16. A circuit comprising: a central processing unit; an interface for accessing a slave device, the interface comprising a transmit buffer for transmitting to the slave device, a first-in-first-out receive buffer for receiving from the slave device, and an underflow mechanism associated with the first-in-first out receive buffer; a data transfer engine coupled between the interface and the central processing unit; and a memory, coupled to the central processing unit, storing code arranged to be executed on the central processing unit to set up the data transfer engine to access the slave device by: based on said set-up, operating the data transfer engine to supply a read request word to a slot of the 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 the 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 use the underflow mechanism of said first-in-first-out receive buffer to determine the last word therein and hence determine said response word.
23. The circuit of claim 16 , wherein said underflow mechanism is configured to output the last validly read word upon underflow of the first-in-first-out receive buffer, and the determination of said last word comprises operating the data transfer engine to read a number of words from the first-in-first-out receive buffer sufficient to cause underflow.