Abstract:
A Viterbi detector receives a signal that represents a binary sequence having groups of no more and no fewer than a predetermined number of consecutive bits each having a first logic level, where the groups are separated from each other by respective bits having a second logic level. The Viterbi detector recovers the binary sequence from the signal by calculating a respective path metric for each of no more than four possible states of the binary sequence, and determining a surviving path from the calculated path metrics, where the binary sequence lies along the surviving path. Or, the Viterbi detector recovers the binary sequence from the signal by calculating respective path metrics for possible states of the binary sequence, calculating multiple path metrics for no more than one of the possible states, and determining the surviving path from the calculated path metrics.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 09/087,364, entitled “METHOD AND APPARATUS FOR READING AND WRITING GRAY CODE SERVO DATA TO A MAGNETIC MEDIUM USING SYNCHRONOUS DETECTION,” which is incorporated by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The invention is related generally to electronic circuits, and more particularly to a Viterbi detector and technique for recovering a binary sequence from a read signal. In one embodiment, a servo channel includes a pruned PR4 Viterbi detector that recovers Gray coded servo data read from a data-storage disk. As compared to other servo channels, this PR4 targeted channel allows synchronous detection of the track ID information without oversampling, which allows a significant increase in the density of the servo data stored on the disk, and thus which allows a significant reduction in the disk area allocated to servo data. More specifically, constructing the servo channel to fit a target PR4 power spectrum (defined by a PR4 polynomial) allows the servo channel to perform a lower level of equalization on the servo signal. Lowering the level of equalization often lowers the level of equalization noise introduced into the servo signal, and thus causes less degradation of the servo signal&#39;s signal-to-noise ratio (SNR). Furthermore, the PR4 Viterbi detector is pruned to match a Gray code coding scheme. This pruning increases the minimum Euclidian distance of error events. Therefore, such a pruned PR4 Viterbi detector can often recover servo information from a servo signal having an SNR that is lower than other Viterbi detectors can tolerate. Consequently, because it can process a servo signal having a lower SNR and because it causes less degradation of the servo signal&#39;s SNR, such a servo channel allows a disk to have a higher servo-data storage density. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 is a plan view of a conventional magnetic data-storage disk  10 . The disk  10  is partitioned into a number—here eight—of disk sectors  12   a - 12   h , and includes a number—typically in the tens or hundreds of thousands—of concentric data tracks  14   a - 14   n . File data is stored in respective data sectors (not shown) within each track  14 . Although the disk  10  is described as having eight disk sectors  12   a - 2   h , it may have more or fewer disk sectors  12 . 
     Referring to FIG. 2, respective servo wedges  16  are located within each track  14  at the beginning of each disk sector  12 . For clarity, only servo wedges  16   a - 16   c  are shown, it being understood that the other servo wedges are similar. The servo wedges  16  contain respective servo data that allows a head position system (FIG. 11) to position a read-write head (FIGS. 4 and 5) over the track  14  to be read from or written to. The manufacturer of a disk drive (FIG. 11) containing the disk  10  typically writes the servo wedges  16  onto the disk  10  before shipping the disk drive to a customer; neither the disk drive nor the customer alters the servo wedges  16  thereafter. 
     FIG. 3 is a diagram of the servo wedge  16   a  of FIG. 2, it being understood that the other servo wedges  16  are similar. Write splices  18   a  and  18   b  respectively separate the servo wedge  16   a  from adjacent data sectors (not shown). A servo address mark (SAM)  20  indicates to the head position system that the read-write head is at the beginning of a servo wedge  16 , and thus at the beginning of a disk sector  12 . A servo preamble  22  synchronizes the sample clock of a servo channel (FIGS.  4  and  5 ), and a servo synchronization mark (SSM)  24  identifies the beginning of a head-location identifier  26 . A data preamble and a data synchronization mark, which are sometimes similar to the servo preamble  22  and the SSM  24 , respectively, are discussed in U.S. patent application Ser. No. 09/410,274, filed Sep. 30, 1999, which is incorporated by reference. The location identifier  26  allows the head position system to coarsely determine and adjust the position of the read-write head with respect to the surface of the disk  10 . More specifically, the location identifier  26  includes a sector identifier  28  and a track identifier  30 , which respectively identify the disk sector  12 —here the sector  12   a —and the data track  14 —here the track  14   a —that contain the servo wedge  16   a . Because the read-write head may read the location identifier  26  even if the head is not directly over the track  14   a , the servo wedge  16   a  also includes bursts  32   a - 32   n , which allow the head position system to finely determine and adjust the position of the read-write head. 
