Putting Data in a Named Segment

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 and 1.5
  

SUMMARY

The /ND compiler switch can be used to name data segments and force certain data into these segments. Named data segments are always far segments, so far pointers and addressing must be used.

Using /ND causes the program to assume that the DS register points to the named segment, not to the default data segment. SS is no longer assumed to be equal to DS, so functions may need to use _loadds (__loadds) to ensure that DS is loaded correctly.

MORE INFORMATION

/ND is used mostly for shared data segments and is really a special purpose option. A better technique for forcing data into a particular segment is to use the _based (__based) keyword with a segment variable name.

Variables that are declared outside of a function and not initialized are communal. That is, they are not placed in the named data segment if /ND is used. The reason for this is because the compiler places data in named segments, but communal items are assigned to segments by the linker. With /ND initialized global data and all static data go into named segments rather than the default segments.

So, when compiling with /ND mydata:

   int a;      /* goes to default segment   */
   int b = 5;  /* goes to MYDATA     */
   static c;   /* goes to MYDATA_BSS */

So, to put the variable in the named data group, either declare the variable static or initialize it (as in either b or c above).

In Microsoft C version 6.0 and later, the _based (__based in C/C++ 7.0 and later) keyword can be used to specify in which segment the data item will reside. This is generally preferred to using /ND. Refer to the manuals for more information on using based variables.