The AXI MMU IP is generally inserted automatically by the AXI Interconnect between an endpoint master device and the Crossbar to perform special address decoding services. There is generally no reason to instantiate the MMU directly in a design.
The Interconnect inserts the MMU whenever the address map view of any endpoint slave device differs among multiple connected master devices. This generally occurs when a master is to have access to a subset (aperture) of the slave's entire address range. The MMU enforces proper address range checking to prevent such masters from accessing beyond the intended address region. Any attempt to access outside of the ranges configured into the MMU will result in the MMU responding with a protocol-compliant response with the BRESP/RRESP field set to the DECERR value (2'b11); such transactions are not propagated through the MMU. The MMU can be configured with up to 256 address ranges of endpoint slave addresses that can be accessed by the master. The MMU can further define any of the address ranges as read-only or write-only. The AXI MMU does not support address remapping.