level languages and executable graphical representations of systems. SCAM Working Conference , 2001–2010. This approach allows for a much more flexible approach to system analysis, dispensing with the requirement for designer to collaborate by publishing a convenient form for understanding and modification. It can also be applied to scenarios where a designer is not needed, like DNA. However, this form of analysis doesn't contemplate a costlier machine-to-machine code analysis than human-to-machine code analysis. History The earliest programs for stored-program computers were entered in binary through the front panel switches of the computer. This first-generation programming language had no distinction between source code and machine code. When IBM first offered software to work with its machine, the source code was provided at no additional charge. At that time, the cost of developing and supporting software was included in the price of the hardware. For decades, IBM distributed source code with its software product licenses, until 1983. Most early computer magazines published source code as type-in programs. Occasionally the entire source code to a large program is published as a hardback book, such as Computers and Typesetting, vol. B: TeX, The Program by Donald Knuth, PGP Source Code and Internals by Philip Zimmermann, PC SpeedScript by Randy Thompson, and µC/OS, The Real-Time Kernel by Jean Labrosse. Organization The source code which constitutes a program is usually held in one or more text files stored on a computer's hard disk; usually, these files are carefully arranged into a directory tree, known as a source tree. Source code can also be stored in a database (as is common for stored procedures) or elsewhere. The source code for a particular piece of software may be contained in a single file or many files. Though the practice is uncommon, a program's source code can be written in different programming languages. For example, a program written primarily in the C programming language, might have portions written in assembly language for optimization purposes. It is also possible for some components of a piece of software to be written and compiled separately, in an arbitrary programming language, and later integrated into the software using a technique called library linking. In some languages, such as Java, this can be done at runtime (each class is compiled into a separate file that is linked by the interpreter at runtime). Yet another method is to make the main program an interpreter for