Problems with Qsys Avalon UART Rx Interrupt

Hi, I have another question concernig the IRq acknowledgement of the UART Core.

Fore examle if there is an Tx Interrupt, then the belonging bit in the status register is set and i have to read the status register. But how do I clear the IRQ or acknowledge it, better said, how do i reset the irq-bit?

In the UART Core Spec. there is the annotation that "Writing zero to the status register clears DCTS, E, TOE, ROE, BRK, FE and PE bits".

At the moment, if there is an Tx IRQ, my IRQ pins goes from Low to High, I read the status register, I set the status register tx-irq-bit to zero (of cource with no effect), and the IRQ Pin remains high and goes never again to Low Level.

Thanks.