Inline Assembly Seems to Incorrectly Access Far Labels

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
  

The following inline assembly code produces a NULL pointer assignment when compiled with the compact or large memory model. The code should store the ss:sp in the huge pointer tsrstack. However, the code generated assumes that the variables are in the segment referenced by the DS register. This is not a correct assumption for a compact or large memory model program. Thus, the values are moved into DGROUP causing the R6001 NULL pointer assignment message.

This is NOT a compiler error. With inline assembly, the compiler makes no changes to the code. Therefore, there is no segment override added to the code to allow for tsrstack residing in another segment (in this case FAR_DATA). It is the developer's responsibility to make sure that the correct code is used for each memory model.

Sample Code

char _huge * tsrstack;
void main (void)

   _asm MOV WORD PTR tsrstack[0], sp
   _asm MOV WORD PTR tsrstack[2], ss

This program was extracted from the larger program example ALARM.C, which is accessible through online help.