Abstract:
A method and system of managing spatially related defects on a data storage media surface in a data storage device includes operations of identifying defect locations on the media surface, determining whether the location of an identified defect is within a predetermined window of another identified defect location on the media surface, if the location is within the predetermined window, characterizing the defects in the window as a scratch. A scratch-tracking table is then generated having a unique entry for each scratch and a start index and an end index for each scratch. Also, a scratch index table is generated that lists each and every defect location on the media along with its defect index and the scratch index associating the particular defect with an identified scratch. These two tables are then utilized to pad the scratches. A variant of the method includes iteratively processing through caches in the event that limited buffer memory is available to the device controller or large numbers of defect locations are identified during certification testing.

Description:
FIELD OF THE INVENTION  
       [0001]     This application relates generally to data storage devices and more particularly to a method and system for efficient management of defects on a data storage medium in a data storage device such as a disc drive.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the field of storage medium defect management, various methods have been utilized to handle defects. Some of these defects may be isolated occurrences on the media. Others may be characterized as scratches. A scratch, as used in this application, is a line of defects on the storage media where data cannot be properly stored and recovered. They are usually caused by some process during manufacture, or handling, and may be continuous or may have breaks in-between. Process and/or reliability problems may be encountered when such scratches grow, i.e. are extended, during normal drive operation. One method utilized for handling potentially large defects such as scratches in the recording medium surface, is called “scratch fill.” One scratch fill method is described in detail in co-pending application Ser. No. 10/003,459, filed Oct. 31, 2001.  
         [0003]     Scratch fill algorithms basically look at the defects identified on the media and fill in gaps between closely spaced defects as these typically are indicative of continuous scratches in the media surface. This process is one method that attempts to anticipate where defects that are passed over during generation of the defect list are likely to occur and essentially fill in the gaps, as well as pad the identified defects. During drive operation, a substantial amount of processing time is utilized in processing data through the defect management algorithms. In addition, there is a potential for the defect list to become full during the scratch fill process as well as failing due to improperly fill due to limitations in the algorithms. In short, such problems may cause the microprocessor to simply run out of memory during the scratch fill process.  
         [0004]     Accordingly there is a need for a robust and efficient method of handling and processing scratches, and handling data that includes fast processing and accessing of defect lists so that minimal processing time is needed for such checks. The present invention provides a solution to this and other problems, and offers other advantages over the prior art.  
       SUMMARY OF THE INVENTION  
       [0005]     Against this backdrop the present invention has been developed. An embodiment of the present invention to reduce the processing time is to load and utilize part of the Primary Defect List (PDL) into fast cache memory or Static Random Access Memory (SRAM). Another scheme may use the Synchronous Dynamic Random Access Memory (SDRAM). In both cases, defect tracking tables are utilized to track the scratches and the buffer memory is used to complement that used by the microcontroller. This results in reduced processing time and elimination of the problem of overloading the available memory.  
         [0006]     A method of managing spatially related defects on a data storage media surface in a data storage device in accordance with an embodiment of the present invention includes operations of identifying defect locations on the media surface, determining whether the location of an identified defect is within a predetermined window of another identified defect location on the media surface, if the location is within the predetermined window, characterizing the defects in the window as a scratch. A scratch-tracking table is then generated having a start index and an end index for each scratch. Also, a scratch index table is generated that lists each and every defect location along with its defect index and the scratch index associating the particular defect with an identified scratch. These two tables are then utilized to pad the scratches as well as being utilized in a buffer during drive operation to facilitate efficient defect location identification when queried by the controller of the data storage device. Another embodiment of the present invention utilizes one or more caches to iteratively develop and process the scratch tracking table and scratch index tables as well as develop the padding of the defects in the event that limited memory is available for use.  
         [0007]     These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a plan view of a disc drive incorporating a preferred embodiment of the present invention showing the primary internal components.  
         [0009]      FIG. 2  is a schematic block diagram of a disc drive control system utilized in control of the disc drive shown in  FIG. 1 .  
         [0010]      FIG. 3  is a basic overall process flow diagram of the method of handling scratches in accordance with a preferred embodiment of the present invention.  
         [0011]      FIG. 4  is an illustration of an exemplary portion of a scratch tracking table in accordance with a preferred embodiment of the present invention.  
         [0012]      FIG. 5  is an illustration of an exemplary portion of a primary defect list scratch index table associated with the scratch-tracking table shown in  FIG. 4 .  
         [0013]      FIG. 6  is a process flow diagram of a routine that generates the tables shown in  FIGS. 4 and 5 .  
         [0014]      FIG. 7  is a further exemplary illustration of a scratch tracking table (STT) in accordance with a preferred embodiment of the present invention.  