     FIG. 4 is a block diagram of a conventional read-write head  34  and a read channel  36 , which recovers the location identifier  26  from the servo wedges  16  of FIGS. 2 and 3 and provides the recovered identifier to the head position system. The channel  36  is typically used to recover both servo and read data, and thus functions as a servo channel while it is recovering servo data. Therefore, the channel  36  is hereinafter called servo channel  36 . 
     The servo channel  36  includes a preamplifier  38 , a continous lowpass filter (LPF)  37 , a gain stage  39 , an analog-to-digital converter (ADC)  40 , a finite-impulse-response (FIR) filter  42 , a Viterbi detector  44 , and a decoder  46 . The head  34  converts the bit sequence that composes the servo wedge  16  into a servo signal, and the preamplifier  38  amplifies the servo signal. The LPF  37  equalizes the servo signal, the gain stage  39  amplifies the signal so as to control the overall gain of the channel  36 , the ADC  40  samples and digitizes the amplified signal, and the FIR filter  42  boosts the power of the signal to better equalize consecutive digitized samples—here two samples at a time—to the target polynomial (e.g., PR4) of the channel  36 . The Viterbi detector  44 , which is designed for the target polynomial, recovers the servo bit sequence from the servo signal by processing the equalized samples—here two samples at a time. The decoder  46  decodes the recovered bit sequence and provides the decoded bit sequence to the head position system. Alternatively, if the servo bit sequence is not coded, then the decoder  46  may be omitted such that the Viterbi detector provides the recovered bit sequence directly to the head position system. Other circuit blocks, which are omitted from FIG. 3 for clarity, detect the SAM  20  and the SSM  24  (FIG. 3) and control the timing and other characteristics of the channel  36 . 
     Referring to FIGS. 1 and 4, the storage capacity of the disk  10  is typically limited by its surface area and the minimum servo-signal SNR specified for the Viterbi detector  44 . Specifically, the diameter of the disk  10 , and thus its surface area, are typically constrained to industry-standard sizes. Therefore, the option of increasing the surface area of the disk  10  to increase its storage capacity is usually unavailable to disk-drive manufacturers. Furthermore, the SNR of the servo signal is a function of the servo-data-storage density on the surface of the disk  10 ; the higher the storage density, the lower the SNR of the servo signal, and vice-versa. Typically, as the SNR of the servo signal decreases, the number of errors that the Viterbi detector  44  introduces into the recovered servo data increases. Unfortunately, an increase in the number of errors may degrade the effective servo-data-recovery speed of a disk drive to unacceptable levels. 
     One way to increase the data-storage capacity of the disk  10  is to decrease radial distance, i.e., the pitch, between adjacent data tracks  14 . This allows the manufacturer to fit more tracks  14 , and thus more data, onto the disk  10 . 
     Unfortunately, decreasing the pitch of the data tracks  14  often decreases the SNR of the servo signal by increasing the inter-symbol interference (ISI) and media noise during reading of the servo data. ISI, media noise, and the affect ISI and media noise have on the SNR of a data read signal such as the servo signal are discussed in U.S. patent application Ser. No. 09/409,923, entitled “PARITY-SENSITIVE VITERBI DETECTOR AND METHOD FOR RECOVERING INFORMATION FROM A READ SIGNAL”, filed Sep. 30,1999, which is incorporated by reference. 
