Some Internal Decisions

Internal Decisions

The Head.aux Field

The ANSI standard on forth requires that a DOES> code changes the CFA-vector of the latest CREATE to the colon's token-list right after the DOES>. However, PFE does actually regard the value in the CFA as the address of an actuall C-routine, so it does simply jump there - the bytes at the DOES> address need to be actual cpu asm code, but PFE can not compile cpu specific code as it would require knowledge about the cpu beyond that of the C compiler.

Actually, the CFA of the last CREATE word is changed to a C routine in the standard .text body that shall start to execute the colon's token-list right after DOES>. However, if the CFA of CREATE points to a common routine the .text section, how shall the DODOES code know where to start execution?

In the forth-83 and earlier forth dialects, the address where to actually jump to was stored in the field directly following the CREATE address, i.e. in the first PFA cell. Therefore they used the word did receive the address of CREATE plus one cell. The forth'94 standard does not allow this - the sequence after DOES> gets the actual PFA address, and

All non-asm implementation of forth do the same trick - they add the extra field before the CFA, in PFE called the AUX-field. It is only used by DOES-words but by its implementation issues it will be allocated for every word. The header size of a word in a non-asm implementation is therefore bigger than in in asm implementations of forth just for these DOES-words.

To sum it up - between the LINK-field and the CODE-field is an AUX-field. All forth languages base on C do this.

(the name aux is from older times where this field was also used for some other information instead of the does-address. it isn't now and the field should perhaps be just named Head.does but feel free to use this field if there is a special need)

The Flag.byte Field

The forth'94 does not do any requirements on the structure of a name-field, it does not even need to exists. The forth'83 implementations had a atleast a header field, and the traditional structure of a name field from FIG-times is a counted string whose upper bits have been (mis-)used for the flag-bits. The highest bit marks it as the the flagfield (it is always set), followed by the immediate-bit and the smudge-bit. This leaves just 5 bit for the actual count of the name in the lower bits, and the names were hence limited to 31 chars.

However, 31 chars is clearly inappropriate for names more than twenty years later - the PFE is often used to interface to C-defined API and whereas old implementations of C had a 32-char limit too, they do not now anymore. The limit had to be raised for PFE, and instead of making extremely radical changes to the name-structure, the trick of F-PC was used - to move the the flags just before the count-byte of the name.

Due to some other implemenation issues (the link-to-name routine looks for the highbit that marks the flag-field) the limit is now 128 chars. As an advantage, the structure of the namefield is now a plain counted string without any flags mangled in. As always, the PFE allows to change back to the traditional ways, and in the source code a macro is used to enhance readability - "_FFA". And the other macro is called "NFACNT" which will return (*nfa&31) for the traditional mode to mask out the flag-bits from the count-value - in the default mode of pfe however, it is just (*nfa). Use these macros to calm down the #ifdef-noise.

To sum it up - the flags live in an extra byte just before the name field which is a normal counted string.

The Head Layout

  structure Head                   structure Head
    1 chars field flags              1 chars field FFA
    x chars field name               x chars field NFA
    1 cells field link               1 cells field LFA
    1 cells field aux                1 cells field AUX
    1 cells field code               1 cells field CFA
    ( followed by body)              ( followed by PFA)
  endstructure                     endstructure

Using `configure` options, both the flags-field and the aux-field can be cut out again to arrive at a header-structure that is FIG-forth compatible. The use of `with-fig` will cut out both fields.

...

(there are a lot more internal decisions to be explained, however they are of not much interest to forth programmers - they will be added to this document later)