         [0015]      FIG. 8  is a further exemplary illustration of a primary defect list scratch index (PSI) table associated with the scratch tracking table shown in  FIG. 7 .  
         [0016]      FIG. 9  is a process flow diagram of the routine that generates the scratch padding in accordance with the present invention.  
         [0017]      FIG. 10  illustrates portions the Scratch Tracking Table and P-List Scratch Index table with associated padding of exemplary Scratch  9 .  
         [0018]      FIG. 11  is a process flow diagram of a routine that generates the STT and PSI tables in accordance with the present invention in which a cache is utilized for both the P-List and the PSI table.  
         [0019]      FIG. 12  is a process flow diagram of a routine that generates the STT and PSI tables in accordance with an embodiment of the present invention in which a cache is utilized for the STT.  
         [0020]      FIG. 13  is a process flow diagram of a routine that pads the defect scratches identified with caching for STT.  
         [0021]      FIG. 14  is a process flow diagram of a routine that is utilized when the PSI table is larger than the size of a buffer space allocated for the PSI table 
     
    
     DETAILED DESCRIPTION  
       [0022]     A disc drive  100  that incorporates a preferred embodiment of the present invention is shown in  FIG. 1 . The disc drive  100  includes abase  102  to which various components of the disc drive  100  are mounted. A top cover  104 , shown partially cut away, cooperates with the base  102  to form an internal, sealed environment for the disc drive in a conventional manner. The components include a spindle motor  106  that rotates one or more discs  108  at a constant high speed. Information is written to and read from tracks on the discs  108  through the use of an actuator assembly  110 , which rotates during a seek operation about a bearing shaft assembly  112  positioned adjacent the discs  108 . The actuator assembly  110  includes a plurality of actuator arms  114  which extend towards the discs  108 , with one or more flexures  116  extending from each of the actuator arms  114 . Mounted at the distal end of each of the flexures  116  is a head  118 , which includes a fluid bearing slider, enabling the head  118  to fly in close proximity above the corresponding surface of the associated disc  108 .  
         [0023]     During a seek operation, the track position of the heads  118  is controlled through the use of a voice coil motor (VCM)  124 , which typically includes a coil  126  attached to the actuator assembly  110 , as well as one or more permanent magnets  128  which establish a magnetic field in which the coil  126  is immersed. The controlled application of current to the coil  126  causes magnetic interaction between the permanent magnets  128  and the coil  126  so that the coil  126  moves in accordance with the well-known Lorentz relationship. As the coil  126  moves, the actuator assembly  110  pivots about the bearing shaft assembly  112 , and the heads  118  are caused to move across the surfaces of the discs  108 .  
         [0024]     The spindle motor  106  is typically de-energized when the disc drive  100  is not in use for extended periods of time. The heads  118  are moved over park zones  120  near the inner diameter of the discs  108  when the drive motor is de-energized. The heads  118  are secured over the park zones  120  through the use of an actuator latch arrangement, which prevents inadvertent rotation of the actuator assembly  110  when the heads are parked.  
         [0025]     A flex assembly  130  provides the requisite electrical connection paths for the actuator assembly  110  while allowing pivotal movement of the actuator assembly  110  during operation. The flex assembly includes a printed circuit board  132  to which head wires (not shown) are connected; the head wires being routed along the actuator arms  114  and the flexures  116  to the heads  118 . The printed circuit board  132  typically includes circuitry for controlling the write currents applied to the heads  118  during a write operation and a preamplifier for amplifying read signals generated by the heads  118  during a read operation; The flex assembly terminates at a flex bracket  134  for communication through the base deck  102  to a disc drive printed circuit board (not shown) mounted to the bottom side of the disc drive  100 .  
         [0026]     Referring now to  FIG. 2 , shown therein is a basic functional block diagram of the disc drive  100  of  FIG. 1 , generally showing the main functional circuits which are resident on the disc drive printed circuit board and used to control the operation of the disc drive  100 . The disc drive  100  is operably connected to a host computer  140  in a conventional manner. Control communication paths are provided between the host computer  140  and a disc drive microprocessor  142 , the microprocessor  142  generally providing top level communication and control for the disc drive  100  in conjunction with programming for the microprocessor  142  stored in microprocessor memory (MEM)  143 . The MEM  143  can include random access memory (RAM), read only memory (ROM) and other sources of resident memory for the microprocessor  142 .  
