<s>
Accent	O
is	O
an	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
,	O
most	O
notable	O
for	O
being	O
the	O
predecessor	O
to	O
the	O
Mach	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
Originally	O
developed	O
at	O
Carnegie	O
Mellon	O
University	O
(	O
CMU	O
)	O
,	O
Accent	O
was	O
influenced	O
by	O
the	O
Aleph	B-Operating_System
kernel	I-Operating_System
developed	O
at	O
the	O
University	O
of	O
Rochester	O
.	O
</s>
<s>
Accent	O
improves	O
upon	O
the	O
older	O
kernel	B-Operating_System
,	O
fixing	O
several	O
problems	O
and	O
re-targeting	O
hardware	O
support	O
for	O
networks	O
of	O
workstation	B-Device
machines	O
(	O
specifically	O
,	O
the	O
Three	B-Device
Rivers	I-Device
PERQ	I-Device
)	O
instead	O
of	O
minicomputers	B-Architecture
.	O
</s>
<s>
Development	O
of	O
Accent	O
led	O
directly	O
to	O
the	O
introduction	O
of	O
Mach	B-Operating_System
,	O
used	O
in	O
NeXTSTEP	B-Application
,	O
GNU	B-Application
Hurd	I-Application
,	O
and	O
modern	O
Apple	O
operating	B-General_Concept
systems	I-General_Concept
including	O
Mac	B-Application
OS	I-Application
and	O
iOS	B-Application
.	O
</s>
<s>
In	O
this	O
respect	O
it	O
was	O
similar	O
in	O
concept	O
to	O
Unix	B-Application
,	O
although	O
the	O
implementation	O
was	O
much	O
different	O
,	O
using	O
messages	O
instead	O
of	O
memory	O
.	O
</s>
<s>
This	O
turned	O
out	O
to	O
have	O
a	O
number	O
of	O
problems	O
,	O
notably	O
that	O
copying	O
memory	O
on	O
their	O
Data	B-Device
General	I-Device
Eclipse	I-Device
was	O
very	O
expensive	O
.	O
</s>
<s>
In	O
particular	O
,	O
Accent	O
targeted	O
workstation	B-Device
machines	O
featuring	O
a	O
MMU	B-General_Concept
,	O
using	O
the	O
MMU	B-General_Concept
to	O
"	O
copy	O
"	O
large	O
blocks	O
of	O
memory	O
via	O
mapping	O
,	O
making	O
the	O
memory	O
appear	O
to	O
be	O
in	O
two	O
different	O
places	O
.	O
</s>
<s>
Only	O
data	O
that	O
was	O
changed	O
by	O
one	O
program	O
or	O
another	O
would	O
have	O
to	O
be	O
physically	O
copied	O
,	O
using	O
the	O
copy-on-write	B-Language
algorithm	O
.	O
</s>
<s>
Under	O
Accent	O
the	O
data	O
simply	O
"	O
appeared	O
"	O
in	O
the	O
user	O
process	O
for	O
the	O
cost	O
of	O
a	O
few	O
instructions	O
sent	O
to	O
the	O
MMU	B-General_Concept
.	O
</s>
<s>
To	O
address	O
this	O
,	O
Accent	O
made	O
the	O
port	O
ID	O
's	O
internal	O
to	O
the	O
kernel	B-Operating_System
only	O
.	O
</s>
<s>
Instances	O
of	O
a	O
program	O
opening	O
ports	O
were	O
handed	O
back	O
different	O
IDs	O
,	O
stored	O
in	O
a	O
mapping	O
in	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
Whenever	O
a	O
message	O
was	O
sent	O
to	O
the	O
kernel	B-Operating_System
for	O
delivery	O
,	O
it	O
would	O
first	O
check	O
that	O
the	O
program	O
had	O
access	O
to	O
the	O
port	O
in	O
question	O
by	O
comparing	O
with	O
the	O
mapping	O
table	O
for	O
that	O
program	O
.	O
</s>
<s>
Guessing	O
port	O
numbers	O
no	O
longer	O
worked	O
,	O
the	O
program	O
's	O
port	O
IDs	O
gave	O
no	O
clue	O
of	O
the	O
"	O
real	O
"	O
IDs	O
in	O
the	O
kernel	B-Operating_System
,	O
and	O
any	O
attempt	O
to	O
talk	O
on	O
one	O
not	O
explicitly	O
handed	O
out	O
by	O
the	O
kernel	B-Operating_System
was	O
an	O
error	O
.	O
</s>
<s>
This	O
kind	O
of	O
capability	O
system	O
,	O
using	O
subject-specific	O
identifiers	O
for	O
capabilities	O
,	O
is	O
called	O
a	O
C-list	B-Data_Structure
system	I-Data_Structure
.	O
</s>
<s>
In	O
the	O
early	O
1980s	O
many	O
felt	O
that	O
future	O
gains	O
in	O
performance	O
would	O
be	O
made	O
by	O
adding	O
more	O
CPUs	B-General_Concept
to	O
machines	O
,	O
something	O
the	O
Accent	B-Operating_System
kernel	I-Operating_System
was	O
not	O
really	O
equipped	O
to	O
handle	O
.	O
</s>
<s>
Adding	O
to	O
the	O
problem	O
was	O
that	O
a	O
new	O
generation	O
of	O
more	O
powerful	O
workstations	B-Device
were	O
appearing	O
,	O
meaning	O
that	O
Accent	O
would	O
likely	O
have	O
to	O
be	O
ported	O
to	O
them	O
anyway	O
.	O
</s>
<s>
Likewise	O
Unix	B-Application
had	O
grown	O
into	O
the	O
operating	B-General_Concept
system	I-General_Concept
of	O
choice	O
for	O
experimental	O
work	O
,	O
both	O
on	O
operating	B-General_Concept
system	I-General_Concept
design	O
,	O
as	O
well	O
as	O
a	O
development	O
platform	O
for	O
user	O
applications	O
.	O
</s>
<s>
The	O
new	O
system	O
would	O
use	O
Accent	O
's	O
ports	O
system	O
within	O
a	O
Unix	B-Application
kernel	B-Operating_System
,	O
creating	O
the	O
famed	O
Mach	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
Ability	O
to	O
run	O
processes	O
with	O
different	O
microcoded	B-Device
instruction	O
sets	O
.	O
</s>
