omf.txt

开放源码的编译器open watcom 1.6.0版的源代码

Version 1.0 Initially written by Alex Brodsky, June 20, 1996

Notes regarding the implementation of the OMF part of ORL.

Due to the 'distinct' nature of the OMF versus ELF or COFF the 
implementation of OMF within the framework involves several peculiarities.  
To begin OMF has not sections.  Since the section paradigm is the basis 
of ELF, COFF and hence ORL the OMF implementation presents the 
information found in OMF in the form of sections.  

The sections are created in the following way.  Each segment or comdat 
is mapped to a section.  Only these sections are 'real' and hence have 
contents.  

All symbols through out the object file are colected and placed in a symbol 
table section.  Only one symbol table section is ever created.  

All relocations (FIXUPPs) are collected and placed into a relocs 
section.  Similarly to a symbol table section, only one is ever created 
and contains all relocs for all 'real' sections.  

All comments are collected and placed into a a single notes section.  
There is also only one of these sections.  All three sections, symbol 
table, relocs, and notes, may be treated in the same way as their ELF 
or COFF equivalent.  The important things to note is that all three of 
these section are virtual and hence have no contents.  Secondly all 
real sections will return the same reloc section when asked for it.  
Consequently care must be taken to avoid rescanning the same reloc 
section multiple times.  

OMF uses two hidden sections which are string tables, one is a 
collection of all LNAMEs and LLNAMEs and the other is a collection of 
all EXTDEFs, and COMDEFs.  These are never seen by the client and 
should never be made visible to the client.  These sections have no 
content and hence are also virtual.

Symbols are treated the same way as in COFF or ELF.  The only extension 
is the group and group symbol which will be discussed later.  

The OMF has a concept of a table.  To facilitate groups the ORL 
interface has been extended to manipulated groups in the same way as 
sections.  Each group consists of 0 or more segments and has a name 
associated with it.  Since COFF and ELF don't have groups, the 
interface will always claim that 0 groups exist if an ELF or COFF file 
is being read.

Under OMF COMDATs are special and can have associated segments.  These 
may be accessed using the provided facilities.

For further details refer to the ORL interface and the OMF code.