Cyclical Redundancy Check Method and Apparatus

1. Field of Invention

This invention relates generally to digital data communication systems, particularly to the detection of transmission errors by the use of codes.

2. Related Art and Other Considerations

In a digital data communication system (including storage and retrieval from electronic memory, optical media or magnetic media) errors may occur as a result of noise, media defects or circuit failure. For reliable communications it is necessary to be reasonably sure of detecting all transmission errors. One way of doing this is by the use of cyclic redundancy check symbols or CRCs. In general CRCs are calculated by treating the data as the coefficients of a polynomial, dividing the data by another polynomial, and keeping the remainder. The data and CRC together make up a codeword which is then transmitted into the communication channel. At a receiving end of the channel a CRC is again. At allocated and compared to the original. A discrepancy between the two CRCs indicates a transmission error.

In some applications the data and CRC together are coded into an error-correcting code by adding error-correcting characters (ECC). After a correction procedure has been used the CRC can be used to detect errors that are either not detected or are miscorrected by the corrector.

The data can be viewed as a sequence of binary bits or a sequence of multi-bit symbols. In general a bit-oriented CRC is desired if the expected errors are random bit errors and a symbol-oriented CRC is desired if the expected errors are burst errors or random symbol errors.

FIG. 1 shows a typical bit-oriented 8-bit CRC generator where the serial input is on the DATA line. The generator in FIG. 1 will generate an 8-bit CRC character which can be used to detect any error which affects at most two bits in the serial data stream.

FIG. 2 shows a typical byte-oriented 8-bit CRC generator where D.sub.0 to D.sub.7 make up the input data byte. The generator in FIG. 2 will generate an 8-bit CRC character which can be used to detect any single-byte error. It cannot, however, detect every two-bit error since the two errors could be in two different bytes.

FIG. 3 shows a data transmission system which includes a dynamic RAM (DRAM) for temporary storage and a communication channel which could be, for example, magnetic storage. The system uses an ECC system to correct errors occurring in the communication channel. It also employs a two-byte CRC to check for errors in data retrieved from the DRAM and to check the results of the corrector. For ECC purposes the data is partitioned into three interleaves. Each interleave makes up one ECC codeword containing two ECC bytes which can correct one byte in each interleave. This is a common technique for making a system capable of correcting a burst error of three bytes but using an ECC capable of correcting only one byte. Conceptually, during ECC generation using the system of FIG. 3, the first data byte goes into ECC generator 1, the second byte goes into ECC generator 2, the third byte goes into ECC generator 3, the fourth byte goes into ECC generator 1, etc. After the data has been input to the generators the first ECC byte comes from the generator which follows the generator which received the last data byte and the remaining ECC bytes are taken from the generators in the same sequence as previously described. All signal lines are 8 bits wide.

The CRC system of FIG. 3 will detect all two-byte errors (and consequently all two-bit errors) in a block of data read from the DRAM. However, regardless of which CRC and ECC polynomials are used, it is possible to have three errors in one interleave which are not detected by the ECC or CRC.

The invention features both one stage and two stage CRC generation systems which include a programmable CRC generator/checker. The CRC generation/checker comprises a segmenting circuit (for segmenting an input stream of user data into a plurality m of sub-streams); a circuit which forms a linear combination of the plurality m of sub-streams and operates upon the linear combination in order to generate CRC bytes; and, a checker circuit for comparing the generated CRC bytes with previously generated CRC bytes.

In connection with preparation of CRC bytes prior to an encoding operation of a two stage system, input user data is first applied to a first CRC generator/checker for the generation of K.sub.1 number of primary CRC bytes. The input user data and associated resultant CRC bytes are then stored in a memory. Prior to encoding, a data block obtained from the memory (including both the user data and the primary CRC bytes) is applied to a second CRC generator/checker. User data from the memory is utilized by the second CRC generator/checker to generate verification CRC bytes. Following generation of the verification CRC information units, during K.sub.1 number of verification clock cycles, the first K.sub.1.sup.th verification CRC bytes are compared with the K.sub.1 number of primary CRC bytes for the purpose of detecting any errors occurring with respect to the memory. After the verification, a number K.sub.2 -K.sub.1 of verification CRC bytes generated by the second CRC generator/checker is optionally applied to the encoder as secondary CRC bytes, thereby adding additional CRC information prior to encoding and storage of a block on a storage medium.

In connection with processing of CRC information after a decoding operation of a two stage system, decoded user data (including CRC bytes) is applied both to the second CRC generator/checker and to the memory. The second CRC generator/checker receives the user data and CRC bytes in much the same manner as it does prior to an encoding operation, generating K.sub.2 number of verification CRC bytes for comparison with a like number of corresponding CRC bytes obtained from the decoded data. When the decoded data is retrieved from the memory for sending to a host, the decoded data is sent to the first CRC generator/checker to check for memory errors.