         [0027]     The discs  108  are rotated at a constant high speed by a spindle motor control circuit  148 , which typically electrically commutates the spindle motor  106  ( FIG. 1 ) through the use of back electromotive force (BEMF) sensing. During a seek operation, wherein the actuator  110  moves the heads  118  between tracks, the position of the heads  118  is controlled through the application of current to the coil  126  of the voice coil motor  124 . A servo control circuit  150  provides such control. During a seek operation the microprocessor  142  receives information regarding the velocity of the head  118 , and uses that information in conjunction with a velocity profile stored in memory  143  to communicate with the servo control circuit  150 , which will apply a controlled amount of current to the voice coil motor coil  126 , thereby causing the actuator assembly  110  to be pivoted.  
         [0028]     Data is transferred between the host computer  140  or other device and the disc drive  100  by way of an interface  144 , which typically includes a buffer to facilitate high-speed data transfer between the host computer  140  or other device and the disc drive  100 . Data to be written to the disc drive  100  is thus passed from the host computer  140  to the interface  144  and then to a read/write channel  146 , which encodes and serializes the data and provides the requisite write current signals to the heads  118 . To retrieve data that has been previously stored in the disc drive  100 , read signals are generated by the heads  118  and provided to the read/write channel  146 , which performs decoding and error detection and correction operations and outputs the retrieved data to the interface  144  for subsequent transfer to the host computer  140  or other device.  
         [0029]     Throughout this specification a number of abbreviations are used that require short definitions. They are as follows:  
         [0030]     SRAM: Static Dynamic Random Access Memory  
         [0031]     SDRAM: Synchronous Dynamic Random Access Memory  
         [0032]     DRAM: Dynamic Random Access Memory.  
         [0033]     TCM: Tightly Coupled Memory.  
         [0034]     P-List: Primary Defect List (PDL). This is a list of all data defects.  
         [0035]     P-List Cache Table: This table is a cache to hold the P-List entries from the SDRAM during data processing.  
         [0036]     PSFT: Primary Servo Flaw Table. This is a table tracking location of all servo defects.  
         [0037]     TA List: Thermal Asperities List. This list contains all identified thermal asperities.  
         [0038]     STT: Scratch-Tracking Table. This table contains one entry for each scratch identified. The STT stores the index of the entries in the P-List and other information.  
         [0039]     PSI: P-List Scratch Index. The PSI is a table having an entry for every P-List entry and each of the 2-byte entry record for the STT index that the P-List entry has been associated with. In other words, the PSI stores the STT index that the corresponding P-List entries belong to.  
         [0040]     BFI: Bytes From Index. This is the distance on a track from the index mark to the defect location.  
         [0041]     Len: Length of the defect.  
         [0042]     In a disc drive data storage device, any defects on the magnetic media fall into one of three categories: data defects, servo defects, and thermal asperities. All identified data defects are kept in the P-List. All servo defects are kept in the PSFT. All thermal asperities identified are kept in the TA list. Both the P-List and the PSFT undergo scratch fill processing. In addition, the defects in the PSFT and TA list are folded in to the P-List at the end of the certification testing prior to release of the drive from production.  
         [0043]     A scratch is typically recognized and identified as such if two defects are detected within a predetermined radial and circumferential window. As an example, a typical window may be 500 bytes circumferentially and 130 cylinders radially. Thus, if two defects are identified in this area they will be characterized as a scratch.  
         [0044]     A basic two-step scratch fill process  200  in accordance with an embodiment of the present invention is shown in  FIG. 3 . Scratch fill begins in operation  202  where the entries in the P-list are classified into different scratches. Each entry in the P-List is evaluated to determine whether it falls within the scratch definition window such as mentioned above. Once the entire P-List has been analyzed, control transfers to operation  204 , where padding of each of the scratches takes place. The padding operation  204  basically adds bytes called “pad defect entries” at either end of the scratch and fills in the middle portion of the identified scratch. Control then transfers to end operation  206 .  
         [0045]     Next, a Scratch Tracking Table  210 , two entries of which are shown in  FIG. 4 , is generated and updated for each entry in the P-list utilizing the process operations  220  shown in  FIG. 6 . In parallel, a P-list Status Index (PSI) table  216  is generated. A portion of the PSI table  216  is illustrated in  FIG. 5 . The PSI table  216  associates each P-list entry  212  with the STT  210  and requires 2 bytes for each PSI entry  214 . Thus there is one PSI entry  214  for every P-list entry  212  and each is a 2-byte entry record. The PSI table  216  is maintained in DRAM, and includes 1024 entries in a cache.  
         [0046]     The scratch tracking table (STT)  210  has one entry per scratch. Each entry lists a number of properties of the identified scratch: Start index  213  (index number associated with the P-list entry  212 ), end index (from the P-list), skew, thickness, end point, and other properties not pertinent to this discussion. Two entries in the STT are shown in  FIG. 4 . Shown are two scratch entries  8  and  9 .  
