Q26006: "Illegal Function Call" from EXE Using Coprocessor

Article: Q26006
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | TAR68499 | mspl13_basic
Last Modified: 31-OCT-1988

When compiled with BC (/o and /d options make no difference) and run
on a machine equipped with a math coprocessor, the following program
generates an "Illegal function call" message at run time.

   SCREEN 1            'fails in any graphics mode
   OPEN "file4" FOR RANDOM AS #4
   CLOSE #4
   WINDOW (-320, -100)-(320, 100)
   FOR x = -202 TO 202 STEP 4
      aspect = ABS(202 / x) * 5 / 12
      IF aspect < 1 THEN
         radius = 20 / aspect
      ELSE radius = 20
      END IF
      y = 40 * SQR(1 - x ^ 2 / 40804)
      CIRCLE (x, y), radius, , , , aspect
      CIRCLE (x, -y), radius, , , , aspect
   NEXT x

In QuickBASIC Version 4.00, if you use the DOS command SET NO87="none"
to turn off the coprocessor, or if you run inside the QuickBASIC
editor, the program executes successfully. The program also runs
properly in QB and QB87 in QuickBASIC Version 3.00.

This is not an error in the QuickBASIC Version 4.00 software.

The behavior of the above program in Version 4.00 is due to a
difference in the way optimization is handled between the editor and
BC. The error occurs in the following line:

   y = 40 * SQR(1 - x ^ 2 / 40804)

When the program is run inside the editor this line is ultimately
evaluated as follows:

   y = 40 * SQR( 0 )

However, as an EXE, the same line results in an attempt to find the
SQR of a negative number; the same number rounded to zero in the
editor.

To work around the problem, assign (1 - x^2 / 40804) to an
intermediate variable, set the intermediate variable to zero if it is
less than zero, and use the intermediate value in the SQR function.

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.