
but	O
it	O
is	O
an	O
entirely	O
new	O
design	O
.	O
</s>
<s>
Unlike	O
the	O
F	O
compiler	B-Language
,	O
it	O
has	O
to	O
perform	O
compile	B-Language
time	O
evaluation	O
of	O
constant	O
expressions	O
using	O
the	O
run-time	O
library	O
,	O
reducing	O
the	O
maximum	O
memory	O
for	O
a	O
compiler	B-Language
phase	O
to	O
28	O
kilobytes	O
.	O
</s>
<s>
A	O
second-time	O
around	O
design	O
,	O
it	O
succeeded	O
in	O
eliminating	O
the	O
annoyances	O
of	O
PL/I	B-Language
F	O
such	O
as	O
cascading	O
diagnostics	O
.	O
</s>
<s>
It	O
was	O
written	O
in	O
S/360	B-Application
Macro	O
Assembler	O
by	O
a	O
team	O
,	O
led	O
by	O
Tony	O
Burbridge	O
,	O
most	O
of	O
whom	O
had	O
worked	O
on	O
PL/I	B-Language
F	O
.	O
Macros	O
were	O
defined	O
to	O
automate	O
common	O
compiler	B-Language
services	O
and	O
to	O
shield	O
the	O
compiler	B-Language
writers	O
from	O
the	O
task	O
of	O
managing	O
real-mode	O
storage	O
,	O
allowing	O
the	O
compiler	B-Language
to	O
be	O
moved	O
easily	O
to	O
other	O
memory	O
models	O
.	O
</s>
<s>
The	O
gamut	O
of	O
program	O
optimization	O
techniques	O
developed	O
for	O
the	O
contemporary	O
IBM	O
Fortran	B-Application
H	O
compiler	B-Language
were	O
deployed	O
:	O
the	O
Optimizer	O
equaled	O
Fortran	B-Application
execution	O
speeds	O
in	O
the	O
hands	O
of	O
good	O
programmers	O
.	O
</s>
<s>
Announced	O
with	O
IBM	B-Device
S/370	I-Device
in	O
1970	O
,	O
it	O
shipped	O
first	O
for	O
the	O
DOS/360	O
operating	B-General_Concept
system	I-General_Concept
in	O
August	O
1971	O
,	O
and	O
shortly	O
afterward	O
for	O
OS/360	B-Application
,	O
and	O
the	O
first	O
virtual	O
memory	O
IBM	O
operating	B-General_Concept
systems	I-General_Concept
OS/VS1	B-Application
,	O
MVS	B-Application
,	O
and	O
VM/CMS	B-Application
.	O
</s>
<s>
(	O
The	O
developers	O
were	O
unaware	O
that	O
while	O
they	O
were	O
shoehorning	O
the	O
code	O
into	O
28	O
kb	O
sections	O
,	O
IBM	O
Poughkeepsie	O
was	O
finally	O
ready	O
to	O
ship	O
virtual	O
memory	O
support	O
in	O
OS/360	B-Application
)	O
.	O
</s>
<s>
This	O
compiler	B-Language
went	O
through	O
many	O
versions	O
covering	O
all	O
mainframe	B-Architecture
operating	B-General_Concept
systems	I-General_Concept
including	O
the	O
operating	B-General_Concept
systems	I-General_Concept
of	O
the	O
Japanese	O
plug-compatible	B-Device
machines	I-Device
(	O
PCMs	O
)	O
.	O
</s>
<s>
The	O
compiler	B-Language
has	O
been	O
superseded	O
by	O
"	O
IBM	O
PL/I	B-Language
for	O
OS/2	B-Application
,	O
AIX	B-Application
,	O
Linux	B-Application
,	O
z/OS	B-Application
"	O
below	O
.	O
</s>
<s>
The	O
PL/I	B-Language
checkout	O
compiler	B-Language
,	O
(	O
colloquially	O
"	O
The	O
Checker	O
"	O
)	O
announced	O
in	O
August	O
1970	O
was	O
designed	O
to	O
speed	O
and	O
improve	O
the	O
debugging	O
of	O
PL/I	B-Language
programs	O
.	O
</s>
<s>
The	O
three-pass	O
design	O
cut	O
the	O
time	O
to	O
compile	B-Language
a	O
program	O
to	O
25%	O
of	O
that	O
taken	O
by	O
the	O
F	O
Compiler	B-Language
.	O
</s>
<s>
It	O
can	O
be	O
run	O
from	O
an	O
interactive	O
terminal	O
,	O
converting	O
PL/I	B-Language
programs	O
into	O
an	O
internal	O
format	O
,	O
"	O
H-text	O
"	O
.	O
</s>
<s>
This	O
format	O
is	O
interpreted	O
by	O
the	O
Checkout	O
compiler	B-Language
at	O
run-time	O
,	O
detecting	O
virtually	O
all	O
types	O
of	O
errors	O
.	O
</s>
<s>
Over	O
time	O
the	O
debugging	O
capability	O
of	O
mainframe	B-Architecture
programming	O
environments	O
developed	O
most	O
of	O
the	O
functions	O
offered	O
by	O
this	O
compiler	B-Language
and	O
it	O
was	O
withdrawn	O
(	O
in	O
the	O
1990s	O
?	O
)	O
</s>
<s>
Perhaps	O
the	O
most	O
commercially	O
successful	O
implementation	O
aside	O
from	O
IBM	O
's	O
was	O
Digital	O
Equipment	O
Corporation	O
's	O
VAX	B-Device
PL/I	B-Language
,	O
later	O
known	O
as	O
DEC	O
PL/I	B-Language
.	O
</s>
<s>
The	O
implementation	O
is	O
"	O
a	O
strict	O
superset	O
of	O
the	O
ANSI	O
X3.4-1981	O
PL/I	B-Language
General	O
Purpose	O
Subset	O
and	O
provides	O
most	O
of	O
the	O
features	O
of	O
the	O
new	O
ANSI	O
X3.74-1987	O
PL/I	B-Language
General	O
Purpose	O
Subset	O
"	O
,	O
and	O
was	O
first	O
released	O
in	O
1988	O
.	O
</s>
<s>
It	O
originally	O
used	O
a	O
compiler	B-Language
backend	O
named	O
the	O
VAX	B-Device
Code	O
Generator	O
(	O
VCG	O
)	O
created	O
by	O
a	O
team	O
led	O
by	O
Dave	O
Cutler	O
.	O
</s>
<s>
The	O
front	O
end	O
was	O
designed	O
by	O
Robert	O
Freiburghouse	O
,	O
and	O
was	O
ported	O
to	O
VAX/VMS	B-Operating_System
from	O
Multics	B-Application
.	O
</s>
<s>
It	O
runs	O
on	O
VMS	B-Application
on	O
VAX	B-Device
and	O
Alpha	B-Device
,	O
and	O
on	O
Tru64	B-Operating_System
.	O
</s>
<s>
During	O
the	O
1990s	O
,	O
Digital	O
sold	O
the	O
compiler	B-Language
to	O
UniPrise	O
Systems	O
,	O
who	O
later	O
sold	O
it	O
to	O
a	O
company	O
named	O
Kednos	O
.	O
</s>
<s>
Kednos	O
marketed	O
the	O
compiler	B-Language
as	O
Kednos	O
PL/I	B-Language
until	O
October	O
2016	O
when	O
the	O
company	O
ceased	O
trading	O
.	O
</s>
<s>
In	O
the	O
late	O
1960s	O
and	O
early	O
1970s	O
,	O
many	O
US	O
and	O
Canadian	O
universities	O
were	O
establishing	O
time-sharing	O
services	O
on	O
campus	O
and	O
needed	O
conversational	O
compiler/interpreters	O
for	O
use	O
in	O
teaching	O
science	O
,	O
mathematics	O
,	O
engineering	O
