Method of prechecking the validity of a write access request

Abstract

A method and apparatus for prechecking (probing) the validity of an access request for writing result data to an external system prior to executing the instruction that generates the result is provided. This allows instruction execution to continue uninterrupted in the event that the write is allowed. The microprocessor's Address Unit issues a 'probe' request to the Memory Management Unit (MMU) via an internal bus while saving the instruction's virtual address in a virtual address buffer local to the Address Unit. The MMU checks the validity of the 'probe' request without converting the virtual address to a physical address and issues an access grant signal which is saved by the microprocessor's Execution Unit for subsequent use. The Execution Unit processes the data in parallel to the MMU checking the validity of the probe request. If the virtual address associated with the probe request resulted in an access grant signal, then the Execution Unit issues a write request while the virtual address previously stored in the Address Unit is sent to the MMU for translation to a physical address. Both the write data and the physical address are stored in a buffer in the microprocessor's Bus Interface Unit (BIU) for subsequent transfer to an external system. The data is then written to the external system at the physical address provided by the BIU.