Patent Publication Number: US-7904604-B2

Title: Expedited and low power command sequence servicing

Description:
FIELD OF THE INVENTION 
     The claimed invention relates generally to the field of computer-based systems and more particularly, but not by way of limitation, to an apparatus and method for servicing commands received in a selected command sequence in an expedited, low power manner. 
     BACKGROUND 
     Computer-based systems enable a wide variety of data processing tasks to be accomplished in a fast and efficient manner. From hand-held consumer products to geographically distributed wide area networks with multi-device data storage arrays, such systems continue to increasingly pervade all areas of society and commerce. 
     Software is often provided to direct the operation of such systems. Software (including firmware) can take a number of forms such as application programs, operating systems, interface and controller routines, and maintenance and housekeeping modules. 
     During system initialization, the software is often initially configured to place the system into an operational ready mode, which can include the loading of an operating system from a peripheral device to a host device. During subsequent operation, each initiated software process, such as a host command request, can result in the operation of a number of other routines to carry out various tasks required to complete the initial process. 
     As systems continue to be provided with ever increasing levels of hardware and software complexity, there is a continual need for improvements in the manner in which a device services a command sequence, such during the loading of an operating system from a data storage device to a host device. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention are accordingly directed to an apparatus and method for servicing commands in a selected command sequence. 
     In accordance with some preferred embodiments, the apparatus generally comprises a controller adapted to, upon receipt of the selected command sequence comprising a first command followed by a second command, determine an elapsed time interval therebetween, and to use the elapsed time interval to subsequently service the first and second commands during a subsequent receipt of the selected command sequence. 
     The apparatus further preferably comprises an interface circuit adapted to communicate with a host device, wherein the selected command sequence is issued by the host device and received by the interface circuit. The apparatus further preferably comprises a storage medium and the first and second commands comprise data transfer commands to transfer data between the medium and the host device. 
     Preferably, the selected command sequence comprises commands associated with a loading operation in which operating system software is transferred from the apparatus to the host device. 
     The controller preferably pre-fetches from the medium to a buffer readback data associated with at least a selected one of the first and second commands to expedite servicing of the commands. The controller further preferably initiates one or more reduced power modes in relation to the magnitude of the elapsed time interval. 
     In accordance with further preferred embodiments, the method preferably comprises steps of receiving a selected command sequence comprising a first command followed by a second command, determining an elapsed time interval between the first and second commands, and using the elapsed time interval to subsequently service the first and second commands during a subsequent receipt of the selected command sequence. 
     As before, the selected command sequence preferably comprises commands associated with a loading operation in which operating system software is transferred from a data storage device to a host device. 
     The using step preferably comprises pre-fetching readback data associated with at least a selected one of the first and second commands prior to receipt of the associated command. The using step also further preferably comprises selectively entering one or more reduced power modes in relation to the elapsed time interval. 
     The method further preferably comprises generating a command history table which stores the first command, the second command and the elapsed time interval, and using the command history table to predict a future time at which the second command will be received during the subsequent receipt of the selected command sequence. 
     In this way, the command sequence can be identified and serviced in a reduced amount of time since command requests are anticipated and requested data can be pre-fetched to the buffer for immediate availability. The command-sequence can further be executed with lower power consumption since the device can intelligently enter power saving modes between successive commands. 
     These and various other features and advantages which characterize the claimed invention will become apparent upon reading the following detailed description and upon reviewing the associated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of a data storage device constructed and operated in accordance with preferred embodiments of the present invention. 
         FIG. 2  is a generalized functional block diagram of the device of  FIG. 1  in conjunction with a host. 
         FIG. 3  provides a time sequence associated with an initialization operation for the system of  FIG. 2 . 
         FIG. 4  illustrates a time interval between commands issued by the host. 
         FIG. 5  provides a format for a command history table (CHT) captured by the device for a sequence of commands from the host, such as during the initialization operation of  FIG. 3 . 