,	O
and	O
computer	O
science	O
.	O
</s>
<s>
Dartmouth	O
was	O
developing	O
BASIC	O
,	O
but	O
PL/I	B-Language
was	O
a	O
popular	O
choice	O
,	O
as	O
it	O
was	O
concise	O
and	O
easy	O
to	O
teach	O
.	O
</s>
<s>
As	O
the	O
IBM	O
offerings	O
were	O
unsuitable	O
,	O
a	O
number	O
of	O
schools	O
built	O
their	O
own	O
subsets	O
of	O
PL/I	B-Language
and	O
their	O
own	O
interactive	O
support	O
.	O
</s>
<s>
In	O
the	O
1960s	O
and	O
early	O
1970s	O
,	O
Allen-Babcock	O
implemented	O
the	O
Remote	O
Users	O
of	O
Shared	O
Hardware	O
(	O
RUSH	O
)	O
time	O
sharing	O
system	O
for	O
an	O
IBM	B-Device
System/360	I-Device
Model	I-Device
50	I-Device
with	O
custom	O
microcode	B-Device
and	O
subsequently	O
implemented	O
IBM	O
's	O
CPS	B-General_Concept
,	O
an	O
interactive	O
time-sharing	O
system	O
for	O
OS/360	B-Application
aimed	O
at	O
teaching	O
computer	O
science	O
basics	O
,	O
offered	O
a	O
limited	O
subset	O
of	O
the	O
PL/I	B-Language
language	O
in	O
addition	O
to	O
BASIC	O
and	O
a	O
remote	O
job	O
entry	O
facility	O
.	O
</s>
<s>
PL/C	B-Language
,	O
a	O
dialect	O
for	O
teaching	O
,	O
a	O
compiler	B-Language
developed	O
at	O
Cornell	O
University	O
,	O
had	O
the	O
unusual	O
capability	O
of	O
never	O
failing	O
to	O
compile	B-Language
any	O
program	O
through	O
the	O
use	O
of	O
extensive	O
automatic	O
correction	O
of	O
many	O
syntax	O
errors	O
and	O
by	O
converting	O
any	O
remaining	O
syntax	O
errors	O
to	O
output	O
statements	O
.	O
</s>
<s>
The	O
language	O
was	O
almost	O
all	O
of	O
PL/I	B-Language
as	O
implemented	O
by	O
IBM	O
.	O
</s>
<s>
PL/C	B-Language
was	O
a	O
very	O
fast	O
compiler	B-Language
.	O
</s>
<s>
(	O
Student	O
Language/1	O
,	O
Student	O
Language/One	O
or	O
Subset	O
Language/1	O
)	O
was	O
a	O
PL/I	B-Language
subset	O
,	O
initially	O
available	O
late	O
1960s	O
,	O
that	O
ran	O
interpretively	O
on	O
the	O
IBM	B-Device
1130	I-Device
;	O
instructional	O
use	O
was	O
its	O
strong	O
point	O
.	O
</s>
<s>
PLAGO	O
,	O
created	O
at	O
the	O
Polytechnic	O
Institute	O
of	O
Brooklyn	O
,	O
used	O
a	O
simplified	O
subset	O
of	O
the	O
PL/I	B-Language
language	O
and	O
focused	O
on	O
good	O
diagnostic	O
error	O
messages	O
and	O
fast	O
compilation	B-Language
times	O
.	O
</s>
<s>
The	O
Computer	O
Systems	O
Research	O
Group	O
of	O
the	O
University	O
of	O
Toronto	O
produced	O
the	O
SP/k	B-Language
compilers	B-Language
which	O
supported	O
a	O
sequence	O
of	O
subsets	O
of	O
PL/I	B-Language
called	O
SP/1	O
,	O
SP/2	O
,	O
SP/3	O
,	O
...	O
,	O
SP/8	O
for	O
teaching	O
programming	O
.	O
</s>
<s>
Programs	O
that	O
ran	O
without	O
errors	O
under	O
the	O
SP/k	B-Language
compilers	B-Language
produced	O
the	O
same	O
results	O
under	O
other	O
contemporary	O
PL/I	B-Language
compilers	B-Language
such	O
as	O
IBM	O
's	O
PL/I	B-Language
F	O
compiler	B-Language
,	O
IBM	O
's	O
checkout	O
compiler	B-Language
or	O
Cornell	O
University	O
's	O
PL/C	B-Language
compiler	B-Language
.	O
</s>
<s>
In	O
a	O
major	O
revamp	O
of	O
PL/I	B-Language
,	O
IBM	O
Santa	O
Teresa	O
in	O
California	O
launched	O
an	O
entirely	O
new	O
compiler	B-Language
in	O
1992	O
.	O
</s>
<s>
The	O
initial	O
shipment	O
was	O
for	O
OS/2	B-Application
and	O
included	O
most	O
ANSI-G	O
features	O
and	O
many	O
new	O
PL/I	B-Language
features	O
.	O
</s>
<s>
Subsequent	O
releases	O
provided	O
additional	O
platforms	O
(	O
MVS	B-Application
,	O
VM	B-Application
,	O
OS/390	B-Application
,	O
AIX	B-Application
and	O
Windows	B-Application
)	O
,	O
but	O
as	O
of	O
2021	O
,	O
the	O
only	O
supported	O
platforms	O
are	O
z/OS	B-Application
and	O
AIX	B-Application
.	O
</s>
<s>
IBM	O
continued	O
to	O
add	O
functions	O
to	O
make	O
PL/I	B-Language
fully	O
competitive	O
with	O
other	O
languages	O
(	O
particularly	O
C	B-Language
and	O
C++	B-Language
)	O
in	O
areas	O
where	O
it	O
had	O
been	O
overtaken	O
.	O
</s>
<s>
The	O
corresponding	O
"	O
IBM	O
Language	O
Environment	O
"	O
supports	O
inter-operation	O
of	O
PL/I	B-Language
programs	O
with	O
Database	O
and	O
Transaction	O
systems	O
,	O
and	O
with	O
programs	O
written	O
in	O
C	B-Language
,	O
C++	B-Language
,	O
and	O
COBOL	B-Application
,	O
the	O
compiler	B-Language
supports	O
all	O
the	O
data	O
types	O
needed	O
for	O
intercommunication	O
with	O
these	O
languages	O
.	O
</s>
<s>
The	O
PL/I	B-Language
design	O
principles	O
were	O
retained	O
and	O
withstood	O
this	O
major	O
extension	O
,	O
comprising	O
several	O
new	O
data	O
types	O
,	O
new	O
statements	O
and	O
statement	O
options	O
,	O
new	O
exception	B-General_Concept
conditions	I-General_Concept
,	O
and	O
new	O
organisations	O
of	O
program	O
source	O
.	O
</s>
<s>
The	O
resulting	O
language	O
is	O
a	O
compatible	O
super-set	O
of	O
the	O
PL/I	B-Language
Standard	O
and	O
of	O
the	O
earlier	O
IBM	O
compilers	B-Language
.	O
</s>
<s>
Major	O
topics	O
added	O
to	O
PL/I	B-Language
were	O
:	O
</s>
<s>
Additional	O
structured	B-Language
programming	I-Language
constructs	O
.	O
</s>
<s>
The	O
latest	O
series	O
of	O
PL/I	B-Language
compilers	B-Language
for	O
z/OS	B-Application
,	O
called	O
Enterprise	O
PL/I	B-Language
for	O
z/OS	B-Application
,	O
leverage	O
code	O
generation	O
for	O
the	O
latest	O
z/Architecture	B-Device
processors	O
(	O
z14	O
,	O
z13	O
,	O
zEC12	O
,	O
zBC12	O
,	O
z196	O
