Apparatus and method for transferring data in a data storage subsystems wherein a multi-sector data transfer order is executed while a subsequent order is issued

PCT No. PCT/GB91/00255 Sec. 371 Date Apr. 2, 1993 Sec. 102(e) Date Apr. 2, 1993 PCT Filed Feb. 19, 1991 PCT Pub. No. WO92/15053 PCT Pub. Date Sep. 3, 1992Described is a technique for transferring data between a device controller and a device in a data storage subsystem. The controller has a data buffer through which passes data being transferred to and from the device. The controller communicates with the device by means of low level multi sector Read/Write orders and the data requested by the controller is transferred between device and controller in the form of 32, 64 or 128 byte packets. The device has a relatively small data buffer which can store one packet of data at a time. While one order is executing on the device, the controller is able to issue subsequent orders. One of these orders is an EXTEND order which orders the device to continue reading beyond the data specified in the read order currently executing on the device. The EXTEND order also allows implementation of 'back to back' writes.