<s>
The	O
Raytheon	B-Device
704	I-Device
is	O
a	O
16-bit	B-Device
minicomputer	B-Architecture
introduced	O
by	O
Raytheon	O
in	O
1970	O
.	O
</s>
<s>
The	O
basic	O
machine	O
contained	O
4kwords	O
(	O
8kB	O
)	O
of	O
memory	O
and	O
a	O
simple	O
arithmetic	B-General_Concept
logic	I-General_Concept
unit	I-General_Concept
(	O
ALU	O
)	O
running	O
at	O
1MHz	O
.	O
</s>
<s>
It	O
was	O
normally	O
operated	O
with	O
a	O
Teletype	B-Application
Model	I-Application
33	I-Application
acting	O
as	O
a	O
computer	B-General_Concept
terminal	I-General_Concept
.	O
</s>
<s>
A	O
key	O
feature	O
of	O
the	O
design	O
was	O
the	O
ability	O
to	O
expand	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
using	O
plug-in	O
cards	O
.	O
</s>
<s>
Options	O
included	O
a	O
hardware	O
multiply/divide	O
unit	O
,	O
an	O
8-level	O
vectored	B-Architecture
interrupt	I-Architecture
controller	O
,	O
a	O
DMA	B-General_Concept
controller	I-General_Concept
,	O
among	O
others	O
.	O
</s>
<s>
Memory	O
was	O
based	O
on	O
an	O
18-bit	O
word	O
,	O
not	O
16-bit	B-Device
,	O
with	O
the	O
extra	O
bits	O
for	O
use	O
with	O
an	O
optional	O
parity	B-Error_Name
check	I-Error_Name
card	O
.	O
</s>
<s>
Another	O
unique	O
feature	O
was	O
that	O
general	O
input/output	B-General_Concept
expansion	O
was	O
external	O
,	O
using	O
a	O
daisy	B-Application
chained	I-Application
cable	O
system	O
known	O
as	O
DIO	O
.	O
</s>
<s>
The	O
PDP-8/I	B-Device
,	O
of	O
1968	O
,	O
cost	O
$	O
12,800	O
for	O
a	O
similar	O
4kWord	O
machine	O
,	O
but	O
used	O
smaller	O
12-bit	O
words	O
and	O
thus	O
had	O
6kB	O
of	O
memory	O
compared	O
to	O
the	O
704	O
's	O
16-bit	B-Device
words	O
where	O
the	O
same	O
4kWord	O
memory	O
was	O
8kB	O
.	O
</s>
<s>
The	O
704	O
was	O
also	O
faster	O
,	O
running	O
at	O
1MHz	O
rather	O
than	O
the	O
PDP-8	B-Device
'	O
s	O
600kHz	O
.	O
</s>
<s>
Another	O
machine	O
aimed	O
more	O
squarely	O
at	O
the	O
704	O
's	O
instrumentation	O
market	O
was	O
the	O
HP	B-Device
2116A	I-Device
,	O
another	O
16-bit	B-Device
design	O
that	O
listed	O
at	O
$	O
22,000	O
.	O
</s>
<s>
In	O
spite	O
of	O
these	O
advantages	O
,	O
the	O
704	O
faced	O
stiff	O
competition	O
from	O
other	O
newly-introduced	O
machines	O
like	O
the	O
Data	B-Device
General	I-Device
Nova	I-Device
,	O
which	O
had	O
a	O
similar	O
feature	O
set	O
but	O
was	O
less	O
expensive	O
,	O
with	O
a	O
similar	O
configuration	O
costing	O
$	O
7,999	O
.	O
</s>
<s>
The	O
Nova	B-Device
was	O
slower	O
than	O
the	O
704	O
,	O
but	O
this	O
was	O
addressed	O
in	O
the	O
SuperNOVA	O
,	O
released	O
in	O
1970	O
for	O
$	O
11,700	O
.	O
</s>
<s>
Like	O
most	O
minicomputers	B-Architecture
of	O
the	O
late	O
1960s	O
,	O
the	O
704	O
was	O
designed	O
to	O
be	O
mounted	O
in	O
a	O
rack	B-Application
mount	I-Application
case	O
,	O
with	O
the	O
CPU	O
being	O
tall	O
.	O
</s>
<s>
The	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
used	O
16-bit	B-Device
words	O
,	O
although	O
it	O
also	O
included	O
