<s>
Pilot	O
is	O
a	O
single-user	O
,	O
multitasking	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
designed	O
by	O
Xerox	O
PARC	O
in	O
early	O
1977	O
.	O
</s>
<s>
Pilot	O
was	O
written	O
in	O
the	O
Mesa	B-Language
programming	I-Language
language	I-Language
,	O
totalling	O
about	O
24,000	O
lines	O
of	O
code	O
.	O
</s>
<s>
Pilot	O
was	O
designed	O
as	O
a	O
single	O
user	O
system	O
in	O
a	O
highly	O
networked	O
environment	O
of	O
other	O
Pilot	O
systems	O
,	O
with	O
interfaces	O
designed	O
for	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	O
)	O
across	O
the	O
network	O
via	O
the	O
Pilot	O
stream	O
interface	O
.	O
</s>
<s>
Pilot	O
combined	O
virtual	B-Architecture
memory	I-Architecture
and	O
file	O
storage	O
into	O
one	O
subsystem	O
,	O
and	O
used	O
the	O
manager/kernel	O
architecture	O
for	O
managing	O
the	O
system	O
and	O
its	O
resources	O
.	O
</s>
<s>
Its	O
designers	O
considered	O
a	O
non-preemptive	O
multitasking	B-Operating_System
model	O
,	O
but	O
later	O
chose	O
a	O
preemptive	O
(	O
run	O
until	O
blocked	O
)	O
system	O
based	O
on	O
monitors	O
.	O
</s>
<s>
Pilot	O
included	O
a	O
debugger	O
,	O
Co-Pilot	O
,	O
that	O
could	O
debug	O
a	O
frozen	O
snapshot	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
written	O
to	O
disk	O
.	O
</s>
<s>
A	O
typical	O
Pilot	O
workstation	O
ran	O
3	O
operating	B-General_Concept
systems	I-General_Concept
at	O
once	O
on	O
3	O
different	O
disk	O
volumes	O
:	O
Co-Co-Pilot	O
(	O
a	O
backup	O
debugger	O
in	O
case	O
the	O
main	O
operating	B-General_Concept
system	I-General_Concept
crashed	O
)	O
,	O
Co-Pilot	O
(	O
the	O
main	O
operating	B-General_Concept
system	I-General_Concept
,	O
running	O
under	O
co-pilot	O
and	O
used	O
to	O
compile	O
and	O
bind	O
programs	O
)	O
and	O
an	O
inferior	O
copy	O
of	O
Pilot	O
running	O
in	O
a	O
3rd	O
disk	O
volume	O
,	O
that	O
could	O
be	O
booted	O
to	O
run	O
test	O
programs	O
(	O
that	O
might	O
crash	O
the	O
main	O
development	O
environment	O
)	O
.	O
</s>
<s>
This	O
architecture	O
was	O
unique	O
because	O
it	O
allowed	O
the	O
developer	O
to	O
single-step	O
even	O
operating	B-General_Concept
system	I-General_Concept
code	O
with	O
semaphore	O
locks	O
,	O
stored	O
on	O
an	O
inferior	O
disk	O
volume	O
.	O
</s>
<s>
However	O
,	O
as	O
the	O
memory	O
and	O
source	O
code	O
of	O
the	O
D-series	O
Xerox	O
processors	O
grew	O
,	O
the	O
time	O
to	O
checkpoint	O
and	O
restore	O
the	O
operating	B-General_Concept
system	I-General_Concept
(	O
known	O
as	O
a	O
"	O
world	O
swap	O
"	O
)	O
grew	O
very	O
high	O
.	O
</s>
<s>
It	O
could	O
take	O
60-120	O
seconds	O
to	O
run	O
just	O
one	O
line	O
of	O
code	O
in	O
the	O
inferior	O
operating	B-General_Concept
system	I-General_Concept
environment	O
.	O
</s>
<s>
Pilot	O
was	O
used	O
as	O
the	O
operating	B-General_Concept
system	I-General_Concept
for	O
the	O
Xerox	B-Operating_System
Star	I-Operating_System
workstation	O
.	O
</s>