,	O
z114	O
)	O
via	O
the	O
use	O
of	O
ARCHLVL	O
parm	O
control	O
passed	O
during	O
compilation	B-Language
,	O
and	O
was	O
the	O
second	O
High	O
level	O
language	O
supported	O
by	O
z/OS	B-Application
Language	O
Environment	O
to	O
do	O
so	O
(	O
XL	O
C/C	O
++	O
being	O
the	O
first	O
,	O
and	O
Enterprise	O
COBOL	B-Application
v5	O
the	O
last	O
.	O
)	O
</s>
<s>
The	O
ordinal	O
facilities	O
are	O
like	O
those	O
in	O
Pascal	B-Application
,	O
</s>
<s>
The	O
DEFINE-statement	O
(	O
see	O
below	O
)	O
allows	O
additional	O
TYPEs	O
to	O
be	O
declared	O
composed	O
from	O
PL/I	B-Language
'	O
s	O
built-in	O
attributes	O
.	O
</s>
<s>
The	O
UNION	O
attribute	O
(	O
equivalent	O
to	O
CELL	O
in	O
early	O
PL/I	B-Language
specifications	O
)	O
permits	O
several	O
scalar	O
variables	O
,	O
arrays	O
,	O
or	O
structures	O
to	O
share	O
the	O
same	O
storage	O
in	O
a	O
unit	O
that	O
occupies	O
the	O
amount	O
of	O
storage	O
needed	O
for	O
the	O
largest	O
alternative	O
.	O
</s>
<s>
Compound	O
assignment	O
operators	O
a	O
la	O
C	B-Language
e.g.	O
</s>
<s>
Additional	O
parameter	O
descriptors	B-Data_Structure
and	O
attributes	O
were	O
added	O
for	O
omitted	O
arguments	O
and	O
variable	O
length	O
argument	O
lists	O
.	O
</s>
<s>
A	O
number	O
of	O
vendors	O
produced	O
compilers	B-Language
to	O
compete	O
with	O
IBM	O
PL/I	B-Language
F	O
or	O
Optimizing	O
compiler	B-Language
on	O
mainframes	B-Architecture
and	O
minicomputers	O
in	O
the	O
1970s	O
.	O
</s>
<s>
In	O
1974	O
Burroughs	O
Corporation	O
announced	O
PL/I	B-Language
for	O
the	O
B6700	O
and	O
B7700	O
.	O
</s>
<s>
UNIVAC	O
released	O
a	O
UNIVAC	O
PL/I	B-Language
,	O
and	O
in	O
the	O
1970s	O
also	O
used	O
a	O
variant	O
of	O
PL/I	B-Language
,	O
PL/I	B-Language
PLUS	O
,	O
for	O
system	B-Application
programming	I-Application
.	O
</s>
<s>
From	O
1978	O
Data	O
General	O
provided	O
PL/I	B-Language
on	O
its	O
Eclipse	B-Device
and	O
Eclipse	B-Device
MV	I-Device
platforms	O
running	O
the	O
AOS	O
,	O
AOS/VS	O
&	O
AOS/VS	O
II	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
A	O
number	O
of	O
operating	B-General_Concept
system	I-General_Concept
utility	O
programs	O
were	O
written	O
in	O
the	O
language	O
.	O
</s>
<s>
CDC	O
delivered	O
an	O
optimizing	O
subset	O
PL/I	B-Language
compiler	B-Language
for	O
Cyber	O
70	O
,	O
170	O
and	O
6000	O
series	O
.	O
</s>
<s>
Fujitsu	O
delivered	O
a	O
PL/I	B-Language
compiler	B-Language
equivalent	O
to	O
the	O
PL/I	B-Language
Optimizer	O
.	O
</s>
<s>
Stratus	B-General_Concept
Technologies	I-General_Concept
PL/I	B-Language
is	O
an	O
ANSI	O
G	O
implementation	O
for	O
the	O
VOS	B-General_Concept
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
IBM	B-Device
Series/1	I-Device
PL/I	B-Language
is	O
an	O
extended	O
subset	O
of	O
ANSI	O
Programming	O
Language	O
PL/I	B-Language
(	O
ANSI	O
X3.53-1976	O
)	O
for	O
the	O
IBM	B-Device
Series/1	I-Device
Realtime	O
Programming	O
System	O
.	O
</s>
<s>
In	O
2011	O
,	O
Raincode	O
designed	O
a	O
full	O
legacy	O
compiler	B-Language
for	O
the	O
Microsoft	O
.NET	B-Application
and	O
.NET	B-Application
Core	I-Application
platforms	O
,	O
named	O
The	O
Raincode	O
PL/I	B-Language
compiler	B-Language
.	O
</s>
<s>
In	O
the	O
1970s	O
and	O
1980s	O
Digital	O
Research	O
sold	O
a	O
PL/I	B-Language
compiler	B-Language
for	O
CP/M	B-Application
(	O
PL/I	B-Language
-80	O
)	O
,	O
CP/M	B-Operating_System
-86	I-Operating_System
(	O
PL/I	B-Language
-86	O
)	O
and	O
Personal	O
Computers	O
with	O
DOS	B-Device
.	O
</s>
<s>
It	O
was	O
based	O
on	O
Subset	O
G	O
of	O
PL/I	B-Language
and	O
was	O
written	O
in	O
PL/M	B-Language
.	O
</s>
<s>
Micro	O
Focus	O
implemented	O
Open	O
PL/I	B-Language
for	O
Windows	B-Application
and	O
UNIX/Linux	O
systems	O
,	O
which	O
they	O
acquired	O
from	O
Liant	O
.	O
</s>
<s>
IBM	O
delivered	O
PL/I	B-Language
for	O
OS/2	B-Application
in	O
1994	O
,	O
and	O
PL/I	B-Language
for	O
AIX	B-Application
in	O
1995	O
.	O
</s>
<s>
Iron	O
Spring	O
PL/I	B-Language
for	O
OS/2	B-Application
and	O
later	O
Linux	B-Application
was	O
introduced	O
in	O
2007	O
.	O
</s>
<s>
PL/S	B-Language
,	O
a	O
dialect	O
of	O
PL/I	B-Language
,	O
initially	O
called	O
BSL	O
was	O
developed	O
in	O
the	O
late	O
1960s	O
and	O
became	O
the	O
system	B-Language
programming	I-Language
language	I-Language
for	O
IBM	O
mainframes	B-Architecture
.	O
</s>
<s>
Almost	O
all	O
IBM	O
mainframe	B-Architecture
system	O
software	O
in	O
the	O
1970s	O
and	O
1980s	O
was	O
written	O
in	O
PL/S	B-Language
.	O
</s>
<s>
It	O
differed	O
from	O
PL/I	B-Language
in	O
that	O
there	O
were	O
no	O
data	O
type	O
conversions	O
,	O
no	O
run-time	O
environment	O
,	O
structures	O
were	O
mapped	O
differently	O
,	O
and	O
assignment	O
was	O
a	O
byte	O
by	O
byte	O
copy	O
.	O
</s>
<s>
PL/S	B-Language
was	O
succeeded	O
by	O
PL/AS	B-Language
,	O
and	O
then	O
by	O
PL/X	B-Language
,	O
which	O
is	O
the	O
language	O
currently	O
used	O
for	O
internal	O
work	O
on	O
current	O
operating	B-General_Concept
systems	I-General_Concept
,	O
OS/390	B-Application
and	O
now	O
z/OS	B-Application
.	O
</s>
<s>
It	O
is	O
also	O
used	O
for	O
some	O
z/VSE	B-Application
and	O
z/VM	B-Application
components	O
.	O
</s>
<s>
IBM	B-Application
Db2	I-Application
for	O
z/OS	B-Application
is	O
also	O
written	O
