<s>
The	O
Mass	B-Device
Storage	I-Device
Control	I-Device
Protocol	I-Device
(	O
MSCP	O
)	O
is	O
a	O
protocol	O
that	O
was	O
designed	O
by	O
Digital	O
Equipment	O
Corporation	O
of	O
Maynard	O
,	O
Massachusetts	O
for	O
the	O
purposes	O
of	O
controlling	O
their	O
high-end	O
mass	B-Device
storage	I-Device
options	O
.	O
</s>
<s>
First	O
implemented	O
in	O
the	O
HSC50	B-Operating_System
hierarchical	O
storage	O
controller	O
,	O
the	O
protocol	O
quickly	O
spread	O
throughout	O
the	O
entire	O
line	O
of	O
mass	B-Device
storage	I-Device
controllers	O
built	O
by	O
DEC	O
.	O
</s>
<s>
The	O
UDA50	B-Device
is	O
an	O
implementation	O
of	O
MSCP	O
built	O
on	O
a	O
Unibus	B-Device
card	O
;	O
other	O
implementations	O
(	O
for	O
example	O
,	O
the	O
RQDX	B-Device
)	O
stretch	O
down	O
to	O
the	O
Q-bus	B-Architecture
and	O
small	O
,	O
5	O
megabyte	O
disk	O
drives	O
and	O
even	O
diskettes	B-Device
.	O
</s>
<s>
Designed	O
to	O
minimize	O
the	O
amount	O
of	O
CPU	B-General_Concept
involvement	O
,	O
the	O
protocol	O
depends	O
upon	O
two	O
queues	O
.	O
</s>
<s>
Into	O
one	O
queue	O
are	O
placed	O
packets	B-Protocol
which	O
fully	O
describe	O
the	O
commands	O
to	O
be	O
executed	O
by	O
the	O
mass	B-Device
storage	I-Device
subsystem	O
.	O
</s>
<s>
To	O
initiate	O
an	O
I/O	O
request	O
,	O
the	O
CPU	B-General_Concept
has	O
only	O
to	O
create	O
a	O
small	O
data	O
structure	O
in	O
memory	O
,	O
append	O
it	O
to	O
a	O
"	O
send	O
"	O
queue	O
,	O
and	O
if	O
that	O
is	O
the	O
first	O
packet	B-Protocol
in	O
the	O
send	O
queue	O
,	O
wake	O
the	O
MSCP	O
controller	O
.	O
</s>
<s>
After	O
the	O
command	O
has	O
been	O
executed	O
,	O
an	O
appropriate	O
status	O
packet	B-Protocol
is	O
placed	O
into	O
the	O
second	O
queue	O
to	O
be	O
read	O
by	O
the	O
CPU	B-General_Concept
.	O
</s>
<s>
Interrupts	O
to	O
the	O
CPU	B-General_Concept
(	O
a	O
costly	O
operation	O
)	O
are	O
not	O
needed	O
so	O
long	O
as	O
further	O
command	O
packets	B-Protocol
remain	O
in	O
the	O
command	O
queue	O
and	O
the	O
response	O
queue	O
is	O
not	O
in	O
danger	O
of	O
over-flowing	O
.	O
</s>
<s>
Because	O
MSCP	O
packets	B-Protocol
were	O
deliberately	O
designed	O
to	O
resemble	O
the	O
packets	B-Protocol
exchanged	O
on	O
the	O
VMScluster	B-Operating_System
interconnects	O
,	O
it	O
is	O
a	O
very	O
inexpensive	O
operation	O
to	O
ship	O
storage	O
requests	O
around	O
a	O
VMScluster	B-Operating_System
for	O
remote	O
execution	O
;	O
this	O
greatly	O
facilitates	O
the	O
creation	O
of	O
large-scale	O
VMSclusters	B-Operating_System
.	O
</s>
<s>
The	O
dependence	O
upon	O
in-memory	O
packets	B-Protocol
and	O
the	O
minimization	O
of	O
interrupts	O
and	O
I/O	O
-space	O
reads	O
and	O
writes	O
greatly	O
facilitates	O
remote	O
operations	O
.	O
</s>
