August 10, 2010

Source Code, Machine Code & Legacy Code: Introduction to Codes

Da Vinci Code Cryptex HDR
Source code
In computer science, source code (commonly just source) is any collection of statements or declarations written in some human-readable computer programming language. Source code allows the programmer to communicate with the computer using a reserved number of instructions.

The source code which constitutes a program is usually held in one or more text files, sometimes stored in databases as stored procedures and may also appear as code snippets printed in books or other media. A large collection of source code files may be organized into a directory tree, in which case it may also be known as a source tree.

A computer program's source code is the collection of files needed to convert from human-readable form to some kind of computer-executable form. The source code may be converted into an executable file by a compiler, or executed on the fly from the human readable form with the aid of an interpreter.

The code base of a programming project is the larger collection of all the source code of all the computer programs which make up the project.

Legacy code
Legacy code is source code that relates to a no-longer supported or manufactured operating system or other computer technology. The term can also mean code inserted into modern software for the purpose of maintaining an older or previously supported feature — for example supporting a serial interface even though many modern systems don't have a serial port. It may also be in the form of supporting older file formats that may have been encoding in non-ASCII characters, such as EBCDIC

In practice, most source code has some dependency on the platform for which it is designed—unless a programmer uses a platform-independent programming language like Java, it is hard to write a large, useful program that is totally independent of its environment. When the manufacturer upgrades a platform (or the platform is superseded), the code may no longer work without changes, and becomes legacy code. A large part of the task of a software engineer is to continually alter code to prevent this.

While the term usually refers to source code, it can also apply to executable code that no longer runs on a later version of a system, or requires a compatibility layer to do so. An example would be a classic Macintosh application which will not run natively on Mac OS X, but runs inside the Classic environment, or a Win16 application running on Windows XP using the Windows on Windows feature in XP.

Machine code
Machine code or machine language is a system of instructions and data executed directly by a computer's central processing unit. Machine code may be regarded as a primitive (and cumbersome) programming language or as the lowest-level representation of a compiled and/or assembled computer program. Programs in interpreted languages  are not represented by machine code however, although their interpreter (which may be seen as a processor executing the higher level program) often is. Machine code is sometimes called native code when referring to platform-dependent parts of language features or libraries. Machine code should not be confused with so called "bytecode", which is executed by an interpreter.

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