in	O
PL/X	B-Language
.	O
</s>
<s>
PL/C	B-Language
,	O
is	O
an	O
instructional	O
dialect	O
of	O
the	O
PL/I	B-Language
computer	O
programming	O
language	O
,	O
developed	O
at	O
Cornell	O
University	O
in	O
the	O
1970s	O
.	O
</s>
<s>
Two	O
dialects	O
of	O
PL/I	B-Language
named	O
PL/MP	O
(	O
Machine	O
Product	O
)	O
and	O
PL/MI	O
(	O
Machine	O
Interface	O
)	O
were	O
used	O
by	O
IBM	O
in	O
the	O
system	O
software	O
of	O
the	O
System/38	B-Device
and	O
AS/400	B-Device
platforms	O
.	O
</s>
<s>
PL/MP	O
was	O
used	O
to	O
implement	O
the	O
so-called	O
Vertical	O
Microcode	B-Device
of	O
these	O
platforms	O
,	O
and	O
targeted	O
the	O
IMPI	O
instruction	O
set	O
.	O
</s>
<s>
PL/MI	O
targets	O
the	O
Machine	O
Interface	O
of	O
those	O
platforms	O
,	O
and	O
is	O
used	O
in	O
the	O
Control	B-Operating_System
Program	I-Operating_System
Facility	I-Operating_System
and	O
the	O
XPF	O
layer	O
of	O
OS/400	B-Application
.	O
</s>
<s>
The	O
PL/MP	O
code	O
was	O
mostly	O
replaced	O
with	O
C++	B-Language
when	O
OS/400	B-Application
was	O
ported	O
to	O
the	O
IBM	B-Device
RS64	I-Device
processor	O
family	O
,	O
although	O
some	O
was	O
retained	O
and	O
retargeted	O
for	O
the	O
PowerPC/Power	O
ISA	O
architecture	O
.	O
</s>
<s>
The	O
PL/MI	O
code	O
was	O
not	O
replaced	O
,	O
and	O
remains	O
in	O
use	O
in	O
IBM	B-Application
i	I-Application
.	O
</s>
<s>
PL/8	B-Language
(	O
or	O
PL.8	O
)	O
,	O
so-called	O
because	O
it	O
was	O
about	O
80%	O
of	O
PL/I	B-Language
,	O
was	O
originally	O
developed	O
by	O
IBM	O
Research	O
in	O
the	O
1970s	O
for	O
the	O
IBM	B-Device
801	I-Device
architecture	O
.	O
</s>
<s>
It	O
later	O
gained	O
support	O
for	O
the	O
Motorola	B-Device
68000	I-Device
and	O
System/370	B-Device
architectures	O
.	O
</s>
<s>
millicode	B-General_Concept
and	O
firmware	B-Application
for	O
z/Architecture	B-Device
systems	O
)	O
and	O
has	O
been	O
re-engineered	O
to	O
use	O
a	O
64-bit	B-Device
gcc-based	O
backend	O
.	O
</s>
<s>
Honeywell	O
,	O
Inc	O
.	O
developed	O
PL-6	B-Language
for	O
use	O
in	O
creating	O
the	O
CP-6	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Prime	O
Computer	O
used	O
two	O
different	O
PL/I	B-Language
dialects	O
as	O
the	O
system	B-Language
programming	I-Language
language	I-Language
of	O
the	O
PRIMOS	B-General_Concept
operating	B-General_Concept
system	I-General_Concept
:	O
PL/P	B-Language
,	O
starting	O
from	O
version	O
18	O
,	O
and	O
then	O
SPL	O
,	O
starting	O
from	O
version	O
19	O
.	O
</s>
<s>
XPL	B-Language
is	O
a	O
dialect	O
of	O
PL/I	B-Language
used	O
to	O
write	O
other	O
compilers	B-Language
using	O
the	O
XPL	B-Language
compiler	B-Language
techniques	O
.	O
</s>
<s>
XPL	B-Language
added	O
a	O
heap	O
string	O
datatype	O
to	O
its	O
small	O
subset	O
of	O
PL/I	B-Language
.	O
</s>
<s>
HAL/S	B-Language
is	O
a	O
real-time	O
aerospace	O
programming	O
language	O
,	O
best	O
known	O
for	O
its	O
use	O
in	O
the	O
Space	O
Shuttle	O
program	O
.	O
</s>
<s>
HAL/S	B-Language
was	O
implemented	O
in	O
XPL	B-Language
.	O
</s>
<s>
IBM	O
and	O
various	O
subcontractors	O
also	O
developed	O
another	O
PL/I	B-Language
variant	O
in	O
the	O
early	O
1970s	O
to	O
support	O
signal	O
processing	O
for	O
the	O
Navy	O
called	O
SPL/I	O
.	O
</s>
<s>
SabreTalk	B-Language
,	O
a	O
real-time	O
dialect	O
of	O
PL/I	B-Language
used	O
to	O
program	O
the	O
Sabre	B-Application
airline	O
reservation	O
system	O
.	O
</s>
<s>
PL/I	B-Language
implementations	O
were	O
developed	O
for	O
mainframes	B-Architecture
from	O
the	O
late	O
1960s	O
,	O
mini	O
computers	O
in	O
the	O
1970s	O
,	O
and	O
personal	O
computers	O
in	O
the	O
1980s	O
and	O
1990s	O
.	O
</s>
<s>
Although	O
its	O
main	O
use	O
has	O
been	O
on	O
mainframes	B-Architecture
,	O
there	O
are	O
PL/I	B-Language
versions	O
for	O
DOS	B-Device
,	O
Microsoft	B-Application
Windows	I-Application
,	O
OS/2	B-Application
,	O
AIX	B-Application
,	O
OpenVMS	B-Operating_System
,	O
and	O
Unix	B-Application
.	O
</s>
<s>
It	O
has	O
been	O
widely	O
used	O
in	O
business	O
data	B-General_Concept
processing	I-General_Concept
and	O
for	O
system	O
use	O
for	O
writing	O
operating	B-General_Concept
systems	I-General_Concept
on	O
certain	O
platforms	O
.	O
</s>
<s>
Very	O
complex	O
and	O
powerful	O
systems	O
have	O
been	O
built	O
with	O
PL/I	B-Language
:	O
</s>
<s>
The	O
SAS	B-Language
System	I-Language
was	O
initially	O
written	O
in	O
PL/I	B-Language
;	O
the	O
SAS	B-Language
data	O
step	O
is	O
still	O
modeled	O
on	O
PL/I	B-Language
syntax	O
.	O
</s>
<s>
The	O
pioneering	O
online	O
airline	O
reservation	O
system	O
Sabre	B-Application
was	O
originally	O
written	O
for	O
the	O
IBM	O
7090	O
in	O
assembler	O
.	O
</s>
<s>
The	O
S/360	B-Application
version	O
was	O
largely	O
written	O
using	O
SabreTalk	B-Language
,	O
a	O
purpose	O
built	O
subset	O
PL/I	B-Language
compiler	B-Language
for	O
a	O
dedicated	O
control	O
program	O
.	O
</s>
<s>
The	O
Multics	B-Application
operating	I-Application
system	I-Application
was	O
largely	O
written	O
in	O
PL/I	B-Language
.	O
</s>
<s>
PL/I	B-Language
was	O
used	O
to	O
write	O
an	O
executable	O
formal	O
definition	O
to	O
interpret	O
IBM	O
's	O
System	B-Architecture
Network	I-Architecture
Architecture	I-Architecture
.	O
</s>
<s>
PL/I	B-Language
did	O
not	O
fulfill	O
its	O
