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→History: Refactoring is not a style of organizing procedural code; it is a method to safely edit a program. |
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One solution is to use an interpreter which reads the symbolic language a bit at a time, and calls functions to perform the actions. As the source code is typically much [[code density|denser]] than the resulting machine code, this can reduce overall memory use. This was the reason [[Microsoft BASIC]] is an interpreter:{{efn|[[Dartmouth BASIC]], upon which MS is ultimately based, was a compiler that ran on mainframe machines.}} its own code had to share the 4 kB memory of machines like the [[Altair 8800]] with the user's source code. A compiler translates from a source language to machine code, so the compiler, source, and output must all be in memory at the same time. In an interpreter, there is no output. Code is created a line at a time, executed, and then discarded.
Threaded code is a formatting style for compiled code that minimizes memory use. Instead of writing out every step of an operation at its every occurrence in the program, as was common in [[macro assembler]]s for instance, the compiler writes each common bit of code into a subroutine. Thus, each bit exists in only one place in memory (see "[[Don't repeat yourself]]"). The top-level application in these programs may consist of nothing but subroutine calls. Many of these subroutines, in turn, also consist of nothing but lower-level subroutine calls
Mainframes and some early microprocessors such as the [[RCA 1802]] required several instructions to call a subroutine. In the top-level application and in many subroutines, that sequence is constantly repeated, with only the subroutine address changing from one call to the next. This means that a program consisting of many function calls may have considerable amounts of repeated code as well.
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