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P-Code

As already mentioned, the executable files produced by earlier versions of VB don’t consist of native code. For computers to run these programs, it is clear that they must entail some extra overhead for the computer to understand them. Because you have just taken a brief look at how interpreters work, you might fairly expect to discover that there is an interpreter at the heart of VB. VB does not, however, rely only on an interpretive mechanism to perform this decoding at runtime.

Every time you type a new line of code in the VB Code Editor, VB tokenizes it as a symbol that represents a series of machine instructions. That is, VB doesn’t compile directly to machine code, but it produces a series of tokens that are a sort of shorthand for particular operations.

The process of tokenization occurs when an object is regularly represented by a particular set of signs or symbols. The most familiar form of tokenization is language. Words are not identical with the things for which they stand—for instance, the word apple isn’t the same thing as an actual apple—but words are understood to be tokens for the things they represent.

A programming language such as Visual Basic is a special form of language. Unlike natural languages, the expressions formed by a programming language literally can be transformed into the things that they represent. Even though the computer can’t understand words, programs can be thought of as instructions for a computer, and certain tools (compilers and linkers) turn words into actual computer instructions. Think about turning the word apple into an actual apple, and you begin to see how a programming language is distinct from a natural language such as English.

When you compile your program, the source code compiles to P-Code, which consists of a series of these symbols. When you turn your program into an EXE file, VB builds an executable file that contains the P-Code and the necessary executable header and startup code. When VB code is compiled, however, you don’t immediately get actual computer instructions. P-Code itself isn’t executable. If a computer were told to take the contents of the P-Code literally, it wouldn’t know what to do.


  

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