================================================================
This is the pseudocode for a circular buffer in which the writer 
blocks if the buffer is full, and the reader blocks if the buffer
is empty. 

B: array (0 .. N-1) of Integer;
In_Ptr, Out_Ptr: Integer := 0; 
El: Semaphore := 0; 
Sp: Semaphore := N; 

task body Producer is
  I: Integer; 
begin
  loop
    Produce(I);
    Wait (Sp); 
    B(In_Ptr) := I; 
    In_Ptr := (In_Ptr + 1) mod N;
    Signal (El); 
  end loop;
end Producer; 

task body Consumer is
  I: Integer; 
begin
  loop
    Wait (El); 
    I := B(Out_Ptr); 
    Out_Ptr := (Out_Ptr + 1) mod N; 
    Signal (Sp); 
    Consume (I); 
  end loop; 
end Consumer; 

================================================================
This is the pseudocode for a circular buffer in which the writer 
drops the data (without blocking) if the buffer is full, 
and the reader blocks if the buffer is empty. 
This is most likely what you have to use in the RAP implementation. 

B: array (0 .. N-1) of Integer;
In_Ptr, Out_Ptr: Integer := 0; 
El: Semaphore := 0; 
Sp: Semaphore := N; 

task body Producer is
  I: Integer; 
begin
  loop
    Produce(I);
    if TryWait (Sp) = EAGAIN: 
      <do nothing, the element is dropped>
    else: 
      B(In_Ptr) := I; 
      In_Ptr := (In_Ptr + 1) mod N;
      Signal (El); 
    end if 
  end loop;
end Producer; 

task body Consumer is
  I: Integer; 
begin
  loop
    Wait (El); 
    I := B(Out_Ptr); 
    Out_Ptr := (Out_Ptr + 1) mod N; 
    Signal (Sp); 
    Consume (I); 
  end loop; 
end Consumer;