         [0047]      FIG. 5  shows an exemplary portion of a PSI table  216 . Note that in this figure, each of the scratches with PSI of  0  through  7  corresponds to single defects and therefore the PSI table entry index number  213  (left column) and the PSI value  214  (center column) are the same. This circumstance is purely coincidental in this simplified example. Since these defects do not form a scratch with any priorentries, they are assigned to different scratch numbers. Referring to  FIG. 5 , Scratch No.  8 , begins at cylinder  1289 , head  0  and BFI of  352 , 481  and ends at cylinder  1290 , head  0 , and BFI of  352 , 425 . Similarly, Scratch Number  9  begins at cylinder  2362 , head  0 , and BFI of  242 , 256 . Scratch Number  9  ends at cylinder  2365 , head  0  and BFI of  242 , 270 .  
         [0048]     The operational flow diagram of the process  202  of characterizing the scratches on the disc is shown in  FIG. 6 . Process  202  begins in start operation  222  upon the completion of generation of the P-list. Control then transfers to operation  224 .  
         [0049]     Operation  224  loads a first entry from the P-List. Control then transfers to query operation  226  that asks whether the loaded P-list entry fits the scratch size window of any of the last P-List entry of the existing STT scratch entries, and thus can be classified in the current STT entries. In other words, this query operation examines whether the loaded P-List entry fits the criteria defining the scratch window. As mentioned above, a typical predetermined radial and circumferential window may be 500 bytes circumferentially and 130 cylinders radially. Thus, if the current defect is identified as falling into such an area encompassing the last defect of any STT entry, both entries will form a scratch or part of a scratch. If the answer is no, control transfers to operation  228 . In operation  228 , a new STT entry is created for the loaded P-list entry, as the defect is not part of an identified scratch at this point. Control then transfers to operation  230 .  
         [0050]     If the answer in query operation  226  is yes, control transfers to operation  232 . Here the relevant STT entry is updated to the loaded P-list entry value. This, in essence, identifies the defect as part of the scratch identified in the relevant STT entry. Control then transfers to operation  230 .  
         [0051]     Control operation  230  updates the PSI entry  214  (i.e. the scratch number) of the P-List entry  212  in the PSI table  216  and then transfers to query operation  234 . Operation  234  checks whether the operation has reached the end of the P-List and, if the answer is yes, control transfers to end operation  236  and process control returns to the host. If on the other hand, the answer is no, there are more P-List entries, then control transfers back to operation  224  where a next P-List entry is loaded, and operations  224  through  234  are repeated until the last P-list entry is processed. In this manner, the STT  210  and PSI table  216  are both generated.  
         [0052]      FIGS. 7 and 8  illustrate this process  202  in action in more detail.  FIG. 7  shows four scratches, Nos.  0 ,  8 ,  9  and  10  as illustrative examples. Since, during the first time through the routine, in operation  224 , there are no entries in the STT  210 , the process takes the first entry through query operation  226 , in which the answer is no, and goes to operation  228 , in which a new entry is created that starts with index  0 . The start and end index will be  0  at this point. The defect location information,  347 / 1 / 177 , 144  are thus inserted in the STT  210  for Scratch No.  0  as the end point in operation  228 . The PSI value of  0  is entered in the PSI table  216  in operation  230 . The identical steps described above, i.e. operations  224 ,  226 ,  228 , and  230  are repeated for the next 7 entries since they do not form scratches with any other points in the P-list, and thus new STT entries are generated, rather than prior entries updated.  
         [0053]     Entry indices  8  and  9  of the P-List, however, actually do form a scratch. First, the corresponding information for index  8 , as in index  0 , is copied to entry  8  of the STT. The end point of Scratch  8  initially will be  1 , 289 / 0 / 352 , 381  in operation  228 . Then, when index  9  of the P-List is processed through operation  224  to operation  226 , i.e., the P-List entry for index  9  is checked against the endpoint of the previous STT  210  entries, it meets the criteria to be a part of Scratch  8 . Thus the answer to the query in operation  226  is yes. Control then transfers to operation  232 . The STT  210  entry for Scratch No.  8  is updated to end at index  9 , and the ending point is updated to  1 , 290 / 0 / 352 , 425 . Thus the STT  210  and PSI table  216  are updated with the relevant information with scratch  8  ending at P-list entry  9 , as is also shown in  FIG. 4 .  
         [0054]     Now, note that in the PSI table  216  in  FIG. 8 , P-list entries  10 ,  11 , and  13  are all very closely related on head  0 . They are all thus within a window and are part of a scratch number  9 . Indices  10  and  11  are processed the same way as  8  and  9  discussed above. Their information is stored under Scratch No.  9  in STT  210 .  
