OpenCL code results a 'GPU device not Responding' Error...Any ideas...???

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Cloo;
using System.Runtime.InteropServices;
using System.Diagnostics;
 
namespace PI_with_Cloo
{
    class Program
    {
        static ComputePlatform platform = ComputePlatform.Platforms[0];
        #region Kernel Source
        static string kernelSource = @"
#pragma OPENCL EXTENSION cl_khr_byte_addressable_store : enable
int mul_mod(int a, int b, int m)
{
 return (int)(((long)a) * (long)(b)) % m;
}
 
#pragma OPENCL EXTENSION cl_khr_byte_addressable_store : enable
int inv_mod(int x, int y)
{                                 
 int q,u,v,a,c,t;
 
 u=x;
 v=y;
 c=1;
 a=0;
 
 q=v/u;
 
 t=c;
 c=a-q*c;
 a=t;
 
 t=u;
 u=v-q*u;
 v=t;
 while (u!=0)
 {
   q=v/u;
 
   t=c;
   c=a-q*c;
   a=t;
 
   t=u;
   u=v-q*u;
   v=t;
 }
 
 a=a % y;
 
 if (a<0) a=y+a;
 
 return a;
}
 
#pragma OPENCL EXTENSION cl_khr_byte_addressable_store : enable
int powmod(int a, int b, int m)
{
 int r, aa;
 
 r=1;
 aa=a;
 
 while (1)
  {
   if ((b & 1) != 0) 
    {
     r = mul_mod(r, aa, m);
    }
 
   b = b >> 1;
 
   if (b == 0) break;
 
   aa = mul_mod(aa, aa, m);
  }
 
 return r;
}
 
// return true if n is prime
#pragma OPENCL EXTENSION cl_khr_byte_addressable_store : enable
bool is_prime(int n)
{
 int r;
 
 if ((n % 2) == 0) 
  {
   return false;
  }
 
 r = (int)sqrt((float)n);
 
 for (int i = 3; i <= r; i += 2)
  {
   if ((n % i) == 0) 
    {
     return false;
    }
  }
 
 return true;
}
 
// return the prime number immediatly after n
#pragma OPENCL EXTENSION cl_khr_byte_addressable_store : enable
int next_prime(int n)
{
  n++;
  while (is_prime(n)==false)
  {
	n++;
  }
 
 return n;
}
 
#pragma OPENCL EXTENSION cl_khr_byte_addressable_store : enable
float frac(float d)
{
 if (d>0)
  {
   return d-((int)d);
  } 
   else
  {
   return d+floor(d);
  } 
}
 
//start from digit n, return 9 digits
#pragma OPENCL EXTENSION cl_khr_byte_addressable_store : enable
float CalculatePiDigits(int n)
{
 int av, vmax, num, den, s, t;
 int nb;
 float sum;
 int a;
 int i;
 int k,v,kq,kq2;
 
 nb = (int) ((n + 20) * log(10.0) / log(2.0)) ;
 
 sum = 0;
 
 a=3;
 
 for (a = 3; a <= (2 * nb); a = next_prime(a)) 
  {
   vmax = (int) ((log((float)(2 * nb)) / log((float)a)));
 
   av = 1;
 
   for (i= 0; i<vmax; i++)
    {
     av = av * a;
    }
 
   s = 0;
   num = 1;
   den = 1;
   v = 0;
   kq = 1;
   kq2 = 1;
 
   for (k=1; k<=nb; k++)
    {
     t = k;
 
     if (kq >= a) 
      {
       t = t/a;
       v--;
       while ((t % a) == 0)
       {
		t = t/a;
        v--;
       }
 
       kq = 0;
      }
 
     kq++;
     num = mul_mod(num, t, av);
 
     t = 2 * k - 1;
 
     if (kq2 >= a)
      {
       if (kq2 == a) 
        {
         t = t/a;
         v++;
         while ((t % a) == 0)
         {
          t = t/a;
          v++;
         }
        }
       kq2 = kq2-a;
      }
 
     den = mul_mod(den, t, av);
     kq2 = kq2 + 2;
 
     if (v > 0) 
      {
       t = inv_mod(den, av);
       t = mul_mod(t, num, av);
       t = mul_mod(t, k, av);
 
       for (i=v; i<vmax; i++)
        {
         t = mul_mod(t, a, av);
        }
 
       s = s + t;
 
       if (s >= av)
        {
         s = s-av;
        }
      }
 
    }
 
   t = powmod(10, n - 1, av);
   s = mul_mod(s, t, av);
   sum = frac(sum + (float) s / (float) av);
  }
  return sum;
}
 
