Data sector mark generation for a headerless disk drive architecture5940862Abstract A disk sequencer uses control words to identify starts and splits in headerless data sectors. Each control word indicates an event-count, an initial event, and a terminal event. The initial event, which can be an EOS pulse, a sector pulse, or a data sync pulse, synchronizes the start of a count until the terminal event. The terminal event causes either generation of a sector pulse indicating a start of a data sector or an interrupt indicating a split in a data sector. A disk sequencer includes an event counter and a data segment sequencer. The event counter starts counting at the initial event, counts the event-count, and generates the sector pulse or interrupt at the end of the event-count. A sector pulse starts a data segment sequencer processing a first data segment of a data sector. The interrupt causes the data segment sequencer to execute an interrupt routine for fields surrounding a servo sector. After an EOS pulse and completion of the interrupt routine, the data segment sequencer resumes processing the data sector by processing a second or subsequent data segments in the data sector. Claims We claim: Description BACKGROUND
TABLE 1
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CDR Format Control Word Syntax
Bits Label Definition
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15 PARITY PARITY is set or cleared so that the
control word has even parity.
14 SKIP SKIP enables or disables data
transfer for control word.
13 LAST LAST indicates whether the control
word is the last control word for a
data sector.
12:0 EVENTC EVENTC indicates a number of byte
clock cycles associated with the
control word.
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In accordance with the CDR format, each control word indicates: an event-count (or number) of byte clock cycles until an event within a data frame; whether a data sector associated with the control word should be skipped (i.e. not read or written); and whether the control word is the last for a data sector. In accordance with an aspect of the invention, Table 2 shows a control word format referred to herein as ECW (event control word) format. In accordance with the ECW format, each control word indicates: an event-count (or number) of byte clock cycles between initial and terminal events within a data frame; whether a data sector associated with the control word should be skipped; whether the beginning of the event-count should be synchronized with an EOS pulse, a sector pulse, or data synchronization pulse; and whether a sector pulse or a data transfer interrupt should be generated following the end of the event-count.
TABLE 2
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ECW Format
Bits Label Definition
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15 PARITY Set or cleared so that the control
word has even parity.
14 SKIP SKIP is 0 or 1 to respectively
enable or disable data transfer
during an event-count.
13 SEOS When GSEC = 1 and SEOS = 1, the event-
count is synchronized with an EOS
pulse. When GSEC = 1 and SEOS = 0, the
event-count is synchronized with a
sector pulse. When GSEC = 0 and
SEOS = 0, the event-count is
synchronized with a data sync
signal. When GSEC = 0 and SEOS = 1, the
control word is ignored.
12 GSEC When GSEC = 1, a sector pulse is
generated at the end of the event-
count. When GSEC = 0 and SEOS = 0, an
interrupt is generated at the end of
the event-count.
11:0 EVENTC The event-count indicating the
number of byte clock cycles before
an event (e.g. a sector pulse or an
interrupt signal).
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Bits SEOS and GSEC form an encoded field having four encoding: synchronize to an EOS pulse and generate a sector pulse at the end of count EVENTC; synchronize to a previous sector pulse and generate another sector pulse at the end of count EVENTC; synchronize to a data synchronization pulse and interrupt data transfer at the end of count EVENTC; and ignore this control word. The sector pulses start processing of a data segment and possibly start the next event-count. An interrupt stops data transfer while servo sequencer 130 handles a servo sector. Table 3 shows an embodiment of a track format table using control words of Table 2 to indicate the track format illustrated in FIG. 3.
TABLE 3
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Format Table for Multiple Sectors per Frame
PARITY SKIP SEOS GSEC EVENTC COMMENT
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1 0 1 1 008h EOS0 -> SEC251
0 0 0 1 255h SEC251 -> SEC252
0 0 0 1 255h SEC252 -> SEC0
0 0 0 1 255h SEC0 -> SEC1
1 0 0 0 089h DSYNC1 -> SPLIT
1 0 1 1 1D4h EOS1 -> SEC2
0 0 0 1 255h SEC2 -> SEC3
0 0 0 1 255h SEC3 -> SEC4
1 0 0 0 112h DSYNC4 -> SPLIT
1 0 1 1 146h EOS2 -> SEC5
0 0 0 1 255h SEC5 -> SEC6
0 0 0 1 255h SEC6 -> SEC7
1 0 0 0 196h DSYNC7 -> SPIT
1 0 1 1 0C2h EOS3 -> SEC8
0 0 0 1 255h SEC8 -> SEC9
0 0 0 1 255h SEC9 -> SEC10
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Table 3 indicates the control words for the first four data frames FRAME 0 to FRAME 3 of the track illustrated in FIG. 3. Repeatedly looping through Table 3 provides control words for frames FRAME 4 to FRAME 77. Table 4 shows a format table using the control words of Table 2 for the track format illustrated in FIG. 4.
