Q69012: Use of OFFSET and SEG on Local Variables in Inline Assembly

Article: Q69012
Product(s): See article
Version(s): 6.00 6.00a | 6.00 6.00a
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | s_quickc | mspl13_c
Last Modified: 6-FEB-1991

Inline assembly allows you to use the SEG and OFFSET operators to get
the segment and offset address of variables. However, if you use the
SEG directive on a local variable, you will get the following error
message in C versions 6.00 and 6.00a or QuickC versions 2.50 and 2.51:

   error C2415: improper operand type

You will not get this error with QuickC versions 2.00 and 2.01, and
incorrect code will be generated. In all versions, using the OFFSET
directive on a local variable will not directly give you a near
pointer to the variable.

It is important to remember that a local variable is placed on the
stack at run time. Therefore, the compiler cannot determine its
address at compile time. The value returned by OFFSET applied to a
local variable actually is that variable's position on the stack
relative to the BP register. Thus, adding the BP register to the
OFFSET value will create a near pointer into the stack segment.

Because the local variable is on the stack, the segment value of a
local variable is simply the stack segment (SS) register. The
following two sample programs demonstrate the incorrect and correct
method to access a local variable address:

Sample Code
-----------

// These programs show how you might try to load an address
// of a local variable into the dx, ax register combination.

// This is the wrong way to get a local variable address.

void main(void)
{
   int foo;
   _asm
   {
      mov ax, OFFSET foo
      mov dx, SEG foo
   }
}

// This is the right way to get a local variable address.

void main (void)
{
   int foo;
   _asm
   {
      mov ax, OFFSET foo
      add ax, bp
      mov dx, ss
   }
}

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