Q28613: Object Module Format Extensions

Article: Q28613
Product(s): See article
Version(s): 5.00 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 17-NOV-1988

The following are Microsoft extensions to the Intel object module
format that were incorporated in Versions 5.00 and 5.10 of the C
compiler. For a complete description of the object module format, you
can contact Microsoft System Languages Technical Support at (206)
454-2030 and request the Object Module Format Application Note.

Microsoft OMF Extensions

Local Symbols (C 5.00) -- B4h, B6h, B7h, B8h

There will be three new record types to handle local symbols:

1. LEXTDEF = 0xb4 (180)
   Format is identical to EXTDEF.

2. LPUBDEF = 0xb6 (182) or 0xb7 (183)
   Format is identical to PUBDEF.

3. LCOMDEF = 0xb8 (184)
   Format is identical to COMDEF.

LEXTDEF, LPUBDEF, and LCOMDEF are to be used exactly as EXTDEF,
PUBDEF, and COMDEF, except that the symbols are not visible outside
the module where they are defined--for example, C statics.

New OMF Comment -- Class A1h

A comment record with class A1h indicates that the obsolete method of
communal representation through TYPDEF and EXTDEF pairs is not used,
and that COMDEF records can be interpreted. The meaning of this record
is undergoing revision.

IMPDEF (OS2, Windows) -- Comment Class A0h, Subtype 1

The IMPort DEFinition (IMPDEF) record takes the form of a COMENT
record with class 0xa0:

    DB      88h         ; COMENT
    DW      reclen      ; record length
    DB      00h         ; for Intel compatibility
    DB      A0h         ; class: OMF extension
    DB      01h         ; subtype: IMPDEF
    DB      fOrd        ; nonzero value means import by ordinal
    ; Internal Name
    DB      n           ; length of name
    DB      n dup(?)    ; ASCII text of name
    ; Module Name
    DB      n           ; length of name
    DB      n dup(?)    ; ASCII text of name
IF import by name (fOrd == 0)
    ;   Imported Name:  if length byte is 0 then imported name is
    ;   identical to internal name.
    ;
    DB      n           ; length of name
    DB      n dup(?)    ; ASCII text of name
ELSE
    DW      ordno       ; ordinal number
ENDIF

    DB      checksum

EXPDEF (C 5.10) -- Comment Class A0h, Subtype 2

The EXPort DEFinition record takes the form of a COMENT record with
class 0xa0:

    DB      88h         ; COMENT
    DW      reclen      ; record length
    DB      00h         ; for Intel compatibility
    DB      A0h         ; class: OMF extension
    DB      02h         ; extension type:  EXPDEF
    DB      flags
        ;   80h = set if ordinal number specified
        ;   40h = set if RESIDENTNAME
        ;   20h = set if NODATA
        ;   1Fh = # of parameter words

    ; Exported name:
    DB      n           ; length of name
    DB      n dup(?)    ; ASCII text of name

    ; IF internal name different from exported name:
    DB      n           ; length of name
    DB      n dup(?)    ; ASCII text of name
    ; ELSE internal name same as exported name:
    DB      00h

    ; IF ordinal number specified
    DW      ordno       ; ordinal number

    DB      checksum

LIBMOD Comment Record -- Class A3h (LIB 3.07 in MASM 5.00)

    DB      88h             ; COMENT
    DW      reclen          ; record length
    DB      00h             ; for Intel compatibility
    DB      A3h             ; class: LIBrary MODule name
    DB      N               ; length of module name
    DB      N dup(?)        ; ASCII text of module name
    DB      checksum

The LIBMOD comment record identifies the name of a library module.
This record allows LIB to preserve the source file name in the THEADR
record and still identify the module name. Since the module name is
the base name of the .OBJ file, the two names may be completely
different.

LIB adds a LIBMOD record when an .OBJ file is added to a library and
strips the LIBMOD record when an .OBJ file is removed from a library,
so this record usually only exists in .LIB files.

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