<s>
The	O
MCP	B-Application
(	O
Master	B-Application
Control	I-Application
Program	I-Application
)	O
is	O
the	O
operating	B-General_Concept
system	I-General_Concept
of	O
the	O
Burroughs	O
B5000/B5500/B5700	O
and	O
the	O
B6500	O
and	O
successors	O
,	O
including	O
the	O
Unisys	O
Clearpath/MCP	O
systems	O
.	O
</s>
<s>
MCP	B-Application
was	O
originally	O
written	O
in	O
1961	O
in	O
ESPOL	B-Language
(	O
Executive	B-Language
Systems	I-Language
Problem	I-Language
Oriented	I-Language
Language	I-Language
)	O
.	O
</s>
<s>
In	O
the	O
1970s	O
,	O
MCP	B-Application
was	O
converted	O
to	O
NEWP	B-Language
which	O
was	O
a	O
better	O
structured	O
,	O
more	O
robust	O
,	O
and	O
more	O
secure	O
form	O
of	O
ESPOL	B-Language
.	O
</s>
<s>
The	O
MCP	B-Application
was	O
a	O
leader	O
in	O
many	O
areas	O
,	O
including	O
:	O
the	O
first	O
operating	B-General_Concept
system	I-General_Concept
to	O
manage	O
multiple	O
processors	O
,	O
the	O
first	O
commercial	O
implementation	O
of	O
virtual	B-Architecture
memory	I-Architecture
,	O
and	O
the	O
first	O
OS	O
written	O
exclusively	O
in	O
a	O
high-level	B-Language
language	I-Language
.	O
</s>
<s>
In	O
1961	O
,	O
the	O
MCP	B-Application
was	O
the	O
first	O
OS	O
written	O
exclusively	O
in	O
a	O
high-level	B-Language
language	I-Language
(	O
HLL	O
)	O
.	O
</s>
<s>
The	O
Burroughs	B-Device
Large	I-Device
System	I-Device
(	O
B5000	O
and	O
successors	O
)	O
were	O
unique	O
in	O
that	O
they	O
were	O
designed	O
with	O
the	O
expectation	O
that	O
all	O
software	O
,	O
including	O
system	O
software	O
,	O
would	O
be	O
written	O
in	O
an	O
HLL	O
rather	O
than	O
in	O
assembly	B-Language
language	I-Language
,	O
which	O
was	O
a	O
unique	O
and	O
innovative	O
approach	O
in	O
1961	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
Burroughs	O
was	O
free	O
to	O
distribute	O
the	O
source	O
code	O
of	O
all	O
software	O
it	O
sold	O
,	O
including	O
the	O
MCP	B-Application
,	O
which	O
was	O
designed	O
with	O
this	O
openness	O
in	O
mind	O
.	O
</s>
<s>
As	O
such	O
,	O
the	O
MCP	B-Application
could	O
be	O
considered	O
one	O
of	O
the	O
earliest	O
open-source	B-Application
projects	O
.	O
</s>
<s>
Now	O
that	O
MCP	B-Application
runs	O
on	O
commodity	O
hardware	O
,	O
some	O
elements	O
of	O
the	O
MCP	B-Application
based	O
software	O
suite	O
are	O
no	O
longer	O
made	O
available	O
in	O
source	O
form	O
by	O
Unisys	O
.	O
</s>
<s>
The	O
MCP	B-Application
was	O
the	O
first	O
commercial	O
OS	O
to	O
provide	O
virtual	B-Architecture
memory	I-Architecture
,	O
which	O
has	O
been	O
supported	O
by	O
the	O
Burroughs	B-Device
large	I-Device
systems	I-Device
architecture	O
since	O
its	O
inception	O
.	O
</s>
<s>
Unisys	O
stopped	O
producing	O
the	O
hardware	O
in	O
the	O
early	O
2010s	O
,	O
and	O
the	O
operating	B-General_Concept
system	I-General_Concept
is	O
now	O
run	O
under	O
emulation	O
.	O
</s>
<s>
The	O
MCP	B-Application
provides	O
a	O
file	B-Application
system	I-Application
with	O
hierarchical	O
directory	B-Application
structures	O
.	O
</s>
<s>
In	O
early	O
MCP	B-Application
implementations	O
,	O
directory	B-Application
nodes	O
were	O
represented	O
by	O
separate	O
files	O
with	O
directory	B-Application
entries	O
,	O
as	O
other	O
systems	O
did	O
.	O
</s>
<s>
However	O
,	O
since	O
about	O
1970	O
,	O
MCP	B-Application
