configuring spi1 nss pin in STM32F405 as external interrupt pin

Hello everyone..
I am working on STM32F405xx controller.
I am configuring SPI1 NSS pin(PA4) as external interrupt pin for falling edge but its not working for me.
in reference manual it is given that it is possible to configure this pin (NSS pin )as general purpose pin while using the SPI1 in Software Select Mode.
I have done the following settings:

  /*!< SPI NSS pin configuration */

  SPI_InitStructure.SPI_Direction         = SPI_Direction_2Lines_FullDuplex;
  SPI_InitStructure.SPI_Mode              = SPI_Mode_Master;
  SPI_InitStructure.SPI_DataSize          = SPI_DataSize_8b;
  SPI_InitStructure.SPI_CPOL              = SPI_CPOL_Low;     //   SPI_CPOL_High        SPI_CPOL_Low
  SPI_InitStructure.SPI_CPHA              = SPI_CPHA_1Edge;   //   SPI_CPHA_1Edge  SPI_CPHA_2Edge
  SPI_InitStructure.SPI_NSS               = SPI_NSS_Soft;
  SPI_InitStructure.SPI_BaudRatePrescaler = baudRatePrescaler;
  SPI_InitStructure.SPI_FirstBit          = SPI_FirstBit_MSB;
  SPI_InitStructure.SPI_CRCPolynomial     = 7;
  SPI_Init(SPI1, &SPI_InitStructure);

void Interrupt_Config(void)
{
  GPIO_InitTypeDef   GPIO_InitStructure;
  NVIC_InitTypeDef   NVIC_InitStructure;
  EXTI_InitTypeDef   EXTI_InitStructure;

  /* Enable GPIOA clock */

  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);/* 13/09/2013 GPIOA */

  /* Enable SYSCFG clock */

  RCC_APB2PeriphClockCmd(RCC_APB2Periph_SYSCFG, ENABLE);

  /* Configure interrupt pin */

  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN;
  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_DOWN;
  GPIO_InitStructure.GPIO_Pin  = GPIO_Pin_4;
  GPIO_Init(GPIOA, &GPIO_InitStructure);

  /* Connect GPIO_Pin_4 */

  SYSCFG_EXTILineConfig(EXTI_PortSourceGPIOA, EXTI_PinSource4);

  /* Configure GPIO_Pin_4 */

  EXTI_InitStructure.EXTI_Line = SID_INTRPT_LINE;
  EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt;
  EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Falling;
  EXTI_InitStructure.EXTI_LineCmd = ENABLE;
  EXTI_Init(&EXTI_InitStructure);

  /* Enable and set GPIO_Pin_4 Interrupt to the lowest priority */

  NVIC_InitStructure.NVIC_IRQChannel = EXTI4_IRQn;    /*   13/09/2013   */
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0x01;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0x01;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);
}



// following is the handler for temporary purpose

void EXTI4_IRQHandler(void)
{

int x;

x++;


}

When i am checking it on simulator by clicking on to pending tab it goes into the handler
but on real hardware it is not coming to handler. On hardware end everything is ok.