When the APB protocol generic is set to apb4, the AXI to APB Bridge core drives m_apb_pstrb and m_apb_pprot signals. s_axi_wstrb is passed to m_apb_pstrb during write transfers. s_axi_arprot is assigned to m_apb_pprot during a read transfer, and s_axi_awprot is assigned to m_apb_pprot during a write transfer.