MECHANISM TO IMPROVE FAULT ISOLATION AND DIAGNOSIS IN COMPUTERS

Abstract: A system and method for improving the isolation and diagnosis of hardware faults in a computing system wherein means are provided for indicating whether unusable data has previously triggered diagnosis of the hardware fault that caused the data to be unusable. If diagnosis has not been performed, the flag is not set. If diagnosis has already been performed, the flag is set. One embodiment comprises an interface which is used to convey data from one subsystem to another. When the interface receives data from the first subsystem, the data is examined to determine whether it contains an uncorrectable error (including missing data). If the data contains an uncorrectable error, the interface examines the flag corresponding to the data to determine whether hardware fault diagnosis has already been initiated. If diagnosis has already been initiated, the data is passed to the second subsystem without initiating further diagnosis. If diagnosis has not been initiated, the interface initiates diagnosis and sets the flag to indicate that diagnosis has already been initiated. The data and corresponding flag are then passed to the second subsystem. If the data contains an uncorrectable error, data error handling procedures will be performed by the subsystem that requested the data, regardless of the value of the corresponding flag.

Description: TITLE: MECHANISM TO IMPROVE FAULT ISOLATION AND DIAGNOSIS IN COMPUTERS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates generally to error detection and handling in computer systems, and more particularly to a method and apparatus for identifying data which is unusable and for initiating or inhibiting diagnosis of the hardware faults that caused the data to be unusable 2. Description of Related Art Computer systems typically include a number of active devices or components such as processors, I O bπdges and graphics devices, as well as a memory system. Any of these devices, or the interconnections between them, can expeπence hardware faults which cause errors in data or difficulty reaching data through the faulty hardware Many error management techniques have been developed to aid diagnosis and limit the effect of these errors. One simple technique is parity checking. Parity checking utilizes a single bit (the parity bit) associated with a piece of data (typically a byte) to determine whether there is a single-bit error m the data. Parity checking cannot detect multiple-bit errors, however, and provides no means for collecting even single-bit errors. A more sophisticated system uses error correction codes (ECC) to detect and even to correct some errors, ("error detection correction" will be used generally herem to identify the systems and bits, or codes, which are used m both error detection and error correction, including ECC codes and parity.) An ECC code consists of a group of bits associated with a piece of data. A typical ECC system may use eight ECC bits (an ECC code) to detect and correct errors in a 64-bit piece of data. The ECC bits provide enough information for an ECC algorithm to detect and correct a single-bit error, or to detect errors in two bits. If this particular system detects errors in two bits, the errors cannot be corrected. If a data error can be corrected (e.g., if the memory system uses a single-bit correcting ECC code and there is a single-bit error), the error is simply corrected and operation of the system continues normally If an error cannot be corrected, it may propagate through the system, causmg additional errors and prompting diagnoses of hardware faults which may not exist. For example, with reference to the system illustrated m Fig. 1, a fault m cache 12 may cause an error m a data value stored m the cache If the data in cache 12 is in a modified or "dirty" state, it may have to be copied out of the cache so that other processors can use it. If an error m the modified cache line cannot be corrected, hardware diagnosis may be initiated to determine the source of the error. The error may then be propagated when it is wπtten by processor 11 to the bus 19 and/or mam memory 18. If another processor (e.g. processor 13) reads the data value and stores the data value in cache 14, it will see the uncorrectable error and may initiate a second hardware diagnosis This second diagnosis may indicate a hardware fault m processor 13, cache 14 or mam memory 18, when the error actually arose m cache 12 The error may propagate throughout the system, including processors and caches (e.g. 15, 16) which are interfaced remotely to the system (e.g. by interface 17). If a data error cannot be corrected (e.g., if the memory system only uses parity checking, or if there are too many bit errors for an ECC system to correct), the data may be referred to as unusable data. When a pπor art system attempts to access a piece of unusable data, the device from which the data is requested may respond in one of several ways. In one instance, a processor accessmg the unusable data may retπeve the data, determine that it is unusable, and append a new ECC code based on the unusable data to allow subsequent eπors to be detected. This would allow the onginal eπor to spread through the system without detection In another instance, the memory storing the unusable data might simply not return any data at all. In contrast to the previously descπbed implementation, this would prevent the spread of the eπors therein In response to the failure to return any data, the device attempting to access the data would tune out and initiate diagnosis to determme the source of the eπor. As mdicated above, however, the eπor may have aπsen pπor to this access, so it is likely that the diagnosis will provide no useful information. In fact, the initiation of the diagnosis may actually confuse the issue of where the eπor arose, smce the hardware