1. A method of accessing a slave device from a circuit comprising a central processing unit, a data transfer engine, and an interface to the slave device; the method comprising: executing code on said central processing unit to set up the data transfer engine to access said slave device; based on said set-up, operating the data transfer engine to supply a read request word to a slot of a transmit buffer of the interface for transmission to the slave device, said slave device generating a corresponding response word related to said read request word and, after return of said corresponding response word to a first-in-first-out receive buffer of the interface, to free said slot of said transmit buffer to disable said first-in-first-out receive buffer from receiving any further data such that the last word therein is assured to be said response word; and using an underflow mechanism of said first-in-first-out receive buffer to determine the last word therein and hence determine said response word.