How to configure an ARM GPIO port (STR9) to an alternate function?
The STR91x family of microcontrollers has very flexible peripherals.  Unfortunately, this flexibility makes them somewhat complicated to set up.  My suggestion is to start by downloading the STR91xFA Firmware Library from ST.com.  Once you have the library linked into your project, you need to do something similar to the following:

void InitGPIO7( void )
{
   GPIO_InitTypeDef GPIO_InitStructure;

   /* Enable the GPIO7 clock */
   SCU_APBPeriphClockConfig(__GPIO7, ENABLE);

   /* Initialize the GPIO port */
   GPIO_DeInit(GPIO7);

   /* Configure pin GPIO7.3 */
   GPIO_InitStructure.GPIO_Direction = GPIO_PinOutput;
   GPIO_InitStructure.GPIO_Pin = GPIO_Pin_3;
   GPIO_InitStructure.GPIO_Type = GPIO_Type_PushPull;
   GPIO_InitStructure.GPIO_IPConnected = GPIO_IPConnected_Disable;
   GPIO_InitStructure.GPIO_Alternate = GPIO_OutputAlt1;
   GPIO_Init (GPIO7, &amp;GPIO_InitStructure);
}


Note that this is untested but it should be pretty close.