Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim savefile As String
        Dim dialresult As DialogResult
        dialresult = OpenFileDialog1.ShowDialog
        If dialresult <> Windows.Forms.DialogResult.OK Then
            Exit Sub
        End If

        savefile = OpenFileDialog1.FileName
        Dim srcbytes() As Byte = File.ReadAllBytes(savefile)

        srcbytes(12) = 0
        srcbytes(13) = 0
        srcbytes(14) = 0
        srcbytes(15) = 0

        Dim checksum(3) As Integer
        For Each b As Byte In checksum
            b = 0
        Next

        Dim boolcarry, temp As Integer
        boolcarry = 0
        temp = 0
        For i As Integer = 0 To srcbytes.Length - 1
            temp = srcbytes(i) + boolcarry
            checksum(0) = checksum(0) * 2 + temp
            checksum(1) = checksum(1) * 2
            If checksum(0) > 255 Then
                checksum(1) = checksum(1) + (checksum(0) - checksum(0) Mod 256) \ 256
                checksum(0) = checksum(0) Mod 256
            End If

            checksum(2) = checksum(2) * 2
            If checksum(1) > 255 Then
                checksum(2) = checksum(2) + (checksum(1) - checksum(1) Mod 256) \ 256
                checksum(1) = checksum(1) Mod 256
            End If

            checksum(3) = checksum(3) * 2
            If checksum(2) > 255 Then
                checksum(3) = checksum(3) + (checksum(2) - checksum(2) Mod 256) \ 256
                checksum(2) = checksum(2) Mod 256
            End If

            If checksum(3) > 255 Then
                checksum(3) = checksum(3) Mod 256
            End If

            If (checksum(3) And 128) <> 0 Then

                boolcarry = 1
            Else
                boolcarry = 0
            End If

        Next

        Dim bytes(3) As Byte

        Dim j As Integer = 0
        For Each i As Integer In checksum
            bytes(j) = CType(i, Byte)
            j += 1
        Next

        TextBox1.Text = String.Format("The sum after modification is:{0:x} {1:x} {2:x} {3:x}", bytes(0), bytes(1), bytes(2), bytes(3))

        Dim writefile As FileStream = File.Open(savefile, FileMode.Open)
        writefile.Seek(12, SeekOrigin.Begin)
        writefile.Write(bytes, 0, 4)
        writefile.Flush()
        writefile.Close()

        lblmsg.Text = "Check sum bytes is successfully written."
    End Sub