<s>
The	O
IBM	B-Device
System/4	I-Device
Pi	I-Device
is	O
a	O
family	O
of	O
avionics	O
computers	O
used	O
,	O
in	O
various	O
versions	O
,	O
on	O
the	O
F-15	O
Eagle	O
fighter	O
,	O
E-3	O
Sentry	O
AWACS	O
,	O
Harpoon	O
Missile	O
,	O
NASA	O
's	O
Skylab	O
,	O
MOL	O
,	O
and	O
the	O
Space	O
Shuttle	O
,	O
as	O
well	O
as	O
other	O
aircraft	O
.	O
</s>
<s>
It	O
descends	O
from	O
the	O
approach	O
used	O
in	O
the	O
System/360	B-Application
mainframe	B-Device
family	O
of	O
computers	O
,	O
in	O
which	O
the	O
members	O
of	O
the	O
family	O
were	O
intended	O
for	O
use	O
in	O
many	O
varied	O
user	O
applications	O
.	O
</s>
<s>
System/4	B-Device
Pi	I-Device
consisted	O
of	O
basic	O
models	O
:	O
</s>
<s>
Connections	O
with	O
System/360	B-Application
:	O
</s>
<s>
The	O
Skylab	O
space	O
station	O
employed	O
the	O
model	O
TC-1	O
,	O
which	O
had	O
a	O
16-bit	B-Device
word	O
length	O
and	O
16,384	O
words	O
of	O
memory	O
with	O
a	O
custom	O
input/output	O
assembly	O
.	O
</s>
<s>
The	O
AP-101	O
,	O
being	O
the	O
top-of-the-line	O
of	O
the	O
System/4	B-Device
Pi	I-Device
range	O
,	O
shares	O
its	O
general	O
architecture	O
with	O
the	O
System/360	B-Application
mainframes	O
.	O
</s>
<s>
It	O
has	O
16	O
32-bit	O
registers	B-General_Concept
,	O
and	O
uses	O
a	O
microprogram	B-Device
to	O
define	O
an	O
instruction	B-General_Concept
set	I-General_Concept
of	O
154	O
instructions	O
.	O
</s>
<s>
Originally	O
only	O
16	B-Device
bits	I-Device
were	O
available	O
for	O
addressing	O
memory	O
;	O
later	O
this	O
was	O
extended	O
with	O
four	O
bits	O
from	O
the	O
program	B-General_Concept
status	I-General_Concept
word	I-General_Concept
register	O
,	O
allowing	O
a	O
directly	O
addressable	O
memory	O
range	O
of	O
1M	O
locations	O
.	O
</s>
<s>
While	O
its	O
specifications	O
are	O
exceeded	O
by	O
most	O
of	O
the	O
modern	O
microprocessors	B-Architecture
,	O
it	O
was	O
considered	O
high-performance	O
for	O
its	O
era	O
as	O
it	O
could	O
process	O
480,000	O
instructions	O
per	O
second	O
(	O
0.48	O
MIPS	O
;	O
compared	O
to	O
the	O
7,000	O
instructions	O
per	O
second	O
(	O
0.007	O
MIPS	O
)	O
of	O
the	O
computer	O
used	O
on	O
Gemini	O
spacecraft	O
,	O
while	O
top-of-the	O
line	O
microprocessors	B-Architecture
as	O
of	O
2020	O
were	O
capable	O
of	O
performing	O
more	O
than	O
2,000,000	O
MIPS	O
)	O
.	O
</s>
<s>
The	O
Space	O
Shuttle	O
AP-101s	O
were	O
augmented	O
by	O
glass	B-Device
cockpit	I-Device
technology	O
.	O
</s>
<s>
The	O
AP-101S	O
upgrade	O
in	O
the	O
early	O
1990s	O
used	O
semiconductor	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
Each	O
AP-101	O
on	O
the	O
Shuttle	O
was	O
coupled	O
with	O
an	O
input-output	B-Device
processor	I-Device
(	O
IOP	O
)	O
,	O
consisting	O
of	O
one	O
Master	O
Sequence	O
Controller	O
(	O
MSC	O
)	O
and	O
24	O
Bus	O
Control	O
Elements	O
(	O
BCEs	O
)	O
.	O
</s>
<s>
The	O
Shuttle	O
's	O
guidance	O
,	O
navigation	O
and	O
control	O
software	O
was	O
written	O
in	O
HAL/S	B-Language
,	O
a	O
special-purpose	O
high-level	B-Language
programming	I-Language
language	I-Language
,	O
while	O
much	O
of	O
the	O
operating	O
system	O
and	O
low-level	O
utility	O
software	O
was	O
written	O
in	O
assembly	B-Language
language	I-Language
.	O
</s>
<s>
AP-101s	O
used	O
by	O
the	O
US	O
Air	O
Force	O
are	O
mostly	O
programmed	O
in	O
JOVIAL	B-Language
,	O
such	O
as	O
the	O
system	O
found	O
on	O
the	O
B-1B	O
bomber	O
.	O
</s>
