When first learning to program, many students find it difficult to think in terms of doing "one thing at a time". The advent of parallel machines relaxed this restriction. Unfortunately, doing several things at the same time is even more difficult than doing one thing at a time. In this section, we describe how a process is structured and some different scheduling mechanisms: batch systems, multitasking, and time slicing.
A program can be viewed as a series of instructions acting on data. When it is executed, the processor must keep track of which instruction(s) is to be executed during the current clock cycle, and what data the instruction refers to. The mechanism used by the processor to keep track of instructions is called a program counter. The idea of an address space is similar, but applicable to data. The move from serial to parallel architectures can be described in terms of the increasing complexity of these mechanisms.