Interruption Handling using zfp compilation flag

["Vinícius Franchini" <viniciusnf@gmail.com>]

Hi guys,
 my name is Vinícius and I'm doing a master degree. My thesis is a simplified on-board computer for an satellite. I must implement some scheduling and package analysis functions. As I need to work with a real time system, I'm using the zfp flag for compilation. I tried to implement some interruptions using sentences like this:

   procedure Receive_USART;

   -- Address the interruption handler		
   pragma Machine_Attribute (Entity => Receive_USART,
                             Attribute_Name => "signal");

   pragma Export (Convention    => C,
                  Entity        => Receive_USART,
                  External_Name => "__vector_26");    --USART0_RXC

Using Proteus (ISIS Professional 6.7 SP2), I'm debugging the algorithm. The initialization of the interruption is right, but when the interruption occur, it isn't handling to the procedure. 

Do I need to do something more to handle the procedure when the interruption occur ? Do you fellows have any other simple way to handle interruptions ?

Thanks very much for your help....