Method and apparatus for sequencing misaligned external bus transactions

Abstract

In accordance with the preferred embodiment of the present invention, a bus interface unit of a microprocessor is provided with a Micro Request Sequencer (EBMRS) disposed between a bus scheduling queue (EBBQ) and external bus control logic (EBCTL). Under normal bus request traffic, the EBMRS is effectively transparent and allows normal communication between the EBCTL and the EBBQ. However, for misaligned bus transactions, which comprise memory accesses that cross a bus width boundary, the EBMRS intercepts such transactions for special sequencing, while blocking any further requests from the EBBQ. The EBMRS separates each misaligned bus transaction request into at least first and second split transaction requests, with each split request forming a memory access that does not cross a data bus width boundary of the external bus. It then issues the first split request to the EBCTL for processing on the external bus. External bus agents involved with processing of the split requests then return first response information regarding the completion of the first split request. If the first response information indicates that the first split request will complete without being deferred or retried, the EBMRS issues the second split request to the EBCTL for processing on the external bus. Upon the receipt of second response information for the second split request indicating that the second split request is guaranteed to complete without being deferred or retried, the EBMRS then issues any further transaction requests received from the EBBQ without jeopardizing the order dependency of the split requests or subsequent bus transaction requests buffered in the EBBQ.