internally	O
uses	O
a	O
'	O
FLAT	O
 '	O
directory	B-Application
listing	O
all	O
file	O
paths	O
on	O
a	O
volume	O
.	O
</s>
<s>
This	O
is	O
because	O
opening	O
files	O
by	O
visiting	O
and	O
opening	O
each	O
directory	B-Application
in	O
a	O
file	O
path	O
was	O
inefficient	O
and	O
for	O
a	O
production	O
environment	O
it	O
was	O
found	O
to	O
be	O
better	O
to	O
keep	O
all	O
files	O
in	O
a	O
single	O
directory	B-Application
,	O
even	O
though	O
they	O
retain	O
the	O
hierarchical	O
naming	O
scheme	O
.	O
</s>
<s>
The	O
only	O
difference	O
visible	O
to	O
users	O
is	O
that	O
an	O
entity	O
file	O
can	O
have	O
the	O
same	O
name	O
as	O
a	O
directory	B-Application
.	O
</s>
<s>
For	O
example	O
,	O
"	O
A/B	O
"	O
and	O
"	O
A/B/C	O
"	O
can	O
both	O
exist	O
;	O
"	O
B	O
"	O
can	O
be	O
both	O
a	O
node	O
in	O
a	O
file	O
and	O
a	O
directory	B-Application
.	O
</s>
<s>
These	O
attributes	O
record	O
all	O
sorts	O
of	O
meta	O
data	O
about	O
a	O
file	O
,	O
most	O
importantly	O
its	O
name	O
and	O
its	O
type	O
(	O
which	O
tells	O
the	O
system	O
how	O
to	O
handle	O
a	O
file	O
,	O
like	O
the	O
more	O
limited	O
four-character	O
file	O
type	O
code	O
on	O
the	O
Macintosh	B-Device
)	O
.	O
</s>
<s>
Other	O
attributes	O
have	O
the	O
file	O
's	O
record	O
size	O
(	O
if	O
fixed	O
for	O
commercial	O
applications	O
)	O
,	O
the	O
block	O
size	O
(	O
in	O
multiples	O
of	O
records	O
that	O
tells	O
the	O
MCP	B-Application
how	O
many	O
records	O
to	O
read	O
and	O
write	O
in	O
a	O
single	O
physical	O
IO	O
)	O
and	O
an	O
area	O
size	O
in	O
multiples	O
of	O
blocks	O
,	O
which	O
gives	O
the	O
size	O
of	O
disk	O
areas	O
to	O
be	O
allocated	O
as	O
the	O
file	O
expands	O
.	O
</s>
<s>
The	O
MCP	B-Application
implements	O
a	O
Journaling	B-Application
file	I-Application
system	I-Application
,	O
providing	O
fault	O
tolerance	O
in	O
case	O
of	O
disk	O
failure	O
,	O
loss	O
of	O
power	O
,	O
etc	O
.	O
</s>
<s>
It	O
is	O
not	O
possible	O
to	O
corrupt	O
the	O
file	B-Application
system	I-Application
(	O
except	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
or	O
other	O
trusted	O
system	O
software	O
with	O
direct	O
access	O
to	O
its	O
lower	O
layers	O
)	O
.	O
</s>
<s>
The	O
file	B-Application
system	I-Application
is	O
case-insensitive	O
and	O
not	O
case-preserving	B-Application
unless	O
quotes	O
are	O
added	O
around	O
the	O
name	O
in	O
which	O
case	O
it	O
is	O
case-sensitive	O
and	O
case-preserving	B-Application
.	O
</s>
<s>
MCP	B-Application
processes	B-Operating_System
are	O
called	O
"	O
Jobs	B-General_Concept
"	O
and	O
"	O
Tasks.	O
"	O
</s>
<s>
A	O
Job	O
contains	O
one	O
or	O
more	O
tasks	B-General_Concept
.	O
</s>
<s>
Tasks	B-General_Concept
within	O
a	O
job	O
can	O
run	O
sequentially	O
or	O
in	O
parallel	O
.	O
</s>
<s>
Logic	O
can	O
be	O
implemented	O
at	O
the	O
Job	O
level	O
,	O
typically	O
in	O
the	O
MCP	B-Application
's	O
job	O
control	O
language	O
WFL	O
,	O
to	O
control	O
the	O
flow	O
of	O
a	O
job	O
.	O
</s>
<s>
Once	O
all	O
tasks	B-General_Concept
in	O
a	O
job	O
are	O
complete	O
,	O
the	O
job	O
itself	O
is	O
completed	O
.	O
</s>
<s>
An	O
MCP	B-Application
Process	O
goes	O
through	O
a	O
life	O
cycle	O
from	O
the	O
time	O
it	O
enters	O
the	O
system	O
until	O
it	O
leaves	O
.	O
</s>
<s>
Tasks	B-General_Concept
within	O
a	O