TABLE 4
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Format Table for Multiple Frame per Sector
PARITY SKIP SEOS GSEC EVENTC COMMENT
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1 0 1 1 008h EOS0 -> SEC30
1 0 0 0 0DCh DSYNC30a -> SPLIT
1 0 0 0 0DCh DSYNC30b -> SPLIT
1 0 1 1 096h EOS2 -> SEC0
1 0 0 0 0DCh DSYNC0a -> SPLIT
1 0 0 0 0DCh DSYNC0b -> SPLIT
. . . . . Repeat the above
. . . . . 14 times for data
. . . . . sectors 2 to 28
1 0 1 1 008h EOS75 -> SEC29
1 0 0 0 0DCh DSYNC29a -> SPLIT
1 0 0 0 0DCh DSYNC29b -> SPLIT
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As an alternative to Table 4, which contains 93 control words, just the first six control would be sufficient if disk sequencer 140 treats the last data sector 29 differently. In particular, looping through the first six control words 15 times provides the control words for data sectors 0 to 28. Only the first three control words of a sixteenth loop are actually usable. If the sixteenth loop wraps to the top of the table after the third control word, disk sequencer 140 only processes the correct control words. Alternatively, after handling the last data sector, disk sequencer 140 can discard all previously loaded control words and reset loading of control words to the top of the format table. Tables 3 and 4 may be stored as lists in non-volatile memory, for example, in ROM, EEPROM, EPROM, or Flash memory (not shown) or on disks 152, for example, in a track having a track format table stored in non-volatile memory so that the tables can be read from the track and into data buffer 120. Less disk space is required to store track format tables than would be required for headers because a single track format table typically applies multiple disks 152 and multiple tracks in an annular zone on each disk. Further, one control word in a track format table such as Table 3 can apply to multiple data sector on each track. However, to store format tables on disk, some other source must provide the format information for reading the track containing the format tables. Disk sequencer 140 uses the format tables to locate boundaries of data sectors, defect information to identify which data sectors are defective or should otherwise be skipped, and request information indicating which data sectors are actually requested for a transfer. Microprocessor 110 prepares this information for disk sequencer 140. In one embodiment of the invention, microprocessor 110 modifies control words from format tables by setting bit SKIP if necessary, to indicate a defective data sector or a data sector to be skipped for other reasons. In this embodiment, microprocessor 110 must duplicate control words from tables such as Table 3 so that each control word corresponds to one specific data sector because skip bits typically differ even for data sectors otherwise having the same format. This can greatly increase the number of control words required per track. Alternatively, disk sequencer 140 uses a repeat count and a shortened format table such as Table 3, and microprocessor 110 separately provides a list of the physical sector numbers of defective data sectors. There are several alternative methods for loading control words into disk sequencer 140. In a first alternative, microprocessor 110 loads the control words into disk sequencer 140 for a data transfer. Loading of control words may be periodic, for example, one set loaded per data frame or per data sector. For a second alternative, microprocessor 110 passes an address of a track format table in data buffer 120 to disk sequencer 140. Disk sequencer 140 then retrieves control words from the table starting at the indicated address. Microprocessor 110 can also pass the addresses of the top and bottom of the table (or equivalent information) to disk sequencer 140 for wrap around from the bottom to the top of the table. Control words from a format table must be synchronized or aligned with the position of data heads 154 relative to disks 152. In particular, when data heads are over a data frame, disk sequencer 140 requires the control words associated with that data frame. Microprocessor 110 or alternatively disk sequencer 140 aligns the control words. When microprocessor 110 is responsible for alignment, firmware determines the frame and sector numbers corresponding to the current position of the data heads and then selects the control word corresponding to those frame and sector numbers. For a format table such as Table 3 or 4, the number of control words per track, frame, or sector varies in a complex manner. Accordingly, selection of the correct control word for alignment can be complex and typically requires sequentially parsing through the table until the control word corresponding to the current data sector is found. This requires a significant amount of processing power. In accordance with an aspect of the invention, control words that are no-ops or fillers are added to track tables so that the number of control word per data sector is constant. Table 5 indicates an alternative format table for the track of FIG. 3.
