A device for PCIe might not be
capable of satisfying a specific read request. For example, if the read request
targets an unsupported address for PCIe, the
completer returns a completion TLP with a completion status of 0b001 - Unsupported Request. The completer that
returns a completion TLP with a completion status of Reserved must be treated as an unsupported request status, according
to the PCI Express Base Specification v3.0. When the slave bridge receives an
unsupported request response, the Slave Unsupported Request (SUR) interrupt is
asserted and the DECERR response is asserted with arbitrary data on the AXI4 memory mapped bus.