         FIG. 6  is a flow chart for a SYSTEM INITIALIZATION routine illustrative of steps carried out in accordance with preferred embodiments of the present invention to use the CHT of  FIG. 5  to reduce power consumption and reduce access time during the servicing of the sequence of commands in the CHT of  FIG. 5 . 
         FIG. 7  is a flow chart for an INTERRUPT subroutine which preferably forms a portion of the routine of  FIG. 6 . 
         FIG. 8  graphically illustrates respective power requirements for different modes of operation for the device  100 . 
         FIG. 9  provides energy consumption curves for each of the different modes of operation of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     To illustrate an exemplary environment in which presently preferred embodiments of the present invention can be advantageously practiced,  FIG. 1  shows an exploded view of a data storage device  100 . The device  100  is preferably characterized as a small form factor disc drive used to store and retrieve user data in a battery-operated, handheld mobile product such as a notebook computer or a digital camera, but such is not limiting to the scope of the claimed subject matter. 
     The device  100  includes a rigid, environmentally controlled housing  101  formed from a base deck  102  and a top cover  104  which are mated together using a plurality of fasteners  106 . A spindle motor  108  is mounted within the housing  101  to rotate a number of data storage media  110  (in this case, two) at a relatively high speed. 
     The media  110  are accessed by a corresponding array of data transducing heads  112 . The heads  112  are supported by an actuator  114  and moved across the media surfaces by a voice coil motor, VCM  116 . A flex circuit assembly  118  facilitates communication between the actuator  114  and control circuitry disposed on a printed circuit board, PCB  120 . The PCB  120  is preferably mounted to an exterior surface of the base deck  102 . 
     As shown in  FIG. 2 , the control circuitry includes an interface circuit  124  which communicates with a host device  126  using a suitable interface protocol (fibre channel, SAS, SCSI, etc.). The interface circuit  124  includes a buffer (cache memory)  128  for the temporary storage of data being transferred to or from the media  110 . A controller  130  provides top level control for the device  100  and is preferably characterized as a programmable, general purpose processor with suitable programming to direct the operation of the device  100 . 
     A read/write channel  132  encodes data to be written to the media  110  during a write operation and reconstructs transduced readback signals from the media  110  to reconstruct previously stored data during a read operation. A preamplifier/driver circuit (preamp)  134  provides head selection circuitry and conditions signals provided to and received from the heads  112 . The preamp  134  is preferably placed in close proximity to the heads  112 , such as on the side of the actuator  114  as shown in  FIG. 1 . 
     A servo circuit  136  provides closed loop positional control for the heads  112 , and preferably includes a digital signal processor (DSP)  138  which operates in accordance with associated programming and in response to control inputs from the top level controller  130 . 
     It is contemplated that the host  126  is characterized as a computer-based system and utilizes a BIOS module  140  (basic input/output system) to control initialization operations, such as power-up or reboot/reset sequences. The BIOS module  140  is preferably provided in non-volatile memory and provides an initial instruction sequence for the host  126 . 
     The BIOS sequence can include a number of steps such as the loading of interrupt handlers and device drivers, the initialization of various registers and power management systems, the initiation of power on self test (POST) routines for various devices attached to the host (including the device  100 ), the detection of which devices are bootable, and the loading of an operating system (OS) during a boot sequence. As will be recognized, upon successful conclusion of the boot sequence the system will be in an operational ready mode, and will carry out tasks in accordance with the loaded operating system and any applications running thereon. 
       FIG. 2  provides a generalized time sequence during the host initialization process with respect to the device  100 . A first elapsed period of time represented in  FIG. 2  at  142  depicts a POST interval, during which the various aforementioned initializations and test routines are carried out. The POST interval  142  is followed by an OS load interval  144  during which components of the host operating system are sequentially requested from the device  100  and loaded into host memory (not shown). 
