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→History: Removing an old myth. There is no code being "created" in most interpreters, and it's not line by line either, rather statement by statement. |
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Early computers had relatively little memory. For example, most [[Data General Nova]], [[IBM 1130]], and many of the first [[microcomputer]]s had only 4 kB of RAM installed. Consequently, a lot of time was spent trying to find ways to reduce a program's size, to fit in the available memory.
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 [[Microsoft BASIC]] 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.
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.
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