A data processing system in which a host processor prepares command streams for causing an accelerator of the data processing system to perform processing tasks for an application executing on the host processor, each command stream including a sequence of commands for implementation by a command stream execution unit of the accelerator. A pre-execution unit is provided that is operable to interpret commands fetched from command stream storage before the command is provided to the command stream execution unit for implementation to determine whether the pre-execution unit is able to perform an action in response to the command and, when the pre-execution unit is able to do so, to perform an action in response to the command.