     The POST interval  142  can be divided into the following sequential stages: a first non-operating stage (NOPS)  146  prior to the device POST, the device POST  148  (during which the host  126  verifies the device  100  to be operational), and a second NOPS interval  150  after the device POST and prior to the OS load interval  144 . For reference, the term “non-operating” is understood to reflect that, generally, no operational requests are being made to the device  100  from the host  126  during these intervals. 
     The lengths of the respective NOPS intervals  146 ,  150  will depend on a number of factors such as the speed of the host processor and the number of devices attached to the host  126 , and can thus comprise several seconds or more. Similarly, the loading of the OS during the OS load interval  144  generally comprises a sequence of read and write commands during which the device  100  is instructed to seek to various locations on the media  100  and read or write data. The OS load interval  144  can thus also nominally require several seconds to complete, depending on the configuration of the system. 
     Accordingly, preferred embodiments of the present invention are generally directed to servicing commands received in a selected command sequence to efficiently transfer data between the storage media  110  and the host device  126 . 
     Preferably, the commands relate to the loading of the OS during the interval  144 , but such is not necessarily limiting to the claimed invention. Readback data associated with the host commands are preferably pre-fetched to the buffer  128 , decreasing the elapsed time required to complete the servicing of the commands. Further, power saving modes are preferably entered as appropriate between selected pairs of the commands to reduce power consumption by the device  100 . 
       FIG. 4  illustrates a time line to show sequential receipt of first and second host commands (denoted at  152 ,  154 ) during operation of the system of  FIG. 2 . A time interval  156  is preferably determined as the elapsed time between the start of the first command  152  and the start of the second command  154 , and so the time interval  156  preferably includes the execution time associated with execution of the first command  152 . 
     Preferably, during an initial OS load sequence the controller  130  ( FIG. 2 ) utilizes a timer  158  to capture the time intervals between each of a sequence of the host commands. The controller  130  preferably uses these time intervals to generate a command history table (CHT)  160  having a format such as depicted generally in  FIG. 5 . Each entry in the CHT  160  includes a command field  162  that identifies the host command and one or more logical block address (LBA) range fields  164  that identify the particular LBAs, or sectors, of data associated with the host command (i.e., those sectors to which data are written or from which data are retrieved during the execution of the command). 
     Each entry in the CHT  160  further preferably includes a sector count field  166  which identifies the number of associated sectors (LBAs), and a time interval (T-INT) field  168  which stores the associated time interval measured between each pair of successive commands (such as the interval  156  in  FIG. 4 ). The particular format and number of entries can be varied as desired, and can include all or a portion of the sequential host commands during a given session. 
     For purposes of the present discussion it will be contemplated that the CHT  160  is configured to hold a total of 100 entries, so that the CHT  160  reflects the first 100 commands issued by the host  126  during the OS load interval  144 . It is further contemplated for the present discussion that these 100 commands do not constitute all of the commands utilized during the OS load interval  144 ; that is, the total number of commands issued by the host during the OS loading process is greater than 100, so that the CHT  160  is an initial subset of the overall process. However, in other preferred embodiments the CHT  160  can be readily configured to list all of the commands in the OS load process, as desired. 
     The CHT  160  is preferably stored in non-volatile memory within the device  100 , such as in one or more reserved sectors on the media  110  which are accessible by the heads  112 , but are not normally utilized to store and retrieve user data. 
       FIG. 6  provides a flow chart for a SYSTEM INITIALIZATION routine  200 , representative of preferred steps carried out by the device  100  to utilize the CHT  160  of  FIG. 5 . The device  100  preferably begins the routine  200  prior to the OS load interval  144 , such as during the second NOPS interval  150  (see  FIG. 3 ). It is further contemplated that the controller  130  initiates and periodically resets the timer  158  during the course of the routine  200 , as will be discussed below. 
     At step  202 , the CHT  160  is first loaded from the reserve sectors into a program area of the buffer  128  or other suitable memory location accessible by the controller  130 . At step  204 , the next sequential entry in the CHT  160  is evaluated. This will comprise the first entry in the CHT  160  during the first time through the routine  200 . Each successive entry will generally be evaluated in turn except as noted below. 