supporters	O
 '	O
hopes	O
that	O
it	O
would	O
displace	O
Fortran	B-Application
and	O
COBOL	B-Application
and	O
become	O
the	O
major	O
player	O
on	O
mainframes	B-Architecture
.	O
</s>
<s>
There	O
cannot	O
be	O
a	O
definitive	O
explanation	O
for	O
this	O
,	O
but	O
some	O
trends	O
in	O
the	O
1970s	O
and	O
1980s	O
militated	O
against	O
its	O
success	O
by	O
progressively	O
reducing	O
the	O
territory	O
on	O
which	O
PL/I	B-Language
enjoyed	O
a	O
competitive	O
advantage	O
.	O
</s>
<s>
First	O
,	O
the	O
nature	O
of	O
the	O
mainframe	B-Architecture
software	O
environment	O
changed	O
.	O
</s>
<s>
Application	O
subsystems	O
for	O
database	O
and	O
transaction	B-General_Concept
processing	I-General_Concept
(	O
CICS	B-Application
and	O
IMS	B-Application
and	O
Oracle	B-General_Concept
on	O
System	B-Device
370	I-Device
)	O
and	O
application	O
generators	O
became	O
the	O
focus	O
of	O
mainframe	B-Architecture
users	O
 '	O
application	O
development	O
.	O
</s>
<s>
Fortran	B-Application
was	O
not	O
used	O
in	O
these	O
application	O
areas	O
,	O
confining	O
PL/I	B-Language
to	O
COBOL	B-Application
's	O
territory	O
;	O
most	O
users	O
stayed	O
with	O
COBOL	B-Application
.	O
</s>
<s>
But	O
as	O
the	O
PC	O
became	O
the	O
dominant	O
environment	O
for	O
program	O
development	O
,	O
Fortran	B-Application
,	O
COBOL	B-Application
and	O
PL/I	B-Language
all	O
became	O
minority	O
languages	O
overtaken	O
by	O
C++	B-Language
,	O
Java	B-Language
and	O
the	O
like	O
.	O
</s>
<s>
Second	O
,	O
PL/I	B-Language
was	O
overtaken	O
in	O
the	O
system	B-Application
programming	I-Application
field	O
.	O
</s>
<s>
The	O
IBM	O
system	B-Application
programming	I-Application
community	O
was	O
not	O
ready	O
to	O
use	O
PL/I	B-Language
;	O
instead	O
,	O
IBM	O
developed	O
and	O
adopted	O
a	O
proprietary	O
dialect	O
of	O
PL/I	B-Language
for	O
system	B-Application
programming	I-Application
.	O
</s>
<s>
–	O
PL/S	B-Language
.	O
</s>
<s>
With	O
the	O
success	O
of	O
PL/S	B-Language
inside	O
IBM	O
,	O
and	O
of	O
C	B-Language
outside	O
IBM	O
,	O
the	O
unique	O
PL/I	B-Language
strengths	O
for	O
system	B-Application
programming	I-Application
became	O
less	O
valuable	O
.	O
</s>
<s>
Third	O
,	O
the	O
development	O
environments	O
grew	O
capabilities	O
for	O
interactive	O
software	O
development	O
that	O
,	O
again	O
,	O
made	O
the	O
unique	O
PL/I	B-Language
interactive	O
and	O
debugging	O
strengths	O
less	O
valuable	O
.	O
</s>
<s>
Fourth	O
,	O
features	O
such	O
as	O
structured	B-Language
programming	I-Language
,	O
character	O
string	O
operations	O
,	O
and	O
object	O
orientation	O
were	O
added	O
to	O
COBOL	B-Application
and	O
Fortran	B-Application
,	O
which	O
further	O
reduced	O
PL/I	B-Language
'	O
s	O
relative	O
advantages	O
.	O
</s>
<s>
On	O
mainframes	B-Architecture
there	O
were	O
substantial	O
business	O
issues	O
at	O
stake	O
too	O
.	O
</s>
<s>
IBM	O
's	O
hardware	O
competitors	O
had	O
little	O
to	O
gain	O
and	O
much	O
to	O
lose	O
from	O
success	O
of	O
PL/I	B-Language
.	O
</s>
<s>
Compiler	B-Language
development	O
was	O
expensive	O
,	O
and	O
the	O
IBM	O
compiler	B-Language
groups	O
had	O
an	O
in-built	O
competitive	O
advantage	O
.	O
</s>
<s>
With	O
no	O
early	O
support	O
for	O
PL/I	B-Language
by	O
other	O
vendors	O
it	O
was	O
best	O
to	O
avoid	O
PL/I	B-Language
.	O
</s>
<s>
This	O
article	O
uses	O
the	O
PL/I	B-Language
standard	O
as	O
the	O
reference	O
point	O
for	O
language	O
features	O
.	O
</s>
<s>
But	O
a	O
number	O
of	O
features	O
of	O
significance	O
in	O
the	O
early	O
implementations	O
were	O
not	O
in	O
the	O
Standard	O
;	O
and	O
some	O
were	O
offered	O
by	O
non-IBM	O
compilers	B-Language
.	O
</s>
<s>
Multithreading	B-Operating_System
,	O
under	O
the	O
name	O
"	O
multitasking	B-Operating_System
"	O
,	O
was	O
implemented	O
by	O
PL/I	B-Language
F	O
,	O
the	O
PL/I	B-Language
Checkout	O
and	O
Optimizing	O
compilers	B-Language
,	O
and	O
the	O
newer	O
AIX	B-Application
and	O
Z/OS	B-Application
compilers	B-Language
.	O
</s>
<s>
It	O
comprised	O
the	O
data	O
types	O
EVENT	O
and	O
TASK	O
,	O
the	O
TASK-option	O
on	O
the	O
CALL-statement	O
(	O
Fork	B-Operating_System
)	O
,	O
the	O
WAIT-statement	O
(	O
Join	B-Operating_System
)	O
,	O
the	O
DELAY(delay-time )	O
,	O
EVENT-options	O
on	O
the	O
record	O
I/O	O
statements	O
and	O
the	O
UNLOCK	O
statement	O
to	O
unlock	O
locked	O
records	O
on	O
EXCLUSIVE	O
files	O
.	O
</s>
<s>
Event	O
data	O
identify	O
a	O
particular	O
event	O
and	O
indicate	O
whether	O
it	O
is	O
complete	O
( 	O
 '	O
1'B	O
)	O
or	O
incomplete	O
( 	O
 '	O
0'B	O
)	O
:	O
task	O
data	O
items	O
identify	O
a	O
particular	O
task	O
(	O
or	O
process	B-Operating_System
)	O
and	O
indicate	O
its	O
priority	O
relative	O
to	O
other	O
tasks	O
.	O
</s>
<s>
The	O
first	O
IBM	O
Compile	B-Language
time	O
preprocessor	B-General_Concept
was	O
built	O
by	O
the	O
IBM	O
Boston	O
Advanced	O
Programming	O
Center	O
located	O
in	O
Cambridge	O
,	O
Mass	O
,	O
and	O
shipped	O
with	O
the	O
PL/I	B-Language
F	O
compiler	B-Language
.	O
</s>
<s>
The	O
DEC	O
and	O
Kednos	O
PL/I	B-Language
compilers	B-Language
implemented	O
much	O
the	O
same	O
set	O
of	O
features	O
as	O
IBM	O
,	O
with	O
some	O
additions	O
of	O
their	O
own	O
.	O
</s>
<s>
IBM	O
has	O
continued	O
to	O
add	O