         [0055]     In particular, the sequence is as follows. P-List entry  10  is loaded in operation  224 . Control transfers to query operation  226 , where the entry is compared to the previous P-List entries to see if it fits within the window for a scratch. As it does not, control transfers to operation  228 , where Scratch No.  9  entry is made in the STT  210 , with start and end values of  10 , and end point of  2 , 362 / 0 / 242 , 256 . Control transfers to operation  230 , where the PSI for entry  10  is updated to reflect Scratch No.  9 . Control then returns to operations  224  and  226  for P-List entry  11 . As the P-List entry  11  is within the window, control transfers to operation  232  where the end value is updated to P-List entry  11  and the end point is updated to  2 , 365 / 0 / 242 , 270 . Control then transfers to operation  230 , where the PSI for entry  11  is set at  9 .  
         [0056]     Control then passes to operation  234 , thence back to operation  224 , where P-List index No.  12  defect is loaded. Control then transfers to operation  226 , where the P-List entry is compared again to the prior entries. This entry is not within the window, so control transfers to operation  228 , where a new entry  10  is assigned in the STT  210 . The start value and end value are set at the P-List entry index of  12 , and the end point is set at  2 , 366 / 1 / 555 , 047 .  
         [0057]     Control then passes through query operation  234  again to operation  224  where P-List entry  13  is loaded. In operation  226 , this entry is compared to the prior P-List entries and found to be within the window of Scratch No.  9 . Thus control transfers to operation  232 . Here, the scratch start value remains the same, but the end value is now updated to  13 . The end point is also updated to  2 , 368 / 0 / 242 , 298 . Control then passes to operation  234 , and, for this example, assuming there are no more entries in the P-List, transfers to end operation  236 , which essentially passes control back to operation  204  in the process  200  shown in  FIG. 3 .  
         [0058]     The above process illustrates that, as each P-List entry is evaluated, the STT  210  is appended to or updated until all P-List entries have been tested against the window criteria for a scratch. This completes the first phase of the process in accordance with the present invention, involving characterization of the defects in the P-List  212 .  
         [0059]     Operation sequence  204 , of padding all the identified scratches in the STT  210 , will now be described with reference to  FIGS. 9 and 10 . Padding of the identified scratches is performed in sequence, starting at the top and working down to the end of the STT  210  via the operational sequence  204  shown in  FIG. 9 . Sequence or routine  204  begins in start operation  240  which initializes counters and registers. Control then passes to operation  242  where the first scratch, Scratch No.  0 , in STT  210  is loaded. The Control then transfers to operation  244 .  
         [0060]     Recall from  FIG. 7 , that Scratch No.  0  includes only one defect. This is merely coincidental, as discussed above. In operation  244 , the PSI table  216  is searched to identify another P-List entry  212  associated with Scratch No.  0 . Since there is only one, control then transfers to operation  246 , where the top of the one defect scratch is padded. The length of the defect is compared against a length parameter set by the user. If the defect length exceeds the value set, a pad of similar length will be added one cylinder above and below the defect. If the defect length equals or is less than the value set, a pad defect entry of the value set by the user is added above and below the defect. Thus the total “tail” size, i.e., the pad at either end of the scratch, in the radial direction, is determined by the user. Control then transfers to operation  248  where a pad is established between the 2 P-List entries. However, in Scratch  0  there is no second P-List entry, therefore control simply passes to query operation  250 , which asks whether the end of the scratch has been reached. In the case of Scratch  0 , the answer is yes, and control transfers to operation  252 , where the bottom of the single defect scratch of Scratch  0  is padded in a manner as above described in operation  246 . Control then transfers to query operation  256 . Query operation  256  asks whether the end of the STT has been reached. In this case, the answer is no, and control transfers back to operation  242 , where the next entry from the STT is loaded. The sequence of operations  242 ,  244 ,  246 ,  248 ,  250 ,  252 , and  256  are then repeated, in the example described and shown in  FIGS. 7 and 8 , for Scratches  1  through  7 .  
         [0061]     Then, for Scratch No.  8 , the STT entry is loaded in operation  242 . Control passes to operation  244  where the PSI is searched for the next P-List entry associated with Scratch No.  8  and loaded. Control then passes to operation  246 , where the top of Scratch No.  8  is padded. Again, the length of the defect is compared against a length parameter set by the user. If the defect length exceeds the value set, a pad of similar length will be added one cylinder above and below the defect. If the defect length equals or is less than the value set, a pad defect entry of the value set by the user is added above and below the defect. Thus the total “tail” size, i.e., the pad at either end of the scratch, in the radial direction, is determined by the user. Control then transfers to operation  248  where a pad is established between the 2 P-List entries. Control then transfers to query operation  250  which asks whether the end of the scratch has been reached. In this case, the answer is yes, so control passes to operation  252  where the bottom of the scratch is padded as previously described. Control then passes to query operation  256  which asks whether the end of the STT has been reached. The answer is no, so control passes back to operation  242  and the next entry, Scratch No.  9 , is loaded.  