TABLE 5
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Track Format Table
PARITY SKIP SEOS GSEC EVENTC COMMENT
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1 0 1 1 008h EOS0 -> SEC251
1 0 1 0 000h Filler
0 0 0 1 255h SEC251 -> SEC252
1 0 1 0 000h Filler
0 0 0 1 255h SEC252 -> SEC0
1 0 1 0 000h Filler
0 0 0 1 255h SEC0 -> SEC1
1 0 0 0 089h DSYNC1 -> SPLIT
1 0 1 1 1D4h EOS1 -> SEC2
1 0 1 0 000h Filler
0 0 0 1 255h SEC2 -> SEC3
1 0 1 0 000h Filler
0 0 0 1 255h SEC3 -> SEC4
1 0 0 0 112h DSYNC4 -> SPLIT
1 0 1 1 146h EOS2 -> SEC5
1 0 1 0 000h Filler
0 0 0 1 255h SEC5 -> SEC6
1 0 1 0 000h Filler
0 0 0 1 255h SEC6 -> SEC7
1 0 0 0 196h DSTBC7 -> SPIT
1 0 1 1 0C2h EOS3 -> SEC8
1 0 1 0 000h Filler
0 0 0 1 255h SEC8 -> SEC9
1 0 1 0 000h Filler
0 0 0 1 255h SEC9 -> SEC10
1 0 1 0 000h Filler
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Table 5 has a fixed number (two) of control words per data sector. Fillers are added, if necessary, following the last control word for data sector so that the total number of control words per data sector is constant. The address of the control word corresponding to a data sector can be determined by adding an offset to the product of the physical sector number for the data sector and the number of control words per data sector. Microprocessor 110 requires very little processing power for such alignment calculations. However, Table 5 is longer than Table 3 and requires more space in data buffer 120 and more bandwidth for transfer of control words. When disk sequencer 140 aligns control words, microprocessor 110 starts disk sequencer 140 at any convenient location (typically the start) of a format table such as Table 3 and disk sequencer 140 automatically selects control words aligned with the position of data heads 154. This has the advantage of reducing the processing power requirements on microprocessor 110 without increasing the length of format tables. FIG. 5 shows a block diagram of an embodiment of disk sequencer 140 in accordance with an embodiment of the invention. Disk sequencer 140 includes a FIFO buffer 510 into which microprocessor 110 or a FIFO filler (not shown) loads control words. In the exemplary embodiment, the control word are 16-bit values in compliance with syntax indicated in Table 2, and FIFO buffer 510 is a 4.times.16-bit FIFO buffer. FIFO buffer 510 has output signals SKIP, SEOS, GSEC, and EVENTC which indicate values of fields of the same names in a first-loaded (top) control word in FIFO buffer 510. A state machine 550 uses signals SKIP, SEOS, GSEC, and EVENTC from control word FIFO 510 to control generation of sector pulses and interrupt signals for a data segment sequencer 540. If a control word is necessary for generation of a sector pulse or an interrupt, signal EVENTC, which represents an event-count from the control word, is stored in register 535 for transfer to a counter 530. The control word is discarded, for example, if the control word is a filler or otherwise not currently necessary. For each control word used, state machine 550 selects an initial event such as an EOS pulse, a sector pulse, or a data sync pulse to start counter 530 decrementing the associated event-count and selects whether counter 530 generates a sector pulse in signal SECTOR or an interrupt in signal CDRINT when the event-count decrements to zero. As described below, the sector pulses and interrupts start and stop data segment sequencer 540 which transfers data between disks 152 and data buffer 120. FIG. 6 shows an embodiment of counter 530 that is a free running counter. Counter 530 includes a register 630 for storing the count value COUNT, a decrementing circuit 640 which subtracts 1 from the count value, and a multiplexer 610 which controls whether the decremented count value COUNT-1 or the value of signal NEXTCNT is the input signal to register 630. When a synchronization signal from a multiplexer 620 is asserted, the value of signal NEXTCNT is loaded into register 630, and counter 530 begins decrementing a new event-count. A signal SEL.sub.-- SYNC from state machine 550 controls which of signals EOSPULSE, SECTOR, or DATASYNC synchronizes loading a new event-count. Signal EOSPULSE includes EOS pulses that have been synchronized with the byte clock. Signal SECTOR is the output signal from pulse generator 650. Signal DATASYNC contains data sync pulses that are synchronized with the byte clock. When the synchronization signal is not asserted, multiplexer 610 selects signal COUNT-1 and the count is decremented. When the count reaches zero, a pulse generator 650 or 660 asserts a one-clock cycle sector pulse in signal SECTOR or asserts an interrupt via signal CDRINT. A signal SEL.sub.-- EVENT from state machine 550 selects which of signals SECTOR or CDRINT is asserted. The sector pulses and interrupts must match the position of data heads 154 relative to disks 152 for data segment sequencer 540 to access desired data sectors on disks 152. Accordingly, control words must be aligned to the position of data heads 154. In accordance with an aspect of the invention, disk sequencer 140 performs automatic alignment to reduce the processing power requirements on microprocessor 110. For automatic alignment, disk sequencer 140 includes counters 560, 570, and 574 for counts CURRSEC, EOSCTR, and EOSCMP, respectively. Count EOSCTR indicates the current position of data heads 154 to a data frame granularity. Counts EOSCMP and CURRSEC indicate the data frame and the data sector associated with the top control word in FIFO buffer 510. A data signal MPU.sub.