Memory card interface method using multiplexed storage protect key to indicate command acceptance

A memory interface between the storage controller and memory card of an S/390 system uses the S/390 Storage Protect (SP) Key as an indication or protocol of storage command acceptance by the memory card. When the SP key is returned, then the command is deemed to be accepted by the memory card and the key will be used by the processor for its storage validation in accordance with the S/390 architecture. In the event that the memory card detected an error associated with the command, it will then return an error response code via a memory status bus. The memory status bus is multiplexed to service the existing architected requirement as well as an indicator of handshaking between the memory controller and the memory card.