     Furthermore, the servo channel  36  may effectively decrease the SNR of the servo signal by heavily equalizing the digitized samples of the signal to a target power spectrum and corresponding target polynomial (e.g., EPR4) that the servo signal does not fit well. The Viterbi detector  44  is often designed for a target polynomial (e.g., EPR4) that requires the FIR filter  42  to heavily equalize the digitized samples of the servo signal so that the filtered samples “fit” the target power spectrum represented by the target polynomial. For example, this may occur when the Viterbi detector  44  is used to recover both servo and read data. Because the storage density of the servo data in a track  14  is typically less than the storage density of the read data within the same track, the servo-data field requires different equalization than the read-data field. For reasons that are omitted here for brevity, this different equalization is often required because the power spectrum of the read signal may be quite different than the power spectrum of the servo signal. Therefore the channel  36  is typically constructed to target the power spectrum of the read data, not the servo data. If one equalizes the servo signal to force it to have the same power spectrum as the read signal, then this equalization typically enhances the noise at the frequencies where there is no signal power for the servo signal. Thus, such equalization often introduces a relatively high level of equalization noise into the filtered samples, thus effectively increasing the noise component, and decreasing the SNR, of the servo signal. 
     Consequently, the servo channel  36  limits the servo-data-storage density, and thus thedata-storage capacity, of the disk  10 . Specifically, the servo-data-storage density of the disk  10  must be low enough such that the total effective SNR of the servo signal (the SNR of the servo read signal reduced by the equalization noise) is greater than or equal to the minimum SNR required by the Viterbi detector  44 . Therefore, the higher the level of equalization performed by the servo channel  36  and the higher the minimum SNR required by the Viterbi detector  44 , the lower the servo-data-storage density of the disk  10  must be. 
     SUMMARY OF THE INVENTION 
     In accordance with an embodiment of the invention, a Viterbi detector receives a signal that represents a binary sequence having groups of no more and no fewer than a predetermined number of consecutive bits each having a first logic level, where the groups are separated from each other by respective bits having a second logic level. The Viterbi detector recovers the binary sequence from the signal by calculating a respective path metric for each of no more than four possible states of the binary sequence, and determining a surviving path from the calculated path metrics, where the binary sequence lies along the surviving path. In a related embodiment, the Viterbi detector recovers the binary sequence from the signal by calculating respective path metrics for possible states of the binary sequence, calculating multiple path metrics for no more than one of the possible states, and determining the surviving path from the calculated path metrics. 
     For a binary sequence coded according to a Gray code coding scheme, such a Viterbi detector can accurately recover the coded binary sequence from a servo signal having an effective SNR that is significantly lower than the minimum SNR required by prior Viterbi detectors. Furthermore, the sampled servo signal can be equalized to a target power spectrum (e.g., PR4) that fits the power spectrum of unequalized servo data being read, and thus can operate with a lower level of equalization than prior servo-data detection schemes require. More specifically, a PR4 Viterbi detector is pruned to match the Gray coded coding scheme, thereby increasing the minimum Euclidian distance of the error events. In addition, the servo channel that incorporates the Viterbi detector equalizes the servo signal to a target PR4 power spectrum, which is the same or approximately the same as the power spectrum of the servo data. Thus, this equalization does not increase the noise power of the servo signal as much as an equalization to another target power spectrum (e.g., EPR4) that is different than the servo-data power spectrum. 
     Therefore, such a Viterbi detector in such a servo channel can recover servo data from a disk having a higher servo-data-storage density than other Viterbi detectors in other servo channels can tolerate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a conventional magnetic data-storage disk having disk sectors and data tracks. 
     FIG. 2 is a close-up view of the servo wedges of the disk of FIG.  1 . 
     FIG. 3 is a diagram of a servo wedge of FIG.  2 . 
     FIG. 4 is a block diagram of a conventional read channel for reading the servo wedges of FIGS. 2 and 3. 
     FIG. 5 is a block diagram of a servo channel for reading servo wedges according to an embodiment of the invention. 
     FIG. 6 is a chart of uncoded words and corresponding coded words used to encode servo data code for coding according to an embodiment of the invention. 
     FIG. 7 is a one-sample-at-a-time trellis diagram for the Viterbi detector of FIG. 5 according to an embodiment of the invention. 
     FIG. 8 is a two-sample-at-a-time trellis diagram for the Viterbi detector of FIG. 5 according to an embodiment of the invention. 
     FIG. 9 is a diagram of the servo-data Gray codes and the corresponding magnetization patterns for adjacent data tracks according to an embodiment of the invention. 
     FIG. 10A is a plot of servo signals corresponding to read-head positions over and between first and second data tracks according to an embodiment of the invention. 
     FIG. 10B is lot of servo signals corresponding to read-head positions over and between second and third data tracks according to an embodiment of the invention. 
