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10 LET X = (7 + 8) / 3

20 PRINT X

30 END

Unlike ALGOL, BASIC didn't require the programmer to specify whether a variable was to be stored as an integer or a floating-point value. Most numbers were stored as floating-point values without the programmer needing to worry about it.

Many subsequent implementations of BASIC have been in the form of interpreters rather than compilers. As I explained earlier, a compiler reads a source-code file and creates an executable file. An interpreter, however, reads source code and executes it directly as it's reading it without creating an executable file. Interpreters are easier to write than compilers, but the execution time of the interpreted program tends to be slower than that of a compiled program. On home computers, BASIC got an early start when buddies Bill Gates (born 1955) and Paul Allen (born 1953) wrote a BASIC interpreter for the Altair 8800 in 1975 and jump-started their company, Microsoft Corporation.

The Pascal programming language, which inherited much of its structure from ALGOL but included record handling from COBOL, was designed in the late 1960s by Swiss computer science professor Niklaus Wirth (born 1934). Pascal was quite popular for IBM PC programmers, but in a very specific form—the product Turbo Pascal, introduced by Borland International in 1983 for the bargain price of $49.95. Turbo Pascal (written by Danish student Anders Hejlsberg, born in 1960) was a version of Pascal that came complete with an integrated development environment. The text editor and the compiler were combined in a single program that facilitated very fast programming. Integrated development environments had been popular on large mainframe computers, but Turbo Pascal heralded their arrival on small machines.

Pascal was also a major influence on Ada, a language developed for use by the United States Department of Defense. The language was named after Augusta Ada Byron, whom I mentioned in Chapter 18 as the chronicler of Charles Babbage's Analytical Engine.

And then there's C, a much-beloved programming language created between 1969 and 1973 largely by Dennis M. Ritchie at Bell Telephone Laboratories. People often ask why the language is called C. The simple answer is that it was derived from an early language called B, which was a simplified version of BCPL (Basic CPL), which was derived from CPL (Combined Programming Language).

I mentioned in Chapter 22 that the UNIX operating system was designed to be portable. Most operating systems at the time were written in assembly language for a specific processor. In 1973, UNIX was written (or rather, rewritten) in C, and since then the operating system and the language have been closely identified.

C is generally a very terse language. For example, instead of the words begin and end used in ALGOL and Pascal to delimit blocks, C uses the curly braces { and }. Here's another example. It's very common for a programmer to add a constant amount to a variable:

i = i + 5;

In C, you can shorten this to

i += 5;

If you only need to add 1 to the variable (that is, to increment it), you can shorten the statement even further:

i++;

On 16-bit or 32-bit microprocessors, such a statement can be carried out by a single machine-code instruction.

I mentioned earlier that most high-level languages don't include bit-shifting operations or Boolean operations on bits, which are features supported by many processors. C is the exception to this rule. In addition, an important feature of C is its support of pointers, which are essentially numeric memory addresses. Because C has operations that parallel many common processor instructions, C is sometimes categorized as a high-level assembly language. More than any ALGOL-like language, C closely mimics common processor instruction sets.

Yet all ALGOL-like languages—which really means most commonly used programming languages—were designed based on von Neumann architecture computers. Breaking out