The first CRC generator/checker and the second CRC generator/checker are configured to generate as many as k number of CRC bytes. However, the number of CRC bytes K.sub.1 and K.sub.2 actually outputted by the respective first and second CRC generator/checkers is programmable, so that K.sub.1 and K.sub.2 may be unequal (e.g., K.sub.2 >K.sub.1) for a specified encoding/decoding operation and changeable from one encoding/decoding operation to another.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

FIG. 1 is a schematic block diagrammatic view of a typical bit-oriented 8-bit CRC generator.

FIG. 2 is a schematic block diagrammatic view of a typical byte-oriented 8-bit CRC generator.

FIG. 3 is a schematic block diagrammatic view of a data path which includes a CRC generator and checker, a DRAM, and a communication channel with ECC protection.

FIG. 4 is a schematic block diagrammatic view of a one stage CRC generation system according to an embodiment of the invention and provided with memory error checking.

FIG. 4A a schematic block diagrammatic view of a one stage CRC generation system of FIG. 4 which uses an adder and a conventional CRC generator.

FIG. 5 a schematic block diagrammatic view of the adder of FIG. 4A.

FIG. 6 is a schematic block diagrammatic view of a two stage CRC generation system according to an embodiment of the invention.

FIG. 7 is a schematic block diagrammatic view of a CRC generator/checker of the CRC generation system of FIG. 6.

FIG. 7A is a detailed schematic block diagrammatic view of an embodiment of the CRC generator/checker of FIG. 7.

FIG. 8 is a flowchart depicting basic steps executed by the CRC generation system of FIG. 6 in connection with two stage CRC generation prior to encoding.

The invention includes both one stage and two stage CRC generation systems. FIG. 4 shows a generalized one stage CRC generation system 400 collaterally provided with memory error checking; FIG. 6 shows a generalized two stage CRC generation system 600 with in-line memory checking. As discussed subsequently, both one stage CRC system 400 and two stage CRC system 600 employ a CRC generator/checker which can be a novel CRC generator/checker of the type basically illustrated in FIG. 7. In the embodiments illustrated in FIG. 4 and FIG. 6 the CRC systems 400 and 600 are used in conjunction with an ECC system which, in the preferred embodiment, is a three-way interleaved Reed-Solomon code.

The invention includes a method and apparatus that is capable of generating and checking CRC information units (e.g., bytes) and which is order-programmable. The CRC bytes are used to detect random errors occurring in DRAM and also to detect uncorrectable errors occurring in the communication channel.

The one stage CRC generation system 400 of FIG. 4 includes an encoding side and a decoding side. The encoding side of system 400 comprises CRC generator 401; memory (e.g., DRAM) 402; a set of ECC generators 420.sub.1, 420.sub.2, 420.sub.3 ; and a CRC checker circuit 422, all under control of encoding controller 426. The encoding side of system 400 applies encoded data (including both CRC and ECC bytes) to a communication channel 430. Communication channel 430 in turn outputs received data to the decoding side. The decoding side of system 400 includes a error correction unit 432 and a CRC checker circuit 434, both under the control of decoding controller 436.

The encoding side of system 400 receives input electrical signals at a port 440 (e.g., a port from a host device). As used herein, a port such as port 440 receives a sequence of electrical signals representing a user data stream, the user data stream comprising bytes where each byte is an element of the finite field GP(2.sup.P) and where a byte can be of any p-bit arbitrary length. The user data stream is organized into blocks of electrical signals representing digital data, each block containing n data symbols, wherein each block is mathematically represented by the polynomial D(x).

The electrical signals from port 440 are applied both to an input port GRC generator 401 and a first input port of a multiplexer 442. An output port of CRC generator 401 is connected to a second input port of multiplexer 442. An output port of multiplexer 442 is connected to a data input port of DRAM 402. A data output port of DRAM 402 is connected to: a first input port of channel multiplexer 444; an input port of CRC checker 422; and (via ECC input multiplexer 446) to a selected one of the ECC generators 420.sub.1, 420.sub.2, 420.sub.3. Output ports of the ECC generators 420 are connected to respective input ports of ECC output multiplexer 448.

Encoding controller 426 controls the sequencing (e.g, timing) and operating of the constituent elements of the encoding side of system 400, including the application of signals (e.g., multiplexing) to and from such elements. Controller 426 is connected to DRAM 402 by an address and control bus 450; to MUXes 442, 444, 446, and 448 by a MUX SELECT bus 452; to CRC generator 401 by CRC generator control bus 454; and, to CRC checker 422 by CRC checker control bus 456. These and other unillustrated connections of controller 426 are understood by the man skilled in the art, such as unillustrated connections for controlling ECC generators 420, for example.

