<s>
OS4000	B-Operating_System
is	O
a	O
proprietary	B-Application
operating	B-General_Concept
system	I-General_Concept
introduced	O
by	O
GEC	B-Device
Computers	I-Device
Limited	I-Device
in	O
1977	O
as	O
the	O
successor	O
to	O
GEC	O
DOS	O
,	O
for	O
its	O
range	O
of	O
GEC	B-Device
4000	I-Device
series	I-Device
16-bit	B-Device
,	O
and	O
later	O
32-bit	O
,	O
minicomputers	B-Architecture
.	O
</s>
<s>
OS4000	B-Operating_System
was	O
developed	O
through	O
to	O
late	O
1990s	O
,	O
and	O
has	O
been	O
in	O
a	O
support-only	O
mode	O
since	O
then	O
.	O
</s>
<s>
The	O
first	O
operating	B-General_Concept
systems	I-General_Concept
for	O
the	O
GEC	B-Device
4000	I-Device
series	I-Device
were	O
COS	O
(	O
Core	O
Operating	B-General_Concept
System	I-General_Concept
)	O
and	O
DOS	O
(	O
Disk	O
Operating	B-General_Concept
System	I-General_Concept
)	O
.	O
</s>
<s>
These	O
were	O
basically	O
single-user	B-Operating_System
multi-tasking	O
operating	B-General_Concept
systems	I-General_Concept
,	O
designed	O
for	O
developing	O
and	O
running	O
Process	O
control	O
type	O
applications	O
.	O
</s>
<s>
OS4000	B-Operating_System
was	O
first	O
released	O
around	O
1977	O
.	O
</s>
<s>
It	O
reused	O
many	O
of	O
the	O
parts	O
of	O
DOS	O
,	O
but	O
added	O
multi-user	B-Operating_System
access	O
,	O
OS4000	B-Operating_System
JCL	O
Command-line	O
interpreter	O
,	O
Batch	B-General_Concept
processing	I-General_Concept
,	O
OS4000	B-Operating_System
hierarchical	O
filesystem	O
(	O
although	O
on-disk	O
format	O
very	O
similar	O
to	O
the	O
non-hierarchical	O
DOS	O
filesystem	O
)	O
.	O
</s>
<s>
OS4000	B-Operating_System
JCL	O
was	O
based	O
on	O
the	O
Cambridge	O
University	O
Phoenix	O
command	O
interpreter	O
.	O
</s>
<s>
OS4000	B-Operating_System
Rel	O
3	O
arrived	O
around	O
1980	O
,	O
and	O
included	O
Linked-OS	O
—	O
support	O
for	O
Linked	O
OS4000	B-Operating_System
operating	B-General_Concept
systems	I-General_Concept
to	O
enable	O
multi-node	O
systems	O
to	O
be	O
constructed	O
.	O
</s>
<s>
The	O
main	O
customer	O
for	O
this	O
was	O
the	O
central	O
computing	O
service	O
of	O
University	O
College	O
London	O
(	O
Euclid	O
)	O
,	O
where	O
a	O
multi-node	O
system	O
consisting	O
of	O
a	O
Hub	O
file	B-Application
server	I-Application
and	O
multiple	O
Rim	O
multi-access	O
compute	O
server	O
systems	O
provided	O
service	O
for	O
over	O
100	O
simultaneous	O
users	O
.	O
</s>
<s>
OS4000	B-Operating_System
Rel	O
4	O
arrived	O
around	O
1983	O
,	O
and	O
upped	O
the	O
maximum	O
number	O
of	O
user	O
modules	O
to	O
150	O
(	O
again	O
,	O
mainly	O
for	O
the	O
University	O
College	O
London	O
Euclid	O
system	O
)	O
,	O
together	O
with	O
an	O
enhanced	O
Batch	B-General_Concept
processing	I-General_Concept
system	O
.	O
</s>
<s>
OS4000	B-Operating_System
Rel	O
5	O
introduced	O
a	O
modified	O
version	O
of	O
the	O
OS4000	B-Operating_System
filesystem	O
called	O
CFSX	O
,	O
in	O
order	O
to	O
allow	O
easier	O
use	O
of	O
larger	O
disks	O
.	O
</s>
<s>
OS4000	B-Operating_System
Rel	O
6	O
introduced	O
support	O
for	O
dual	O
processor	O
systems	O
(	O
GEC	O
4190D	O
)	O
.	O
</s>
<s>
OS4000	B-Operating_System
was	O
developed	O
in	O
the	O
UK	O
at	O
GEC	B-Device
Computers	I-Device
Borehamwood	O
offices	O
in	O
Elstree	O
Way	O
,	O
and	O
at	O
GEC	B-Device
Computers	I-Device
Dunstable	O
Development	O
Centre	O
in	O
Woodside	O
Estate	O
,	O
Dunstable	O
.	O
</s>
<s>
The	O