job	O
do	O
not	O
wait	O
in	O
queue	O
;	O
instead	O
going	O
directly	O
to	O
the	O
'	O
Scheduled	O
 '	O
state	O
when	O
initiated	O
.	O
</s>
<s>
Running	O
processes	B-Operating_System
are	O
those	O
that	O
use	O
a	O
processor	O
resource	O
and	O
are	O
marked	O
as	O
'	O
running	O
 '	O
.	O
</s>
<s>
Processes	B-Operating_System
that	O
are	O
ready	O
to	O
be	O
assigned	O
to	O
a	O
processor	O
,	O
when	O
there	O
is	O
no	O
free	O
processor	O
are	O
placed	O
in	O
the	O
ready	O
queue	O
.	O
</s>
<s>
Processes	B-Operating_System
may	O
be	O
assigned	O
a	O
“	O
Declared	O
”	O
or	O
“	O
Visible	O
”	O
priority	O
,	O
generally	O
50	O
as	O
the	O
default	O
,	O
but	O
can	O
be	O
from	O
0	O
to	O
99	O
for	O
user	O
processes	B-Operating_System
.	O
</s>
<s>
System	O
processes	B-Operating_System
may	O
be	O
assigned	O
the	O
higher	O
values	O
.	O
</s>
<s>
Processes	B-Operating_System
that	O
are	O
directly	O
part	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
called	O
Independent	O
Runners	O
,	O
have	O
the	O
highest	O
priority	O
regardless	O
of	O
numeric	O
priority	O
value	O
.	O
</s>
<s>
Next	O
come	O
processes	B-Operating_System
using	O
an	O
MCP	B-Application
lock	O
,	O
then	O
Message	O
Control	O
Systems	O
such	O
as	O
CANDE	B-Operating_System
.	O
</s>
<s>
Then	O
Discontinued	O
processes	B-Operating_System
.	O
</s>
<s>
Then	O
Work	B-Language
Flow	I-Language
Language	I-Language
jobs	B-General_Concept
.	O
</s>
<s>
Finally	O
come	O
user	O
processes	B-Operating_System
.	O
</s>
<s>
At	O
a	O
lower	O
level	O
,	O
there	O
is	O
a	O
Fine	O
priority	O
intended	O
to	O
elevate	O
the	O
priority	O
of	O
tasks	B-General_Concept
that	O
do	O
not	O
use	O
their	O
full	O
processor	O
slice	O
.	O
</s>
<s>
Processes	B-Operating_System
that	O
are	O
waiting	O
on	O
other	O
resources	O
,	O
such	O
as	O
a	O
file	O
read	O
,	O
wait	O
on	O
the	O
EVENT	O
data	B-General_Concept
structure	I-General_Concept
.	O
</s>
<s>
Thus	O
all	O
processes	B-Operating_System
waiting	O
on	O
a	O
single	O
resource	O
wait	O
on	O
a	O
single	O
event	O
.	O
</s>
<s>
When	O
the	O
resource	O
becomes	O
available	O
,	O
the	O
event	O
is	O
caused	O
,	O
which	O
wakes	O
up	O
all	O
the	O
processes	B-Operating_System
waiting	O
on	O
it	O
.	O
</s>
<s>
Processes	B-Operating_System
may	O
wait	O
on	O
multiple	O
events	O
for	O
any	O
one	O
of	O
them	O
to	O
happen	O
,	O
including	O
a	O
time	O
out	O
.	O
</s>
<s>
Events	O
are	O
fully	O
user	O
programmable	O
–	O
that	O
is	O
,	O
users	O
can	O
write	O
systems	O
that	O
use	O
the	O
generalized	O
event	O
system	O
provided	O
by	O
the	O
MCP	B-Application
.	O
</s>
<s>
Processes	B-Operating_System
that	O
have	O
terminated	O
are	O
marked	O
as	O
completed	O
.	O
</s>
<s>
Operationally	O
,	O
the	O
status	O
of	O
all	O
tasks	B-General_Concept
in	O
the	O
system	O
is	O
displayed	O
to	O
the	O
operator	O
.	O
</s>
<s>
All	O
running	O
and	O
ready	O
processes	B-Operating_System
are	O
displayed	O
as	O
'	O
Active	O
 '	O
tasks	B-General_Concept
(	O
since	O
the	O
system	O
implements	O
preemptive	O
multitasking	O
,	O
the	O
change	O
from	O
ready	O
to	O
running	O
and	O
back	O
is	O
so	O
quick	O
that	O
distinguishing	O
ready	O
and	O
running	O
tasks	B-General_Concept
is	O
pointless	O
because	O
they	O
will	O
all	O
get	O
a	O
slice	O
of	O
the	O
processor	O
within	O
a	O