On the decoding side of system 400, error correction unit 432 receives and corrects data from channel 430. The corrected data is output on data bus 460, which data is also applied to CRC checker 434. Decoding controller 436 is connected by CRC checker control bus 462 to control the operation of CRC checker 434.

In one embodiment, the CRC generator 401, CRC checker 422, and CRC checker 434 take the form of a novel generator/checker subsequently illustrated with respect to FIG. 7. However, in another embodiment (system 400A) illustrated in FIG. 4A, CRC generator 401, CRC checker 422, and CRC checker 434 are conventional CRC units augmented by m-way adder circuits 471, 472, and 474, respectively. That is, m-way adder circuits 471, 472, and 474 are connected upstream of the respective CRC generator/checkers 401, 422, 434. In other respects, the elements of system 400A resemble like-numbered elements of system 400.

FIG. 5 illustrates the configuration of m-way adder circuits 471, 472, and 474. Each m-way adder circuit includes an adder 480 and a register 482. In the system 400B shown in FIG. 4A, each m-way adder circuit is a 3-way adder circuit.

The system 400A shown in FIG. 4A resembles that shown in FIG. 3, except that the input user data from port 440 is segmented into groups of three bytes. The sum of the three bytes in each group is sent to the conventional CRC generator 410. The first byte in each group will ultimately be sent to ECC generator 420.sub.1, the second byte in each group will ultimately be sent to ECC generator 420.sub.2, and the third byte in each group will ultimately be sent to ECC generator 420.sub.3. The CRC generator 401 is only clocked once for each three bytes. In FIG. 5 the register 482 is reset on the third clock of each three-byte group.

FIG. 6 shows a two stage CRC generation system 600 which generally comprises a first CRC generator/checker 601 for generating primary CRC information; a memory (e.g., DRAM) 602; a second CRC generator/checker 603 (for, e.g., generating secondary CRC information); an encoder (such as a Reed Solomon encoder) 604; a data utilization device (such as a disk sub-system) 605; a decoder (such as a Reed Solomon decoder) 606; and, a controller 626.

System 600 also includes a host port 640 connected to an unillustrated host device. Host port 640 is connected to apply an input user data stream to a first input port of DRAM input multiplexer 642 and to a first input port of first CRC generator multiplexer 643. An output port of multiplexer 643 is connected to an input port of first CRC generator/checker 601. An output port of generator/checker 601 is connected to a second input port of DRAM input multiplexer 642. An output port of DRAM multiplexer 642 is connected to a first input port of DRAM 602. A first output port of DRAM 602 is connected both to a first input port of encoder input multiplexer 644 and to a first input port of second CRC generator/checker multiplexer 645. An output port of multiplexer 645 is connected to an input port of second CRC generator/checker 603. An output port of generator/checker 603 is connected to a second input port of encoder input multiplexer 644. An output port of multiplexer 644 is connected to an input port of encoder 604.

Disk sub-system 605 has both a data input port and a data output port. The data input port of disk sub-system 605 is connected to receive data from an output port of encoder 604. The data output port of disk sub-system 605 is connected to apply data to an input port of decoder 606.

An output port of decoder 606 is connected both to a second input port of second CRC generator multiplexer 645 and to a second data input port of DRAM 602. DRAM 602 has a second output port which is connected both to a host port 646 and to a second input port of first CRC generator multiplexer 643.

Controller 626 is connected to control the timing and sequencing of the operation of the constituent elements of system 600 as explained herein and as otherwise understood by the man skilled in the art. For example, controller 626 is connected to multiplexers 642, 643, 644, and 645 by a multiplexer select bus 650; to DRAM 602 by a DRAM address and control bus 652; to first CRC generator/checker 601 by a first CRC generator control bus 654; to the second CRC generator/checker 603 by a second CRC generator control bus 656; to encoder 604 by encoder control bus 690; and, to decoder 606 by decoder control bus 692. Controller 626 has a plurality of input ports 627.sub.1 and 627.sub.2 for obtaining input values for programming system 600. In particular, input port 627.sub.1 receives a value K.sub.1 determinative of the number of CRC bytes to be outputted by first CRC generator/checker 601; input port 627.sub.2 receives a value K.sub.2 determinative of the number of CRC bytes to be outputted by second CRC generator/checker 603.

In system 600, the first CRC generator/checker 601 is connected (via MUX 643) to host port (640) for generating primary CRC information based on input user data received from the host device. Memory 602 stores input user data received from the host device and the primary CRC information generated with respect thereto by the first CRC generator/checker 601. Second CRC generator/checker 603, connected to memory 602 via MUX 645, serves to check data retrieved from memory 602 and (optionally) to generate secondary CRC information based on data obtained from memory 602 including the primary CRC information. Encoder 604 forms encoded information by utilizing the data obtained from memory 602 including the primary CRC information as well as optional secondary CRC information obtained from second CRC generator/checker 603.

