Languages: Difference between revisions
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→But I heard of an OS written in language XYZ, isn't it interpreted?: Typo
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* All or part of the code has been compiled to native code. This may involve using a sub-set of the language with reduced runtime requirements (e.g., [http://www.citeulike.org/user/hamish/article/201596 Pre-Scheme], or [http://www.squeak.org/features/vm.html Slang] - while they have not been used for OS development to date, they do demonstrate this sort of low-level implementation language which can be used this way).
** '''Forth'''-based operating systems are a special case of this. While Forth is usually described as an interpreted language, the [http://en.wikipedia.org/wiki/Threaded_code threaded-code interpreters] many FORTH systems use work differently from most other interpreters; in effect, the interpreter walks through the various Forth 'words' that the code is comprised of until it reaches the low-level words that are implemented in assembly or compiled code, which is what actually gets executed. Furthermore, Forth systems incorporate a special sort of assembler, which produces code specifically meant to be used by the interpreter; also, commonly used 'words' can be compiled into native code as needed. Finally, many embedded Forth systems use special-purpose hardware (see below) to support the language.
** Most '''Lisp''' systems freely mix interpreted and compiled code, and in nearly all 'serious' Lisp systems since the late 1970s, the Listener REPL (the 'command line' for the Lisp system) is not an interpreter, but rather a compile-and-go system - each piece of code is at least partially compiled on the fly. While no significant Lisp OS has been made without also using language-specific hardware support (see below), if one were, it surely would not
* The system ran on specialized hardware and microcode, which acted as hardwired 'interpreter' for it's primary language, or for the portable bytecode which it normally used. This type of system includes the SOAR (Smalltalk On A RISC), the Recursiv System, The Lillith Modula-2 System, and the Burroughs 6500 (a mainframe designed for running Algol-60 in the 1960s). The system programming techniques for these cannot work on stock hardware. For example:
** The '''[http://en.wikipedia.org/wiki/MIT_CADR MIT CADR] Lisp machine architecture''' had an extensive instruction set with hardware support for certain high-level operations such as type-tag checking and GC. It had a [http://en.wikipedia.org/wiki/Tagged_architecture tagged architecture] meaning that a portion of the 36-bit addressing word was designated for type information. Typically these machines had a variety of compilers including one for the system language Lisp which was capable of taking advantage of the additional instruction set.
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