     As next shown by decision steps  206 ,  208  and  210 , the controller  130  preferably determines whether three conditions apply with regard to the selected entry: (1) whether the command associated with the selected entry is a write command, (2) whether the buffer  128  is full, and (3) whether the selected entry is the last entry in the CHT  160 . 
     As will be recognized, the first condition relates to the nature of the command associated with the selected entry; that is, the command will generally comprise a write command to write data to the media  110 , a read command to read data from the media  110 , or some sort of status or other type of command not involving data transfer. The second condition relates to whether the buffer  128  is now full of pre-fetched data (which will be unlikely the first time through the routine  200 ). The third condition relates to whether the selected entry is the last entry in the CHT  160  (again, this will be unlikely the first time through the routine  200 ). 
     Whenever any of these three conditions are satisfied, the routine passes to an interrupt subroutine  212  which will be discussed more fully below with respect to  FIG. 7 . Otherwise, the flow passes to step  214  wherein the device  100  operates to execute the command associated with the selected entry, which can include the retrieval of the associated LBAs from the media  110  and placement of such into the buffer  128  if the selected entry constitutes a read command. 
     Preferably, at this point the routine  200  returns as shown by decision step  216  and continues to evaluate each entry in the CHT  160  in turn until either an actual command is received from the host  126 , or until at least one of the foregoing conditions of steps  206 ,  208  and  210  is satisfied. 
     At this point in the discussion it will be contemplated that a new actual command is in fact received from the host  126 . Accordingly, the routine passes to decision step  218  where an inquiry is made to determine whether the issued command is the same as at least a selected one of the pre-executed commands from the CHT  160 . 
     If so, the device  100  will preferably update the time interval (T-INT) in the CHT  160  to a value of zero (0) seconds, step  220 , and will transfer the associated data from the buffer  128  to the host  126 , step  222 . The device  100  will also preferably update the latest CHT entry in the reserve sector at step  224  and return to evaluate the next entry in the CHT  160  at step  204 . 
     If the new command received at step  218  is not the same as one of the pre-executed commands from the CHT  160 , the flow will continue to decision step  226  where the controller  130  will determine whether the new actual command from the host  126  is a write command. If so, the controller  130  will preferably discard the remaining entries in the CHT  160  and the cached pre-fetched data in the buffer  128 , capture the new command sequence from this point forward and provide an updated CHT  160  to the reserved sectors, as represented by step  228 . The routine then exits (end step  230 ). The new, updated CHT  160  will be used during the next pass through the routine  200  (such as during the next system initialization operation). 
     Returning to step  226 , if the new actual command is a read command, then the remaining entries in the CHT  160  and the cached pre-fetched data may not necessarily need to be discarded. This is because an additional read command would not tend to affect the results of other read commands that have already been executed. 
     Accordingly, in this case the flow passes to step  232  wherein the controller  130  determines whether the selected entry is the last entry in the CHT  160  (in this case, entry no. 100). If so, the routine simply ends at step  230 ; if not, the CHT  160  is updated to reflect this new actual command from the host  126  at step  234 , services this command, and the routine enters a waiting state at step  236  to await the next actual command from the host  126 . In this way, the CHT  160  will continue to be updated until filled, after which the routine will end at step  230 . 
     Returning again to the decision steps  206 ,  208  and  210 , it will be recalled that once at least one of these three conditions are met (i.e., the next entry in the CHT  160  is a write command, the buffer  128  is full of pre-fetched read data, or the last entry in the CHT  160  has been reached), the routine passes to the aforementioned interrupt subroutine  212 . The subroutine  212  includes a number of interrelated, parallel paths, each of which will be covered in detail in turn. 
     As represented in  FIG. 7 , a first preferred step carried out by the subroutine  212  is a temporary halting of further CHT pre-execution activities, as depicted by step  238 . This allows the controller  130  to determine whether it would be appropriate at this time to temporarily enter a power saving mode during which at least some of the operational components of the device  100  are turned off or placed in a less power consuming mode of operation. 