instructions	O
that	O
worked	O
on	O
8-bit	O
data	O
,	O
useful	O
for	O
working	O
with	O
ASCII	B-Protocol
text	I-Protocol
.	O
</s>
<s>
Math	O
was	O
performed	O
in	O
parallel	O
and	O
used	O
two	B-General_Concept
's	I-General_Concept
complement	I-General_Concept
number	O
format	O
.	O
</s>
<s>
A	O
showcase	O
feature	O
was	O
the	O
external	O
input/output	B-General_Concept
system	O
,	O
handled	O
using	O
a	O
separate	O
16-bit	B-Device
wide	O
"	O
data	O
input/output	B-General_Concept
bus	O
"	O
,	O
or	O
DIO	O
,	O
with	O
up	O
to	O
16	O
devices	O
on	O
two	O
physical	O
ports	O
.	O
</s>
<s>
These	O
were	O
connected	O
together	O
with	O
custom	O
DIO	O
cables	O
,	O
daisy	B-Application
chained	I-Application
up	O
to	O
in	O
total	O
.	O
</s>
<s>
The	O
basic	O
unit	O
included	O
4kilowords	O
of	O
18-bit	O
memory	O
,	O
and	O
a	O
DIO	O
controller	O
for	O
an	O
ASCII	B-Protocol
terminal	O
,	O
normally	O
the	O
Teletype	B-Application
Model	I-Application
33	I-Application
or	O
35	O
.	O
</s>
<s>
These	O
included	O
4	O
kilowords	O
core	B-General_Concept
memory	I-General_Concept
modules	O
,	O
with	O
enough	O
room	O
in	O
the	O
case	O
for	O
a	O
total	O
of	O
four	O
modules	O
for	O
a	O
total	O
of	O
16	O
kilowords	O
.	O
</s>
<s>
The	O
purpose	O
of	O
the	O
18-bit	O
memory	O
was	O
to	O
allow	O
the	O
use	O
of	O
an	O
optional	O
parity	B-Error_Name
check	I-Error_Name
module	O
,	O
which	O
offloaded	O
this	O
task	O
from	O
the	O
CPU	O
.	O
</s>
<s>
Two	O
bits	O
were	O
used	O
to	O
store	O
separate	O
parity	B-Error_Name
bits	I-Error_Name
for	O
the	O
two	O
bytes	O
of	O
data	O
in	O
each	O
word	O
.	O
</s>
<s>
The	O
hardware	O
multiply/divide	O
card	O
reduced	O
the	O
time	O
for	O
a	O
16-bit	B-Device
multiply	O
from	O
105µsec	O
to	O
only	O
8	O
,	O
and	O
a	O
divide	O
from	O
193	O
to	O
10	O
.	O
</s>
<s>
The	O
DIO	O
normally	O
had	O
a	O
single	O
level	O
of	O
interrupt	B-Application
,	O
which	O
is	O
fine	O
for	O
simple	O
uses	O
but	O
not	O
in	O
realtime	B-General_Concept
computing	I-General_Concept
and	O
similar	O
roles	O
where	O
rapid	O
hardware	O
handling	O
of	O
data	O
is	O
a	O
major	O
concern	O
.	O
</s>
<s>
For	O
these	O
roles	O
,	O
the	O
Priority	O
Interrupt	B-Application
Expansion	O
added	O
an	O
eight-level	O
interrupt	B-Application
system	O
,	O
and	O
a	O
second	O
card	O
could	O
be	O
used	O
to	O
expand	O
this	O
to	O
16	O
levels	O
.	O
</s>
<s>
Installing	O
this	O
option	O
also	O
activated	O
the	O
front-panel	O
interrupt	B-Application
button	O
,	O
which	O
was	O
normally	O
inactive	O
as	O
the	O
only	O
interrupt	B-Application
,	O
level	O
00	O
,	O
was	O
assigned	O
to	O
the	O
terminal	O
.	O
</s>
<s>
The	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
(	O
DMA	O
)	O
card	O
connected	O
up	O
to	O
six	O
devices	O
to	O
memory	O
,	O
although	O
only	O
one	O
device	O
was	O
active	O
at	O
a	O
time	O
,	O
using	O
a	O
separate	O
dedicated	O
bus	O
that	O
bypassed	O
the	O
CPU	O
and	O
DIO	O
.	O
</s>
<s>
This	O
was	O
normally	O
used	O
