A gateway for interfacing a host with a subsystem for acting as a work accelerator to the host, the gateway enabling the transfer of batches of data to and from the subsystem at pre-compiled data exchange synchronisation points attained by the subsystem. The gateway is configured to: receive from a storage system data determined by the host to be processed by the subsystem; store a number of credits indicating the availability of data for transfer to the subsystem at each pre-compiled data exchange synchronisation point; receive a synchronisation request from the subsystem when it attains a data exchange synchronisation point; and in response to determining that the number of credits comprises a non-zero number of credits: transmit a synchronisation acknowledgment to the subsystem; and cause the received data to be transferred to the subsystem.