Interrupt handling

Hi,
this time I've got a problem with interrupt handling in Pistachio. ;-)
I'm writing a serial port user mode driver without IDL - to understand
how it's working. Here are the code parts:

Low level interrupt handling in SRC_PLAT(intctrl.cc):

void intctrl_t::handle_irq( word_t irq )
{
	this->mask_and_ack( irq );
	handle_interrupt( irq );
}

Roottasks main:

int main (void)
{
	[...]
	hwint_tid = L4_GlobalId( MPSC_INT_RCC, 1 );
	res = L4_AssociateInterrupt( hwint_tid, L4_Myself() );
	if ( !res )
		L4_KDB_Enter( "Couldn't register interrupt." );
	[...]
	echo_tid = start_thread( &echo_server,
&echo_stack[sizeof(echo_stack) - 1] ); /* taken from SDIOS */
	[...]
}

Thread which echoes character back on serial line:

void echo_server (void)
{
	L4_MsgTag_t tag;
	char c;

	while ( true )
	{
		/* wait for ipc */
		tag = L4_Receive( hwint_tid );
		c = inc();
		if ( c )
			outc( c );
		L4_Reply( hwint_tid );
	}
}

What happens is this:

Enable MPSC_INT_RCC    -> only one interrupt active at the moment
Disable MPSC_INT_RCC   -> interrupt received and masked
ACK MPSC_INT_RCC

The interrupt isn't enabled again. How should that happen? I thought a
reply to the interrupt IPC automatically triggers the kernel unmask the
interrupt again?!

Thanks,

Martin.