     The particular boundary conditions utilized during the subroutine  212  to guide these decisions are preferably empirically derived as discussed below. At this point it will be understood that such empirical analysis has resulted in the determination of the various exemplary set points shown in  FIG. 7 . 
     The first set point is represented in decision step  240 . During this step, the controller  130  preferably compares the then existing output of the timer  158  to the time interval (T-INT) for the next entry in the CHT  160  to determine a ΔT value. The ΔT value generally represents an estimate of the amount of time before receipt of the next host command. 
     As shown by step  240 , if the ΔT value is greater than or equal to a first value (in this case 0.3 seconds), it will be presumed that the next host command will not likely occur for at least 0.3 s and the controller  130  will elect to enter a selected power saving mode in the interim. 
     The particular mode will further preferably depend upon the actual magnitude of the ΔT value; as shown by decision step  242 , if the ΔT value is greater than or equal to a second, larger value (in this case 8 s), then the controller  130  places the device  100  into a greater power saving mode (e.g., STANDBY mode) as shown by step  244 . Otherwise, the controller  130  places the device  100  into a lesser power saving mode (e.g., IDLE 2  mode), step  246 . 
     The STANDBY and IDLE 2  modes are merely illustrative of different hierarchies of power management that can be used as desired. For reference, IDLE 2  mode preferably includes the parking of the actuator  114  in a parked position and the deactivating of the VCM  116  and the associated servo circuit  136  (see  FIGS. 1 and 2 ). Because read and write operations are suspended, this mode also allows substantial reductions in power applied to the VCM driver circuitry, servo electronics, preamp  134  and the read/write channel  132 . Portions of the interface circuit  124  and certain routines of the controller  130  can also be deactivated as well because the servo and read/write subsystems are not active. 
     STANDBY mode preferably includes all of the power saving steps of IDLE 2 , plus an additional reduction in power which is achieved by halting further operation of the spindle motor  108  and associated spindle control and driver circuitry. 
     Entering a greater power savings mode generally results in greater amounts of power saved, but at a price both in terms of recovery time and recovery power required to restore the device  100  to active mode. While the device  100  remains in one of the power saving modes of steps  244 ,  246 , the controller  130  continues to monitor for receipt of the next actual command from the host  126 , as shown by decision step  248 . 
     Preferably, upon entering the associated power saving mode, the controller  130  resets or otherwise keys the timer  158  to begin a time out period for the power saving mode, as depicted by decision step  250 . This time out period is associated with the expected time until receipt of the next command. In this way, if an actual command from the host  126  is not received in the interim, the controller  130  will continue to maintain the device in the associated power saving mode until the completion of this time out period, as depicted by the loop formed by steps  248 ,  250  and  252 . 
     At such time that the time out period ends (without receipt of an intervening actual host command), the flow passes from step  250  to step  254  wherein the controller  130  brings the device  100  out of the power saving mode and back into the operational ready mode. The appropriate time out period is preferably selected to place the device  100  in the operational ready mode just prior to the next expected command; thus, more reinitialization time will generally be required for a greater power saving mode (e.g., STANDBY) as compared to a lesser power saving mode (e.g., IDLE 2 ). 
     Continuing with a review of  FIG. 7 , when the device  100  is brought back out of one of the power saving modes of steps  244 ,  246 , or when the initial ΔT value is found to be less than the first value (i.e., ΔT is not equal to or greater than 0.3 s), the routine continues to decision step  256  where the controller  130  checks the CHT  160  for the next entry. If all entries in the CHT  160  have been serviced and thus there are no more commands in the table to be serviced, the subroutine  212  (and hence the routine  200  of  FIG. 6 ) ends at step  258 . 
