<s>
ARX	B-Operating_System
was	O
an	O
unreleased	O
Mach-like	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
written	O
in	O
Modula-2	B-Language
+	I-Language
developed	O
by	O
Acorn	O
Computers	O
Ltd	O
in	O
the	O
Acorn	O
Research	O
Centre	O
(	O
ARC	O
)	O
United	O
Kingdom	O
(	O
UK	O
)	O
and	O
later	O
by	O
Olivetti	O
-	O
which	O
purchased	O
Acorn	O
-	O
for	O
Acorn	O
's	O
new	O
Archimedes	B-Device
personal	O
computers	O
based	O
on	O
the	O
ARM	B-Architecture
architecture	I-Architecture
reduced	B-Architecture
instruction	I-Architecture
set	I-Architecture
computer	I-Architecture
(	O
RISC	B-Architecture
)	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPUs	O
)	O
.	O
</s>
<s>
According	O
to	O
the	O
project	O
Application	O
Manager	O
Richard	O
Cownie	O
,	O
during	O
the	O
project	O
,	O
while	O
Acorn	O
was	O
developing	O
the	O
kernel	O
,	O
it	O
used	O
the	O
C	O
and	O
Acorn	O
Modula	O
Execution	O
Library	O
(	O
CAMEL	O
)	O
in	O
the	O
Acorn	O
Extended	O
Modula-2	B-Language
(	O
AEM2	O
)	O
compiler	O
(	O
ported	O
from	O
Modula-2	B-Language
ETH	O
Zurich	O
(	O
ETH	O
)	O
using	O
Econet	B-Device
hardware	O
)	O
.	O
</s>
<s>
In	O
an	O
effort	O
to	O
port	O
Sun	O
's	O
workstations	B-Device
Sun	O
NeWS	O
to	O
the	O
Archimedes	B-Device
,	O
David	O
Chase	O
developed	O
a	O
compiler	O
based	O
on	O
AEM2	O
for	O
the	O
programming	O
language	O
Modula-3	B-Language
.	O
</s>
<s>
ARX	B-Operating_System
was	O
a	O
preemptive	B-Operating_System
multitasking	B-Operating_System
,	O
multithreading	B-Operating_System
,	O
multi-user	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Much	O
of	O
the	O
OS	O
ran	O
in	O
user	B-Operating_System
mode	I-Operating_System
and	O
as	O
a	O
result	O
suffered	O
performance	O
problems	O
due	O
to	O
switches	O
into	O
kernel	O
mode	O
to	O
perform	O
mutexes	B-Operating_System
,	O
which	O
led	O
to	O
the	O
introduction	O
of	O
the	O
SWP	O
instruction	O
to	O
the	O
instruction	O
set	O
of	O
the	O
ARMv2a	O
version	O
of	O
the	O
ARM	B-Architecture
processor	I-Architecture
.	O
</s>
<s>
It	O
had	O
support	O
of	O
a	O
file	O
system	O
for	O
optical	O
(	O
write	B-Device
once	I-Device
read	I-Device
many	I-Device
(	O
WORM	B-Device
)	O
)	O
disks	O
and	O
featured	O
a	O
window	O
system	O
,	O
a	O
window	O
toolkit	O
(	O
and	O
a	O
direct	O
manipulation	O
user	B-Application
interface	I-Application
(	O
UI	O
)	O
editor	O
)	O
and	O
an	O
Interscript-based	O
text	O
editor	O
,	O
for	O
enriched	O
documents	O
written	O
in	O
Interpress	B-Language
(	O
a	O
HTML	B-Language
precursor	O
)	O
.	O
</s>
<s>
The	O
OS	O
had	O
to	O
be	O
fitted	O
in	O
a	O
512KB	O
read-only	B-Device
memory	I-Device
(	O
ROM	B-Device
)	O
ROM	B-General_Concept
image	I-General_Concept
.	O
</s>
<s>
This	O
suggests	O
that	O
ARX	B-Operating_System
had	O
a	O
microkernel-type	O
design	O
.	O
</s>
<s>
It	O
was	O
not	O
finished	O
in	O
time	O
to	O
be	O
used	O
in	O
the	O
Acorn	B-Device
Archimedes	I-Device
range	O
of	O
computers	O
,	O
which	O
shipped	O
in	O
1987	O
with	O
an	O
operating	B-General_Concept
system	I-General_Concept
named	O
Arthur	O
,	O
later	O
renamed	O
RISC	B-Operating_System
OS	I-Operating_System
,	O
derived	O
from	O
the	O
earlier	O
Machine	B-Operating_System
Operating	I-Operating_System
System	I-Operating_System
(	O
MOS	B-Operating_System
)	O
from	O
Acorn	O
's	O
earlier	O
8-bit	O
BBC	B-Device
Micro	I-Device
range	O
.	O
</s>
<s>
Confusion	O
persisted	O
about	O
the	O
nature	O
of	O
ARX	B-Operating_System
amongst	O
the	O
wider	O
public	O
and	O
press	O
,	O
with	O
some	O
believing	O
that	O
ARX	B-Operating_System
was	O
Acorn	O
's	O
own	O
Unix	O
variant	O
,	O
with	O
this	O
view	O
being	O
refined	O
in	O
time	O
to	O
accommodate	O
ARX	B-Operating_System
as	O
Acorn	O
's	O
own	O
attempt	O
to	O
deliver	O
a	O
"	O
UNIX	O
look-alike	O
"	O
whose	O
development	O
had	O
been	O
abandoned	O
in	O
favour	O
of	O
a	O
traditional	O
Unix	O
version	O
for	O
the	O
Archimedes	B-Device
,	O
which	O
ultimately	O
emerged	O
as	O
RISC	B-Operating_System
iX	I-Operating_System
.	O
</s>