     FIG. 11 is a block diagram of a disk-drive system that incorporates the servo channel of FIG. 5 according to an embodiment of the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 5 is a block diagram of a servo channel  50  according to an embodiment of the invention, where like reference numerals identify components that are common to both the servo channel  50  and the servo channel  36  of FIG.  4 . More specifically, the servo channel  50  uses the same front end (preamplifier  38 , LPF  37 , gain stage  39 , and ADC  40 ) as the servo channel  36  of FIG. 4, but uses a different FIR, Viterbi detector, and decoder. For clarity, the LPF  37  and gain stage  39  are omitted from FIG.  5 . 
     The servo channel  50  often allows a servo signal to have an SNR that is lower than the servo channel  36  (FIG. 4) allows, and thus often allows a disk such as the disk  10  (FIG. 1) to have a higher data-storage capacity than the channel  36  allows. More specifically, the servo data that composes the servo wedges  16  (FIGS. 2 and 3) has a power spectrum that depends on the density of the servo data and the rotational speed of the disk. Consequently, the servo channel  50  includes an FIR filter  52  for equalizing the servo-signal samples to a target power spectrum that is the same as or close to the power spectrum of the servo data, and includes a Viterbi detector  54  constructed for the polynomial that represents this target power spectrum. In one embodiment, a PR4 power spectrum is the same or is close to the power spectrum of the servo data. Therefore, because the the target of the servo channel  50  is similar to the power spectrum of the servo data, the FIR filter  52  can provide a significantly lower level of equalization than it could if the servo-channel target was significantly different (e.g., EPR4) than the servo-data power spectrum. Therefore, lowering the equalization introduces less equalization noise into the servo-signal samples, and thus the channel  50  does not lower the effective SNR of the servo signal as much as the servo channel  36 —which equalizes the servo data to a different target (EPR4)—lowers it. Furthermore, one can prune the Viterbi detector  54  to fit the coding scheme of the servo data, and thus can reduce the minimum servo-signal SNR required by the detector  54 . Thus, by constructing the servo channel  50  to target a power spectrum—here the target power spectrum represented by a PR4 polynomial—that is similar to the servo-data power spectrum, and by pruning the Viterbi detector  54  to fit the servo-data coding scheme, one can significantly decrease the minimum servo-signal SNR that the channel  50  requires, and thus can significantly increase the servo-data storage density, and thus the data-storage capacity, of the disk. 
     In operation, the servo channel  50  reads the servo data from a disk and provides the sector and track identifiers to the head position system, which uses this information to properly position the read-write head  34  with respect to the disk surface. The read-write head  34 , the preamplifier  38 , and the ADC  40  operate as discussed above in conjunction with FIG.  4 . The FIR filter  52  equalizes consecutive digitized samples—here two samples at a time—to the target polynomial, which is a PR4 polynomial in one embodiment. The Viterbi detector  54  recovers the servo bit sequence from the servo read signal by processing the equalized samples—here two samples at a time—and stores the recovered bit sequence in one or more registers  56 . A decoder  58  decodes the recovered bit sequence, which, in one embodiment, is coded as discussed below in conjunction with FIGS. 6 and 9, and provides the decoded bit sequence to the head position system. Other circuit blocks, which are omitted from FIG. 5 for brevity, detect the SAM  20  and the SSM  24  (FIG. 3) and control the timing, gain (e.g., gain stage  39  of FIG.  4 ), and other characteristics of the channel  50 . 
     FIG. 6 is a chart of uncoded words and corresponding Gray code coding words that code the servo data within the servo wedges  16  (FIGS. 2 and 3) according to an embodiment of the invention. The Gray code coding scheme is a 4:12 run-length-limited (RLL) code having d=2, k=10, and having single pairs and only single pairs of logic 1&#39;s. That is, each consecutive set of four uncoded bits is coded as a respective twelve-bit coding word having a minimum of two and a maximum of ten logic 0&#39;s are between consecutive single pairs of logic 1&#39;s. Furthermore, as discussed below in conjunction with FIGS. 9,  10 A, and  10 B, when the read-write head  34  is between data tracks  14  (FIGS.  1  and  2 ), this Gray code coding scheme allows the servo signal to provide accurate head-position information to the head position system. This Gray code coding scheme is further discussed in U.S. patent application Ser. No. 09/087,364, which is heretofore incorporated by reference. 
