1. #@language@# An interactive extensible language using H. Moore in the 1960s. FORTH is highly user-configurable and there are many different implementations, the following description is of a typical default configuration. Forth programs are structured as lists of "words" - FORTH's term which encompasses language keywords, primitives and subroutines to an extreme - nearly everything is a subroutine. A word is any string of characters except the separator which defaults to space. Numbers are treated specially. Words are read one at a time from the input stream and either executed immediately ("interpretive execution") or compiled as part of the definition of a new word. The sequential nature of list execution and the implicit use of the data stack (numbers appearing in the lists are pushed to the stack as they are encountered) imply postfix syntax. Although postfix notation is initially difficult, experienced users find it simple and efficient. Words appearing in executable lists may be " primitives" compiled procedures or other special words. A procedure definition is introduced by ":" and ended with ";" and is compiled as it is read. Most Forth dialects include the source language structures BEGIN-AGAIN, BEGIN-WHILE-REPEAT, BEGIN-UNTIL, DO-LOOP, and IF-ELSE-THEN, and others can be added by the user. These are "compiling structures" which may only occur in a procedure definition. and "ENDCODE" or similar constructs. Forth primitives are mixture. In fact code in-lining is the basis of compilation in some implementations. Once assembled, primitives are used exactly like other words. A significant difference in behaviour can arise, however, from the fact that primitives end with a jump to "NEXT", the entry point of some code called the sequencer, whereas non-primitives end with the address of the "EXIT" primitive. systems so a process can be descheduled after executing a non-primitive, but not after a primitive. Forth implementations differ widely. Implementation processors, macros at various levels, or interpreters written in another language such as C. Some implementations provide real-time response, user-defined data structures, Some Forth systems support virtual memory without specific hardware support like MMUs. However, Forth virtual memory is usually only a sort of extended data space and does not usually support executable code. words for arithmetic, memory access, loops, IF statements, and the user's application. Many Forth systems provide user-declared "vocabularies" which allow the same word to have different meanings in different contexts. Within one vocabulary, re-defining a word causes the previous definition to be hidden from the interpreter (and therefore the compiler), but not from previous definitions. FORTH was first used to guide the telescope at NRAO, Kitt letters in a program name, so FOURTH became FORTH. Versions include fig-FORTH, FORTH 79 and FORTH 83. FORTH Interest Group, Box 1105, San Carlos CA 94070. [Leo Brodie, "Starting Forth"]. [Leo Brodie, "Thinking Forth"]. [Jack Woehr, "Forth, the New Model"]. [R.G. Loeliger, "Threaded Interpretive Languages"]. (1997-04-16)
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