         [0062]     The column on the right side of  FIG. 10  indicates the sequence of pad addition made to the Scratch No.  9 , between defects  2 , 362 / 0 / 242 , 256  and  2 , 365 / 0 / 242 , 270 . The example illustrated in  FIG. 10  is four cylinders in the radial direction for illustrative purposes. This could be as much as 20 or 30 cylinders.  
         [0063]     Control passes to operation  244  where the PSI is searched for the next P-List entry associated with Scratch No.  9  and this second entry is loaded. Control then passes to operation  246 , where the top of Scratch No.  9  is padded. As shown in  FIG. 10 , the padding is four cylinders above. Control then transfers to operation  248  where a pad is established between the 2 P-List entries. In this case, two pad entries are made. Control then transfers to query operation  250  which asks whether the end of the scratch has been reached. In the example shown in  FIG. 10 , the answer is yes, so control passes to operation  252  where the bottom of the scratch is padded as previously described.  
         [0064]     However, if the example of Scratch No.  9  as shown in  FIGS. 7 and 8  were encountered, where there is another PSI entry associated with Scratch No.  9 , the answer to query operation  250  would have been no. In that case, control would transfer to operation  254 . In operation  254 , the earlier of the pair of entries previously loaded in operation  244  is discarded and replaced with the next P-List entry for the scratch. In the case of  FIGS. 7 and 8 , that would be index  13 , ( 2368 / 0 / 242 , 298 ). Control then passes to operation  248  where padding is done between the two loaded entries. For scratches that have a larger number of defects identified, this sequence, of operations  248 ,  250 ,  254  are repeated until the answer in query operation  250  is yes. Control then transfers to operation  252 , where the bottom of the scratch is padded as described above. Control then passes to query operation  256 , which asks whether the end of the STT has been reached. If the answer is now yes, control transfers to return operation  258 , where control returns to the calling program.  
         [0065]     In summary, in the example shown in  FIG. 10 , first, four pad defect entries are added above the scratch defect, i.e., one on each of the four cylinders immediately prior to the start cylinder of the defect (physically, on one side of the defect). Second, a pad defect entry is added to the two cylinders between the start and end cylinders. Third, four pad defect entries are added following the end of the scratch, i.e. one on each of the four cylinders immediately after the end cylinder of the defect (i.e., physically on the other side of the end defect). The BFI number is just the quantum space to be added or subtracted to each preceding or subsequent pad entry. It can be obtained by the difference of the two defect points divided by the number of cylinders in between. In the illustrated example, the difference is 14 and the number of cylinders in between is 3. Similarly, there is padding done in the circumferential direction that is not illustrated in the example shown in  FIG. 10 .  
         [0066]     An alternative method in accordance with an embodiment of the present invention is utilized when dealing with a drive configuration that includes limited Tightly Controlled Memory (TCM). In this case, caching schemes are incorporated into the method  200  of characterizing ( 202 ) and padding ( 204 ) scratches. This method is shown in  FIGS. 11 through 14 .  
         [0067]     TCM currently incorporates only 1024 entries of PSI (2 bytes each entry), 1024 entries of P-List (14 bytes each entry) and 256 entries of STT (48 bytes each). Consequently, a caching scheme must be employed, in which scratch characterization is done in blocks of 1024 entries and padding is done using the PSI, with only selected entries being retrieved. This is facilitated because each P-List entry belongs to only one scratch. One of the advantages of using the PSI, is that it facilitates quick update and retrieval since it only utilizes 2 bytes per entry.  
         [0068]     In the discussion that follows, it may be helpful to note that  FIGS. 11 and 12  demonstrate the characterization algorithm  202  and  FIGS. 13 and 14  demonstrate the overall padding algorithm  204  variations involving the use of caching.  
         [0069]     Turning now to  FIG. 11 , the simplest use of a cache in an embodiment of the present invention is shown. This is the situation when there are less than 1024 entries in the P-List  212  and less than 256 entries in the STT  210 . This involves P-List caching and is shown in routine  300  in  FIG. 11 . If the PSI table exceeds 1024 entries, PSI caching is performed as shown in  FIG. 14 . If the STT  210  exceeds 256 entries, STT caching is performed as shown in  FIG. 12 . If necessary, STT in the padding operations may also be cached, as shown in  FIG. 13 .  