Decoder 606 receives input data and decodes the input data into decoded input data including decoded CRC information. Decoder 606 applies the decoded input data including the decoded CRC information to second CRC generator/checker 603 and to memory 602. Second CRC generator/checker 603 validates the decoded CRC information included in the decoded input data. First CRC generator/checker 601 is connected to receive decoded input data including the decided CRC information stored in memory 602 for validating decided CRC information received from memory 602.

Thus, in an encoding operation, second CRC generator/checker 603 compares the primary CRC information and verification CRC information generated by second CRC generator/checker 603 to determine errors attributable to memory 602. Encoder 604, is also connected to second CRC generator/checker 603 and also utilizes the secondary CRC information to form the encoded information. Conversely, in a decoding operation, first CRC generator/checker 601 determines errors attributable to memory 602.

FIG. 7 generally depicts a CRC generator/checker 700 suitable for use as the CRC generators and CRC checkers described herein. Generator/checker 700 basically includes a segmenting or inter leaving circuit 710; a switch or multiplexer 701; a circuit 712 for forming and using a linear combination of user data sub-streams to generate CRC information; and, an error checker circuit 714. Segmenting circuit 710 serves to segment an input stream of user data into a plurality m of sub-streams for application to switch 701. Switch 701 selects from the plurality of user data sub-streams in accordance with clock cycle and applies the selected user data sub-stream to circuit 712. The resultant CRC information, preferably in the form of CRC bytes, is outputted on parallel CRC bus 716. When generator/checker 700 is employed to check received CRC information previously generated on the basis of received data, the previously generated CRC information is compared by error checker 714 with verification CRC information currently generated (with respect to the same user data) by circuit 712. Should the previously generated and currently generated verification CRC information differ, an error signal is generated on CRC error line 718.

FIG. 7A shows structural details of one particular implementation of CRC generator/checker 700, and particular an implementation wherein m=3. Accordingly, segmenting circuit 710 of FIG. 7A is shown to include three sub-stream lines 707.sub.0, 707.sub.1, 707.sub.2. For reasons hereinafter discussed, sub-stream lines 707.sub.1 and 707.sub.2 have multipliers M.sub.1 and M.sub.2 connected thereon. Switch 701 is a three-input multiplexer having input selected by controller 626 (via a control bus). An output port of multiplexer 701 is connected to apply the sub-stream selected thereby to adder input bus 720.

As shown in FIG. 7A, circuit 712 comprises k number of sub-circuits 722.sub.1 -722.sub.k for generating as many as k bytes of CRC information with respect to the linear combination of sub-streams. As explained below, each subcircuit 722 is a divider having a divisor which is a power of .alpha.. Each sub-circuit 722 comprises an adder input multiplexer 702; an adder 703; a multiplier 704; a register 705 for storing CRC information; and, a register input multiplexer 708. Each adder 703 has a first input port, a second port, and an out put port. The first port of each adder 703 is connected to adder input bus 720 so that the plurality of sub-streams can be multiplexed thereto. Each multiplier 704 has an output port thereof selectively connected (via a first input port of associated multiplexer 702) to the second input port of its associated adder 703. Each register 705 has an input port connected to the output port of associated adder 703 and its output port is selectively connected (via a first input port of associated multiplexer 702) to the second input port of the adder 703. Each register 705 stores CRC information in the form of a CRC byte which is related to an error correcting code. Each of the multiplexers 702 and 708 are controlled by controller 626 (for example, via control bus 654).

From the foregoing it is understood that subcircuits 722 are numbered from a first sub-circuit 722.sub.1 to a k.sup.th sub-circuit 722.sub.k. Registers 705 of the sub-circuits 722 are connected so that CRC information stored therein can be rightwardly shifted. The rightward shift occurs after the n clock cycles executed to generate the CRC bytes in registers 705. After the n clock cycles are executed to generate the CRC bytes, the CRC bytes in registers 705 are shifted rightwardly a desired number of clocks (as input to controller 626), one register (e.g., one sub-circuit) per clock. For example, considering the operation of first CRC generator/checker 601, if an input value of 4 is inputted at input port 627.sub.1 of controller 626, during a first shift clock cycle the CRC byte in register 705.sub.1 is shifted out on CRC bus 716; the CRC byte in register 705.sub.2 is shifted into register 705.sub.1 ; the CRC byte in register 705.sub.3 is shifted into register 705.sub.2 ; and so forth. Such shifting continues for four shift clock cycles, so that the CRC values initially in registers 705.sub.1 -705.sub.4 are ultimately shifted out to CRC bus 716.