second	O
)	O
.	O
</s>
<s>
All	O
active	O
tasks	B-General_Concept
can	O
be	O
displayed	O
with	O
the	O
'	O
A	O
 '	O
command	O
.	O
</s>
<s>
Terminated	O
tasks	B-General_Concept
are	O
displayed	O
as	O
completed	O
tasks	B-General_Concept
with	O
the	O
reason	O
for	O
termination	O
,	O
EOT	O
for	O
normal	O
'	O
end	O
of	O
task	O
 '	O
,	O
and	O
DSed	O
with	O
a	O
reason	O
for	O
a	O
process	O
failure	O
.	O
</s>
<s>
All	O
processes	B-Operating_System
are	O
assigned	O
a	O
mix	O
number	O
,	O
and	O
operators	O
can	O
use	O
this	O
number	O
to	O
identify	O
a	O
process	O
to	O
control	O
.	O
</s>
<s>
Tasks	B-General_Concept
terminated	O
by	O
the	O
operator	O
are	O
listed	O
in	O
the	O
complete	O
entries	O
as	O
O-DS	O
.	O
</s>
<s>
Tasks	B-General_Concept
can	O
also	O
terminate	O
due	O
to	O
program	O
faults	O
,	O
marked	O
as	O
F-DS	O
or	O
P-DS	O
,	O
for	O
faults	O
such	O
as	O
invalid	B-Data_Structure
index	I-Data_Structure
,	O
numeric	B-Algorithm
overflow	I-Algorithm
,	O
etc	O
.	O
</s>
<s>
Completed	O
entries	O
can	O
be	O
listed	O
by	O
the	O
operator	O
with	O
the	O
'	O
C	B-Language
 '	O
command	O
.	O
</s>
<s>
Tasks	B-General_Concept
waiting	O
on	O
a	O
resource	O
are	O
listed	O
under	O
the	O
waiting	O
entries	O
and	O
the	O
reason	O
for	O
waiting	O
.	O
</s>
<s>
All	O
waiting	O
tasks	B-General_Concept
may	O
be	O
listed	O
with	O
the	O
'	O
W	O
 '	O
command	O
.	O
</s>
<s>
Tasks	B-General_Concept
waiting	O
on	O
user	O
input	O
or	O
file	O
reads	O
would	O
not	O
normally	O
be	O
listed	O
as	O
waiting	O
entries	O
for	O
operator	O
attention	O
.	O
</s>
<s>
The	O
MCP	B-Application
provides	O
this	O
level	O
of	O
operator	O
recoverability	O
of	O
tasks	B-General_Concept
.	O
</s>
<s>
Such	O
code	O
may	O
be	O
written	O
in	O
an	O
MCP	B-Application
program	O
when	O
it	O
is	O
not	O
desirable	O
to	O
have	O
a	O
task	O
wait	O
,	O
but	O
because	O
of	O
the	O
operator-level	O
recoverability	O
,	O
this	O
is	O
not	O
forced	O
and	O
therefore	O
makes	O
programming	O
much	O
simpler	O
.	O
</s>
<s>
In	O
addition	O
to	O
general	O
plug-in	O
mechanisms	O
,	O
the	O
new	O
form	O
of	O
dynamic	B-Application
library	I-Application
linkage	O
(	O
Connection	O
Libraries	B-Library
)	O
allows	O
programs	O
to	O
import	O
and	O
export	O
functions	O
and	O
data	O
,	O
and	O
hence	O
one	O
program	O
runs	O
code	O
supplied	O
by	O
another	O
.	O
</s>
<s>
In	O
a	O
misguided	O
attempt	O
at	O
compatibility	O
,	O
it	O
was	O
named	O
after	O
the	O
then-proposed	O
C++	O
language	O
construct	O
of	O
the	O
same	O
name	O
.	O
</s>
<s>
Operator	O
(	O
or	O
user	O
)	O
DS	O
is	O
not	O
recoverable	O
except	O
by	O
privileged	O
tasks	B-General_Concept
using	O
an	O
UNSAFE	O
form	O
of	O
try	O
.	O
</s>
<s>
MCP	B-Application
thus	O
provides	O
a	O
very	O
fault-tolerant	O
environment	O
,	O
not	O
the	O
crash-and-burn	O
core-dump	O
of	O
other	O
systems	O
.	O
</s>
<s>
As	O
with	O
file	O
attributes	O
,	O
tasks	B-General_Concept
have	O
attributes	O
as	O
well	O
,	O
such	O
as	O
the	O
task	O
priority	O
(	O
which	O
is	O
assigned	O
at	O
compile	O
time	O
or	O
execution	O
time	O
,	O
or	O
can	O
be	O
changed	O
while	O
the	O
task	O
is	O
running	O
)	O
,	O
processor	O
time	O
,	O
wait	O
time	O
,	O
status	O
,	O
etc	O
.	O
</s>
<s>
Memory	O
may	O
be	O
SAVE	O
(	O
i.e.	O
,	O
memory	O
resident	O
)	O
,	O
OVERLAYABLE	O