         [0070]     The method  300 , involving P-List caching, is built on top of the characterization method as in the first embodiment described above with reference to  FIG. 6 . Operation  304  is in effect operation  202  with the entries loaded from the P-list cache instead of direct from the P-List  212  and the PSI entries updated to the PSI cache instead of direct to the PSI table  216 . Operation  304  will also support caching of the STT that will be described later but not important in the description of the operations where the PSI and P-List are cached.  
         [0071]     With this modification, the operations  302 ,  306 ,  308 ,  310  and  312  are just involved in loading the P-List entries  212  from the P-List to the cache and updating PSI entries  214  from the cache to the PSI Table  216 . The operation  300  begins in operation  302  where the 1024 P-List entries are loaded to the cache. The P-List cache is then transferred to operation  304  which has been described in detail by operation  202  with the exception that the P-List entries are obtained from the cache and PSI entries are updated to the cache.  
         [0072]     When operation  304  has completed for all the cached P-List entries, control is transferred to operation  306 , that will determine if the end of the P-List in the DRAM has been reached. If the answer is no, control will transfer to operation  308  where the updated PSI entries are transferred to the PSI table  216  in DRAM before returning to operation  302  to load the P-List cache with the next 1024 entries from the DRAM. If the answer to the question in operation  306  is yes, control will be transferred to operation  310 .  
         [0073]     Operation  310  will determine if there are any PSI entries updated to the DRAM. If the answer is no, the PSI information will be used directly from the cache and no other operations is necessary and the control is returned to the calling function via operation  314 . If the answer is yes, the PSI entries in the cache is updated to the PSI table  216  in DRAM before control is returned to the call function via operation  314 . A different situation exists when the STT  210  exceeds 256 entries. In this case, all entries in the STT cache are saved to the DRAM and only active entries are kept in the TCM. In this case the STT cache is loaded from the DRAM starting with the first active entry. An active entry is defined as one with the cylinder of the last entry within the radial window of the current entry. This is because the P-List is arranged in an ascending order of cylinder, head and BFI. So, if the last entry of an STT is outside this window, it would never be active again. Thus each cached set of the full STT overlaps, but eliminates any need to include the inactive entries.  
         [0074]     Briefly, the P-List entry is checked against the cache STT  210  as in operation  226 , described above. If an update is possible, the relevant scratch is updated. If an update is not possible, the query is made whether or not it is the end of the STT  210 . If not, load the new entry and repeat. If it is the end of the STT  210 , a new scratch will be created in the STT and the cache information is updated. The entire STT in DRAM is then updated and the number of active STT entries is counted. If more than 256 entries are counted, the first active STT index is recorded. Otherwise, only active entries will be loaded into the cache.  
         [0075]     Referring now to  FIG. 12 , characterization (as in operation  304  in routine  300 ) with STT caching is more fully explained. The routine  400  begins in operation  402  where the query is made whether the active STT  210  has more than 256 active entries. If the answer to query operation  402  is no, process control continues as in routine  300 . In other words, control bypasses operation  404  and transfers to operation  406 , where the P-List entry is checked against the cached STT  210  to determine whether the defect entry is within the predetermined window of any entry in the STT  210 . If the answer to query operation  402  is yes, the active STT is greater than 256 entries, process control proceeds to operation  404  where the cache is loaded with those STT  210  active entries from DRAM, starting with the first active entry, and then control transfers to operation  406 . Referring back to  FIG. 8 , for example, the radial window is 130 cylinders. The last current entry is  13  ( 2 , 368 / 0 / 242 , 298 ). Scratches  0  to  8  are inactive. Only STT Scratches  9  and  10  are active, thus only Scratches  9  and  10  would be loaded into the cache. Then control transfers to operation  408 .  
         [0076]     Query operation  408  asks whether an entry in the cache can possibly be updated. If the answer is no, no update is possible, control transfers to query operation  410 . If an update is possible, control transfers to operation  414 .  
         [0077]     Query operation  410  asks whether the end of the active STT  210  has been reached. If the answer is yes, then control transfers to operation  412  where a new STT entry is generated, since an existing scratch can&#39;t be updated. If the answer in query operation  410  is no, there is more to the active STT  210 , then control transfers back to operation  404 , where the next set of the STT  210  entries is loaded into the cache. Then operations  406  and  408  and  410  are repeated until the end of the active STT  210  is reached, where control is passed to operation  412  then to operation  414  or if the answer to query  408  is yes, where control is passed straight to operation  414 .  