The values of the multipliers 704 are chosen so that the multiplier multiplies by a power of a field element .alpha., the power being r+1 for the multiplier of the first sub-circuit 722.sub.1 and r+k for the multiplier of the k.sup.th sub-circuit 722.sub.k.

The output port of adder 703.sub.1 is connected to error checker circuit 714 and to MUX 708.sub.k. As illustrated in FIG. 7A, error checker circuit 714 comprises an 8-input OR gate 709 which is connected to the output port of adder 703.sub.1. Error checker circuit 714 further includes and AND gate 730 and an R/S flip-flop or latch 732. An output pin of OR gate 709 is connected to a first input pin of AND gate 730, a second input pin of AND gate 730 being connected to receive a signal CRC.sub.-- TIME from controller 626. An output pin of AND gate 730 is connected to a set pin of flip-flop 732; a reset pin of flip-flop 732 receives a signal RST from controller 626. Since adder 703.sub.1 is involved in verifying previously-generated CRC information with currently generated CRC information in the manner hereinafter described, sub-circuit 722.sub.1 is denominated the "comparison" sub-circuit.

FIG. 8 depicts basic steps executed by system 600 of FIG. 6 in connection with two stage CRC generation prior to encoding. Prior to actual CRC generation, as depicted by step 800 of FIG. 8, controller 626 is programmed with values K.sub.1 and K.sub.2 for determining the amount (e.g., number of bytes ranging from 1 to k) of CRC information to be shifted out of first CRC generator 601 and second CRC generator 602, respectively.

In connection with encoding aspects of the CRC generation of system 600 of FIG. 6, bytes of user data from the host are sent to first CRC generator/checker 601 and are written into DRAM 602. Step 802 of FIG. 8 shows first CRC generator/checker 601 obtaining n bytes of user data from the host and using the n bytes of user data to form CRC bytes C.sub.j (j=1,2, . . . k) during n clock cycles. After the n clock cycles for generating CRC bytes C.sub.j (j=1,2, . . . k), during the next K.sub.1 clock cycles (step 804) first CRC generator/checker 601 shifts CRC bytes

also known as the primary CRC bytes, into DRAM 602.

In connection with the execution of step 802, and specifically with reference to generator/checker 700 illustrated in FIG. 7, during the first of the n clock cycles multiplexer 701 switches the first of the n user data bytes onto adder input bus 720. Concerning such first byte obtained on line 707.sub.0 of segmenting circuit 710, M.sub.0 for segmenting circuit 710 may always be selected as a .alpha..sup.0 and hence no multiplier is needed for M.sub.0. During the second of the n clock cycles multiplexer 701 switches the second of the n user data bytes (multiplied by M.sub.1) onto adder input bus 720; during the third of the n clock cycles multiplexer 701 switches the third of the n user data bytes (multiplied by M.sub.2) onto adder input bus 720.

Thus, during a data portion of generation or checking depicted by step 802, MUX 701 supplies the dividers (e.g., sub-circuits 722) with the input user data multiplied by M.sub.0 (from lead 707.sub.0) on the first clock and on every third clock following the first, it supplies the dividers 722 with the input user data multiplied by M.sub.1 (from lead 707.sub.1) on the second clock and on every third clock following the second and it supplies the dividers 722 with the input user data multiplied by M.sub.2 (from lead 707.sub.2) on the third clock and on every third clock following the third. In execution of step 802, MUXes 702 supply adders 703 with the outputs of constant multipliers 704 on the first clock and on every third clock following the first. MUXes 702 supply the adders 703 with the outputs of registers 705 on the other clocks. MUXes 708 supply the adder outputs to the inputs of registers 705.

During execution of step 804, which is a CRC portion of CRC generation or checking, i.e. when CRC bytes are being output, MUXes 708 connect the registers 705 into one shift register. During generation this allows the programmed number (K.sub.1) of CRC bytes which reside in the registers 705 to be shifted out onto CRC bus 716. K.sub.1 clock cycles are required in step 804 to shift out the K.sub.1 number of desired CRC bytes. After the K.sub.1 clock cycles, all registers 705 are re-initialized at zero, so that CRC bytes K.sub.1 . . . k are discarded.

As mentioned before, step 804 shows the shifting of the primary CRC bytes C.sub.j (j=1, . . . K.sub.1) into DRAM 602. At some later time (depicted by step 806), the block of data in DRAM 602 and the primary CRC bytes C.sub.j (j=1, . . . K.sub.1) are read from DRAM 602 and sent both to Reed-Solomon encoder 604 (via MUX 644) and to second GRC generator/checker 603 (via MUX 645). The data from the block in DRAM 602 is applied to second CRC generator/checker 603 in a manner such that n number of user data bytes are applied followed by the K.sub.1 number of primary CRC bytes C.sub.j (j=1, . . . K.sub.1) generated by generator 601 with respect to those n bytes.