architecture	O
of	O
OS4000	B-Operating_System
is	O
very	O
heavily	O
based	O
around	O
the	O
features	O
of	O
the	O
platform	O
it	O
runs	O
on	O
,	O
the	O
GEC	B-Device
4000	I-Device
series	I-Device
minicomputers	B-Architecture
,	O
and	O
these	O
are	O
rather	O
unusual	O
.	O
</s>
<s>
They	O
include	O
a	O
feature	O
called	O
Nucleus	O
,	O
which	O
is	O
a	O
combination	O
of	O
a	O
hardware	B-Architecture
-	O
and	O
firmware-based	O
kernel	O
,	O
which	O
cannot	O
be	O
altered	O
under	O
program	O
control	O
.	O
</s>
<s>
This	O
means	O
that	O
many	O
of	O
the	O
features	O
typically	O
found	O
in	O
operating	B-General_Concept
system	I-General_Concept
kernels	O
do	O
not	O
need	O
to	O
be	O
included	O
in	O
OS4000	B-Operating_System
,	O
as	O
the	O
underlying	O
platform	O
performs	O
these	O
functions	O
instead	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Consequently	O
,	O
there	O
is	O
no	O
provision	O
for	O
running	O
privileged	B-Operating_System
mode	I-Operating_System
code	O
on	O
the	O
platform	O
—	O
all	O
OS4000	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
code	O
runs	O
as	O
processes	O
.	O
</s>
<s>
OS4000	B-Operating_System
lives	O
entirely	O
within	O
these	O
processes	O
.	O
</s>
<s>
Nucleus	O
implements	O
a	O
segmented	O
memory	O
system	O
,	O
with	O
processes	O
having	O
their	O
access	O
to	O
memory	O
segments	O
defined	O
by	O
the	O
system	O
tables	O
,	O
which	O
is	O
maintained	O
by	O
OS4000	B-Operating_System
.	O
</s>
<s>
OS4000	B-Operating_System
provides	O
a	O
memory	O
system	O
which	O
handles	O
both	O
store-resident	O
memory	O
,	O
and	O
virtual	O
memory	O
backed	O
by	O
disk	O
which	O
is	O
known	O
as	O
overlay	O
,	O
with	O
overlaying	O
being	O
performed	O
at	O
the	O
segment	O
level	O
.	O
</s>
<s>
OS4000	B-Operating_System
also	O
inherited	O
grouped	O
segments	O
from	O
DOS	O
,	O
where	O
a	O
group	O
of	O
segments	O
were	O
to	O
be	O
overlaid	O
and	O
retrieved	O
as	O
a	O
single	O
group	O
,	O
but	O
this	O
feature	O
was	O
very	O
little	O
used	O
in	O
OS4000	B-Operating_System
.	O
</s>
<s>
A	O
process	O
may	O
use	O
any	O
mixture	O
of	O
resident	O
and	O
overlayable	O
segments	O
,	O
although	O
a	O
process	O
performing	O
real-time	B-General_Concept
tasks	O
would	O
normally	O
be	O
designed	O
to	O
only	O
use	O
resident	O
segments	O
.	O
</s>
<s>
OS4000	B-Operating_System
supports	O
a	O
fully	O
mixed	O
set	O
of	O
process	O
scheduling	O
within	O
the	O
same	O
system	O
,	O
from	O
hard	O
real-time	B-General_Concept
processes	O
,	O
through	O
soft	O
real-time	B-General_Concept
,	O
time-shared	O
,	O
and	O
background	O
.	O
</s>
<s>
Given	O
that	O
OS4000	B-Operating_System
also	O
includes	O
full	O
program	O
development	O
and	O
test/debug	O
facilities	O
,	O
this	O
made	O
OS4000	B-Operating_System
ideal	O
for	O
developing	O
and	O
deploying	O
real-time	B-General_Concept
applications	I-General_Concept
such	O
as	O
process	O
control	O
and	O
high	O
speed	O
(	O
at	O
the	O
time	O
)	O
data	O
communications	O
all	O
within	O
one	O
system	O
.	O
</s>
<s>
OS4000	B-Operating_System
uses	O
its	O
own	O
proprietary	B-Application
filesystem	O
.	O
</s>
<s>
OS4000	B-Operating_System
JCL	O
limits	O
characters	O
in	O
file	O
path	O
components	O
to	O
upper	O
case	O
letters	O
and	O
numbers	O
only	O
.	O
</s>
<s>
There	O
must	O
be	O
one	O
disk	O
mounted	O
with	O
a	O
volume	O
name	O
of	O
SYSTEM	O
which	O
contains	O
specific	O
files	O
