ExoSegDSAlias()
 Creates a data descriptor for the given code segment
------------------------------------------------------------------------------
 C Prototype

     void *ExoSegDSAlias(void *pm_ptr)

 Returns

     This function returns a data pointer to the same linear address as
     pm_ptr, or NULL if an error occurred.

 Description

     This function creates a new data descriptor with the same base as the
     segment specified by pm_ptr.  After the call to ExoSegDSAlias(), the
     descriptor indicated in pm_ptr still exists and retains its original
     segment type.  The offset of the alias will ALWAYS equal the offset of
     the original pointer.

     ExoFreeSelector() MUST be called to free the selector allocated with
     ExoSegDSAlias().  Selectors are a limited resource and repeated calls to
     ExoSegDSAlias() as well as ExoSegCSAlias() and ExoProtectedPtr() could
     use up all the available selectors causing the program to crash.  Once
     you are done with a pointer created with ExoSegDSAlias(), free the
     selector unless you will be using the pointer throughout the program.

 Examples

     The example below consists of two modules: one in assembly language and
     one in C. The int_instruction label in the assembly code marks the
     location of an instruction signaling Interrupt 0h.  The C code
     overwrites this instruction so that a different interrupt is signaled.

     The C code calls ExoSegDSAlias() to get an alias data pointer to
     int_instruction. The example then increments the pointer by one byte to
     point to the location of "0h" in the instruction. The example overwrites
     this location with "0x88" and frees the data descriptor with
     ExoFreeSelector() since it is no longer needed.

     public _do_int, int_instruction
     _do_int:
     ...
     int_instruction:
     int 0h
     ...

     char *data;
     extern int_instruction();

     /* create data segment alias and write interrupt number 0x88 */
     data = ExoSegDSAlias(int_instruction);
     *(data + 1) = 0x88;

     /* mark code segment as resident so the VMM system won't swap
        it out and lose the changes made to it */
     ExoReside(int_instruction);

     /* free the selector created by ExoSegDSAlias() since we don't
        need it anymore */
     ExoFreeSelector(FP_SEG(data));

 Files   Library is EXOSPACE.LIB, header file is Exospace.api.