(	O
i.e.	O
,	O
virtual	B-Architecture
memory	I-Architecture
)	O
or	O
STICKY	O
(	O
meaning	O
memory	O
resident	O
,	O
but	O
movable	O
)	O
.	O
</s>
<s>
GEORGE	O
is	O
the	O
procedure	O
that	O
decides	O
which	O
process	O
is	O
the	O
next	O
one	O
to	O
receive	O
CPU	O
resources	O
and	O
is	O
thus	O
one	O
of	O
the	O
few	O
processes	B-Operating_System
that	O
uses	O
the	O
MoveStack	O
instruction	O
.	O
</s>
<s>
While	O
the	O
task	O
is	O
ALIVE	O
,	O
MCP	B-Application
functions	O
are	O
run	O
on	O
top	O
of	O
that	O
particular	O
process	O
,	O
thus	O
CPU	O
resources	O
are	O
automatically	O
charged	O
to	O
the	O
task	O
causing	O
the	O
MCP	B-Application
overhead	O
.	O
</s>
<s>
Also	O
,	O
much	O
of	O
the	O
MCP	B-Application
work	O
is	O
being	O
performed	O
with	O
that	O
particular	O
stack	O
's	O
security	O
rights	O
.	O
</s>
<s>
Only	O
before	O
BIRTH	O
and	O
after	O
DEATH	O
does	O
the	O
MCP	B-Application
need	O
to	O
be	O
operating	O
out	O
of	O
some	O
other	O
stack	O
.	O
</s>
<s>
MCP	B-Application
libraries	B-Library
provide	O
a	O
way	O
of	O
sharing	O
data	O
and	O
code	O
between	O
processes	B-Operating_System
.	O
</s>
<s>
The	O
article	O
on	O
Burroughs	B-Device
large	I-Device
systems	I-Device
looks	O
at	O
the	O
way	O
dependent	O
processes	B-Operating_System
could	O
be	O
asynchronously	O
run	O
so	O
that	O
many	O
processes	B-Operating_System
could	O
share	O
common	O
data	O
(	O
with	O
the	O
mechanisms	O
to	O
provide	O
synchronized	O
update	O
)	O
.	O
</s>
<s>
Such	O
a	O
family	O
of	O
related	O
processes	B-Operating_System
had	O
to	O
be	O
written	O
as	O
a	O
single	O
program	O
unit	O
,	O
processing	O
procedures	O
at	O
higher	O
lex	O
levels	O
as	O
the	O
asynchronous	O
processes	B-Operating_System
,	O
which	O
could	O
still	O
access	O
global	O
variables	O
and	O
other	O
variables	O
at	O
lower	O
lex	O
levels	O
.	O
</s>
<s>
Libraries	B-Library
completely	O
inverted	O
this	O
scenario	O
with	O
the	O
following	O
advantages	O
:	O
</s>
<s>
Libraries	B-Library
were	O
introduced	O
to	O
MCP	B-Application
systems	O
in	O
the	O
early	O
1980s	O
,	O
having	O
been	O
developed	O
by	O
Roy	O
Guck	O
and	O
others	O
at	O
Burroughs	O
.	O
</s>
<s>
They	O
are	O
very	O
much	O
like	O
C	B-Language
.	O
A	O
.	O
R	O
.	O
Hoare	O
's	O
monitors	O
and	O
provide	O
the	O
opportunity	O
for	O
controlled	O
mutual	O
exclusion	O
and	O
synchronization	O
between	O
client	O
processes	B-Operating_System
,	O
using	O
MCP	B-Application
EVENTs	O
and	O
the	O
Dahm	O
locking	O
technique	O
.	O
</s>
<s>
Libraries	B-Library
offer	O
procedural	O
entry-points	O
to	O
the	O
client	O
,	O
which	O
are	O
checked	O
for	O
a	O
compatible	O
interface	O
(	O
all	O
parameters	O
and	O
return	O
types	O
of	O
imported	O
procedures	O
checked	O
)	O
before	O
the	O
client	O
is	O
linked	O
to	O
the	O
library	O
.	O
</s>
<s>
Libraries	B-Library
are	O
more	O
sophisticated	O
forms	O
of	O
libraries	B-Library
on	O
other	O
systems	O
such	O
as	O
DLLs	B-Application
.	O
</s>
<s>
MCP	B-Application
libraries	B-Library
can	O
be	O
'	O
shared	O
by	O
all	O
 '	O
,	O
‘	O
shared	O
by	O
rununit’	O
or	O
'	O
private	O
 '	O
.	O
</s>
<s>
The	O
private	O
case	O
is	O
closest	O
to	O
libraries	B-Library
on	O
other	O
systems	O
–	O
for	O
each	O
client	O
a	O
separate	O
copy	O
of	O
the	O
library	O
is	O
invoked	O
and	O
