3. SYSTEM FUNCTIONS

Forth is more than a programming language. The earliest versions of Forth ran standalone on primitive minicomputers and microprocessors in the 1970s, providing an integrated system, language, and application functions in a single package. This heritage persists in the Forth virtual machine, even though today it is frequently implemented on top of a conventional operating system.

This section describes words used to load, organize, and manage Forth applications, as well as standard system devices (disk, terminal, and clock). But before considering details of various Forth system functions, the next section will present a fundamental concept commonly used to implement system functions.

3.1 VECTORED ROUTINES

It is often desirable to modify or re-direct system functions, because of changing hardware or application requirements, without recompiling the system kernel. Forth facilitates this by providing execution vectors containing the addresses of the current versions of these functions. There are two groups of vectored routines: system-wide functions and terminal-dependent functions (i.e., those whose behavior differs between different kinds of CRT or between keyboard/display and printer). For each vectored function, there are at least three Forth words: the function itself (which performs a @ EXECUTE on the vector), the vector itself, and at least one routine to be executed.

Table 5 summarizes the vectored routines controlled on a system-wide basis on some Forth systems.

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Table 5: Common system-level vectored routines

* See references, below, for details.

In addition, some routines in multitasking systems are vectored through user variables for differing task-specific functions. Typically, these control CRTs, printers, and other serial devices, or application functions such as databases. Refer to your product documentation for specific details of vectored system functions on your implementation.

References BLOCK and BUFFER, Section 3.4

CREATE, Section 4.2.1

Support of special terminal functions, Section 3.3.3

TYPE, Section 3.3.2

Vectored execution, Section 2.5

3.2 SYSTEM ENVIRONMENT

Standard Forth systems provide a mechanism for inquiring about the configuration and parameters of a particular system, either interactively at the keyboard or within program code. The word ENVIRONMENT? expects to find on the stack the address and length of a text string referring to an option or parameter, and returns either a single false flag (parameter/option is unknown), or a true flag (known) on top of the stack, with a second flag or data value beneath. The word S" (see Section 4.1.5.2), which returns the address and length of a string, is often used with ENVIRONMENT?. For example, the string STACKCELLS is defined as indicating the maximum number of cells in the data stack. You might type at the keyboard, or include in a definition, the phrase:

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S" STACK-CELLS" ENVIRONMENT?

which might return

256 -1

where the -1 (true) indicates that the system recognized the STACK-CELLS string, and the 256 shows that the maximum size of the stack is 256 cells. Table 6 gives the standard strings available for environmental queries and the data values they may return. The data type is the type of the associated data or second flag.

Most word sets contain a basic part and extensions, which may be tested for individually. For example, in this table, BLOCK and BLOCK-EXT separately test for the presence of the basic block word set and the block extensions word set.

Table 6: Environmental query strings and associated data

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Table 6: Environmental query strings and associated data (continued)

Because a system may load options in any order, some environmental queries could return either false or true, depending on when they were executed. The Standard Forth requirements are:

!If a query returns false (unknown) in response to a string, subsequent queries with that string may return true, because additional capabilities may have been

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