SIMPLY FORTRAN THREADING CODE
This means that a program consisting of many function calls may have considerable amounts of repeated code as well. 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. Mainframes and some early microprocessors such as the RCA 1802 required several instructions to call a subroutine. Many of these subroutines, in turn, also consist of nothing but lower-level subroutine calls. The top-level application in these programs may consist of nothing but subroutine calls. Thus, each bit exists in only one place in memory (see " Don't repeat yourself"). Instead of writing out every step of an operation at its every occurrence in the program, as was common in macro assemblers for instance, the compiler writes each common bit of code into a subroutine. Threaded code is a formatting style for compiled code that minimizes memory use. Code is created a line at a time, executed, and then discarded. 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. This was the reason Microsoft BASIC is an interpreter: its own code had to share the 4 kB memory of machines like the Altair 8800 with the user's source code. As the source code is typically much denser than the resulting machine code, this can reduce overall memory use.
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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. Consequently, a lot of time was spent trying to find ways to reduce a program's size, to fit in the available memory. For example, most Data General Nova, IBM 1130, and many of the first microcomputers had only 4 kB of RAM installed. Thus, only the interpreter must be compiled.Įarly computers had relatively little memory. The interpreter instantiates the virtual machine environment and executes the instructions. A different approach is to generate instructions for a virtual machine and to use an interpreter on each hardware platform. The resulting executable is typically fast but, because it is specific to a hardware platform, it isn't portable.
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The common way to make computer programs is to use a compiler to translate source code (written in some symbolic language) to machine code. ( February 2020) ( Learn how and when to remove this template message) Statements consisting only of original research should be removed. Please improve it by verifying the claims made and adding inline citations. This section possibly contains original research. Threaded code is best known for its use in many compilers of programming languages, such as Forth, many implementations of BASIC, some implementations of COBOL, early versions of B, and other languages for small minicomputers and for amateur radio satellites.
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Small programs may also be faster at thread switching, when other programs have filled the cache. However, a program that is small enough to fit in a computer processor's cache may run faster than a larger program that suffers many cache misses. In cached architectures, it may execute slightly slower.
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Threaded code has better density than code generated by alternative generation techniques and by alternative calling conventions. The code may be processed by an interpreter or it may simply be a sequence of machine code call instructions. It is often used in compilers, which may generate code in that form or be implemented in that form themselves. In computer science, threaded code is a programming technique where the code has a form that essentially consists entirely of calls to subroutines. Not to be confused with multi-threaded programming.