Excel 2.20: Using Floating Point Values in a Code Resource

SUMMARY

When creating a code resource to be called from Excel 2.20, steps should be taken to ensure correct manipulation of floating point values.

Many compilers, such as Think's Lightspeed C, treat typed floating point values as string constants. To correctly execute floating point manipulations, the following three steps must be taken:

1. Any typed floating point values must be cast as an 8-byte double (for example, short double in Lightspeed C).

   For example, in Lightspeed C, to find the product of a floating point value pointed to by "a" and the number 2.5, you could use the following code segment:

         *b = *a * (short double) 2.5;
   

2. The A4 address must be adjusted to ensure the values are returned from the correct memory location.

   Because the code is compiled as a resource rather than a program segment, values are manipulated in Excel's memory addresses rather than the code's own addresses. Because of this, the addresses for the code resource must be adjusted to comply with Excel's addresses.

   In Lightspeed C, this can be done using predefined macros from SetUpA4.h, as follows:

   a. Insert the following as a declaration at the beginning of your

         code segment to include Lightspeed C's Setup macro library:
   

            #include "SetUpA4.h"
   
      b. As the first two lines of your executable code, insert the
         following:
   
            RememberA0();
            SetUpA4();
   
      c. As the last line of executable code before the return statement,
         enter the following:
   
            RestoreA4();
   
      You can also do this without using existing macros by using the
      following method:
   
      a. Declare a long variable for temporarily storing A4:
   
            Long A4Sav;
   
      b. In the first two lines of executable code, swap addresses by
         inserting the following:
   
            asm {move.l  a4, A4Sav};
            asm {move.l  a0, a4};
   
      c. Restore the contents of A4 in the last line of executable code
         before the return statement by entering the following:
   
            asm {move.l  A4Sav, a4};
   
   

3. The return value must be specified as its own variable.

A sample code segment to multiply a passed value by 9 would be as follows:

   pascal short double *main(a,b)
         short double *a, *b;
   {
         long A4Sav;
           asm {move.l  a4, A4Sav};
           asm {move.l  a0, a4};
           *b = *a * (short double) 3.141592654;
           asm {move.l  A4Sav, a4};
           return (b);
   }

     | A
   ----------------------------------
   1 | =REGISTER("trig","pi","EBE")
   2 | =CALL(A1,7.5,)
   3 | =UNREGISTER(A1)
   4 | =RETURN()