there	O
is	O
no	O
data	O
sharing	O
between	O
processes	B-Operating_System
.	O
</s>
<s>
It	O
was	O
designed	O
specifically	O
for	O
COBOL	B-Application
,	O
where	O
a	O
rununit	O
is	O
defined	O
as	O
the	O
original	O
initiating	O
client	O
program	O
and	O
all	O
the	O
libraries	B-Library
it	O
has	O
linked	O
to	O
.	O
</s>
<s>
This	O
is	O
the	O
only	O
dynamic	O
implementation	O
of	O
COBOL	B-Application
rununits	O
.	O
</s>
<s>
If	O
this	O
was	O
the	O
first	O
invocation	O
of	O
the	O
library	O
,	O
the	O
library	O
would	O
run	O
its	O
main	O
program	O
(	O
outer	O
block	O
in	O
an	O
ALGOL	B-Language
program	O
)	O
to	O
initialize	O
its	O
global	O
environment	O
.	O
</s>
<s>
This	O
allows	O
many	O
clients	O
to	O
be	O
running	O
the	O
same	O
routine	O
at	O
the	O
same	O
time	O
,	O
being	O
synchronized	O
by	O
the	O
library	B-Library
routine	I-Library
,	O
which	O
accesses	O
the	O
data	O
in	O
the	O
global	O
environment	O
of	O
the	O
library	O
stack	O
.	O
</s>
<s>
Permanent	O
meant	O
that	O
the	O
library	O
remained	O
available	O
for	O
further	O
clients	O
even	O
if	O
the	O
client	O
count	O
dropped	O
to	O
zero	O
–	O
permanent	O
libraries	B-Library
could	O
be	O
unfrozen	O
by	O
an	O
operator	O
with	O
a	O
THAW	O
command	O
.	O
</s>
<s>
Libraries	B-Library
could	O
also	O
be	O
accessed	O
'	O
by	O
title	O
 '	O
and	O
'	O
by	O
function	O
 '	O
.	O
</s>
<s>
'	O
By	O
function	O
 '	O
was	O
an	O
indirect	O
method	O
where	O
a	O
client	O
would	O
just	O
specify	O
the	O
function	O
name	O
of	O
the	O
library	O
,	O
for	O
example	O
'	O
system_support	O
 '	O
and	O
the	O
actual	O
location	O
of	O
the	O
library	O
is	O
found	O
in	O
a	O
table	O
previously	O
set	O
up	O
by	O
an	O
operator	O
with	O
'	O
SL	O
 '	O
(	O
system	B-Library
library	I-Library
)	O
commands	O
,	O
for	O
example	O
'	O
SL	O
system_support	O
=	O
*	O
system/library/support	O
'	O
.	O
</s>
<s>
MCP	B-Application
's	O
fault	O
tolerant	O
attitude	O
also	O
works	O
here	O
–	O
if	O
a	O
client	O
tries	O
accessing	O
a	O
library	O
that	O
is	O
not	O
present	O
,	O
the	O
client	O
is	O
put	O
in	O
the	O
'	O
waiting	O
 '	O
tasks	B-General_Concept
and	O
the	O
library	O
could	O
be	O
made	O
present	O
,	O
or	O
the	O
request	O
redirected	O
.	O
</s>
<s>
Libraries	B-Library
can	O
also	O
be	O
updated	O
on	O
the	O
fly	O
,	O
all	O
that	O
needs	O
to	O
be	O
done	O
is	O
to	O
'	O
SL	O
 '	O
the	O
new	O
version	O
.	O
</s>
<s>
Function	B-Library
libraries	I-Library
also	O
implemented	O
a	O
very	O
important	O
security	O
feature	O
,	O
linkage	O
classes	O
.	O
</s>
<s>
All	O
normal	O
libraries	B-Library
have	O
a	O
linkage	O
class	O
of	O
zero	O
.	O
</s>
<s>
Libraries	B-Library
used	O
by	O
the	O
MCP	B-Application
or	O
other	O
privileged	O
system	O
modules	O
may	O
not	O
be	O
usable	O
from	O
normal	O
programs	O
.	O
</s>
<s>
The	O
entire	O
database	O
system	O
is	O
implemented	O
with	O
libraries	B-Library
providing	O
very	O
efficient	O
and	O
tailored	O
access	O
to	O
databases	O
shared	O
between	O
many	O
clients	O
.	O
</s>
<s>
In	O
the	O
mid-1990s	O
a	O
new	O
type	O
of	O
library	O
was	O
made	O
available	O
:	O
Connection	O
Libraries	B-Library
.	O
</s>
<s>
For	O
example	O
,	O
the	O
networking	O
component	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
is	O
available	O
as	O
a	O
connection	O
library	O
,	O
