Introduction to Register Machines and Basic Design Principles

Key Insight

The limitations of existing Lisp evaluators, which fail to fully elucidate control mechanisms or differentiate between iterative and recursive process evaluations, necessitate a more primitive computational model. Register machines address this by describing processes through the step-by-step operation of a traditional computer, manipulating fixed storage elements called registers. The approach taken is that of a hardware architect, designing specific machines to execute Lisp procedures, including implementing fundamental programming constructs like recursion.

The design of a register machine involves defining its data paths, which include registers and the primitive operations that manipulate their contents. For instance, in Euclid's Algorithm for Greatest Common Divisor (GCD), registers 'a', 'b', and a temporary 't' store numbers. Operations include testing if a register's content is zero or computing a remainder. Data-path diagrams visually represent these components: rectangles for registers, trapezoids for operations, and circles for tests, with arrows showing data flow and assignment points.

Crucially, the operation sequence is dictated by a controller. Controller diagrams, separate from data paths, specify the order of actions: pushing data-path buttons to assign values, performing tests, and making decisions based on test results. Rectangular boxes in the controller diagram indicate data-path button presses, while diamonds represent decision points, guiding execution flow based on a test's outcome. Together, data paths and the controller completely describe a machine's behavior, executing a specific algorithm.