In step 806, second CRC generator/checker 603 receives the user data from DRAM 602 in groups of n number of input bytes. Using the n number of input bytes, during n number of clock cycles the second CRC generator/checker 603 generates k number of verification CRC bytes C.sub.j ' in similar manner as first CRC generator/checker 601 generated the primary CRC bytes C.sub.j. If the user data bytes obtained from DRAM 602 in step 806 are identical to the user data bytes shifted into DRAM 602 from the host as occurs when DRAM is operating properly, then the verification CRC bytes C.sub.j ' will equal the respective primary CRC bytes C.sub.j.

After second CRC generator/checker 603 has generated the verification CRC bytes C.sub.j ' during the n clock cycles of step 806, at step 808 secondary CRC generator/checker 603 performs a checking operation to compare primary CRC bytes C.sub.j (j=1, . . . K.sub.1) with the verification CRC bytes C.sub.j ' (j=1, . . . K.sub.1). In so doing, MUX 701 is operated to apply the primary CRC bytes C.sub.j (j=1, . . . K.sub.1) from line 707.sub.0 of segmenting circuit 710 to the first input port of ADDER 703.sub.1, the application being made one byte per clock cycle for each of K.sub.1 number of verification clock cycles. During a first of the verification clock cycles, the first primary CRC byte C.sub.1 is applied to the first input port of ADDER 703.sub.1 and the first verification CRC byte C.sub.1 ' stored in register 705.sub.1 is applied to the second input port of ADDER 703.sub.1, so that the result of the addition operation of ADDER 703.sub.1 is applied to check circuit 714. With each verification clock cycle, the verification CRC bytes C.sub.j ' in registers 705.sub.j are shifted rightwardly by one sub-circuit 722 (e.g, to the next rightwardly adjacent register), as in the manner of CRC byte shift/readout described hereinbefore, so that after the first verification clock cycle register 705.sub.1 will contain the verification CRC byte C.sub.2 '. Similarly, during a second of the verification clock cycles, the second primary CRC byte C.sub.2 is applied to the first input port of ADDER 703.sub.1 and the second verification CRC byte C.sub.2 ' now stored in register 705.sub.1 is applied to the second input port of ADDER 703.sub.1, again with the result of the addition operation of ADDER 703.sub.1 is applied to check circuit 714. Such shifting and adding continues for each of the K.sub.1 number of verification clock cycles, so that CRC bytes C.sub.j are compared with corresponding bytes C.sub.j ' for J=1, . . . K.sub.1.

During execution of step 808, checker circuit 714 is effectively enabled by a signal CRC.sub.-- TIME from controller 626. When checker circuit 714 is enabled, any inequality between a primary CRC byte C.sub.j and its corresponding verification byte C.sub.j ' causes a non-zero output of OR gate 709 to be gated through AND gate 730 for application to flip-flop 732. The detected inequality causes flip-flop 732 to output a CRC ERROR signal to controller 626 on line 718.

If the value K.sub.2 inputted to controller 626 with respect to second CRC generator/checker 603 is greater than the value K.sub.1, then at step 810 second CRC generator/checker 603 will shift out (on line 716) a number (K.sub.2 -K.sub.1) of secondary CRC bytes, specifically CRC bytes C.sub.j ' (j=K.sub.1 +1, . . . K.sub.2). In this regard., after execution of step 808, registers 705.sub.1, 705.sub.2, 705.sub.3, etc. will contain

The shifting out of secondary CRC bytes C.sub.j ' (j=K.sub.1 +1, . . . K.sub.2) occurs in K.sub.2 -K.sub.1 clock cycles, and is accomplished in the hereinbefore described manner of controlling multiplexers 708 for the rightward shift operation.

After a block of data including both the primary CRC bytes C.sub.j (j=1, . . . K.sub.1) and the secondary CRC bytes C.sub.j '(j=K.sub.1, . . . K.sub.2) is encoded by encoder 604, the encoded block is written to disk subsystem 605. It will be appreciated by the man skilled in the art that encoder 604 generates ECC bytes for each interleave of data, preferably m number of interleaves using a ECC generator polynomial.

Thus, second CRC generator/checker 603 checks whether errors occurred in DRAM 602 and also may generate additional CRC bytes, known as secondary CRC bytes. These secondary CRC bytes may be written to the disk along with the primary CRC bytes. Inclusion of the secondary CRC bytes can be important because the data read from disk may require more protection than the DRAM data owing to the fact that data from disk is more susceptible to error than data from DRAM 602.

