FIX: Optimization Removes SS Override from Inline Assembly

The information in this article applies to:

• The Microsoft C/C++ Compiler (CL.EXE), included with: Microsoft Visual C++ for Windows, version 1.0

SYMPTOMS

If optimizations are used, the version 8.0 C/C++ compiler may remove the use of the stack segment register (SS) override in a pointer reference in an inline assembly block.

RESOLUTION

To get around this problem, do one of the following:

• Disable optimization using the /Od compiler switch.

  -or-

• Use the fast compiler option, /f.

  -or-

• Disable optimization during the function where the problem occurs, using #pragma optimize before and after the function.

  -or-

• In the case of the sample code (in the "MORE INFORMATION" section, below), remove the line of code before the assembly block to allow the compiler to also generate correct code.

STATUS

Microsoft has confirmed this to be a problem with the C/C++ compiler version 8.0. The problem was corrected in C/C++, version 8.0c, included with Visual C++ for Windows, version 1.5.

MORE INFORMATION

The sample code below can be used to reproduce the problem. Use the /Fc compiler option to generate a source/assembly listing of the code.

Sample Code

/* Compile options needed: /c /Ox (or any optimization)
*/
      unsigned char tbl[256];

      static void Bug(unsigned char _far *Font, unsigned char c)
      {
         unsigned char _far *font_data = Font + c*33;

         __asm
         {
            push ds
            mov al, byte ptr ss:[bx]
            mov bl, es:[di]
         }
      }

      main()
      {
         Bug((unsigned char _far *)0, 0);
      }