-- DATA from microprocessor 110 initializes counters 560, 570, and 574. A comparator 576 compares counts EOSCTR and EOSCMP to determine whether the control words are properly aligned. Frame counter 570 may be initialized with a frame number during start-up of hard drive 100 and is updated for each EOS pulse. For example, during start-up, an index pulse from servo sequencer 130 resets counter 570 to the frame number (typically zero) of the data frame following the index pulse. Alternatively, servo sequencer 130 reads a servo wedge number that microprocessor 110 writes to counter 570. Counter 570 increments count EOSCTR for each EOS pulse from servo sequencer 130 and automatic wraps count EOSCTR to the first data frame number (zero) in response to count EOSCTR exceeding a maximum data frame number EOSMAX. Servo sequencer 130 preferably provides EOS pulses even during a seek so that count EOSCTR remains an accurate position indicator and does not need to be changed after a seek. During a seek to a track, microprocessor 110, without performing any alignment checking, prepares or selects a format table and passes the starting address and a repeat count for the format table to disk sequencer 140. Microprocessor 110 also writes to counters 560 and 574 the sector and data frame numbers associated with the first control word in the format table and writes to a register 572 the maximum data frame number EOSMAX to control wrap-around functions of counters 570 and 574. A maximum sector number SECMAX is stored in a register 562 and controls wrap-around functions of counter 560. For the track of FIG. 3, for example, microprocessor 110 sets count EOSCMP in counter 574 to zero, count CURRSEC in counter 560 to 250, maximum sector number SECMAX in register 562 to 252, and maximum data frame number EOSMAX in register 572 to 77. Control words are loaded into FIFO buffer 510 beginning at the start of the format table, and state machine 550 removes control words from FIFO buffer 510 as the control words are processed or discarded. Sequential loading of control words from the format table into FIFO buffer 510 loops from the bottom to the top of the table as required to keep filling FIFO buffer 510 as control words as processed. The control words in FIFO buffer 510 and counts EOSCMP and CURRSEC initially have no set relation to the current data head position as indicated by count EOSCTR. Comparator 576 compares count EOSCTR from counter 570 to count EOSCMP and generates a signal EOSCMPEQ1 indicating whether count EOSCMP is equal to EOSCTR+1. If so, the top control word in FIFO buffer 510 corresponds to the start of the next data frame and counts EOSCMP and CURRSEC correctly indicate the data frame number and physical sector number at the start of the next data frame. The event-count from FIFO buffer 510 is loaded into register 535 to await the next EOS pulse marking the start of the next data frame. If count EOSCMP is not equal to count EOSCTR+1, state machine 550, based on fields SEOS and GSEC, determines whether to increment count EOSCMP or CURRSEC or neither before popping a control word from FIFO buffer 510. Count CURRSEC is incremented if the top control word indicates generation of a sector pulse at the end of the event-count. Count EOSCMP is incremented if the top control word indicates generation of an interrupt or generation of a sector pulse after a count synchronized with an EOS pulse. Comparator 576 compares the new value of count EOSCMP to count EOSCTR, while another control word becomes the top control word in FIFO buffer 510 and signals SKIP, SEOS, GSEC, and EVENTC change according to the new top control word. State machine 550 sequentially removes control words from FIFO buffer 510 in above fashion until count EOSCMP is equal to EOSCTR+1 indicating that the control words in FIFO buffer 510 are aligned for the next EOS pulse. Once the control words are properly aligned, event-counts from the control word are processed through counter 530. Counter 530 generates the sector pulses and interrupts necessary for starting data segment sequencer 540 at the beginning of a data sector and halting data segment sequencer 540 at a split caused by a servo sector. The example of processing the control words of Table 3 starting at an EOS pulses EOS0 is now described. For this example, control words are processed starting from the first control word in Table 3. Before servo sequencer 130 generates EOS pulse EOS0, a first event-count (8) from the first control word in Table 3 is loaded into register 535 and state machine 550 selects signal EOSPULSE for synchronization of the first event-count. The skip field value is loaded into a register 525. At EOS pulse EOS0, the first event-count transfers from register 535 to counter 530. Counter 530 decrements the first event-count while an EJF field passes data heads 154. While decrementing the event-count for the top control word, the top control word is removed from FIFO buffer 510, and another control word becomes the new top control word in FIFO buffer 510. State machine 550 maintains signal SEL.sub.-- EVENT for the removed control word while beginning to process the next control word. For the example of Table 3, state machine 550 loads a skip value from the second control word of Table 3 into register 525, loads an event-count 597 (255 h) from the second control word into register 535, and selects signal SECTOR as the triggering event for the new event-count because fields SEOS and GSEC of the second control word indicate that the second event-count is synchronized with a sector pulse. Eight byte clock cycles after EOS pulse EOS0, when the first event-count decrements to zero, counter 530 generates a sector pulse SEC251 and begins decrementing the second event-count. The sector pulse starts data segment sequencer 540 and cause the value of the skip field from register to be loaded into a latch 520. Data segment sequencer starts data transfer for a data segment if the current sector is requested, is not defective, and control words for the current sector do not indicate the data sector should be skipped. To identify that a sector is requested for a transfer, disk sequencer 140 includes a counter 564 into which microprocessor 110 writes information indicating a range of requested data sectors. Counter 564 generates a signal indicating the physical sector number of the next data sector to be transferred. Counter 564 may be a counter that increments a sector number from the start to the end of a request range, or any other circuit which provides a signal identifying requested data sectors. A comparator 568 compares the sector number from counter 564 to signal CURRSEC from counter 560. If signal CURRSEC matches the sector number from counter 564, comparator 568 asserts a signal REQSECEQ to indicate the current sector is requested for a transfer. Microprocessor 110 stores in a defect RAM 565 an ordered list of the physical sector number of defective data sectors in the current track. The list is matched with the requested sectors and begins with the physical sector number of the first defective sector following the first requested data sector. A comparator 566 compares a sector number from defect RAM 565 to the current sector number indicated by signal CURRSEC and asserts a signal DEFSECEQ if there is a match indicating the current