Characteristics of Data
Fundamentally, computers follow a sequence of instructions they are given in the form of data. A set of instructions to perform a given task (or tasks) is called a "program". In the nominal case, the program, as executed by the computer, will consist of binary machine code. The elements of storage manipulated by the program, but not actually executed by the CPU, are also data. The Marvellous twist is that program instructions; and data that the program manipulates, are both stored in exactly the same way. Therefore it is possible for computer programs to operate on other computer programs, by manipulating their programmatic data.
The line between program and data can become blurry. An interpreter, for example, is a program. The input data to an interpreter is itself a program, just not one expressed in native machine language. In many cases, the interpreted program will be a human-readable text file, which is manipulated with a text editor program (more normally associated with plain text data). Metaprogramming similarly involves programs manipulating other programs as data. Programs like compilers, linkers, debuggers, program updaters, virus scanners and such use other programs as their data.
To store data bytes in a file, they have to be serialized in a "file format". Typically, programs are stored in special file types, different from those used for other data. Executable files contain programs; all other files are also data files. However, executable files may also contain "in-line" data which is "built-in" to the program. In particular, some executable files have a data segment, which nominally contains constants and initial values (both data).
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