Option -coff: Select Output Format COFF
Option -coff selects output format COFF. The most common use for -coff is to produce modules for 32-bit Windows. However, it's not mandatory that the output will be 32-bit with -coff; depending on what cpu is selected when the .MODEL directive will be parsed, the output may be 32- or 64-bit.For more information about COFF, see COFF Output Format.
Chapter Output Formats lists all available output formats.