sector is defective. Signal DEFSECEQ being asserted disables data transfer even if the current data sector is a requested data sector. Data transfer is also disabled if signal SKIPSEC is asserted because control words for the current data sector indicate that the data sector should be skipped. Sector pulses trigger latch 520 so that latch 520 only loads skip field value SKIPNEXT from register 525 once for a data sector. Accordingly, signal SKIPSEC is only effected by field SKIP of one control word per data sector. At sector pulse SEC251, signal SKIPSEC is set to indicate the value of the skip field from the second control word of Table 3. Since the second control word of Table 3 indicates synchronization with a sector pulse, sector pulse SEC251 causes counter 530 to load and immediately begin decrementing the second event-count. When the event-count transfers to counter 530, state machine 550 selects the output signal for the second skip count and removes the second control word from FIFO buffer 510, and the event-count and skip field for the third control word are stored in registers 535 and 525. Data segment sequencer 540 controls data transfers and uses signals SECTOR and CDRINT to identify boundaries of data segments. FIG. 7 illustrates an exemplary embodiment of data segment sequencer 540 in which a random access memory (RAM) 730 holds a sequencer program. Microprocessor 110 loads RAM 730 with a sequencer program that implements a read vector and a write data vector for transferring data segments and an interrupt subroutine. The read and write vectors are adapted to the particular data segment format and use byte counts to identify boundaries of fields within a data sector. In an exemplary embodiment of the invention, configuration registers in disk sequencer 140 store byte counts for the various overhead fields, the largest data field, and frame fields such as EJF field 220 and ISG field 280 in FIG. 2. For execution of the sequencer program, data segment sequencer 540 includes an address counter 720 that holds an instruction address for RAM 730 and an instruction latch 735 to hold the instruction read from that address. An instruction decoder 740 executes the instruction held by latch 735. A multiplexer 715, which is coupled to an adder 725, an address latch 745, a vector address register 710, and instruction decoder 740, selects the address of the next instruction to be executed. Adder 725 increments the instruction address in counter 720 for sequential execution of the sequencer program. Address latch 740 holds a saved address, for example, a return address, that instruction decoder 740 selects when executing a return or return from interrupt instruction. Instruction decoder 740 calculates the next address for branch or call instructions. Register 710 holds addresses for subroutines such as the read vector, the write vector, and the interrupt procedures. A branch select circuit 750 is triggered by signals CDRINT and SECTOR and selects the appropriate routine to be executed in response to those signals. In executing the sequencer program, instruction decoder 740 selects byte counts that are loaded into a counter 760 and generates signals CNTRLS that control circuitry (not shown) for processing segment fields. Each cycle of the byte clock, the count in counter 760 changes in one of three ways depending on the signal that a multiplexer 755 selects. The count in counter 760 can be set to sequence count SEQCNT from instruction decoder 740, decremented by an adder 765, or set to a count previously saved in a register 770. In response to a sector pulse, instruction decoder 740 loads counter 760 with the counts for the first leading overhead field such as VFO field 230, and counter 760 decrements that byte count each cycle of the byte clock until the count is decremented to zero signaling the end of the first overhead. During the count for the first overhead field, instruction decoder 740 performs control operations that depend on whether the data transfer is enabled for the data sector and whether the transfer is a read or a write. Once the count for the first overhead field is complete, instruction decoder 740 loads counter 760 with the next byte count, for example, for training field 240 or data sync field 250 if no training field is used. Decrementing the counts for the overhead fields controls timing of processing of overhead fields. After a data sync field, counter 760 is loaded with the data count for a full data sector if the current data segment is the first data segment in a data sector. A sector pulse marks the start (i.e. the first data segment) of a data sector. Decrementing of the data count controls the timing of data transfer and may be synchronized with a data sync signal generated by HDA 150 or disk sequencer 140 during processing of the overhead fields. Unless an interrupt is asserted through signal CDRINT, the data count decrements to zero before counts for trailing overhead fields such as pad field 270 start. Counter 530 asserting an interrupt signal CDRINT causes the data count from counter 760 to be saved in latch 770, halts data transfer, and starts an interrupt routine in data segment sequencer 540. The interrupt routine handles the trailing overhead fields, waits for an EOS pulse, and processes initial fields in the next data frame before returning to the interrupted procedure. How the interrupt procedure handles the fields at the end of one data frame and the start of the next depends on whether the operation interrupted was a read or a write. For a write, the interrupt procedure writes trailing fields (e.g. pad bytes) of a data segment, waits for an EOS pulse, and writes leading overhead fields (e.g. VFO, TF, and SYNC fields) before returning. For a read, the interrupt procedure resets the read gate, waits for an EOS pulse before setting the read gate, and then waits for a data sync signal before returning. Upon returning from the interrupt, the saved data count from latch 770 is reloaded into counter 760 and further decremented when the interrupted procedure is resumed. For the track format of FIG. 3, sector pulse 251 starts data segment sequencer 540 handling data segment 251 when counter 530 begins decrementing the second event-count (597 or 255 h). Since the second event-count is equal to the byte count for a single-segment data sector plus the byte count for an inter-sector gap, data segment sequencer 540 completes an entire data sector 