The s_axi_user_k_tx_tready signal is asserted by the core and is prioritized by CC, CB, NFC, and UFC messages. After asserting s_axi_user_k_tx_tdata , the USER-K block number and s_axi_user_k_tx_tvalid is asserted. If required, the user application can change s_axi_user_k_tx_tdata when s_axi_user_k_tx_tready is asserted ( This Figure ). This action enables the Aurora 64B/66B core to select the appropriate USER-K BTF from the nine USER-K blocks. The data available during assertion of s_axi_user_k_tx_tready is always serviced.