6.1 OMF Output Format

The OMF format fully supports the segmented architecture of the x86 cpu. The specification has been extended around 1990 to support 32-bit, including the FLAT memory model. However, since it's a bit old now, there's no support of some of the new relocations introduced for 64-bit.

• OMF won't support the IMAGEREL and SECTIONREL operators.
• JWasm currently won't emit OMF LIDATA records ( unlike Masm). This means that if the assembly source contains lots of arrays defined with DUP, the object module might become significantly larger than a similiar object module written by Masm.
• JWasm emits a few COMENT records that Masm won't. These are remnants of Open Watcom's WASM; they may be suppressed with commandline options -zlc and -zld. They shouldn't do any harm, though.

The OMF object module consists of records of certain types. JWasm writes those records in the following order:

Type		Description
THEADR	1	Marks the start of an object module, contains the name of the source file.
COMENT, class A1h	1	MS extension; tells linker the CodeView version number. Emitted if -Zi option is active.
COMENT, class E9h	n	Borland-style auto-dependency record(s). Emitted if -Zd or -Zi options are active.
COMENT, class 9Eh	1	MS extension; directs the linker to use a "standardized" segment ordering. Emitted if the .DOSSEG directive is found in the source.
COMENT, class 9Fh	n	Default library search name(s). Emitted if INCLUDELIB directives are used.
LNAMES	n	Defines names for segments and groups ( optionally also for externals, communals and publics ).
SEGDEF	n	Defines segment(s).
COMENT, class FEh	n	Tells WLink to do FARCALL optimization for a specific segment. Suppressed with -zld.
GRPDEF	n	Defines group(s).
EXTDEF	n	Defines external(s).
COMENT, class A8h	n	Defines weak external(s). Emitted if EXTERN directive is used with "altname".
COMDEF	n	Defines communal(s).
ALIAS	n	Defines alias(es).
PUBDEF	n	Defines public(s).
COMENT, class A0h	n	Defines export(s). Emitted if the EXPORT attribute is used in a PROC directive.
COMENT, class A2h	1	Link Pass Separator. Emitted if no entry point is defined in this object module.
LEDATA	n	Data record; defines segment content(s).
FIXUPP	n	Contains information to make the linker resolve references between object modules.
LINNUM	n	Line number information. Emitted if -Zd or -Zi option is active.
COMENT, class FDh	n	Tells disassembler that a code segment contains data. Suppressed with -zlc.
MODEND	1	Marks the end of the object module; optionally defines the entry point.

LEDATA, FIXUPP and LINNUM records usually are intermixed; a FIXUPP and/or a LINNUM record appears immediately after the LEDATA record to which it refers.
Since v2.11, multiple THEADR records are written if cmdline options -Zd or -Zi are set and line number information is written for more than one source file; this conforms to MASM behavior.