251 before counter 530 decrements the second event-count to zero. As indicated by fields GSEC of the second control word, a sector pulse SEC252 is generated when the second event-count decrements to zero. Sector pulse SEC252 starts counter 530 decrementing the third event-count which is also 597 (255 h) and starts data segment sequencer 540 transferring data segment 252 if data segment 252 is requested and not skipped or defective. The third and fourth control words of Table 3, being identical to the second control word of Table 3, are processed in the same manner as described above for the second event control. When counter 530 decrements the event-count from the fourth control word to zero, counter 530 generates sector pulse SEC1, and state machine 550 loads the event-count from the fifth control word into counter 530. If sector 1 is requested and not skipped or defective, sector pulse SEC1 starts data segment sequencer 540. The fifth control word has an event-count of 137 or 89 h and indicates that the even-count is synchronized with a data sync signal. At the data sync signal, counter 530 begins decrementing the fifth event-count (137) and sequencer counter 760 begins decrementing the data count for a full data field (547). The fifth event-count decrements to zero, and counter 530 generates an interrupt before the data count decrements to zero (i.e. when the data count decrements to 410). The interrupt causes data segment sequencer 540 to execute the interrupt routine before resuming decrementing of the data count for the first data segment in data frame 1. FIGS. 8, 9, and 10 illustrate the states of an exemplary embodiment of state machine 550. State machine 550 controls alternative modes of operation beginning from a state S0 shown in FIG. 8. The different modes are distinguished by whether disk sequencer 140 must align control words and/or generate sector pulses. A signal SECGEN is asserted high if disk sequencer 140 generates sector pulses. A signal AUTOALIGN is asserted high if disk sequencer 140 generates must align the control words. If signal SECGEN is asserted and signal AUTOALIGN is not assert, state machine 550 transitions to a state S6 in FIG. 9 for operating modes referred to herein as SGA and ADSA. If signal AUTOALIGN is assert, state machine 550 transitions to a state S20 in FIG. 10 to align the control words. FIG. 8 illustrates the states for operating modes where an external circuit generates sector pulses (i.e. signal SECGEN is not asserted) and the control words are aligned (i.e. signal AUTOALIGN is not asserted). For a first operating mode, referred to herein as DSA/DFA mode, a signal ENHANCEDSA is not asserted which restricts state machine 550 to three states S0, S1, and S3. In state S0, state machine 550 waits for FIFO buffer 510 to assert a signal RDY indicating FIFO buffer 510 contains a valid control word. When signal RDY is asserted and signals SECGEN and AUTOALIGN are not, state machine 550 moves to state S1 and asserts as signal READCDRF which stores the value signal EVENTC in register 535 and removes and the control word from FIFO buffer 510. State machine 550 then transitions to state S3. For DSA/DFA mode, a synchronized version of an externally generated sector pulse or an EOS starts data segment sequencer 540, and counter 530 begins counting down the event-count from register 535 when signal DATASYNC or signal RETURN is asserted. Upon decrementing to zero, counter 530 asserts signal CDRINT to interrupt data segment sequencer 540. State machine 550 waits in state S3 until counter 530 asserts signal CDRINT. State machine 550 then transitions back to state S0 for the next control word. For an enhanced mode, referred to herein as EDSA mode, state machine 550 uses field LAST of the CDR format when discarding control words. State machine 550 waits in state S0 until FIFO buffer 510 signals a valid control word is ready before transitioning to state S1 and writing field EVENTC to register 535 and removing the control word. State machine 550 then transitions to state S3 and waits until signal CDRINT or LOADCDRDET is asserted. As in DSA/DFA mode, signal CDRINT being asserted indicates an event-count from another control word is required. Signal LOADCDRDET being asserted indicates a control word associated with a current data sector is to be discarded. Control words for a data sector may be discarded if, for example, the data sector is defective or the control words are dummies that maintain a fixed number of control words per data sector in a format table. In the EDSA mode, if signal LOADCDRDET or CDRINT is asserted, state machine 550 transitions from state S3 to state S4 if field LAST of the current control word indicates that the current control word is not the last associated with the current data sector (i.e. the control word in FIFO buffer 510 is associated with the current data sector.) State machine 550 waits in state S4 until a valid control word is ready in FIFO buffer 510 and then in a state S5, reads and removes the control word from FIFO buffer 510. From state S5, state machine 550 transfers back to state S3. If signal LOADCDRDET remains or is newly asserted and the control word removed was not the last control word for the data sector, state machine 550 immediately transitions back to state S4 and continues to loop through states S3, S4, and S5 until the last control word for the current data sector is removed. Otherwise, state machine 550 waits in state S3 until signal CDRINT is asserted indicating counter 530 has counted down to zero and requires another event-count in register 535. In the EDSA mode, state machine 550 transitions from state S3 to state S2 if signal CDRINT or LOADCDRDET is asserted for a control word having field LAST set to indicate the control word is the last associated with a data sector. If signal LOADCDRDET was asserted, state machine 550 passes through state S2 to state S0. If signal CDRINT was asserted, state machine 550 waits in state S2 for signal LOADCDRDET which signals the finish of a data sector. For the ECW format where disk sequencer 140 generates sector pulses, state machine 550 transitions from state S0 to state S20 to perform automatic alignment of control words or to state S6 if control words are already aligned for a data head's position relative to the track. FIG. 9 illustrates the states of state machine 550 once control words are aligned (automatically or otherwise). State machine 550 waits in state S6 until signal RDY is asserted indicating signals SKIP, SEOS, GSEC, and EVENTC are valid for the top control word in FIFO buffer 510. State machine 550 then transitions to a state S7, S10, S13, S15, or S17 according to the values of signals SEOS and GSEC. If signals SEOS and GSEC respectively have values 1 and 0, the top control word in FIFO buffer 510 is a filler. State machine 550 transitions from state S6 to state S17 and asserts a signal POPCDR to remove the top control word from FIFO buffer 510. State machine 550 then returns to state S6. If signals SEOS and GSEC are both 1, state machine 550 transitions from state S6 to state S10, loads an event-count into register 535, selects signal EOSPULSE for synchronization of the event-count, and sets a value CDRZERO to indicate signal SECTOR is asserted when counter 530 decrements the event-count to zero. State machine 550 sets signal SEL.sub.