preprocessor	B-General_Concept
features	O
to	O
its	O
compilers	B-Language
.	O
</s>
<s>
The	O
preprocessor	B-General_Concept
treats	O
the	O
written	O
source	O
program	O
as	O
a	O
sequence	O
of	O
tokens	O
,	O
copying	O
them	O
to	O
an	O
output	O
source	O
file	O
or	O
acting	O
on	O
them	O
.	O
</s>
<s>
When	O
a	O
%	O
token	O
is	O
encountered	O
the	O
following	O
compile	B-Language
time	O
statement	O
is	O
executed	O
:	O
when	O
an	O
identifier	O
token	O
is	O
encountered	O
and	O
the	O
identifier	O
has	O
been	O
DECLAREd	O
,	O
ACTIVATEd	O
,	O
and	O
assigned	O
a	O
compile	B-Language
time	O
value	O
,	O
the	O
identifier	O
is	O
replaced	O
by	O
this	O
value	O
.	O
</s>
<s>
+	O
)	O
,	O
as	O
are	O
the	O
values	O
of	O
ACTIVATEd	O
compile	B-Language
time	O
expressions	O
.	O
</s>
<s>
Thus	O
a	O
compile	B-Language
time	O
variable	O
PI	O
could	O
be	O
declared	O
,	O
activated	O
,	O
and	O
assigned	O
using	O
%PI='3	O
.14159265	O
'	O
.	O
</s>
<s>
Conditional	O
compiling	B-Language
and	O
iterative	O
generation	O
of	O
source	O
code	O
,	O
possible	O
with	O
compile-time	O
facilities	O
,	O
was	O
not	O
supported	O
by	O
the	O
standard	O
.	O
</s>
<s>
Structured	B-Language
programming	I-Language
additions	O
were	O
made	O
to	O
PL/I	B-Language
during	O
standardization	O
but	O
were	O
not	O
accepted	O
into	O
the	O
standard	O
.	O
</s>
<s>
These	O
features	O
were	O
all	O
included	O
in	O
IBM	O
's	O
PL/I	B-Language
Checkout	O
and	O
Optimizing	O
compilers	B-Language
and	O
in	O
DEC	O
PL/I	B-Language
.	O
</s>
<s>
PL/I	B-Language
F	O
had	O
offered	O
some	O
debug	O
facilities	O
that	O
were	O
not	O
put	O
forward	O
for	O
the	O
standard	O
but	O
were	O
implemented	O
by	O
others	O
notably	O
the	O
CHECK(variable-list )	O
condition	O
prefix	O
,	O
CHECK	O
on-condition	O
and	O
the	O
SNAP	O
option	O
.	O
</s>
<s>
The	O
IBM	O
Optimizing	O
and	O
Checkout	O
compilers	B-Language
added	O
additional	O
features	O
appropriate	O
to	O
the	O
conversational	O
mainframe	B-Architecture
programming	O
environment	O
(	O
e.g.	O
</s>
<s>
With	O
the	O
growth	O
of	O
classes	O
in	O
programming	O
theory	O
,	O
approaches	O
to	O
this	O
became	O
possible	O
on	O
a	O
PL/I	B-Language
base	O
UNION	O
,	O
TYPE	O
etc	O
.	O
</s>
<s>
have	O
been	O
added	O
by	O
several	O
compilers	B-Language
.	O
</s>
<s>
PL/I	B-Language
had	O
been	O
conceived	O
in	O
a	O
single-byte	O
character	O
world	O
.	O
</s>
<s>
Time	O
and	O
date	O
handling	O
were	O
overhauled	O
to	O
deal	O
with	O
the	O
millennium	B-Error_Name
problem	I-Error_Name
,	O
with	O
the	O
introduction	O
of	O
the	O
DATETIME	O
function	O
that	O
returned	O
the	O
date	O
and	O
time	O
in	O
one	O
of	O
about	O
35	O
different	O
formats	O
.	O
</s>
<s>
Though	O
the	O
language	O
is	O
easy	O
to	O
learn	O
and	O
use	O
,	O
implementing	O
a	O
PL/I	B-Language
compiler	B-Language
is	O
difficult	O
and	O
time-consuming	O
.	O
</s>
<s>
A	O
language	O
as	O
large	O
as	O
PL/I	B-Language
needed	O
subsets	O
that	O
most	O
vendors	O
could	O
produce	O
and	O
most	O
users	O
master	O
.	O
</s>
<s>
The	O
compile	B-Language
time	O
facilities	O
,	O
unique	O
to	O
PL/I	B-Language
,	O
took	O
added	O
implementation	O
effort	O
and	O
additional	O
compiler	B-Language
passes	O
.	O
</s>
<s>
A	O
PL/I	B-Language
compiler	B-Language
was	O
two	O
to	O
four	O
times	O
as	O
large	O
as	O
comparable	O
Fortran	B-Application
or	O
COBOL	B-Application
compilers	B-Language
,	O
and	O
also	O
that	O
much	O
slower	O
—	O
supposedly	O
offset	O
by	O
gains	O
in	O
programmer	O
productivity	O
.	O
</s>
<s>
This	O
was	O
anticipated	O
in	O
IBM	O
before	O
the	O
first	O
compilers	B-Language
were	O
written	O
.	O
</s>
<s>
Some	O
argue	O
that	O
PL/I	B-Language
is	O
unusually	O
hard	O
to	O
parse	O
.	O
</s>
<s>
The	O
PL/I	B-Language
keywords	O
are	O
not	O
reserved	O
so	O
programmers	O
can	O
use	O
them	O
as	O
variable	O
or	O
procedure	O
names	O
in	O
programs	O
.	O
</s>
<s>
Because	O
the	O
original	O
PL/I	B-Language
( F	O
)	O
compiler	B-Language
attempts	O
auto-correction	O
when	O
it	O
encounters	O
a	O
keyword	O
used	O
in	O
an	O
incorrect	O
context	O
,	O
it	O
often	O
assumes	O
it	O
is	O
a	O
variable	O
name	O
.	O
</s>
<s>
This	O
leads	O
to	O
"	O
cascading	O
diagnostics	O
"	O
,	O
a	O
problem	O
solved	O
by	O
later	O
compilers	B-Language
.	O
</s>
<s>
Program	O
optimization	O
(	O
needed	O
to	O
compete	O
with	O
the	O
excellent	O
program	O
optimization	O
carried	O
out	O
by	O
available	O
Fortran	B-Application
compilers	B-Language
)	O
is	O
unusually	O
complex	O
owing	O
to	O
side	O
effects	O
and	O
pervasive	O
problems	O
with	O
aliasing	B-Application
of	O
variables	O
.	O
</s>
<s>
Unpredictable	O
modification	O
can	O
occur	O
asynchronously	O
in	O
exception	B-General_Concept
handlers	I-General_Concept
,	O
which	O
may	O
be	O
provided	O
by	O
"	O
ON	O
statements	O
"	O
in	O
(	O
unseen	O
)	O
callers	O
.	O
</s>
<s>
In	O
typical	O
use	O
,	O
however	O
,	O
user-written	O
error	B-General_Concept
handlers	I-General_Concept
(	O
the	O
ON-unit	O
)	O
often	O
do	O
not	O
make	O
assignments	O
to	O
variables	O
.	O
</s>
<s>
In	O
spite	O
of	O
the	O
aforementioned	O
difficulties	O
,	O
IBM	O
produced	O
the	O
PL/I	B-Language
Optimizing	O
Compiler	B-Language
in	O
1971	O
.	O
</s>
<s>
PL/I	B-Language
contains	O
many	O
rarely	O
used	O
features	O
