The Mass Storage Control Protocol (MSCP) is a
protocol that was designed by
Digital Equipment Corporation
Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president unti ...
of
Maynard,
for the purposes of controlling their high-end
mass storage
In computing, mass storage refers to the storage of large amounts of data in a persisting and machine-readable fashion. In general, the term is used as large in relation to contemporaneous hard disk drives, but it has been used large in relati ...
options.
First implemented in the
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
The Unibus was the earliest of several computer bus and backplane designs used with PDP-11 and early VAX systems manufactured by the Digital Equipment Corporation (DEC) of Maynard, Massachusetts. The Unibus was developed around 1969 by Gordon ...
card; other implementations (for example, the RQDX) stretch down to the
Q-bus
The Q-bus,Schmidt, Atlant G.,Unibus,Q-Bus and VAXBI Bus, in ''Digital bus handbook'', Di Giacomo Joseph Ed., McGraw Hill, 1990 also known as the LSI-11 Bus, is one of several bus technologies used with PDP and MicroVAX computer systems previous ...
and small, 5 megabyte disk drives and even
diskette
A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined wi ...
s.
Designed to minimize the amount of
CPU
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, an ...
involvement, the protocol depends upon two queues. Into one queue are placed
packets 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
A VMScluster, originally known as a VAXcluster, is a computer cluster involving a group of computers running the OpenVMS operating system. Whereas tightly coupled multiprocessor systems run a single copy of the operating system, a VMScluster is lo ...
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.
External links
MSCP Basic Disk Functions ManualRelated patent
{{Digital Equipment Corporation
OpenVMS
DEC hardware