PMS Notation
and the Question,
What is a Computer?

"PMS" refers to Processors-Memories-Switches: a computer is composed of a processor, primary memory (as opposed to secondary memory such as a disk or subcomponent memory such as registers), and a bus (or switch) that connects them (and, usually, other hardware for I/O). PMS notation is used in some textbooks to create formalized block diagrams describing how computer systems are composed of subsystems).

The following defintions are from [Bell et al. 1972, p17].

• L or Link: Transmits data from one place to another without modification.
• T or Transducer: Changes the representation of data, either from a non-digital representation (external to the digital system) or from one bit representation to another (within the system). Does not modify the information content.
• M or Memory: Stores data over time without modification.
• D or Data operation: Produces new information (in some bit representation) from a set of input data.
• K or Control: Evokes the operation of some component in a digital system in response to various input conditions and its current status.
• S or Switch: Changes the links that connect other components in a digital system, so as to reroute data.
• P or Processor: A digital system consisting of a set of operating components (D's, M's, K's, T's, S's, and L's) with a control (called the interpreter) that both reads an instruction to determine what operations to perform, and determines what next instruction to obtain, thus running autonomously from a memory that holds a program of instructions (called the primary memory, Mp).
• C or Computer: A digital sytem that includes at least one processor and its primary memory.

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