242 Chapter 8 High-Level Programming Languages and disciplined, that they should use only selected control structures. This article and the others published with it introduced the era of structured programming . 9 Each logical unit of a program should have just one entry and one exit. The program should not jump randomly in and out of logical modules. Although programs could be designed in this way in assembly language using instructions that branch to other parts of the program, high-level languages introduced control constructs that made this discipline easy to follow. These constructs are selection statements, looping state-ments, and subprogram statements. Unrestricted branching statements were no longer necessary. With the advent of windows on the screen and input using a mouse, a fifth basic control structure was introduced: asynchronous processing. In the next sections we introduce these concepts at the logical level.
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