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CHAPTER 5

Structured programming

Before IBM released FORTRAN in 1957, almost all programming was done in assembly
language. Part of the reason for this is that nobody knew how to design a good high-level lan-
guage, nor did they know how to write a compiler to generate efficient code. Early attempts at
high-level languages resulted in languages that were not well structured, difficult to read, and
difficult to debug. The first release of FORTRAN was not a particularly elegant language by
today’s standards, but it did generate efficient machine code.

In the 1960s, a new paradigm for designing high-level languages emerged. This new paradigm
emphasized grouping program statements into blocks of code that execute from beginning
to end. These basic blocks have only one entry point and one exit point. Control of which
basic blocks are executed, and in what order, is accomplished with highly structured flow con-
trol statements. The structured program theorem provides the theoretical basis of structured
programming. It states that there are three ways of combining basic blocks: sequencing, selec-
tion, and iteration. These three mechanisms are sufficient to express any computable function.
It has been proven that all programs can be written using only basic blocks, the pre-test loop,
and if-then-else structure. Although most high level languages provide additional flow con-
trol statements for the convenience of the programmer, they are just “syntactical sugar.” Other
structured programming concepts include well-formed functions and procedures, pass-by-
reference and pass-by-value, separate compilation, and information hiding.
These structured programming languages enabled programmers to become much more pro-
ductive. Well-written programs that adhere to structured programming principles are much
easier to write, understand, debug, and maintain. Most successful high-level languages are de-
signed to enforce, or at least facilitate, good programming techniques. This is not generally
true for assembly language. The burden of writing well-structured code lies with the program-
mer, and not the language.

The best assembly programmers rely heavily on structured programming concepts. Failure to
do so results in code that contains unnecessary branch instructions and, in the worst cases, re-
sults in something called spaghetti code. Consider a code listing where a line has been drawn
from each branch instruction to its destination. If the result looks like someone spilled a plate
of spaghetti on the page, then the listing is spaghetti code. If a program is spaghetti code, then
the flow of control is difficult to follow. Spaghetti code is much more likely to have bugs, and

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