,	O
such	O
as	O
multitasking	B-Operating_System
support	O
(	O
an	O
IBM	O
extension	O
to	O
the	O
language	O
)	O
which	O
add	O
cost	O
and	O
complexity	O
to	O
the	O
compiler	B-Language
,	O
and	O
its	O
co-processing	O
facilities	O
require	O
a	O
multi-programming	B-Operating_System
environment	O
with	O
support	O
for	O
non-blocking	O
multiple	O
threads	B-Operating_System
for	O
processes	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Compiler	B-Language
writers	O
were	O
free	O
to	O
select	O
whether	O
to	O
implement	O
these	O
features	O
.	O
</s>
<s>
This	O
"	O
implicit	O
declaration	O
"	O
is	O
no	O
different	O
from	O
FORTRAN	B-Application
programs	O
.	O
</s>
<s>
For	O
PL/I	B-Language
( F	O
)	O
,	O
however	O
,	O
an	O
attribute	O
listing	O
enables	O
the	O
programmer	O
to	O
detect	O
any	O
misspelled	O
or	O
undeclared	O
variable	O
.	O
</s>
<s>
Many	O
programmers	O
were	O
slow	O
to	O
move	O
from	O
COBOL	B-Application
or	O
Fortran	B-Application
due	O
to	O
a	O
perceived	O
complexity	O
of	O
the	O
language	O
and	O
immaturity	O
of	O
the	O
PL/I	B-Language
F	O
compiler	B-Language
.	O
</s>
<s>
Programmers	O
were	O
sharply	O
divided	O
into	O
scientific	O
programmers	O
(	O
who	O
used	O
Fortran	B-Application
)	O
and	O
business	O
programmers	O
(	O
who	O
used	O
COBOL	B-Application
)	O
,	O
with	O
significant	O
tension	O
and	O
even	O
dislike	O
between	O
the	O
groups	O
.	O
</s>
<s>
PL/I	B-Language
syntax	O
borrowed	O
from	O
both	O
COBOL	B-Application
and	O
Fortran	B-Application
syntax	O
.	O
</s>
<s>
So	O
instead	O
of	O
noticing	O
features	O
that	O
would	O
make	O
their	O
job	O
easier	O
,	O
Fortran	B-Application
programmers	O
of	O
the	O
time	O
noticed	O
COBOL	B-Application
syntax	O
and	O
had	O
the	O
opinion	O
that	O
it	O
was	O
a	O
business	O
language	O
,	O
while	O
COBOL	B-Application
programmers	O
noticed	O
Fortran	B-Application
syntax	O
and	O
looked	O
upon	O
it	O
as	O
a	O
scientific	O
language	O
.	O
</s>
<s>
Both	O
COBOL	B-Application
and	O
Fortran	B-Application
programmers	O
viewed	O
it	O
as	O
a	O
"	O
bigger	O
"	O
version	O
of	O
their	O
own	O
language	O
,	O
and	O
both	O
were	O
somewhat	O
intimidated	O
by	O
the	O
language	O
and	O
disinclined	O
to	O
adopt	O
it	O
.	O
</s>
<s>
Another	O
factor	O
was	O
pseudo-similarities	O
to	O
COBOL	B-Application
,	O
Fortran	B-Application
,	O
and	O
ALGOL	B-Language
.	O
</s>
<s>
These	O
were	O
PL/I	B-Language
elements	O
that	O
looked	O
similar	O
to	O
one	O
of	O
those	O
languages	O
,	O
but	O
worked	O
differently	O
in	O
PL/I	B-Language
.	O
</s>
<s>
Such	O
frustrations	O
left	O
many	O
experienced	O
programmers	O
with	O
a	O
jaundiced	O
view	O
of	O
PL/I	B-Language
,	O
and	O
often	O
an	O
active	O
dislike	O
for	O
the	O
language	O
.	O
</s>
<s>
An	O
early	O
UNIX	B-Application
fortune	O
file	O
contained	O
the	O
following	O
tongue-in-cheek	O
description	O
of	O
the	O
language	O
:	O
</s>
<s>
Speaking	O
as	O
someone	O
who	O
has	O
delved	O
into	O
the	O
intricacies	O
of	O
PL/I	B-Language
,	O
I	O
am	O
sure	O
that	O
only	O
Real	O
Men	O
could	O
have	O
written	O
such	O
a	O
machine-hogging	O
,	O
cycle-grabbing	O
,	O
all-encompassing	O
monster	O
.	O
</s>
<s>
Multiply	O
a	O
character	O
string	O
times	O
a	O
bit	B-Data_Structure
string	I-Data_Structure
and	O
assign	O
the	O
result	O
to	O
a	O
float	B-Algorithm
decimal	O
?	O
</s>
<s>
On	O
the	O
positive	O
side	O
,	O
full	O
support	O
for	O
pointers	O
to	O
all	O
data	O
types	O
(	O
including	O
pointers	O
to	O
structures	O
)	O
,	O
recursion	O
,	O
multitasking	B-Operating_System
,	O
string	O
handling	O
,	O
and	O
extensive	O
built-in	O
functions	O
meant	O
PL/I	B-Language
was	O
indeed	O
quite	O
a	O
leap	O
forward	O
compared	O
to	O
the	O
programming	O
languages	O
of	O
its	O
time	O
.	O
</s>
<s>
However	O
,	O
these	O
were	O
not	O
enough	O
to	O
persuade	O
a	O
majority	O
of	O
programmers	O
or	O
shops	O
to	O
switch	O
to	O
PL/I	B-Language
.	O
</s>
<s>
The	O
PL/I	B-Language
F	O
compiler	B-Language
's	O
compile	B-Language
time	O
preprocessor	B-General_Concept
was	O
unusual	O
(	O
outside	O
the	O
Lisp	B-Language
world	O
)	O
in	O
using	O
its	O
target	O
language	O
's	O
syntax	O
and	O
semantics	O
(	O
e.g.	O
</s>
<s>
as	O
compared	O
to	O
the	O
C	B-Language
preprocessor	I-Language
's	O
"	O
#	O
"	O
directives	O
)	O
.	O
</s>
<s>
PL/I	B-Language
provides	O
several	O
'	O
storage	O
classes	O
 '	O
to	O
indicate	O
how	O
the	O
lifetime	O
of	O
variables	O
 '	O
storage	O
is	O
to	O
be	O
managed	O
STATIC	O
,	O
AUTOMATIC	O
,	O
CONTROLLED	O
,	O
and	O
BASED	O
.	O
</s>
<s>
The	O
simplest	O
to	O
implement	O
is	O
STATIC	O
,	O
which	O
indicates	O
that	O
memory	O
is	O
allocated	O
and	O
initialized	O
at	O
load-time	O
,	O
as	O
is	O
done	O
in	O
COBOL	B-Application
"	O
working-storage	O
"	O
and	O
early	O
Fortran	B-Application
.	O
</s>
<s>
PL/I	B-Language
'	O
s	O
default	O
storage	O
class	O
for	O
INTERNAL	O
variables	O
is	O
AUTOMATIC	O
,	O
similar	O
to	O
that	O
of	O
other	O
block-structured	O
languages	O
influenced	O
by	O
ALGOL	B-Language
,	O
like	O
the	O
"	O
auto	O
"	O
storage	O
class	O
in	O
the	O
C	B-Language
language	I-Language
,	O
and	O
default	O
storage	O
allocation	O
in	O
Pascal	B-Application
and	O
"	O
local-storage	O
"	O
in	O
IBM	O
COBOL	B-Application
.	O
</s>
<s>
The	O
