<s>
The	O
8X300	O
is	O
a	O
microprocessor	B-Architecture
produced	O
and	O
marketed	O
by	O
Signetics	O
starting	O
1976	O
as	O
a	O
second	O
source	O
for	O
the	O
SMS	B-General_Concept
300	I-General_Concept
by	O
Scientific	B-General_Concept
Micro	I-General_Concept
Systems	I-General_Concept
,	O
Inc	O
.	O
</s>
<s>
Although	O
SMS	O
developed	O
the	O
SMS	B-General_Concept
300	I-General_Concept
,	O
Signetics	O
was	O
the	O
sole	O
manufacturer	O
of	O
this	O
product	O
line	O
.	O
</s>
<s>
In	O
1978	O
Signetics	O
purchased	O
the	O
rights	O
to	O
the	O
SMS	B-General_Concept
300	I-General_Concept
series	O
and	O
renamed	O
it	O
8X300	O
.	O
</s>
<s>
It	O
was	O
designed	O
to	O
be	O
a	O
fast	O
microcontroller	B-Architecture
and	O
signal	O
processor	O
,	O
and	O
because	O
of	O
this	O
differs	O
considerably	O
from	O
conventional	O
NMOS	B-Algorithm
logic	I-Algorithm
microprocessors	B-Architecture
of	O
the	O
time	O
.	O
</s>
<s>
Another	O
unusual	O
feature	O
is	O
that	O
rather	O
than	O
execute	O
mask	O
,	O
rotate	O
,	O
shift	O
and	O
merge	O
instructions	O
in	O
the	O
arithmetic	B-General_Concept
logic	I-General_Concept
unit	I-General_Concept
(	O
ALU	O
)	O
,	O
as	O
is	O
the	O
case	O
with	O
most	O
microprocessors	B-Architecture
,	O
the	O
8X300	O
has	O
separate	O
mask	O
,	O
rotate	O
,	O
shift	O
and	O
merge	O
units	O
.	O
</s>
<s>
Two	O
virtual	O
write	B-General_Concept
only	I-General_Concept
registers	I-General_Concept
IVL	O
(	O
R7	O
)	O
and	O
IVR	O
(	O
R15	O
)	O
are	O
used	O
to	O
put	O
an	O
address	O
on	O
the	O
IV	O
bus	O
,	O
and	O
two	O
sets	O
of	O
eight	O
virtual	O
registers	O
(	O
R16-R23	O
and	O
R24-R31	O
)	O
are	O
used	O
to	O
transfer	O
data	O
to	O
or	O
from	O
the	O
IV	O
bus	O
.	O
</s>
<s>
The	O
three	O
most	O
significant	O
bits	O
of	O
the	O
instruction	O
define	O
the	O
opcode	B-Language
,	O
and	O
divide	O
the	O
instructions	O
into	O
eight	O
classes	O
:	O
</s>
<s>
The	O
use	O
of	O
the	O
remaining	O
13	O
bits	O
of	O
the	O
instruction	O
depends	O
on	O
the	O
opcode	B-Language
:	O
</s>
<s>
Rotate	O
Unit	O
:	O
This	O
unit	O
will	O
Rotate	O
the	O
data	O
to	O
the	O
RIGHT	O
by	O
the	O
number	O
of	O
places	O
specified	O
in	O
the	O
opcode	B-Language
.	O
</s>
<s>
Mask	O
Unit	O
:	O
This	O
unit	O
will	O
mask	O
off	O
(	O
set	O
to	O
zero	O
)	O
the	O
upper	O
bits	O
of	O
the	O
data	O
so	O
as	O
to	O
retain	O
the	O
number	O
of	O
lower	O
bits	O
specified	O
in	O
the	O
opcode	B-Language
.	O
</s>
<s>
Shift	O
Unit	O
:	O
This	O
unit	O
will	O
shift	O
the	O
data	O
to	O
the	O
LEFT	O
the	O
number	O
of	O
places	O
specified	O
in	O
the	O
opcode	B-Language
.	O
</s>
<s>
Merge	O
Unit	O
:	O
This	O
unit	O
will	O
Merge	O
(	O
replace	O
)	O
the	O
number	O
of	O
bits	O
specified	O
by	O
the	O
opcode	B-Language
in	O
the	O
IV-bus	O
buffer	O
with	O
the	O
data	O
before	O
placing	O
the	O
contents	O
of	O
the	O
buffer	O
on	O
the	O
IV	O
bus	O
.	O
</s>
<s>
In	O
an	O
extensive	O
application	O
note	O
,	O
Signetics	O
showed	O
how	O
to	O
use	O
the	O
8X300	O
as	O
a	O
floppy	B-Device
disk	I-Device
controller	I-Device
.	O
</s>
<s>
Olivetti	O
used	O
the	O
8X300	O
and	O
later	O
8X305	O
as	O
the	O
controller	O
for	O
the	O
ST506	B-Device
hard	O
disk	O
in	O
the	O
M20	B-Device
personal	O
computer	O
series	O
.	O
</s>
<s>
Western	O
Digital	O
used	O
the	O
8X300	O
in	O
their	O
WD1000	O
ST506	B-Device
hard	O
disk	O
controller	O
and	O
the	O
8X305	O
in	O
their	O
WD1001	O
ECC	O
hard	O
disk	O
controller	O
.	O
</s>
