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.
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. 2. The method of claim 1 , wherein: said transmit buffer is a first-in-first-out buffer; said operation of the data transfer engine comprises, based on said set up, processing a sequence of descriptors in the data transfer engine; and the processing of the sequence of descriptors comprises: processing a first descriptor operating the data transfer engine to input said read request word into the first-in-first-out transmit buffer; processing one or more subsequent descriptors operating the data transfer engine to input a number of additional words into the first-in-first-out transmit buffer behind the read request word, the number being at least sufficient to fill said first-in-first-out transmit buffer to an upper stall threshold and arranged so as to generate a stall until said response word has been returned from the slave device; and following said subsequent descriptors in the sequence and synchronized to release of said stall of the first-in-first-out transmit buffer, processing one or more control descriptors operating the data transfer engine to perform said disabling of the first-in-first-out receive buffer.