     FIG. 7 is a pruned trellis diagram that illustrates the operation of the Viterbi detector  54  (FIG. 5) according to an embodiment of the invention. The Viterbi detector  54  is constructed for a PR4 target polynomial B k =A k −A k−2 , where B k  is the digitized sample of the servo signal at sample time k, A k  is the logic value (0 or 1) of the sampled bit of the coded sequence at sample time k, and A k−2  is the logic value of the sampled bit of the coded sequence at sample time k− 2 . Therefore, the trellis has four states that represent four possible states of the coded sequence: S 0  (00 or −−), S 1  (01 or −+), S 2  (10 or +−), and S 3  (11 or ++). Because the Gray coded servo data is constrained as discussed above in conjunction with FIG. 6, the Viterbi detector  54  can be “pruned” such that the number of branches between the states S 0 -S 3  at consecutive sample times k is reduced from eight branches (two branches per state S 0 -S 3 ) to five branches. Thus, only the state S 0  has more than one—here two—incoming branches. The combination of the servo data being constrained according to the Gray code coding scheme and the Viterbi detector  54  being pruned to match the coding scheme increases the minimum squared distance error by a factor of two with respect to a combination of uncoded servo data and a full-state (eight branches) Viterbi detector. This increase in the minimum squared distance reduces by 6 dB the minimum servo-signal SNR required by the detector  54 , and thus makes recovery of the servo data more reliable for a given servo-signal SNR. The minimum squared distance event, i.e., the only possible trellis path that the Gray coded servo data can follow, is shown in solid line. Viterbi detectors and trellis diagrams are further discussed in U.S. patent application Ser. No. 09/409,923, entitled “PARITY-SENSITIVE VITERBI DETECTOR AND METHOD FOR RECOVERING INFORMATION FROM A READ SIGNAL”, and U.S. patent application Ser. No. 09/410,274, entitled “CIRCUIT AND METHOD FOR RECOVERING SYNCHRONIZATION INFORMATION FROM A SIGNAL”, which are heretofore incorporated by reference. 
     FIG. 8 is the pruned trellis diagram of FIG. 7 modified to reflect the Viterbi detector  54  (FIG. 5) processing two samples of the servo signal at a time. Therefore, each branch represents two sample values. For example, “1,−1” indicates that B k−1 =1 and B k =−1. The dashed branch lines indicate that the transitions to states (+,−) and (+,+) are forced, i.e., there is only one respective state from which each of these transitions can originate. 
     FIG. 9 is a diagram of the magnetization patterns corresponding to the 12-bit Gray code coding words stored in the track ID sections of the servo wedges for eighteen adjacent tracks  14  according to an embodiment of the invention. As discussed below in conjunction with FIGS. 10A and 10B, the Gray code coding scheme of FIG. 6 allows the head position system to identify the track over which the read-write head  34  (FIG. 5) is located within +/−1 track, even if the head  34  is positioned between tracks. To obtain this result, the Gray code coding scheme constrains code changes between adjacent tracks  14  to either a 1-bit shift in the position of a pair of logic 1&#39;s, replacement of a pair of 1&#39;s with a pair of logic 0&#39;s, or replacement of a pair of 0&#39;s with a pair of 1&#39;s. For example, the only change in the code words between tracks  1  and  2  is that bits  7  and  8 , which are logic 1&#39;s in track  1 , are replaced with logic 0&#39;s in track  2 . Similarly, the only change between tracks  2  and  3  is that bits  11  and  12 , which are 1&#39;s in track  2 , are “shifted left” such that that bits  10  and  11  are logic 1&#39;s in track  3 . 
     FIGS. 10A and 10B are plots of servo read signals corresponding to tracks  1 - 3  of FIG. 9 according to an embodiment of the invention. As discussed below, the magnetization patterns of FIG. 9 are such that if the read head  34  is in between two tracks  14 , the Viterbi detector  54  recovers the code word corresponding to one of the two tracks. Consequently, the head position system can determine the location of the head  34  within +/−1 track. 