#pragma OPENCL EXTENSION cl_khr_byte_addressable_store : enable
__kernel void PI(__global int *dval ,__global float *sumval)
{
 int gid = get_global_id(0);
 int d = dval[gid];
 
 float sum = CalculatePiDigits(d);
 sumval[gid] = sum;
}
";
        #endregion
 
        static void Main(string[] args)
        {
            ComputeContextPropertyList pList = new ComputeContextPropertyList(platform);
            ComputeContext context = new ComputeContext(ComputeDeviceTypes.Gpu, pList, null, IntPtr.Zero);
 
            Console.WriteLine("Platform Vendor: " + platform.Vendor);
            Console.WriteLine("Platform Name  : " + platform.Name);
            Console.WriteLine("Device Name    : " + context.Devices[0].Name);
            Console.WriteLine();
 
            // Get the Input Value...
            Console.Write("Enter an Integer for PI calculation: ");
            int digits = int.Parse(Console.ReadLine());
 
            Stopwatch sw = new Stopwatch();
            sw.Start();
 
            digits = (digits - 1) / 9;
            digits++;
 
            Console.WriteLine("Calculating " + (digits * 9 + 1) + " PI digits....");
 
            char[] pival = new char[digits * 9 + 1];
            pival[digits * 9] = '0';
 
            int[] dval = new int[digits];
            for (int i = 0; i < digits; i++)
            {
                dval[i] = 1 + (i * 9);
            }
 
            ComputeBuffer<int> PI_dval = new ComputeBuffer<int>(context, ComputeMemoryFlags.ReadOnly | ComputeMemoryFlags.CopyHostPointer, dval);
            ComputeBuffer<float> PI_sumval = new ComputeBuffer<float>(context, ComputeMemoryFlags.WriteOnly, digits);
 
            // Calculate PI
            ComputeProgram program = new ComputeProgram(context, new string[] { kernelSource });
            program.Build(null, null, null, IntPtr.Zero);
 
            ComputeKernel kernel = program.CreateKernel("PI");
            kernel.SetMemoryArgument(0, PI_dval);
            kernel.SetMemoryArgument(1, PI_sumval);
 
            ComputeCommandQueue commands = new ComputeCommandQueue(context, context.Devices[0], ComputeCommandQueueFlags.None);
 
            ComputeEventList events = new ComputeEventList();
 
            commands.Execute(kernel, null, new long[] { digits }, null, events);
 
            float[] sumval = new float[digits];
            GCHandle arrCHandle = GCHandle.Alloc(sumval, GCHandleType.Pinned);
 
            commands.Read(PI_sumval, false, 0, digits, arrCHandle.AddrOfPinnedObject(), events);
            commands.Finish();
 
            arrCHandle.Free();
 
            char[] s;
            string str;
            for (int i = 0; i < digits; i++)
            {
                int d = 1 + i * 9;
 
                str = string.Format("{0:000000000}", (int)(sumval[i] * 1e9));
                s = str.ToCharArray(0, 9);
 
                for (int j = 0; j < 9; j++)
                {
                    pival[d - 1 + j] = s[j];
                }
            }
            sw.Stop();
 
            Console.WriteLine("Calculation completed successfully.\nCompute Time: " + sw.Elapsed.ToString());
            Console.Write("The Value is: 3.");
            for (int i = 0; i < pival.Length; i++)
            {
                Console.Write(pival[i].ToString());
            }
            Console.ReadLine();
        }
    }
}