     On the other hand, if entries remain in the CHT  160 , the subroutine waits for the next incoming actual command from the host  126  until such command is received (step  260 ). At decision step  262 , the controller determines whether the actual command from the host  126  is the same as the command associated with the selected entry in the CHT  160 ; if not, then a discrepancy is noted and the subroutine  212  returns back to the flow of  FIG. 6  at step  264  to carry out the aforementioned operations set forth by steps  226  through  236  in  FIG. 6 . 
     When the actual command from the host  126  matches the CHT  160 , as before the time interval (T-INT) is updated to zero seconds at step  266  and the CHT  160  is updated at step  268 . The subroutine then returns to step  240  to once again consider whether it would be appropriate to enter a power saving mode of operation until receipt of the next actual command from the host  126 . 
     It was mentioned previously that at such times that the device  100  enters one of the selected power saving modes, the controller  130  monitors for receipt of an actual command from the host during the associated time out period. It will now be understood that when such occurs, the flow in  FIG. 7  passes from decision step  248  to decision step  270  which determines whether the last entry of the CHT  160  has been serviced and further processing takes place accordingly as previously described. Although not shown in  FIG. 7 , if an actual host command is received during a time out period, it will be understood that the controller  130  will resume normal operation for the device  100 , as required. 
     It is contemplated that the routines of  FIGS. 6 and 7  will tend to reduce power consumption and access time required during the servicing of the host commands associated with the CHT sequence. While the actual characteristics of a given device will depend on the construction thereof,  FIG. 8  generally illustrates various power requirements found for different modes of operation for a particular type of the device  100 . A first bar chart set  300  represents operation of the device  100  during normal, operational ready mode, a second bar chart set  302  represents operation during IDLE 2  mode, and a third bar chart set  304  represents operation during STANDBY mode. Each of these respective sets  300 ,  302  and  304  are shown with respect to a given time interval (T-INT) between receipt of first and second host commands (C 1  and C 2 , respectively). 
     As shown by set  300 , an average steady-state power consumption level of about 1.35 watts was determined for operation of the device  100  in the normal, operational ready mode. 
     As shown by set  302 , placing the device  100  in IDLE 2  mode between the commands C 1 , C 2  resulted in a reduction of steady-state power consumption to a lower level of 0.80 watts for most of the time interval T-INT. A recovery period comprising 0.04 seconds during which a peak power level of 2.30 watts was found to be required to bring the device  100  back into the operational ready mode in time to service the C 2  command. 
     Set  304  depicts operation of the device in STANDBY mode between the commands. In this mode, steady state power consumption was significantly reduced to a yet further lower level of 0.26 watts. However, a peak power level of 3.00 watts was required during a recovery period of 1.37 seconds in order to bring the device  100  back into the operational ready mode. As will be recognized, one factor which led to this significantly longer recovery period (and higher power requirement) was the time required to accelerate the spindle motor  108  from rest and back to operational speed. 
     It follows that the power savings achieved from entering a given power savings mode will be offset by the energy required to bring the device back into the operational ready mode.  FIG. 9  provides a generalized graphical representation of various energy consumption curves obtained for the various modes of  FIG. 8 : curve  310  corresponds to operation of the device  100  in the operational ready mode, curve  312  corresponds to IDLE 2  mode and curve  314  corresponds to STANDBY mode. 
     From the data of  FIGS. 8 and 9 , appropriate set points can be selected to determine when the various power saving modes can be efficiently entered to enhance power savings. It has been found that the energy requirements by the device  100  to complete a given sequence can be reduced by about 60% or more using the foregoing approach (i.e., the routines  200 ,  212  were found to require only 40% as much power as a baseline approach to load the OS to the host  126 ). 
     Such power savings can advantageously result in less heat generation and other losses associated with the operation of the device  100 . More significantly, such power savings can advantageously operate to extend the operational life of the system when batteries or other limited power sources are employed to provide system power. 
