MASM2DJG 1.02
-------------

MASM2DJG is a command line utility that patches Microsoft COFF format object
files, enabling them to be linked with the DJGPP linker 'ld'.

MASM2DJG enables assembler routines built with Microsoft MASM version 6 or
later to be linked with C sources built using DJGPP.

Syntax:

masm2djg [options] <file>
  -h  : Help
  -u  : Convert gnu objects to Microsoft format
  -m  : Convert Microsoft objects to gnu format (default)
  -v  : Increase verbosity of output
  -t  : Decrease verbosity of output
  -r  : Report patches but don't change the file
  -p  : Apply all changes (default)

so to fixup a.obj type:

masm2djg -t a.obj


Why is MASM2DJG necessary?
--------------------------

MASM produces a COFF output file that contains relocation fixups that are
incompatible with DJGPP's ld.

1. A call to an external subroutine is assembled as an 0E8h opcode  followed by
a relative offset (initially 0).  A REL32 relocation record is created pointing
to the offset.  'ld' incorrectly calculates the displacement to the target
because it ignores the offset of the fixup within the section.  This patch
subtracts the offset within the section of all REL32 fixups.

2. References to local static data are assembled with a section relative
address and a DIR32 relocation record.  'ld' incorrectly calculates the virtual
address because it subtracts the relative section offset from it.  This patch
adds the section offset of the fixup to all 'static' type DIR32 fixups.

3. References to externally visible but locally defined functions are
assembled with a 0 relative address and a DIR32 relocation record with
an IMAGE_SYM_CLASS_EXTERNAL storage class.  'ld' incorrectly calculates
the virtual address because it subtracts the relative section offset
from it.  This patch adds the section offset of the fixup to all
'extern' type DIR32 fixups.


Notes on using MASM with DJGPP
------------------------------

MASM 6 and later will produce a COFF format object file if supplied the /coff
switch on the comand line.

The only source code change required is to add the following directives:

  .386
  .MODEL FLAT

; Start the data segment
  .DATA

; Start the code segment
  .CODE

; If the source uses DGROUP then define it:
  DGROUP EQU flat


Building the source
-------------------

There's a makefile for gcc (DJGPP, cygwin etc), a project file (masm2djg.dsp)
for Visual C 6 and a project for Visual C 1.51 (masm2djg.mak) which built the
16 bit executable provided.

Last revised 2001-04-30 lvr

Lawrence Rust lvr@softsystem.co.uk
Software Systems
Tel: +33 5 49 72 79 63
www.softsystem.co.uk