required	O
by	O
OS4000	B-Operating_System
.	O
</s>
<s>
Files	O
in	O
an	O
OS4000	B-Operating_System
filesystem	O
are	O
typed	O
,	O
which	O
means	O
that	O
the	O
filesystem	O
can	O
hold	O
several	O
different	O
types	O
of	O
file	O
,	O
and	O
understands	O
how	O
the	O
contents	O
are	O
structured	O
.	O
</s>
<s>
Most	O
common	O
are	O
logical	O
files	O
which	O
contain	O
a	O
record	B-General_Concept
structure	I-General_Concept
.	O
</s>
<s>
Additionally	O
,	O
there	O
is	O
a	O
Logical	O
Indexed	O
Sequential	O
(	O
LIS	O
)	O
filetype	O
,	O
which	O
is	O
an	O
ISAM	B-Application
file	O
and	O
always	O
appears	O
to	O
be	O
sorted	O
on	O
its	O
key	O
field	O
,	O
and	O
a	O
Byte	O
stream	O
(	O
BYT	O
)	O
filetype	O
,	O
which	O
was	O
added	O
in	O
Rel	O
6.5	O
to	O
better	O
support	O
the	O
OS4000	B-Operating_System
NFS	B-Protocol
server	O
.	O
</s>
<s>
Support	O
for	O
Unix	O
style	O
symlinks	O
(	O
arbitrary	O
text	O
stored	O
in	O
a	O
catalogue	O
)	O
was	O
added	O
in	O
Rel	O
6.5	O
to	O
better	O
support	O
the	O
OS4000	B-Operating_System
NFS	B-Protocol
server	O
,	O
but	O
symlinks	O
can	O
only	O
be	O
created	O
and	O
are	O
only	O
visible	O
from	O
NFS	B-Protocol
clients	O
.	O
</s>
<s>
OS4000	B-Operating_System
also	O
provides	O
a	O
non-hierarchical	O
temporary	O
filesystem	O
.	O
</s>
<s>
When	O
a	O
user	O
logs	O
in	O
,	O
the	O
OS4000	B-Operating_System
JCL	O
command	O
interpreter	O
SYS.COMM	O
is	O
loaded	O
into	O
the	O
user	O
's	O
COMM	O
process	O
and	O
started	O
.	O
</s>
<s>
Real-time	B-General_Concept
Process	O
Control	O
accounts	O
for	O
over	O
half	O
of	O
all	O
the	O
OS4000	B-Operating_System
systems	O
deployed	O
.	O
</s>
<s>
The	O
earlier	O
of	O
these	O
Real-time	B-General_Concept
Process	O
Control	O
systems	O
were	O
upgraded	O
from	O
DOS	O
to	O
OS4000	B-Operating_System
.	O
</s>
<s>
X.25	B-Protocol
Packet	B-Protocol
Switches	I-Protocol
account	O
for	O
a	O
significant	O
proportion	O
of	O
systems	O
(	O
although	O
earlier	O
GEC	O
X.25	B-Protocol
Packet	B-Protocol
Switches	I-Protocol
ran	O
a	O
special	O
operating	B-General_Concept
system	I-General_Concept
called	O
NOS	O
which	O
was	O
a	O
cut	O
down	O
operating	B-General_Concept
system	I-General_Concept
halfway	O
between	O
DOS	O
and	O
OS4000	B-Operating_System
)	O
.	O
</s>
<s>
Prestel	B-General_Concept
(	O
UK	O
)	O
and	O
the	O
public	O
Videotex	B-Operating_System
systems	O
used	O
in	O
many	O
other	O
countries	O
,	O
and	O
many	O
private	O
Viewdata	B-Application
systems	O
.	O
</s>
<s>
Multi-User	B-Operating_System
Minicomputers	B-Architecture
,	O
used	O
in	O
many	O
Education	O
and	O
Research	O
establishments	O
.	O
</s>
<s>
OS4000	B-Operating_System
was	O
ported	O
to	O
the	O
GEC	B-Device
Series	I-Device
63	I-Device
minicomputer	B-Architecture
where	O
it	O
was	O
known	O
as	O
OS6000	O
.	O
</s>
<s>
This	O
required	O
the	O
addition	O
of	O
a	O
software	O
Nucleus	O
emulation	O
,	O
as	O
this	O
was	O
not	O
a	O
feature	O
of	O
the	O
GEC	B-Device
Series	I-Device
63	I-Device
hardware	B-Architecture
.	O
</s>
<s>
GEC	B-Device
Computers	I-Device
dropped	O
OS6000	O
,	O
and	O
the	O
source	O
code	O
was	O
given	O
to	O
Daresbury	O
Laboratory	O
who	O
was	O
the	O
main	O
user	O
of	O
it	O
,	O
and	O
they	O
continued	O
to	O
keep	O
it	O
in	O
step	O
with	O
OS4000	B-Operating_System
releases	O
for	O
the	O
lifetime	O
of	O
their	O
two	O
GEC	B-Device
Series	I-Device
63	I-Device
systems	O
.	O
</s>