     Similarly, the routines  200 ,  212  were also found to provide significant time savings in servicing the host command sequence. Because the routines preferably carry out many of the various data transfer operations prior to receipt of the actual host commands, many of the data request commands can be immediately serviced from the pre-fetched data in the buffer  128 , resulting in significant reductions in access time. It has been found that the elapsed time required to complete the given sequence can be reduced by about 95% or more (i.e., the routines  200 ,  212  required only 5% of the time as compared to the baseline approach to load the OS during the interval  144 ). 
     Such time savings can advantageously reduce the time required to place the system in an operational ready mode, increasing the availability of the system to the user. Moreover, the time savings can promote further power savings at the system level; that is, the system itself can be placed into a low power mode more often on the basis that, when the system is again needed, the recovery time required to reinitialize the system is significantly reduced. 
     Although preferred embodiments presented herein have been generally directed to servicing the host command sequence associated with the OS load interval  144 , it is clear that such merely constitutes a preferred application and is not limiting; rather, any number of different command sequences, including sequences issued during operational ready mode, can be captured and thereafter more effectively executed utilizing the various preferred embodiments presented herein. 
     It will now be appreciated that preferred embodiments of the present invention are generally directed to an apparatus and method for servicing commands such as the type issued by a host device (such as  126 ) to load an operating system from an associated data storage device (such as  100 ). 
     In accordance with some preferred embodiments, the apparatus comprises a controller (such as  130 ) adapted to, upon receipt of a selected command sequence comprising a first command (such as  152 ) followed by a second command (such as  154 ), determine an elapsed time interval (such as  156 ) between said first and second commands and to use the elapsed time interval to subsequently service said first and second commands during a subsequent receipt of the selected command sequence (such as by  200 ). 
     The apparatus further preferably comprises an interface circuit (such as  124 ) adapted to communicate with a host device (such as  126 ), wherein the selected command sequence is issued by the host device and received by the interface circuit. The apparatus further preferably comprises a storage medium (such as  110 ) wherein the first and second commands comprise data transfer commands to transfer data between the medium and the host device. 
     Preferably, the selected command sequence comprises commands associated with a loading operation in which operating system software is transferred from the apparatus to the host device. 
     The controller preferably pre-fetches from the medium to a buffer (such as  128 ) readback data associated with at least a selected one of the first and second commands to expedite servicing of the commands (such as by step  214 ). The controller further preferably initiates a reduced power mode (such as by steps  244 ,  246 ) in relation to the elapsed time interval. 
     In accordance with further preferred embodiments, the method preferably comprises steps of receiving a selected command sequence comprising a first command (such as  152 ) followed by a second command (such as  154 ), determining an elapsed time interval (such as  156 ) between said first and second commands, and using the elapsed time interval to subsequently service said first and second commands during a subsequent receipt of the selected command sequence (such as by  200 ). 
     As before, the selected command sequence preferably comprises commands associated with a loading operation in which operating system software is transferred from a data storage device (such as  100 ) to a host device (such as  126 ). 
     The using step preferably comprises pre-fetching readback data associated with at least a selected one of the first and second commands prior to receipt of the said at least a selected one of the first and second commands, and further preferably comprises selectively entering a reduced power mode in relation to the elapsed time interval (such as by steps  244 ,  246 ). 
     The method further preferably comprises generating a command history table which stores the first command, the second command and the elapsed time interval, and using the command history table to predict a future time at which the second command will be received during the subsequent receipt of the selected command sequence (such as by step  240 ). 
     For purposes of the appended claims, the recited first means will be understood to correspond to the disclosed controller  130  of  FIG. 2  programmed to carry out the routines of  FIGS. 6 and 7 . 
     It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this detailed description is illustrative only, and changes may be made in detail, especially in matters of structure and arrangements of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. For example, the particular elements may vary depending on the particular processing environment without departing from the spirit and scope of the present invention. 
     In addition, although the embodiments described herein are directed to a data storage device used to load an operating system to a host device, it will be appreciated by those skilled in the art that the claimed subject matter is not so limited and various other processing systems can be utilized without departing from the spirit and scope of the claimed invention.