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{{Notability|date=June 2025}}
The '''Mass Storage Control Protocol''' ('''MSCP''') is a [[Protocol (computing)|protocol]] that was designed by [[Digital Equipment Corporation]] of [[Maynard, Massachusetts|Maynard]], [[Massachusetts]] for the purposes of controlling their high-end [[mass storage]] options.<ref name="n402">{{cite book | last=South | first=David W. | title=The Computer and Information Science and Technology Abbreviations and Acronyms Dictionary | publisher=CRC Press | publication-place=Boca Raton | date=1994-05-06 | isbn=978-0-8493-2444-4 | page=149}}</ref>

First implemented in the [[VMScluster|HSC50]] hierarchical storage controller, the protocol quickly spread throughout the entire line of mass storage controllers built by DEC. The '''UDA50''' is an implementation of MSCP built on a [[Unibus]] card; other implementations (for example, the '''RQDX''') stretch down to the [[Q-bus]] and small, 5 megabyte disk drives and even [[diskette]]s.

Designed to minimize the amount of [[Central processing unit|CPU]] involvement, the protocol depends upon two queues. Into one queue are placed [[packet (information technology)|packet]]s which fully describe the commands to be executed by the mass storage subsystem. To initiate an I/O request, the CPU has only to create a small data structure in memory, append it to a "send" queue, and if that is the first packet in the send queue, wake the MSCP controller. After the command has been executed, an appropriate status packet is placed into the second queue to be read by the CPU.

Interrupts to the CPU (a costly operation) are not needed so long as further command packets remain in the command queue and the response queue is not in danger of over-flowing. I/O-space reads and writes to the MSCP controller, a less-expensive but still-costly operation, are similarly minimized.

Because MSCP packets were deliberately designed to resemble the packets exchanged on the [[VMScluster]] interconnects, it is a very inexpensive operation to ship storage requests around a VMScluster for remote execution; this greatly facilitates the creation of large-scale VMSclusters. The dependence upon in-memory packets and the minimization of interrupts and I/O-space reads and writes greatly facilitates remote operations.<ref name="n296">{{cite book | last=Schulz | first=Greg | title=Resilient Storage Networks | publisher=Elsevier | publication-place=Amsterdam Boston | date=2004-06-10 | isbn=978-0-08-049225-4 | page=101}}</ref>

==References==
{{Reflist}}

==External links==

* [http://vt100.net/manx/details/1,5421 MSCP Basic Disk Functions Manual]
* [https://patents.google.com/patent/US4449182 Related patent]

{{Digital Equipment Corporation}}

[[Category:OpenVMS]]
[[Category:DEC hardware]]

Mass Storage Control Protocol - Revision history

imported>Coldupnorth: update template #JUN25