-- SYNC to select signal EOSPULSE for synchronization of counter 530 and then transitions to a state S11. State machine 550 waits in state S11 until an EOS pulse is asserted (i.e. the event-count from register 535 transfers to counter 530) and transitions from state S11 to a state S9 if signals SEOS and GSEC are asserted (i.e. always if state machine 550 transitioned from state S10 to S11). In state S9, state machine 550 changes signal SEL.sub.-- EVENT to according to value CDRZERO, stores signal SKIP in register 525, and asserts signal POPCDR to remove the control word from FIFO buffer 510 before transitioning back to state S6. The skip value in register 525 is loaded into latch 520 when counter 530 asserts a sector pulse and is associated with the data sector following the event-count. If signals SEOS and GSEC are respectively 0 and 1, indicating both initial and terminal events are sector pulses, state machine 550 transitions from state S6 to state S7. State machine 550 then stores an event-count indicated by signal EVENTC in register 535, selects signal SECTOR for synchronization of the event-count, sets value CDRZERO to identify a sector pulse is generated at the end of the event-count, and transitions from state S7 to a state S8. State machine 550 waits in state S8 for a sector pulse that occurs when the current event-count in counter 530 decrements to zero. At the sector pulse, the next event-count from register 535 transfers to counter 530, and state machine 550 selects signal SECTOR to be pulsed when the event-count next reaches zero. Simultaneous with the current event-count decrementing to zero, the sector pulse which is generated loads the skip value from register 525 into latch 520 and starts counter 530 decrementing the new event-count. From state S8, state machine 550 transitions to stage S9 to perform the actions described for state S9. If signals SEOS and GSEC are both zero, indicating synchronization with signal DATASYNC and assertion of signal CDRINT, state machine 550 transitions from state S6 to state S13 or S15 after a signal SECOUNTING is deasserted to indicate that counter has stopped counting. Waiting for signal SECOUNTING avoids the chance that assertion signal DATASYNC for a current data segment will start a new event-count (i.e. load a new value from register 535 in counter 530) before counter 530 completes the current event-count. A control word having fields SEOS and GSEC equal to zero is read in the time between counter 530 decrementing to zero and the next data sync pulse. Whether state machine 550 transitions from state S6 to S13 or S15 depends on whether the data sector associated with the top control word in FIFO buffer 510 is to be skipped. Signal SKIPSEC, which changes only for the first control word of a data sector, indicates whether the current data sector is to be skipped and applies to the control word in FIFO buffer 510. Accordingly, a signal SECOK, which is equal to .sup..about. (SKIPSEC.vertline.DEFSECEQ), is zero only if the data sector associated with the control word in FIFO buffer 510 should be skipped. If none of signals SEOS, GSEC, and SECOK are asserted, the data sector is skipped, and an event-count for interrupt is not required because data segment sequencer 540 is not started for skipped data sectors. State machine 550 transitions from state S6 to state S13 and pops a control word from FIFO buffer 510. State machine 550 transitions from state S13 to a state S14 and waits until output signals for the next control word are ready. The order of control words in a format table is restricted so that the next control word after the control word removed in state S13 cannot have fields SEOS and GSEC respectively equal to 1 and 0 or 0 and 1. SEOS and GSEC respectively equal to 1 and 0 would indicate a filler inserted between two control words for the same data sector and is forbidden by the table syntax that inserts fillers only as the last control words. SEOS and GSEC respectively equal to 0 and 1 would indicate synchronization to a sector pulse, but the removed control word would have generated an interrupt to data segment sequencer 540, not a sector pulse. Accordingly, if only one of signals SEOS and GSEC is 1 when output signals from FIFO buffer 510 are ready, state machine 550 transitions from state S14 to a state S21 (FIG. 10) which causes an error interrupt. If in state S14, signals SEOS, GSEC, and SECOK remain unasserted, the next control word is for the same skipped data sector as the control word removed in state S13, and state machine 550 transitions from state S14 to state S11 and waits for an EOS pulse before transitioning to a state S12 and removing another control word from FIFO buffer 510 and then returning to state S6. If in state S14, signals SEOS and GSEC both become 1, the next event-count is synchronized with an EOS pulse and terminates with a sector pulse. State machine 550 transitions to state S10 and proceeds from state 9 as described above. If signals SEOS and GSEC are unasserted but signal SECOK is asserted, counter 530 should interrupt data segment sequencer 540 at a split in a data sector. For this case, state machine 550 transitions from state S6 or S14 to state S15 to load register 535 with an event-count indicated by signal EVENTC, select signal DATASYNC for synchronization of counter 530, and set value CDRZERO to indicate signal CDRINT asserts an interrupt at the end of the count. State machine 550 then transitions to a state S16 and waits for signal DATASYNC which starts counter 530 and causes state machine 550 to set signal SEL.sub.