fig1 shows a data storage system including two disk drives 10 , 12 connected via two full duplex serial links 30 , 31 ; 32 , 33 to a disk drive controller 20 . ( additional disk drives , such as disk drive 10 &# 39 ;, may be respectively connected via a full duplex serial link 32 &# 39 ;, 33 &# 39 ; to the disk drive controller 20 .) the controller is in turn attached to a host system ( not shown ) from which it receives commands and data . the controller acts on these commands to issue order to the attached disk drives to control the transfer of data between the disk drives and the host . in the following description , the synchronization technique of the present invention will be discussed in relation to a pair of disk drives but it will be appreciated that the technique is readily extendable to a larger number of drives . referring now to fig2 there is shown in block diagrammatical form , the main components of the controller . microprocessor chip 200 performs the functions of dma controller and also a general system controller , operating in this latter respect under the control of program code stored in rom 220 and ram 210 , both of which are connected to the microprocessor via bus 225 . the microprocessor is also connected to a relatively large dram buffer 202 which is used primarily as a temporary staging post for read / write data being transferred between host and disk drives . the controller microprocessor is further connected by means of a dma bus 230 and an i / o bus 240 to a controller link chip ( clc ) 260 . the dma bus caries the read / write data between the dram buffer and the clc . the clc controls the operation of the serial links over which disk drive and controller communicate . each serial link comprises an inbound link 31 and an outbound link 30 . for reasons of clarity , only one serial link connecting the controller to dasd 12 is shown in fig2 . it will be appreciated that another similar serial link ( comprising inbound and outbound portions ) connects the controller via a second clc to disk drive 10 . the clc 260 of fig2 comprises a frame buffer 262 which is common to both inbound and outbound sides of the link . the outbound portion of the clc comprises frame generator 264 connected to crc generator and 8 / 10 encoder 266 which is in turn connected to serializer 268 . frames are the primary means employed to transfer information over a serial link and will be described in greater detail below . the inbound portion of the clc comprises deserializer 270 which is connected to 8 / 10 decoder and crc checker 272 which is in turn connected to frame decoder 274 . referring now to fig3 there is shown in block diagrammatical form , the major functional components of the disk drive electronics . the inbound and outbound serial links from the controller attach to the drive via an interface protocol chip ( ipc ) 302 . in broad terms , the ipc is the counterpart to the clc in that the inbound portion includes a deserializer , crc checker , 8 / 10 decoder and frame decoder . the outbound portion includes a frame generator , crc generator , 8 / 10 encoder and serializer . data to be read to the disks is routed via the disk controller 304 , prml data channel 306 and arm electronics modules 308 . disk drive orders received over the serial link from the controller are decoded by the ipc and routed to a file control microprocessor ( fcm ) 310 . the fcm operates under the control of program code which is stored in ram 312 and rom 314 . also associated with the fcm is a counter 316 and a clock 318 which are used in the synchronization technique as will be described below . the disk drive electronics also include a tms servo processor 320 that performs the servo control functions . the tms processor is connected to motor speed control logic 322 which controls the speed of the motor which drives the data disks 326 and servo disk 328 . servo information on disk 328 is read by servo head 370 and directed to demodulator 332 . demodulated servo information is used by the tms processor to control the speed of the disks . associated with the tms processor are ram 334 and rom 336 . ram 334 includes an area 338 in which a target phase value is stored . this target phase value is supplied to the ram by the controller . having described the major functional components of the controller and disk drive , it is necessary at this stage to describe those features of the serial link communication protocol which are useful in understanding the remainder of the description . information is transferred over the serial link between the disk drives and controller in the form of frames . the format of a frame is shown in fig4 and consists of a sequence of three or four fields namely control field , address field , data field ( optional ) and a crc field . flags at the beginning and end of a frame are employed to separate one frame from another . there are two basic types of frame namely control frame and application frame . control frames are typically used to , e . g ., recover from link errors and to abort link orders . control frames are employed in the synchronization technique described below . a control frame is distinguished from an application frame in that the most significant six bits of the control field have a non - zero value . an application frame has all zeroes in the most significant six bits of the control field and is thus distinguished from a control frame . there are two main types of application frame namely message frames and data frames . the address field in an application frame specifies the destination address of the frame . message frames are used to communicate between the microprocessors in the controller and the disk drive , e . g ., to send orders to the disk drive and to receive status back from the drive . the format of a message frame is essentially the same as the frame format shown in fig4 but in a message frame , the data field contains a message . the first byte of the data field is a code which identifies the function required and the length of the message field is variable depending on the particular message . the various disk drive orders ( e . g ., read , write , etc .) are sent from the controller as messages in a message frame . each of these orders is fully contained in a single frame . data frames are used to transfer data ( i . e ., read / write data ) between the controller and the disk drive . the length of the data field is variable and a single data transfer will typically require multiple data frames . when the controller is in communication with a disk drive , the required type of frame ( control , message or data ) is generated in the controller link chip frame generator 264 under the control of the microprocessor 200 . the crc is generated and included in the frame , the frame is encoded and then driven over the outbound serial link to the attached disk drive . the ipc interprets the frames received over the link from the controller in order to determine whether the frame is a message , data or control frame . for a message or data frame , the ipc routes the frame to the destination specified in the address field . a control frame which is distinguished by the value of the most significant six bits in the control field is routed to fcm 310 . next will be described , with reference to fig3 the technique by which the speed of the spindle motor is controlled using the phase detection logic . there are a number of different ways of controlling disk speed known in the prior art , and it should be noted that the precise technique is not critical to the disk synchronization technique used in the present invention . during operation of the disk drive , servo information is read from the dedicated servo disk 328 by the servo head 370 and servo pulses are generated by the demodulator 332 . these pulses are compared against bit pulses from a crystal oscillator in comparator logic ( not shown ). in a manner well known in the art , comparator logic calculates the phase difference between the oscillator pulses and the servo pulses . the value of phase is compared to the target phase 338 and the resulting phase error is acted on by the motor servo control algorithm in a manner also well known in the art to cause a temporary adjustment to the spindle motor drive , thereby to reduce the phase error between the detected phase difference and the phase target . for more detail of the phase detection logic , further reference should be made to an ibm technical disclosure bulletin ( vol . 32 no . 58 october 1989 pp 322 - 324 ) entitled ` phase error detect logic for disk file speed control .` this process can be repeated at regular predefined intervals to keep the phase error between the crystal and the spindle motor to within predetermined limits . the spindle motor drive will be changed in progressive steps in order that the speed of the motor does not vary outside predetermined limits ( e . g ., 0 . 01 %) during read or write operations . the fcm hardware counter 316 is reset once per revolution of the disk at index , and counts up at a fixed rate under the control of a high frequency clock ( e . g ., 20 mhz ). the fcm counter will normally operate even when disk synchronization is not required though it would be possible to only activate the counter immediately prior to the initiation of disk synchronization operations . the controller initiates synchronization by sending a control frame over the serial link to the drives to be synchronized . it may be feasible to issue a control frame to the drives simultaneously though it is more likely that the frames will be issued at different times , albeit only separated by a short time period . if not simultaneous , the controller will need to keep a record of the difference in the times at which the frames are issued . a predefined bit ( sync bit ) of the control field in each control frame is set to 1 which defines the frame as a sync frame . receipt and identification of the sync frame at the disk drive causes an interrupt in the fcm which freezes the fcm hardware counter . the frozen value of the counter indicates the elapsed time from the detection of the most recent index pulse to the receipt of the control frame and thus provides a snapshot of the angular position of the disk at the time the sync frame is received . the contents of the counter are read by the file control microprocessor and multiplied by a constant to generate consistent units for the controller ( e . g ., 100 ns ). the fcm then stores the result in its ram address space , in the location spindle phase 339 . at some later time , the controller issues a message frame to each of the disk drives specifying an order to read the fcm ram ( read ram order ). the value of the spindle phase counter stored in ram is read and a status message frame generated by the ipc which includes the value of the counter . receipt of the read ram order at the disk drive causes the counter to reset . in an alternative technique , the counter is interrogated directly by a read position order issued by the controller with receipt of the read position order causing the counter to reset . the controller receives the status message frame from each of the addressed disk drives over the serial link . the frames are identified as status frames and the microprocessor extracts the spindle phase counter values contained in the frames and stores them in ram . the controller microprocessor , under control of a particular task defined by the program code , then determines the relative angular positions of the drives from the received counter values . depending on the synchronization configuration required , the microprocessor calculates the spindle ` error ` in angular position from the average value of the two spindle phase counters and causes the clc to generate a control frame to be issued to each disk drive . the units of the position error contained within the control frame are the same as those of the counter values sent to the controller ( 100 ns ). for the simple case where it is desired to lock the phases of the two drives , the controller calculates the average of the two spindle phase values and issues an order to each drive specifying the spindle error of each drive from the average value . this position error value represents the change in angular position which is required to achieve phase synchronization . it is a simple matter to factor into the spindle error calculation any difference in the times at which the sync frames were issued to the disk drives . it will be appreciated that the technique is readily extendable to the synchronization of three or more disk drives . instead of calculating the average of all the received spindle phases , the controller calculates the average of the maximum and minimum spindle phases and determines the spindle error of each drive from this average . this spindle error value received by each drive is processed by the ipc and the fcm extracts the value and multiplies it to restore it to its internal units . the fcm then causes the tms processor to write a target phase value ( in the internal units ) to a specified portion of the tms ram . this target phase value 338 is then used by the spindle motor servo control algorithm in order to calculate the desired change in motor drive . the spindle motor servo algorithm operates to drive the difference between the spindle error and the target phase to zero . the means by which this is done are well described in the art .