Checksum for Object Module Records Can Be Zero

MS-DOS                 | OS/2       | WINDOWS

The information in this article applies to:

• The Microsoft C/C++ Compiler (CL.EXE), included with:

      - Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
      - Microsoft C for OS/2, versions 6.0 and 6.0a
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, and
        1.52
  

The Microsoft Object Module Format (OMF) consists of records that all have the same basic layout. At the end of each record is a byte that can be used as a checksum. The checksum byte can be set so that adding all bytes in the record, including the checksum byte, results in a sum mod 256 or zero.

In Microsoft C 6.0, 7.0, and Visual C++ 1.0 the quick/fast compile (/qc or /f) feature produces .OBJ files that have the checksum byte set to zero. In both C 6.0, 7.0, and Visual C/C++ 1.0 the optimizing compiler produces .OBJ files that have the checksum set. Both techniques are valid ways of setting the value of the checksum byte.