6.2 COFF Output Format
The COFF object module format is suitable for flat, non-segmented memory models. In most cases
the COFF format is used for 32-bit code. However, it's possible to create a 64-bit COFF module
if the current cpu is
.x64 when the .MODEL directive is parsed
( note that
-win64
is the usual way to create modules for 64-bit Windows ). To some extent 16-bit code
is also supported with COFF, although
OMF will always be the
better choice then.
- The -safeseh cmdline option and the .SAFESEH directive are supported by COFF only.
- Not supported by COFF are:
- the GROUP directive
- the directives to control segment order: .ALPHA, .DOSSEG, .SEQ
- code that will generate "far" fixups (i.e. direct FAR calls or jumps ).
- A few, common segment names are translated in the output module when COFF is selected:
- name _TEXT is translated to .text
- name _DATA is translated to .data
- name CONST is translated to .rdata
- name _BSS is translated to .bss
This also affects simplified segment directives, since _TEXT, _DATA, CONST and _BSS segments
are internally generated when directives .CODE, .DATA, .CONST or .DATA? are detected.
- If COFF is selected, Masm will ignore segment combine type STACK and treat stack segments just like normal
data segments ( the MS linker and compatibles will set the stack's size with commandline options ).
JWasm DOES NOT ignore combine type STACK for the COFF output format; additionally, unless the segment contains
initialized data, JWasm will reset the size of stack segments to 0.
- Directive EXTERN name (altname) : <type> will define a weak external with
characteristics IMAGE_WEAK_EXTERN_SEARCH_LIBRARY ( see MS PE and COFF specification ).
- Directive ALIAS <aliasname>=<actual name> will define a weak external with
characteristics IMAGE_WEAK_EXTERN_SEARCH_ALIAS ( see MS PE and COFF specification ).
- There are some cmdline options to control the extent of symbols that are written to the COFF
output module: -zlf, -zlp and -zls.