<s>
The	O
REX	O
Operating	B-General_Concept
System	I-General_Concept
(	O
Real-time	O
Executive	O
Operating	B-General_Concept
System	I-General_Concept
)	O
is	O
a	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
(	O
RTOS	B-Operating_System
)	O
developed	O
by	O
Qualcomm	O
for	O
the	O
ARM	B-Architecture
processor	I-Architecture
based	O
mobile	O
phone	O
Dual-Mode	O
Subscriber	O
Station	O
(	O
DMSS	O
)	O
or	O
Advanced	O
Mode	O
Subscriber	O
Software	O
(	O
AMSS	O
)	O
development	O
.	O
</s>
<s>
REX	O
is	O
a	O
combination	O
of	O
two	O
operating	B-General_Concept
systems	I-General_Concept
:	O
L4A	O
Pistachio	O
embedded	O
microkernel	B-Operating_System
and	O
Iguana	B-Operating_System
,	O
with	O
extensive	O
modifications	O
and	O
extensions	O
by	O
Qualcomm	O
and	O
HTC	B-Application
.	O
</s>
<s>
Originally	O
developed	O
for	O
the	O
Intel	B-Device
80186	I-Device
,	O
it	O
was	O
then	O
ported	O
to	O
ARM	B-Architecture
,	O
and	O
x86	B-Operating_System
architectures	I-Operating_System
are	O
no	O
longer	O
supported	O
.	O
</s>
<s>
REX	O
is	O
not	O
POSIX	O
compliant	O
or	O
open-source	B-Application
software	I-Application
and	O
requires	O
a	O
proprietary	B-Application
software	I-Application
license	I-Application
from	O
Qualcomm	O
for	O
use	O
.	O
</s>
<s>
REX	O
is	O
an	O
RTOS	B-Operating_System
with	O
the	O
following	O
features	O
:	O
</s>
<s>
REX	O
provides	O
no	O
memory	B-General_Concept
protection	I-General_Concept
features	O
but	O
does	O
have	O
memory	B-General_Concept
management	I-General_Concept
abilities	O
.	O
</s>
