PRB: VB5 SP2 and SP3 Give Incorrect Floating Point Results (178527)

RESOLUTION

Run a simple calculation, such as D = Int(D), to reset the floating point control registers prior to doing floating point calculations. Note that you may have to do this prior to each calculation sequence, depending on what external calls are made between calculation sequences.

MORE INFORMATION

Steps to Reproduce Behavior

Install Visual Basic 5.0 (without Visual Studio 97 Service Packs 2 or 3) on a clean machine.

Make a project with two CommandButton and the following code:

      Private Sub Command1_Click()
      Dim K0 As Double, K1 As Double, K2 As Double, N As Double
        K0 = 0.029204
        K1 = -1.0508
        K2 = -0.34226
        N = 0#
        Do While N <= 2.75
          N = N + 0.02
        Loop
        MsgBox -(K1 + 2# * K2 * N) * Exp(K0 + K1 * N + K2 * N ^ 2) - 0.012
      End Sub

      Private Sub Command2_Click()
        Dim D As Double
        D = Int(D)
      End Sub

Run the program in the IDE and click the CommandButtons in the following order:

Command1
Command2
Command1

The result of the calculation in each case is: 2.80181002613759E-04.
Install Visual Basic 5.0 with the Visual Studio 97 SP2 or SP3, and rerun the program. Note that the result of the calculation is different:
1. Prior to clicking Command2: 2.80181002613761E-04
2. After clicking Command2: 2.80181002613759E-04

Notes

Simply setting N to 2.76 does not cause a problem with the calculation; it must be iterated for the problem to occur.
Running a compiled EXE seems to give the correct results, but this may be due to different start-up sequences and is no guarantee of repeatable results if the floating point control registers are altered by a DLL or other call.