Q65818: Inline Assembly Incorrectly Accesses Far Labels

Article: Q65818
Product(s): See article
Version(s): 6.00   | 6.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | s_quickc | mspl13_c
Last Modified: 24-OCT-1990

The following inline assembly code produces a NULL pointer assignment
when compiled with the compact or large memory models. The code should
store the ss:sp in the huge pointer tsrstack. However, the code
generated assumes that the variables will be 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 example program ALARM.C,
which is accessible through online help.

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.