As described above, second CRC generator/checker 603 allows the incoming primary CRC bytes to be checked one at a time. It also shifts out as many of the k-K.sub.1 remaining CRC bytes as are specified by the input value K.sub.2, these further shifted bytes being secondary CRC bytes). This facilitates a situation in which only the first K.sub.1 number of primary CRC bytes are written into the DRAM 602 but K.sub.2 are written to disk (K.sub.2 >K.sub.1). The checking of the primary CRC bytes is accomplished by adding the incoming primary CRC bytes to the newly generated verification CRC bytes by adder 703.sub.1 and then detecting a non-zero byte with OR gate 709. While CRC bytes are being input to the checker the CRC.sub.-- TIME signal is ON and then any miscompares will set CRC.sub.-- ERROR latch 732. The order of the generator is programmable since the CRC bytes are calculated independently of each other and hence only the desired number of CRC bytes need be shifted out.

In connection with a decoding operation executed by system 600 of FIG. 6, a data block is read from the disk by disk sub-system 605 and is sent to Reed-Solomon decoder 606. Corrected data obtained by decoder 606 (including CRC bytes) is sent both to second CRC generator/checker 603 (to check for uncorrectable errors) and to DRAM 602.

Second CRC generator/checker 603 receives data bytes and CRC bytes from the decoded block in much the same manner as it does prior to an encoding operation. Upon receiving each group of n data bytes, second CRC generator/checker 603 generates K.sub.2 number of verification CRC bytes for comparison with a like number of corresponding CRC bytes obtained from the decoded data. Operation of the second CRC generator/checker 603 in connection with post-decode CRC processing is analogous to steps 806 and 808 above-discussed, and understand with reference thereto. Second CRC generator/checker 603 performs a verification with respect to CRC bytes obtained from the disk sub-system 605.

In connection with post-decode CRC processing, when the data block is :retrieved from DRAM 602 for sending to the host, the data block is again sent to first CRC generator/checker 601 to check for DRAM errors. Operation of the first CRC generator/checker 601 in connection with post-decode CRC processing is also analogous to steps 806 and 808 above-discussed, and understand with reference thereto, with the proviso to that K.sub.1 number of CRC bytes are processed by the first CRC generator/checker 601.

In the illustrated embodiments, the data stream which feeds the CRC generator/checker 700 is segmented into groups of three bytes. A linear combination of the three bytes is then sent to a collection of circuits (e.g., sub-circuits 722) which divide by X+.alpha..sup.r+j for j=1, 2, . . . , k where .alpha. is a primitive element of the field representation of the data bytes. In the preferred embodiment the field is GF(256=2.sup.8). The linear combination may be a simple sum, leading to the embodiment shown in FIG. 4A.

While a simple linear combination may be simple to implement, it is possible to have two bytes each with a single bit error in a block of data read from the DRAM which are not detectable owing to the fact that if the same error is in two bytes of a three-byte group then the two errors will cancel when summed together before being sent to the CRC generator/checker 700. Alternately the linear combination can be chosen so that if the three bytes in the group each have no more than a single "1" in each byte, then the linear combination will be non-zero. In that case, if every byte in error in a block of data contains only a single bit in error, then k CRC bytes can be used to detect k bytes in error even if some of the bytes in error are in the same three-byte group.

Using a linear combination of every three input bytes allows the CRC bytes to be generated in such a way as to increase the minimum distance of the ECC code by one for each CRC byte. This can be done because the ECC consists of three interleaves.

The CRC bytes are generated according to the following rule: Let the input bytes be denoted as D.sub.0 to D.sub.n-1. These bytes make up the coefficients of three polynomials, F.sup.0 (x), F.sup.1 (x) and F.sup.2 (x), thusly: ##EQU1## for s=0, 1, 2 where t.sub.s is the largest integer that is less than (n-s)/3. These polynomials represent the division of D(x) into three groups, e.g. D.sub.0 is in F.sup.0, D.sub.1 is in F.sup.1, D.sub.2 is in F.sup.2, D.sub.3 is in F.sup.0, etc. These polynomials are precisely the same polynomials that are processed by the Reed-Solomon encoder 604 to form the three Reed-Solomon code words. G(x) is defined as a linear combination of F.sup.0 (x), F.sup.1 (x) and F.sup.2 (x) thusly: ##EQU2##

The system 400A in FIG. 4A corresponds to the case where the multipliers, M.sub.s, are all 1. The system 600 in FIG. 6 corresponds to the case where the M.sub.s are chosen so that aM.sub.0 +bM.sub.1 +cM.sub.3 is non-zero as long as a, b and c each contain a single "1". The coefficients of G(x) are linear combinations of three consecutive input bytes, e.g the first coefficient of G(x) is D.sub.0 M.sub.0 +D.sub.1 M.sub.1 +D.sub.2 M.sub.2, the second coefficient of G(x) is D.sub.3 M.sub.0 +D.sub.4 M.sub.1 +D.sub.5 M.sub.2, etc.