compiler	B-Language
and	O
runtime	O
system	O
allocate	O
memory	O
for	O
a	O
stack	O
frame	O
to	O
contain	O
them	O
and	O
other	O
housekeeping	O
information	O
.	O
</s>
<s>
This	O
was	O
not	O
supported	O
by	O
many	O
early	O
IBM	O
compilers	B-Language
.	O
</s>
<s>
These	O
usages	O
are	O
safe	O
and	O
machine	B-Operating_System
independent	I-Operating_System
.	O
</s>
<s>
Record	O
I/O	O
and	O
list	B-Language
processing	I-Language
produce	O
situations	O
where	O
the	O
programmer	O
needs	O
to	O
fit	O
a	O
declaration	O
to	O
the	O
storage	O
of	O
the	O
next	O
record	O
or	O
item	O
,	O
before	O
knowing	O
what	O
type	O
of	O
data	O
structure	O
it	O
has	O
.	O
</s>
<s>
If	O
the	O
records	O
in	O
a	O
data	O
set	O
,	O
or	O
the	O
items	O
in	O
a	O
list	O
of	O
data	O
structures	O
,	O
are	O
organised	O
this	O
way	O
they	O
can	O
be	O
handled	O
safely	O
in	O
a	O
machine	B-Operating_System
independent	I-Operating_System
way	O
.	O
</s>
<s>
PL/I	B-Language
implementations	O
do	O
not	O
(	O
except	O
for	O
the	O
PL/I	B-Language
Checkout	O
compiler	B-Language
)	O
keep	O
track	O
of	O
the	O
data	O
structure	O
used	O
when	O
storage	O
is	O
first	O
allocated	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
problem	O
of	O
wild	O
references	O
and	O
buffer	O
overruns	O
,	O
issues	O
arise	O
due	O
to	O
the	O
alignment	O
and	O
length	O
for	O
data	O
types	O
used	O
with	O
particular	O
machines	O
and	O
compilers	B-Language
.	O
</s>
<s>
With	O
the	O
prevalence	O
of	O
C	B-Language
and	O
its	O
free	O
and	O
easy	O
attitude	O
to	O
pointer	O
arithmetic	O
,	O
recent	O
IBM	O
PL/I	B-Language
compilers	B-Language
allow	O
pointers	O
to	O
be	O
used	O
with	O
the	O
addition	O
and	O
subtraction	O
operators	O
to	O
giving	O
the	O
simplest	O
syntax	O
(	O
but	O
compiler	B-Language
options	O
can	O
disallow	O
these	O
practices	O
where	O
safety	O
and	O
machine	B-Operating_System
independence	I-Operating_System
are	O
paramount	O
)	O
.	O
</s>
<s>
When	O
PL/I	B-Language
was	O
designed	O
,	O
programs	O
only	O
ran	O
in	O
batch	O
mode	O
,	O
with	O
no	O
possible	O
intervention	O
from	O
the	O
programmer	O
at	O
a	O
terminal	O
.	O
</s>
<s>
PL/I	B-Language
exception	B-General_Concept
handling	I-General_Concept
,	O
via	O
ON-units	O
,	O
allowed	O
the	O
program	O
to	O
stay	O
in	O
control	O
in	O
the	O
face	O
of	O
hardware	O
or	O
operating	B-General_Concept
system	I-General_Concept
exceptions	O
and	O
to	O
recover	O
debugging	O
information	O
before	O
closing	O
down	O
more	O
gracefully	O
.	O
</s>
<s>
As	O
a	O
program	O
became	O
properly	O
debugged	O
,	O
most	O
of	O
the	O
exception	B-General_Concept
handling	I-General_Concept
could	O
be	O
removed	O
or	O
disabled	O
:	O
this	O
level	O
of	O
control	O
became	O
less	O
important	O
when	O
conversational	O
execution	O
became	O
commonplace	O
.	O
</s>
<s>
Computational	O
exception	B-General_Concept
handling	I-General_Concept
is	O
enabled	O
and	O
disabled	O
by	O
condition	O
prefixes	O
on	O
statements	O
,	O
blocks	O
(	O
including	O
ON-units	O
)	O
and	O
procedures	O
.	O
</s>
<s>
(	O
SIZE	O
,	O
NOSUBSCRIPTRANGE	O
)	O
:	O
A(I )	O
=	O
B(I )	O
*	O
C	B-Language
;	O
.	O
</s>
<s>
Operating	B-General_Concept
system	I-General_Concept
exceptions	O
for	O
Input/Output	O
and	O
storage	O
management	O
are	O
always	O
enabled	O
.	O
</s>
<s>
to	O
help	O
debug	O
the	O
exception	B-General_Concept
handlers	I-General_Concept
.	O
</s>
<s>
The	O
ON	O
ERROR	O
SYSTEM	O
;	O
statement	O
allows	O
a	O
nested	O
error	O
trap	O
;	O
if	O
an	O
error	O
occurs	O
within	O
an	O
ON-unit	O
,	O
control	O
might	O
pass	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
where	O
a	O
system	O
dump	O
might	O
be	O
produced	O
,	O
or	O
,	O
for	O
some	O
computational	O
conditions	O
,	O
continue	O
execution	O
(	O
as	O
mentioned	O
above	O
)	O
.	O
</s>
<s>
The	O
PL/I	B-Language
RECORD	O
I/O	O
statements	O
have	O
relatively	O
simple	O
syntax	O
as	O
they	O
do	O
not	O
offer	O
options	O
for	O
the	O
many	O
situations	O
from	O
end-of-file	O
to	O
record	O
transmission	O
errors	O
that	O
can	O
occur	O
when	O
a	O
record	O
is	O
read	O
or	O
written	O
.	O
</s>
<s>
The	O
existence	O
of	O
exception	B-General_Concept
handling	I-General_Concept
ON-units	O
can	O
have	O
an	O
effect	O
on	O
optimization	O
,	O
because	O
variables	O
can	O
be	O
inspected	O
or	O
altered	O
in	O
ON-units	O
.	O
</s>
<s>
PL/I	B-Language
has	O
counterparts	O
for	O
COBOL	B-Application
and	O
FORTRAN	B-Application
's	O
specialized	O
GO	O
TO	O
statements	O
.	O
</s>
<s>
Syntax	O
for	O
both	O
COBOL	B-Application
and	O
FORTRAN	B-Application
exist	O
for	O
coding	O
two	O
special	O
two	O
types	O
of	O
GO	O
TO	O
,	O
each	O
of	O
which	O
has	O
a	O
target	O
that	O
is	O
not	O
always	O
the	O
same	O
.	O
</s>
<s>
ALTER	O
(	O
COBOL	B-Application
)	O
,	O
ASSIGN	O
(	O
FORTRAN	B-Application
)	O
:	O
</s>
<s>
There	O
are	O
other/helpful	O
restrictions	O
on	O
these	O
,	O
especially	O
"	O
in	O
programs	O
...	O
RECURSIVE	O
attribute	O
,	O
in	O
methods	O
,	O
or	O
..	O
THREAD	B-Operating_System
option.	O
"	O
</s>
<s>
PL/I	B-Language
has	O
statement	O
label	O
variables	O
(	O
with	O
the	O
LABEL	O
attribute	O
)	O
,	O
which	O
can	O
store	O
the	O
value	O
of	O
a	O
statement	O
label	O
,	O
and	O
later	O
be	O
used	O
in	O
a	O
GOTO	O
statement	O
.	O
</s>