         [0078]     In the former instance, the operation is a “pass through” since the STT  210  was just updated by virtue of adding a new entry. Control then transfers to operation  416   
         [0079]     Query operation  416  again asks whether there are more than 256 active STT entries. This query is necessary to determine if the new STT entry must be updated to the cache and then DRAM or if only an update to the cache is necessary. If the answer in query operation  416  is yes, then control transfers to operation  418 , where the STT in DRAM is updated and the active STT count is updated. If the answer in query operation  416  is no, then control transfers to query operation  420 .  
         [0080]     Query operation  420  asks whether the STT cache is out of space. If not, control transfers to end operation  428 , which transfers control back to the calling program. If the answer in query operation is yes, the STT cache is out of space, control transfers to operation  418 . Again, in operation  418 , the STT in the DRAM is updated and the active STT  210  count is updated. Note that the active STT may have shrunk if the end points of active STT entries passed beyond the radial window of the current P-List entry so that they are unable to form a scratch or part of a scratch with any subsequent P-List entries. Control then transfers to query operation  422 .  
         [0081]     Query operation  422  again asks whether the active STT is greater than 256. If so, control transfers to operation  426 . If the answer in query opration  422  is no, control transfers to operation  424 .  
         [0082]     Operation  424  loads the active entries to the cache and control transfers to end operation  428 , where control returns operation  304  for completion of the characterization algorithm, update of the PSI (operation  306 ) until the end of the P-List  212  is reached in operation  308 . Operation  426 , on the other hand, records the first active STT  210  entry index and then returns to the calling program  300  in operation  428 , specifically operations  304 - 310 .  
         [0083]     The padding portion  500  of the method in accordance with the alternative embodiments of the invention involving caching are best understood while referring to  FIGS. 13 and 14 .  
         [0084]      FIG. 13  shows the process  500  where caching has been utilized, such as where the STT  210  has greater than 256 entries. Here, operation begins in operation  502 , where the cache is loaded from the STT  210 . Control then transfers to padding algorithm operation  504  which implements the operations described previously, in operations  240  through  250  with reference to  FIGS. 9 and 10  in which, for example, pads are added above, in between, and below the identified scratch. After each scratch in the cache is padded through this series of operations, control transfers to query operation  506 , which asks whether the end of the DRAM STT has been reached. If not, control transfers to operation  502  where the next portion of the STT in DRAM is loaded and the padding process in operation  504  is repeated. Finally, when the end of the DRAM STT is reached, control passes to end operation  508  in which overall process control returns to the calling program.  
         [0085]     The process  244  may be slightly different if the PSI table  216  is too large for the TCM, i.e., there are more than 1024 P-List entries. In this case, each time a request to search or update the PSI table  216  is made, such as the operation  244  where the PSI table  216  is searched, a routine  510  as is shown in  FIG. 14  must be implemented.  
         [0086]     Routine  510  begins in operation  512  in which the query is made whether the PSI table  216  is greater than the available cache size, and thus cannot be loaded all at once. If the PSI table  216  is less than the cache size, the PSI table  216  is already in the cache and process continues as above described. If the PSI table is to large, control passes to operation  514 . In operation  514 , the PSI DRAM address is set to the STT start index. Control passes to operation  516  where the first 1024 entries of the PSI table are transferred into the cache. Control then transfers to operation  518  where the cache is searched for entries associated with the scratch. Control then transfers to query operation  520 . Query operation asks whether there are entries associated with a scratch found. If so, control transfers to return operation  524 , in which control returns to the place in the routine asking for the PSI table  216 , such as the operation  244  carried out in the padding routine operation  504  in routine  500 . If, on the other hand, no matching entries were found in operation  518 , control passes to operation  522 . The DRAM addresses are incremented in operation  522  and the next 1024 entries in the DRAM PSI table are loaded into the cache and control returns through operation  516  to search operation  518 . This process repeats until the required P-List entry is found.  
         [0087]     Thus, for a disc drive  100  utilizing a limited buffer size, such as TCM, if the size of the P-List, the STT, and the PSI table are each too large to be immediately accommodated, e.g., several thousand, the process  200  may well involve use of each of the routines  300 ,  400 ,  500 , and  510  described with reference to  FIGS. 11 through 14  iteratively, in order to process all of the scratches identified on the disc media.  
         [0088]     It will be clear that the present invention is well adapted to attain the ends and advantages mentioned as well as those inherent therein. While a presently preferred embodiment has been described for purposes of this disclosure, various changes and modifications may be made which are well within the scope of the present invention. For example, the routines  200 ,  300 ,  400 ,  500  and  512  may be incorporated in drive firmware and/or may be externally controlled during the manufacture of the disc drive  100 . The size of the scratches may be predefined or established by the user. Different padding schemes may be implemented other than the ones specifically described herein. Numerous other changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the invention disclosed and as defined in the appended claims.