-- EVENT. State machine 550 the transitions to state S13 and proceeds from state S13 as described above. FIG. 10 illustrates an automatic alignment process in accordance with an embodiment of the invention. The automatic alignment process would typically be performed during a seek to a new zone so that the necessary track format information is available when the data heads reach the track sought. For automatic alignment, state machine 550 sequentially removes control words from FIFO buffer 510 and updates counts EOSCMP and/or CURRSEC in counters 574 and 560 for each removed control words indicating an EOS pulse and/or a sector pulse. When count EOSCMP is equal to count EOSCTR+1, the control word aligned for the next data frame, and at the next EOS pulse, state machine 550 transistions to state S6 to identify boundaries of data segments as described in reference to FIG. 9. At the start of the alignment process, state machine 550 transitions from state S0 of FIG. 6 to state S20 of FIG. 10 in response to assertion of a signal AUTOALIGN when signal SECGEN or ADVANCEDSA is asserted. Signal SECGEN being asserted indicates disk sequencer 140 generates sector pulses and uses control words that identify the timing of the sector pulses. Signal ADVANCEDSA indicates the control words have a syntax that identifies ends of data sectors and data frames even if sector pulses are generated externally. The automatic alignment process of FIG. 10 requires that the top control word in FIFO buffer 510 indicate synchronizing an event-count to an EOS pulse and generating a sector pulse. Accordingly, if both signals SEOS and GSEC are not asserted when state machine 550 is in state S20, there is an error, and state machine 550 transitions from state S20 to state S21 to assert a signal CCWERR and stop disk sequencer 140. Disk sequencer 140 remains stopped and state machine 550 remains in state S21 until being reset. If the top control word indicates an event-count from an EOS pulse to a sector pulse, state machine 550 transitions from state S20 to a state S22. If signal EOSPULSE is asserted, the automatic alignment process is suspended in state S22 until the frame count EOSCTR is stable (i.e. when signal EOSPULSE is deasserted), and then state machine 550 checks signal EOSCMPEQ1 to determine whether count EOSCMP is equal to count EOSCTR+1. If count EOSCMP is equal to count EOSCTR+1 and signal EOSPULSE is not asserted, the control words are correctly aligned for the next data frame, and state machine 550 transitions to a state S29. If count EOSCMP is not equal to count EOSCTR+1, the control words are not aligned to the position of the data heads relative to the track being accessed. State machine 550 decodes signals SEOS and GSEC and transitions from state S22 to a state S23, S24, or S21 unless signal EOSPULSE is asserted. In state S22, signals SEOS and GSEC should have values appropriate for the first control word of a data frame and cannot have only one of fields SEOS and GSEC set. Accordingly, if only one of signals SEOS and GSEC is asserted, an error occurred, and state machine 550 transitions from state 22 to state 21. If signals SEOS and GSEC are both unasserted for the first control word of a data frame, the control word indicates a data segment occupying an entire data frame. State machine 550 transitions from state S22 to state S23, increments count EOSCMP, and removes the control word from FIFO buffer 510. When state machine 550 returns from state S23 to S22, the top control word then in FIFO buffer 510 is again the first control word for the data frame indicated by count EOSCMP. If both signals SEOS and GSEC are asserted when state machine 550 is in state S22, the control word indicates an event-count from the start of a data frame (i.e. an EOS pulse) to a sector pulse. State machine 550 transitions from state S22 to S24, increments count EOSCMP for the start of the new data frame, increments count CURRSEC for the start of the new data sector, and removes the top control word from FIFO buffer 510. State machine 550 then transitions to a state S25 to decode any remaining control words for the data frame. If the top control word then in FIFO buffer 510 has fields SEOS and GSEC set to indicate synchronization with an EOS pulse at the start of a data frame, the control word is the first for the next frame, and state machine 550 transitions from state S25 back to state S22. If the top control word indicates a full data sector within a data frame, signals SEOS and GSEC are respectively 0 and 1; and state machine 550 transitions from state S25 to a state S28, increments count CURRSEC, and removes the control word from FIFO buffer 510 before returning to state S25. If the next control word is a filler, signals SEOS and GSEC are respectively 1 and 0; and state machine 550 transitions from state S25 to a state S27 and removes the control word from FIFO buffer 510 before returning to state S25. If the top control word indicates an event-count to an interrupt marking the end of the data frame, signals SEOS and GSEC are both 0; and state machine 550 transitions from state S25 to a state S26 and removes the control word from FIFO buffer 510 before returning to state S22. The control words are aligned if in state S22, count EOSCMP is equal to count EOSCTR+1 while signal EOS is not asserted. State machine 550 transitions from state S22 to state S29 and waits for an EOS pulse marking the start of the data frame aligned with the top control word in FIFO buffer 510. Data transfer can then proceed through the states described above in regard to FIG. 9. Although the invention has been described with reference to particular embodiments, the description is only an example of the invention's application and should not be taken as a limitation. In particular, the specific syntaxes for control words and format tables can be varied in a variety of ways in keeping with the invention. Further, the circuitry and counting processes disclosed can be varied. For example, even though preceding discussion described counts that are decremented to zero to identify the timing of a terminal event, counts can end at other cut-off values such as -1 and timing can be performed by incrementing a count until the count reaches a cut-off which varies according to the event-count. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.
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