<s>
Not	B-Operating_System
Another	I-Operating_System
Completely	I-Operating_System
Heuristic	I-Operating_System
Operating	I-Operating_System
System	I-Operating_System
,	O
or	O
Nachos	O
,	O
is	O
instructional	O
software	O
for	O
teaching	O
undergraduate	O
,	O
and	O
potentially	O
graduate	O
level	O
operating	B-General_Concept
systems	I-General_Concept
courses	O
.	O
</s>
<s>
Originally	O
written	O
in	O
C++	B-Language
for	O
MIPS	B-Device
,	O
Nachos	O
runs	O
as	O
a	O
user-process	O
on	O
a	O
host	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
A	O
MIPS	B-Device
simulator	O
executes	O
the	O
code	O
for	O
any	O
user	O
programs	O
running	O
on	O
top	O
of	O
the	O
Nachos	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
The	O
goal	O
of	O
Nachos	O
is	O
to	O
introduce	O
students	O
to	O
concepts	O
in	O
operating	B-General_Concept
system	I-General_Concept
design	O
and	O
implementation	O
by	O
requiring	O
them	O
to	O
implement	O
significant	O
pieces	O
of	O
functionality	O
within	O
the	O
Nachos	O
system	O
.	O
</s>
<s>
In	O
Nachos	O
 '	O
case	O
,	O
Operating	B-General_Concept
System	I-General_Concept
simulator	O
simply	O
means	O
that	O
you	O
can	O
run	O
an	O
OS	O
(	O
a	O
guest	O
OS	O
)	O
on	O
top	O
of	O
another	O
one	O
(	O
the	O
host	O
OS	O
)	O
,	O
similar	O
to	O
Bochs/VMware	O
.	O
</s>
<s>
That	O
means	O
that	O
you	O
can	O
write	O
programs	O
for	O
Nachos	O
,	O
compile	O
them	O
with	O
a	O
real	O
compiler	O
(	O
an	O
old	O
gcc	O
compiler	O
that	O
produces	O
code	O
for	O
MIPS	B-Device
)	O
and	O
run	O
them	O
.	O
</s>
<s>
Nachos	O
has	O
various	O
modules	O
implementing	O
the	O
functionality	O
of	O
a	O
basic	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
NachOS	O
Machine	O
-	O
Nachos	O
simulates	O
a	O
machine	O
that	O
roughly	O
approximates	O
the	O
MIPS	B-Device
architecture	I-Device
.	O
</s>
<s>
The	O
Nachos/MIPS	O
machine	O
is	O
implemented	O
by	O
the	O
Machine	O
object	O
,	O
an	O
instance	O
of	O
which	O
is	O
created	O
when	O
Nachos	O
starts	O
up	O
.	O
</s>
<s>
Nachos	O
can	O
run	O
any	O
MIPS	B-Device
binary	O
,	O
assuming	O
that	O
it	O
restricts	O
itself	O
to	O
only	O
making	O
system	O
calls	O
that	O
Nachos	O
understands	O
.	O
</s>
<s>
As	O
Nachos	O
has	O
not	O
been	O
in	O
active	O
development	O
for	O
a	O
number	O
of	O
years	O
,	O
and	O
possesses	O
a	O
number	O
of	O
recognized	O
flaws	O
(	O
particularly	O
with	O
regards	O
to	O
portability	O
:	O
Nachos	O
relies	O
on	O
MIPS	B-Device
assembly	I-Device
code	O
,	O
and	O
requires	O
porting	O
to	O
run	O
on	O
x86	B-Operating_System
architecture	I-Operating_System
)	O
,	O
successor	O
projects	O
have	O
been	O
initiated	O
.	O
</s>
<s>
In	O
2004	O
,	O
Stanford	O
University	O
created	O
Pintos	B-Application
,	O
a	O
Nachos-inspired	O
system	O
written	O
in	O
C	B-Language
and	O
designed	O
to	O
run	O
on	O
actual	O
x86	B-Operating_System
hardware	O
.	O
</s>
<s>
In	O
2000	O
,	O
Dan	O
Hettena	O
at	O
UC	O
Berkeley	O
ported	O
Nachos	O
to	O
Java	B-Language
as	O
Nachos	O
5.0j	O
,	O
in	O
an	O
effort	O
to	O
make	O
Nachos	O
more	O
portable	O
,	O
more	O
accessible	O
to	O
undergraduates	O
,	O
and	O
less	O
susceptible	O
to	O
subtle	O
bugs	O
in	O
student	O
code	O
that	O
had	O
in	O
earlier	O
versions	O
often	O
dominated	O
student	O
project	O
development	O
time	O
.	O
</s>
<s>
Another	O
Java-based	O
version	O
was	O
created	O
by	O
Professor	O
Peter	O
Druschel	O
at	O
Rice	O
University	O
.	O
</s>
<s>
It	O
was	O
later	O
adapted	O
by	O
Professor	O
Eugene	O
Stark	O
at	O
Stony	O
Brook	O
University	O
in	O
2003	O
and	O
applied	O
in	O
the	O
Operating	B-General_Concept
System	I-General_Concept
course	O
.	O
</s>
<s>
At	O
Graz	O
University	O
of	O
Technology	O
(	O
Austria	O
)	O
,	O
a	O
system	O
called	O
SWEB	O
(	O
"	O
Schon	O
wieder	O
ein	O
Betriebssystem	O
"	O
)	O
has	O
been	O
implemented	O
and	O
is	O
used	O
to	O
teach	O
operating	B-General_Concept
system	I-General_Concept
principles	O
.	O
</s>
