3.3. Interrupts without Tasks

A protected operation that is attached to an interrupt must be a parameterless protected procedure. This is enforced by the pragma Attach_Handler and by the type Parameterless_Handler from package Ada.Interrupts. For interrupt handlers that have pragma Interrupt_Handler and are not attached to an interrupt is it convenient to allow both parameters and protected functions. The XGC compiler supports this as a legal extension to the Ada language.

In the special case where all the operations on a protected type are interrupt level operations, the XGC compiler will generate run-time system calls that avoid the use of the tasking system. Then only if tasks are required will the tasking system be present. This saves about 6K bytes of memory and reduces the amount of unreachable (and untestable) code.

with Ada.Interrupts.Names;

package body Example_Pack is
   use Ada.Interrupts.Names;

   protected UART_Handler is
      procedure Handler;
      pragma Attach_Handler (Handler, UART_A_Rx_Tx);
      --  Must be a parameterless procedure

      procedure Read (Buf : String; Last : Natural);
      pragma Interrupt_Handler (Read);
      --  Runs at interrupt level, may have parameters

      function Count return Integer;
      pragma Interrupt_Handler (Count);
      --  Runs at interrupt level, may be a function
   end UART_Handler;

   protected body UART_Handler is 
      ...
   end UART_Handler;

end Example_Pack;