Mass Storage Control Protocol
Encyclopedia
The Mass Storage Control Protocol (MSCP) was a protocol designed by Digital Equipment Corporation
of Maynard
, Massachusetts
for the purposes of controlling their high-end mass storage
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 was an implementation of MSCP built on a Unibus
card; other implementations (for example, the RQDX) stretched down to the Q-bus
and small, 5 megabyte disk drives and even diskettes.
Designed to minimize the amount of CPU
involvement, the protocol depended upon two queues. Into one queue were placed packets which fully described the commands to be executed by the mass storage subsystem. To initiate an I/O request, the CPU had only to create a small data structure in memory, append it to a "send" queue, and if that was the first packet in the send queue, wake the MSCP controller. After the command was executed, an appropriate status packet would be placed into the second queue to be read by the CPU.
Interrupts to the CPU (a costly operation) were not needed so long as further command packets remained in the command queue and the response queue was not in danger of over-flowing. I/O-space reads and writes to the MSCP controller, a less-expensive but still-costly operation, were similarly minimized.
Because MSCP packets were deliberately designed to resemble the packets exchanged on the VMScluster
interconnects, it was a very inexpensive operation to ship storage requests around a VMScluster for remote execution; this greatly facilitated 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 facilitated remote operations.
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
of Maynard
Maynard, Massachusetts
Maynard is a town in Middlesex County, Massachusetts, United States. As of the 2010 census, the town population was 10,106.- History :Maynard, located on the Assabet River, was incorporated as an independent municipality in 1871. Prior to that it was known as 'Assabet Village' but was legally...
, Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
for the purposes of controlling their high-end mass storage
Mass storage
In computing, mass storage refers to the storage of large amounts of data in a persisting and machine-readable fashion. Devices and/or systems that have been described as mass storage include tape libraries, RAID systems, hard disk drives, magnetic tape drives, optical disc drives, magneto-optical...
options.
First implemented in the HSC50
VMScluster
A VMScluster 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 loosely coupled: each machine runs its own copy of OpenVMS, but the disk storage, lock...
hierarchical storage controller, the protocol quickly spread throughout the entire line of mass storage controllers built by DEC. The UDA50 was an implementation of MSCP built on a Unibus
Unibus
The Unibus was the earliest of several computer bus technologies used with PDP-11 and early VAX systems manufactured by the Digital Equipment Corporation of Maynard, Massachusetts.-History:...
card; other implementations (for example, the RQDX) stretched down to the Q-bus
Q-Bus
The Q-bus was one of several bus technologies used with PDP and MicroVAX computer systems manufactured by the Digital Equipment Corporation of Maynard, Massachusetts....
and small, 5 megabyte disk drives and even diskettes.
Designed to minimize the amount of CPU
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
involvement, the protocol depended upon two queues. Into one queue were placed packets which fully described the commands to be executed by the mass storage subsystem. To initiate an I/O request, the CPU had only to create a small data structure in memory, append it to a "send" queue, and if that was the first packet in the send queue, wake the MSCP controller. After the command was executed, an appropriate status packet would be placed into the second queue to be read by the CPU.
Interrupts to the CPU (a costly operation) were not needed so long as further command packets remained in the command queue and the response queue was not in danger of over-flowing. I/O-space reads and writes to the MSCP controller, a less-expensive but still-costly operation, were similarly minimized.
Because MSCP packets were deliberately designed to resemble the packets exchanged on the VMScluster
VMScluster
A VMScluster 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 loosely coupled: each machine runs its own copy of OpenVMS, but the disk storage, lock...
interconnects, it was a very inexpensive operation to ship storage requests around a VMScluster for remote execution; this greatly facilitated 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 facilitated remote operations.