V4N4 - Complang

discussion will consider only the more
familiar indirect threaded code. Figure
3(a) shows the proFORTH'" defini-
tion structure for a system constant -
that is, a constant which is not refer-
enced in high-level target code. Except
for the location of the LFA, it is essen-
tially identical to the traditional struc-
ture, in that the entire definition is
compiled into the active dictionary
(in this case, dictionary 0). The header
begins at the link field address or LFA,
and consists of the link and the defini-
tion's name. The body immediately
follows the header in memory and
consists in this case of a pointer to a
common support procedure named
(CONST) located at the CFA or code
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field address, followed by the parame-
ter field at the parameter field address
of PFA. In the example the parameter
field contains the constant's value,
which the procedure (CONST) will put
on the stack when it executes. In pro-
FORTH'" the procedure (CONST)
resides in target ROM so that it is also
available for supporting target con-
stants.
Consider now the structure for a
target constant shown in Figure 3@).
Notice that the definition body is now
located in target ROM. In order to be
able to access the body, a pointer is
added to the header at the definition
field address DFA, and the CFA of the
definition is stored there.
Continued on next page
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FIGURE 3(a) - proFORTH DICTIONARY STRUCTURE FOR SYSTEM CONSTANT
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