allowing	O
other	O
programs	O
to	O
use	O
its	O
services	O
by	O
exporting	O
and	O
importing	O
functions	O
.	O
</s>
<s>
Thus	O
,	O
if	O
you	O
open	O
a	O
network	O
connection	O
(	O
e.g.	O
,	O
using	O
TCP	B-Protocol
)	O
,	O
when	O
data	O
arrives	O
for	O
you	O
to	O
read	O
,	O
the	O
networking	O
component	O
can	O
directly	O
call	O
your	O
function	O
to	O
consume	O
it	O
,	O
without	O
having	O
to	O
first	O
copy	O
the	O
data	O
to	O
a	O
buffer	O
and	O
do	O
a	O
context	O
switch	O
.	O
</s>
<s>
Connection	O
Libraries	B-Library
allow	O
a	O
significant	O
degree	O
of	O
control	O
over	O
linkages	O
.	O
</s>
<s>
Another	O
technique	O
for	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	O
)	O
is	O
port	O
files	O
.	O
</s>
<s>
They	O
are	O
like	O
Unix	B-Operating_System
pipes	I-Operating_System
,	O
except	O
that	O
they	O
are	O
generalized	O
to	O
be	O
multiway	O
and	O
bidirectional	O
.	O
</s>
<s>
Since	O
these	O
are	O
an	O
order	O
of	O
magnitude	O
slower	O
than	O
other	O
IPC	O
techniques	O
such	O
as	O
libraries	B-Library
,	O
it	O
is	O
better	O
to	O
use	O
other	O
techniques	O
where	O
the	O
IPC	O
is	O
between	O
different	O
processes	B-Operating_System
on	O
the	O
same	O
machine	O
.	O
</s>
<s>
Port	O
files	O
were	O
introduced	O
with	O
BNA	O
(	O
Burroughs	O
Network	O
Architecture	O
)	O
,	O
but	O
with	O
the	O
advent	O
of	O
standard	O
networking	O
technologies	O
such	O
as	O
OSI	O
and	O
TCP/IP	O
,	O
port	O
files	O
can	O
be	O
used	O
with	O
these	O
networks	O
as	O
well	O
.	O
</s>
<s>
The	O
MCP	B-Application
also	O
provides	O
a	O
sophisticated	O
yet	O
simple	O
operator	O
environment	O
.	O
</s>
<s>
Tasks	B-General_Concept
are	O
entered	O
in	O
the	O
program	O
'	O
mix	O
 '	O
and	O
identified	O
by	O
mix	O
numbers	O
,	O
as	O
are	O
libraries	B-Library
.	O
</s>
<s>
Complete	O
listing	O
of	O
these	O
displays	O
are	O
given	O
by	O
the	O
'	O
A	O
 '	O
(	O
active	O
)	O
,	O
'	O
W	O
 '	O
(	O
waiting	O
)	O
,	O
'	O
C	B-Language
 '	O
(	O
completed	O
)	O
,	O
and	O
'	O
MSG	O
 '	O
(	O
message	O
commands	O
)	O
.	O
</s>
<s>
The	O
OK	O
command	O
can	O
also	O
be	O
used	O
when	O
an	O
operator	O
has	O
made	O
a	O
resource	O
available	O
for	O
a	O
task	O
,	O
although	O
more	O
frequently	O
than	O
not	O
,	O
the	O
MCP	B-Application
will	O
detect	O
that	O
resources	O
have	O
become	O
available	O
,	O
CAUSE	O
the	O
EVENT	O
that	O
processes	B-Operating_System
have	O
been	O
waiting	O
on	O
without	O
further	O
operator	O
intervention	O
.	O
</s>
<s>
As	O
well	O
as	O
tasks	B-General_Concept
and	O
processes	B-Operating_System
,	O
operators	O
also	O
have	O
control	O
over	O
files	O
.	O
</s>
<s>
The	O
operating	O
environment	O
of	O
the	O
MCP	B-Application
is	O
powerful	O
,	O
yet	O
simple	O
and	O
usually	O
only	O
requires	O
a	O
fraction	O
of	O
the	O
number	O
of	O
operators	O
of	O
other	O
systems	O
.	O
</s>
<s>
An	O
important	O
part	O
of	O
the	O
operations	O
environment	O
is	O
the	O
high-level	O
Work	B-Language
Flow	I-Language
Language	I-Language
.	O
</s>
<s>
All	O
significant	O
program	O
actions	O
are	O
optionally	O
logged	O
in	O
a	O
system	O
log	O
and	O
a	O
program	O
log	O
,	O
for	O
example	O
BOJ	O
for	O
beginning	O
of	O
a	O
WFL	O
job	O
,	O
BOT	O
for	O
beginning	O
of	O
a	O
task	O
within	O
a	O
WFL	O
job	O
,	O
EOT	O
and	O
EOJ	O
for	O
end	O
of	O
tasks	B-General_Concept