     FIG. 10A is a plot of servo signals corresponding to the head  34  being over track  1  or track  2  or in between tracks  1  and  2  of FIG.  9 . For clarity, the servo signals are ideal, i.e., have no noise component (other than ISI). The y axis is the amplitude of the servo signal in units of the PR4 sample values −1, 0, and +1, and the x axis is time in units of the samples k of the servo signal. For example, sample time k=3 corresponds to the sample of the servo signal taken when the read head  34  is aligned with the third bit position (bit  3 ) of the Gray coded coding words stored in tracks  1  and  2  (FIG.  9 ). The head  34  generates the read signal  60  when it is directly over track  1 . Similarly, the head  34  generates the read signal  62  when it is directly over track  2 . And the head  34  generates the read signal  64  when it is halfway between tracks  1  and  2 . 
     Still referring to FIG. 10A, because the magnetization patterns of tracks  1  and  2  are identical for bits  1 - 6 , the signals  60 ,  62 , and  64  are virtually identical from sample time k=2 to k=6. At sample times  7 - 10 , the signals  60 ,  62 , and  64  are different. Specifically, the signal  62  transitions from +1, +1 (samples  7  and  8 ) to −1, −1 (samples  9  and  10 ) due to bits  7  and  8  of track  1  being logic 1 and bits  9  and  10  being logic 0. Conversely, because bits  7 - 10  of track  2  are logic 0, there are no flux changes so the signal  62  levels out at 0. The signal  64  transitions from +0.5, +0.5 to −0.5, −0.5, and thus is halfway in between the signals  60  and  62 . In actuality, the signal  64  is likely to be closer to one of the signals  60  and  62  than to the other. Therefore, the Viterbi detector  54  recovers the coding word associated with the track—track  1  or track  2  in this example—closest to the head  34 . The head position system then uses the information provided by the bursts  32  (FIG. 3) in tracks  1  and  2  to precisely position the head  34  over the desired track—track  1  or track  2  in this example. 
     FIG. 10B is a plot of read signals corresponding to the head  34  being over track  2  or track  3  or in between tracks  2  and  3  of FIG.  9 . The read signal  62 —which is identical to the read signal  62  of FIG.  10 A—is the signal that the head  34  generates when it is directly over track  2 . Similarly, the head  34  generates the read signal  66  when it is directly over track  3 . The head  34  generates the read signal  68  when it is halfway between tracks  2  and  3 . As discussed above in conjunction with FIG. 10A, the Viterbi detector  54  recovers the coding word associated with the track—track  2  or track  3  in this example—closest to the head  34 , and thus allows the head position system to locate the head  34 . 
     FIG. 11 is a block diagram of a disk-drive system  100  according to an embodiment of the invention. Specifically, the disk-drive system  100  includes a disk drive  102 , which incorporates the servo channel  50  of FIG.  5 . The disk drive  102  includes the read-write head  34 , a write channel  106  for generating and driving the head  34  with a write signal, and a write controller  108  for interfacing the write data to the write channel  106 . The disk drive  102  also includes a read channel  110  for receiving a read signal from the head  34  and for recovering data from the read signal, and includes a read controller  114  for organizing the read data. The read channel includes the servo channel  50 , which receives the servo signal from the head  34 , recovers the servo data from the servo signal, and provides the recovered servo data to a head position system  120 . The disk drive  102  further includes a storage medium such as one or more disks  116 , each of which may contain data on one or both sides. The head  34  writes/reads the data stored on the disks  116  and is connected to a movable support arm  118 . The head position system  120  determines the position of the head  34  as discussed above in conjunction with FIGS. 9,  10 A, and  10 B, and provides a control signal to a voice-coil motor (VCM)  122 , which positionally maintains/moves the arm  118  so as to positionally maintain/radially move the head  34  over the desired data tracks on the disks  116 . A spindle motor (SPM)  124  and a SPM control circuit  126  respectively rotate the disks  116  and maintain them at the proper rotational speed. 
     The disk-drive system  100  also includes write and read interface adapters  128  and  130  for respectively interfacing the write and read controllers  108  and  114  to a system bus  132 , which is specific to the system used. Typical system busses include ISA, PCI, S-Bus, Nu-Bus, etc. The system  100  also typically has other devices, such as a random access memory (RAM)  134  and a central processing unit (CPU)  136  coupled to the bus  132 . 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.