The j.sup.th CRC byte, C.sub.j, is defined as:

for j=1, 2, . . . , k.

If only k or less of the input bytes D.sub.i were non-zero, and the non-zero bytes each consisted of a single "1", then G(x) would have at most k non-zero coefficients and G(x) could not be zero. All of the CRC bytes are obtained by dividing G(x) by consecutive powers of .alpha., and therefore if there are k CRC bytes then at least one of the CRC bytes would be non-zero. This is equivalent to saying that any set of k or less single-bit errors would be detected.

The Reed-Solomon code to be used in conjunction with the new CRC generator has a generator polynomial which has, as roots, v consecutive powers of .alpha., from .alpha..sup.r-v+1 to .alpha..sup.r. Let E(x) be an error polynomial which has been added to one of the ECC interleaves, i.e. one of the Reed-Solomon code words. If the error is. not detectable by the ECC, then E(x) must have roots which include .alpha..sup.r-v+1 to .alpha..sup.r. Since the factors used in CRC generation have as roots the k consecutive powers of .alpha., from .alpha..sup.r+1 to .alpha..sup.r+k, for the error not to be detected by the CRCs, M.sub.S E(x) must have roots which include .alpha..sup.r+1 to .alpha..sup.r+k. This implies that E(x) must have roots which include .alpha..sup.r-v+1 to .alpha..sup.r+k, which means that the error has more than k+v bytes in error.

The set of M.sub.1 and M.sub.2 constants which cause a +bM.sub.1 +cM.sub.2 to be non-zero for all a, b and c which have a single "1" is not unique. Therefore M.sub.1 and M.sub.2 are picked to satisfy that condition and to also cause linear combinations of other classes to be non-zero. For example, in the preferred embodiment the field generator polynomial is X.sup.8 +X.sup.4 +X.sup.3 +X.sup.2 +1. M.sub.1 is chosen to be .alpha..sup.25 and M.sub.2 is chosen to be .alpha..sup.112. This choice cause a +bM.sub.1 +cM.sub.2 to be non-zero for the case where only two of the three bytes a, b and c are non-zero and the two bytes each have a single 3-bit or shorter non-zero bit sequence. After adding this condition the choice of M.sub.1 and M.sub.2 is still not unique, therefore an additional condition is added which is to maximize the number of two-byte combinations where each byte can have a single 4-bit or shorter non-zero bit sequence. The M.sub.1 and M.sub.2 multipliers each consist of a set of XOR gates such that the output bits are linear combinations, i.e. XOR sums, of certain subsets of the input bits and these subsets are listed in Table I and Table II.

For example, the CRC generator/checker of this invention has been illustrated in conjunction with a three-way interleaved Reed-Solomon code. It may alternately be employed with, for example, a two-way or four-way interleaved code. The principles taught with the illustrated CRC can be applied to a CRC for use in conjunction with encoders not limited to the Reed-Solomon design. The method of this invention can be used for codeword generation, codeword checking or both. It can be used between a computer host and a DRAM, between a DRAM and a magnetic disk, or between two ends of any communication channel. The same CRC can be used in more than one communication channel of a system, for example both between the host and a DRAM and between the DRAM and storage disk.

The CRC of this invention can be programmed to generate k CRC bytes. In conjunction with a Reed-Solomon code the CRC allows detection of k more errors than the Reed-Solomon code is callable of detecting. The number k can be tailored to the propensity for errors of a given communication channel. When the same CRC is used for more than one communication channel, different values of k can be selected for each channel. The CRC of this invention can also be utilized in a non-programmable embodiment having a fixed number of CRC bytes.

A hardware implementation of this invention has been illustrated. The CRC can also be implemented with software. Specific circuitry to implement the block diagram of FIG. 7 of this invent:ion can be readily designed by a routiner in the electronic arts. Variations on the block diagram will also be apparent to those skilled in the art. Specific examples of a field generator polynomial and of the constant multipliers M.sub.S have been supplied. Other constants and polynomials can be chosen, as taught herein. For example, instead of dividing by two eight bit polynomials, division can instead occur with one sixteen bit polynomial.

U.S. patent application Ser. No. 08/326,126 simultaneously filed by Zook et al., entitled "ERROR CORRECTION METHOD AND APPARATUS", and incorporated herein by reference, provides one non-limiting example of a specific utilization of the present invention.

While the preferred embodiment has been disclosed and described in detail herein, it will be obvious to those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope thereof.