The PMS notation provides a structural representation of a digital computer system as a graph which has the system's components as the nodes and information flows along the branches. These aspects of a digital computer system level provide a description of the gross structure, including the amounts of information held in various components, the distribution of control that accomplishes these flows, and other interesting parameters (e.g., technology, function, cost, reliability). Only those aspects of the notation that are used in this book are described; a complete description is given in Bell and Newell [1971].

In PMS there are seven basic component types, each distinguished by the kinds of operations it performs:

Memory, M. A component that holds or stores information (i.e., i-units) over time. Its operations are reading i-units out of the memory and writing i-units into the memory. Each memory that holds more than a single i-unit has associated with it an addressing system by means of which particular i-units can be designated or selected. A memory can also be considered as a switch to a number of submemories. The i-units are not changed in any way by being stored in a memory.

Link, L. A component that transfers information (i.e., i-units) from one place to another in a computer system. It has fixed ports. The operation is that of transmitting an i-unit (or a sequence of them) from the component at one port to the component at the other. Again, except for the change in spatial position, there is no change of any sort in the i-units.