and	O
jobs	B-General_Concept
.	O
</s>
<s>
Logging	O
many	O
events	O
contributes	O
to	O
an	O
apparent	O
slowness	O
of	O
the	O
MCP	B-Application
operating	O
environment	O
compared	O
to	O
systems	O
like	O
Unix	O
,	O
since	O
everything	O
is	O
logged	O
with	O
forced	O
physical	O
writes	O
to	O
the	O
program	O
log	O
after	O
every	O
record	O
,	O
which	O
is	O
what	O
systems	O
like	O
Unix	O
do	O
n't	O
do	O
,	O
even	O
though	O
they	O
too	O
keep	O
many	O
things	O
in	O
the	O
system	O
logs	O
.	O
</s>
<s>
Also	O
a	O
normal	O
program	O
dump	O
,	O
where	O
just	O
one	O
program	O
was	O
dumped	O
,	O
contains	O
information	O
on	O
source-code	O
sequence	B-Protocol
number	I-Protocol
and	O
variable	O
names	O
.	O
</s>
<s>
Beyond	O
the	O
many	O
technical	O
innovations	O
in	O
the	O
MCP	B-Application
design	O
,	O
the	O
Burroughs	B-Device
Large	I-Device
Systems	I-Device
had	O
many	O
management	O
innovations	O
now	O
being	O
used	O
by	O
the	O
internet	O
community	O
at	O
large	O
.	O
</s>
<s>
The	O
system	O
software	O
was	O
shipped	O
to	O
customers	O
inclusive	O
of	O
source	O
code	O
and	O
all	O
the	O
editing	O
and	O
compilation	O
tools	O
needed	O
to	O
generate	O
new	O
versions	O
of	O
MCP	B-Application
for	O
customers	O
.	O
</s>
<s>
Many	O
customers	O
developed	O
niche	O
expertise	O
on	O
the	O
inner	O
workings	O
of	O
the	O
MCP	B-Application
,	O
and	O
customers	O
often	O
sent	O
in	O
the	O
'	O
patches	O
 '	O
(	O
fragment	O
pieces	O
of	O
source	O
code	O
with	O
sequence	B-Protocol
numbers	I-Protocol
)	O
as	O
suggestions	O
of	O
new	O
enhanced	O
features	O
or	O
fault	O
corrections	O
(	O
FTR	O
-	O
field	O
trouble	O
reports	O
)	O
.	O
</s>
<s>
Many	O
of	O
the	O
suggested	O
patches	O
were	O
included	O
by	O
the	O
systems	O
developers	O
and	O
integrated	O
into	O
the	O
next	O
version	O
of	O
the	O
MCP	B-Application
release	O
.	O
</s>
<s>
Unisys	O
MCP	B-Application
has	O
had	O
several	O
generations	O
of	O
compilers	O
in	O
its	O
history	O
supporting	O
a	O
wide	O
variety	O
of	O
programming	O
languages	O
,	O
including	O
:	O
</s>
<s>
ALGOL	B-Language
,	O
including	O
DCALGOL	O
,	O
BDMSALGOL	O
and	O
DMALGOL	O
.	O
</s>
<s>
Compilers	O
previously	O
existed	O
for	O
ESPOL	B-Language
,	O
COBOL(68 )	O
,	O
Fortran(66 )	O
,	O
APL	B-Language
,	O
and	O
PL/I	B-Language
.	O
</s>
<s>
There	O
is	O
no	O
assembler	B-Language
on	O
the	O
Unisys	O
MCP	B-Application
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
MCP	B-Application
was	O
the	O
first	O
OS	O
developed	O
exclusively	O
in	O
a	O
high-level	B-Language
language	I-Language
.	O
</s>
<s>
Over	O
its	O
50-year	O
history	O
,	O
it	O
has	O
had	O
many	O
firsts	O
in	O
a	O
commercial	O
implementation	O
,	O
including	O
virtual	B-Architecture
memory	I-Architecture
,	O
symmetric	O
multiprocessing	O
,	O
and	O
a	O
high-level	O
job	O
control	O
language	O
(	O
WFL	O
)	O
.	O
</s>
<s>
It	O
has	O
long	O
had	O
many	O
facilities	O
that	O
are	O
only	O
now	O
appearing	O
in	O
other	O
widespread	O
operating	B-General_Concept
systems	I-General_Concept
,	O
and	O
together	O
with	O
the	O
Burroughs	B-Device
large	I-Device
systems	I-Device
architecture	O
,	O
the	O
MCP	B-Application
provides	O
a	O
very	O
secure	O
,	O
high	O
performance	O
,	O
multitasking	O
and	O
transaction	O
processing	O
environment	O
.	O
</s>