for	O
hard	B-Device
disk	I-Device
and	O
magnetic	O
tape	O
support	O
.	O
</s>
<s>
The	O
CPU	O
ran	O
lock-step	O
with	O
the	O
core	B-General_Concept
memory	I-General_Concept
,	O
which	O
was	O
a	O
typical	O
design	O
of	O
the	O
era	O
.	O
</s>
<s>
Instructions	O
were	O
generally	O
one	O
word	O
long	O
,	O
in	O
strong	O
contrast	O
to	O
typical	O
designs	O
of	O
the	O
era	O
which	O
used	O
variable	O
length	O
instructions	O
depending	O
on	O
the	O
addressing	B-Language
mode	I-Language
.	O
</s>
<s>
Most	O
instructions	O
used	O
the	O
single	O
user-visible	O
16-bit	B-Device
accumulator	B-General_Concept
ACR	O
and	O
a	O
16-bit	B-Device
memory	O
buffer	O
register	O
(	O
MBR	O
)	O
used	O
to	O
temporarily	O
hold	O
operands	O
for	O
two-operand	O
instructions	O
(	O
the	O
other	O
being	O
ACR	O
)	O
.	O
</s>
<s>
Addresses	O
could	O
also	O
be	O
offset	O
using	O
the	O
16-bit	B-Device
index	B-General_Concept
register	I-General_Concept
IXR	O
.	O
</s>
<s>
The	O
purpose	O
of	O
the	O
15-bit	O
addresses	O
in	O
a	O
16-bit	B-Device
machine	O
was	O
to	O
set	O
aside	O
one	O
bit	O
in	O
the	O
instruction	O
format	O
to	O
indicate	O
that	O
the	O
address	O
was	O
relative	O
,	O
normally	O
bit	O
4	O
.	O
</s>
<s>
On	O
top	O
of	O
both	O
,	O
the	O
index	B-General_Concept
register	I-General_Concept
would	O
be	O
added	O
if	O
the	O
index	O
bit	O
was	O
turned	O
on	O
in	O
the	O
instruction	O
.	O
</s>
<s>
Math	O
instructions	O
,	O
and	O
,	O
always	O
worked	O
on	O
16-bit	B-Device
values	O
,	O
as	O
did	O
logical	O
operations	O
–	O
,	O
(	O
"	O
inclusive	O
"	O
OR	O
)	O
and	O
(	O
exclusive	O
OR	O
)	O
.	O
</s>
<s>
Interesting	O
additions	O
were	O
to	O
invert	O
the	O
value	O
and	O
to	O
perform	O
the	O
two	B-General_Concept
's	I-General_Concept
complement	I-General_Concept
.	O
</s>
<s>
There	O
were	O
three	O
primary	O
comparison	O
operators	O
,	O
to	O
compare	O
words	O
and	O
to	O
compare	O
bytes	O
,	O
and	O
to	O
compare	O
a	O
byte	O
in	O
the	O
accumulator	B-General_Concept
with	O
a	O
literal	O
byte	O
(	O
constant	O
)	O
in	O
the	O
address	O
section	O
of	O
the	O
instruction	O
word	O
.	O
</s>
<s>
The	O
only	O
other	O
support	O
for	O
literals	O
was	O
which	O
loaded	O
a	O
constant	O
byte	O
value	O
into	O
the	O
accumulator	B-General_Concept
.	O
</s>
<s>
For	O
instance	O
,	O
would	O
skip	O
the	O
next	O
instruction	O
if	O
the	O
value	O
in	O
the	O
accumulator	B-General_Concept
was	O
zero	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
accumulator	B-General_Concept
,	O
skip	O
instructions	O
were	O
provided	O
that	O
read	O
the	O
index	B-General_Concept
register	I-General_Concept
,	O
the	O
results	O
of	O
a	O
comparison	O
and	O
for	O
four	O
of	O
the	O
front-panel	O
switches	O
.	O
</s>
<s>
This	O
was	O
essentially	O
a	O
vector	B-Operating_System
processor	I-Operating_System
dedicated	O
to	O
performing	O
fast	O
Fourier	O
transforms	O
(	O
FFT	O
)	O
,	O
with	O
arrays	O
from	O
2	O
to	O
8192	O
complex	O
numbers	O
.	O
</s>
