Structures Wording

[ RfDs/CfVs | Other proposals ]

Poll Standings

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Systems

[ ] I agree with the proposed alteration to the document text.
Gerry Jackson
Hans Bezemer
Bernd Paysan
[ ] I do not agree with the proposed alteration to the document text.

Informal Results

Problem

The X:structures extension defines FIELD: as: 
10.6.2.xxxx FIELD:          "field-colon"              FACILITY EXT
            ( -- )                                     X:structures
The semantics of FIELD: are identical to the execution semantics of the phrase:
ALIGNED 1 CELLS +FIELD
See:
10.6.2.---- +FIELD, 10.6.2.---- BEGIN-STRUCTURE and 10.6.2.---- END-STRUCTURE.
While this is a very concise definition, there are a few difficulties with it:
  1. The stack description is incorrect, it should be: ( n1 "<spaces>name" -- n2 )
  2. It defines a child word which is not described. Normally this is described in a "name Execution" clause.
  3. There is no actual description of the word, simply what amounts to a reference implementation.
This applies to CFIELD:, DFFIELD:, FFIELD: and SFFIELD: as well.

Solution

Replace the definition of FIELD: and friends with: 
10.6.2.xxxx FIELD:          "field-colon"              FACILITY EXT
            ( n1 "<spaces>name" -- n2 )                X:structures
Skip leading space delimiters. Parse name delimited by a space. Offset is the first cell aligned value greater than or equal to n1. n2 = offset + 1 cells.

Create a definition for name with the execution semantics given below.

name Execution: ( a-addr1 -- a-addr2 )
Add the offset calculated during the compile time action to a-addr1 giving the cell aligned address a-addr2.
See:
10.6.2.---- BEGIN-STRUCTURE, A.10.6.---- BEGIN-STRUCTURE, A.10.6.2.---- FIELD:

Proposal

Modify the definition of the following words in accordance with the text given in the Solution, above, with the modification of the alignment as follows:
FIELD: cell aligned
CFIELD: character aligned
FFIELD: float aligned
SFFIELD: single-float aligned
DFFIELD: double-float aligned
Add the following as the Rationale for 10.6.2.---- FIELD:
A.10.6.2.xxxx FIELD:

Create an aligned single cell field in a data structure.

The various xFIELD: words provide for different alignment and size allocation.

The xFIELD: words could be defined as: 

: FIELD:      ALIGNED 1 CELLS    +FIELD ; \ cell aligned
: CFIELD:             1 CHARS    +FIELD ; \ character aligned
: FFIELD:    FALIGNED 1 FLOATS   +FIELD ; \ float aligned
: SFFIELD:  SFALIGNED 1 SFLOATS  +FIELD ; \ single-float aligned
: DFFIELD:  DFALIGNED 1 DFLOATS  +FIELD ; \ double-float aligned

Change History

2010-08-31
Revised into a documentation change (RfC)
2010-03-17 (not published)
Corrected spelling of FILED:
Revised wording at Rostock meeting
Introduction of rationale text.
2010-02-24
Drafting of RfD
2009-09-03
Original wordsmithing of solution during Exeter meeting.

Credits

Peter Knaggs <pjk@bcs.org.uk>
Engineering, Mathematics and Physical Sciences,
University of Exeter, Exeter, Devon EX4 7QF, England
web: www.rigwit.co.uk

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