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5.29.1 Header fields

In Gforth 1.0 we switched to a new word header layout. For a detailed description, read: Bernd Paysan and M. Anton Ertl. The new Gforth header. In 35th EuroForth Conference, pages 5-20, 2019. Since this paper was published, xt and nt have been changed to point to the parameter field, like the body, but otherwise it is still up-to-date.

This section explains just the data structure and the words used to access it. A header has the following fields:

name
>f+c
>link
>cfa
>namehm
>body

Currently Gforth has the names shown above for getting from the xt/nt/body to the field, but apart from the standard >body they are not stable Gforth words. Instead, we provide access words. Note that the documented access words have survived the reorganization of the header layout.

Some of the words expect an nt, some expect an xt. Given that both nt and xt point to the body of a word, what is the difference? For most words, the xt and nt use the same header, and with nt=xt, they point to the same place. However, for a synonym (see Aliases) there is a difference; consider the example

create x
synonym y x
synonym z y

In this case the nt of z points to the body of z, while the xt of z points to the body of x. Words defined with alias or forward (see Forward) also have different nts and xts.

The name field is variable-length and is accessed with name>string (see Name token).

The >f+c field contains flags and the name length (count). You read the count with name>string, and the flags with

compile-only? ( nt – flag  ) gforth-1.0 “compile-only?”

true if nt is marked as compile-only.

The >link field contains a link to the previous word in the same word list. You can read it with

name>link ( nt1 – nt2 / 0  ) gforth-1.0 “name-to-link”

For a word nt1, returns the previous word nt2 in the same wordlist, or 0 if there is no previous word.

The name, >f+c and >link fields are not present for noname words, but name>string and name>link work nevertheless, producing 0 0 and 0, respectively.

The >cfa field (aka code field) contains the code address used for executeing the word; you can read it with >code-address and write it with code-address! (see Threading Words).

The >namehm field contains the address of the header methods table, described below. You access it by performing or accessing header methods (see Header methods).

The >body (aka parameter) field contains data or threaded code specific to the word type; its length depends on the word type. E.g., for a constant it contains a cell with the value of the constant. You can access it through >body (see The gory details of CREATE..DOES>), but this is only standard for words you defined with create.


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