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TheMass Storage Control Protocol (MSCP) is aprotocol that was designed byDigital Equipment Corporation ofMaynard,Massachusetts for the purposes of controlling their high-endmass storage options.[1]
First implemented in theHSC50 hierarchical storage controller, the protocol quickly spread throughout the entire line of mass storage controllers built by DEC. TheUDA50 is an implementation of MSCP built on aUnibus card; other implementations (for example, theRQDX) stretch down to theQ-bus and small, 5 megabyte disk drives and evendiskettes.
Designed to minimize the amount ofCPU involvement, the protocol depends upon two queues. Into one queue are placedpackets 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 theVMScluster 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.[2]
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