Forth

implementation/

• data stack - one-cell wide
• control-flow stack

• return stack - one or more cells

  • rules:

    A program shall not access values on the return stack (using R@, R>, 2R@, 2R> or NR>) that it did not place there using >R, 2>R or N>R;

    A program shall not access from within a do-loop values placed on the return stack before the loop was entered;

    All values placed on the return stack within a do-loop shall be removed before I, J, LOOP, +LOOP, UNLOOP, or LEAVE is executed;

    All values placed on the return stack within a definition shall be removed before the definition is terminated or before EXIT is executed.

    Forth words are organized into a structure called the dictionary. While the form of this structure is not specified by the standard, it can be described as consisting of three logical parts: a name space, a code space, and a data space. The logical separation of these parts does not require their physical separation.

    A program shall not fetch from or store into locations outside data space. An ambiguous condition exists if a program addresses name space or code spac

    names = 1-31 chars

    The relationship between code space and data space is implementation dependent.

    Data space is the only logical area of the dictionary for which standard words are provided to allocate and access regions of memory. These regions are: contiguous regions, variables, text-literal regions, input buffers, and other transient regions, each of which is described in the following sections. A program may read from or write into these regions unless otherwise specified.

  • text-interpreter (repl) = 3.4
  • syntax

  Forth has a simple, operator-ordered syntax. The phrase A B C returns values as if A were executed first, then B and finally C. Words that cause deviations from this linear flow of control are called control-flow words. Combinations of control-flow words whose stack effects are compatible form control-flow structures. Examples of typical use are given for each control-flow word in Annex A.

  Forth syntax is extensible; for example, new control-flow words can be defined in terms of existing ones. This standard does not require a syntax or program-construct checker.