involved m the access may not have caused the eπor. This implementation can also suffer substantial performance losses, smce each device that attempts to access the data can time out and initiate a diagnosis, both of which waste otherwise useful processmg power. In another instance, the memory containing the faulty data may return the data and the associated ECC code as they were stored (i.e., with eπors). In this situation, the processor accessmg the data would initiate hardware diagnosis, which would likely turn out to be futile and confusing. In another instance, the memory may return a predetermmed ECC code which indicates a multiple-bit eπor. It could be difficult, however, for some devices accessmg the data to distmguish between this predetermmed ECC code (which indicates generally that the data is corrupted) and an ECC code which represents an actual, multiple-bit eπor. This difficulty could be mcreased if a subsequent smgle-bit eπor occuπed m the transmission path between the memory and the accessmg device. Whatever the response of the device from which the data was requested, an access to unusable data usually results in one of two standard responses by the computer itself. The first of the standard responses is for the computer to interrupt its operation and reboot itself. This response, of course, results m the termination of all applications executing on the computer and the loss of all work performed by the applications up to that pomt The applications have to be re-started and any lost work must be performed agam One of the significant problems with this response is that even those applications that did not access the unusable data, and would not have accessed this data, are nevertheless terminated. The second of the standard responses is to provide an indication of the unusable data whenever the data is accessed This may be accomplished by simply failing to provide the data (which typically causes the device requesting the data to time out,) or by providing the data along with an ECC code which indicates that the data is unusable This second response resolves the problem of indiscriminately terminating all applications, as only those applications that access the data are aware of the eπor and have to handle the eπor (e.g by terminating themselves) The device that receives the data, however, is aware only that it has not received eπor-free data It may therefore be difficult for this device to determme how the eπor arose. Consequently, eπor reports may be generated each time the data is accessed, which may lead to unnecessary diagnoses or diagnoses of hardware failures which may not actually have occuπed. In a system which uses a specific ECC code to mdicate that unusable data has been detected earlier, the occuπence of an additional, later, single-bit eπor may further confuse the situation. Additionally, as mdicated above, waiting for the requesting device to time out increases the average latency of memory accesses and degrades the performance of the system SUMMARY OF THE INVENTION The problems outlined above may in large part be solved by a system and method for improving the isolation and diagnosis of hardware faults m a computing system Generally speaking, the system provides a mechanism for indicating whether unusable data has previously tπggered diagnosis of the hardware fault that caused the data to be unusable. The mechanism employs a flag associated with the data that indicates whether diagnosis has been performed. If diagnosis has not been performed, the flag is not set (i.e., the flag has a "false" value.) If diagnosis has already been performed, the flag is set (i.e., the flag has a "true" value.) Data may be unusable because it contains eπors which are not coπectable by available eπor coπecπon mechanisms, or because it is missing (I e., it is never received.) In one embodiment, whenever data is requested but not received, and whenever received data contains uncoπectable eπors, status information is captured and diagnosis of the hardware fault that caused the eπor is mitiated. The unusable data is passed on, along with a flag which is set to mdicate that the data is unusable. If the received, unusable data had a flag which was not set, the flag is set. If the received, unusable data did not have a flag, the flag is generated and passed on with the unusable data. If the passed-on data is covered by eπor detection/coπection, a new eπor detection/coπection code is generated to cover the unusable data as well as possibly the flag so that any further eπors can be detected. Thus, any uncoπectable eπors or missing data will be mdicated by the flag, but the fault that caused the data to be unusable will not be re-diagnosed each time the unusable data is passed on. One embodiment compπses an interface which is used to convey data from one component or subsystem to another When the interface receives data from the first subsystem, the data is examined to determme whether it contains an uncoπectable eπor (including missing data ) If the data contains an uncoπectable eπor, the interface examines the flag coπesponding to the data to determme whether hardware fault diagnosis has already been mitiated. If diagnosis has already been mitiated, the data is passed to the second subsystem without initiating further diagnosis. If diagnosis has not been mitiated, or if the flag itself is missing or unusable so that it is not clear whether diagnosis has or has not been performed, the interface initiates diagnosis and sets the flag to mdicate that diagnosis has already been performed The data (and coπespondmg flag) are then passed to the second subsystem. If the data contains an uncoπectable eπor, data eπor handlmg procedures will be performed, regardless of the value of the coπespondmg flag In one embodiment, the interface compπses a circuit that accepts data which does not mclude a flag and produces a coπespondmg flag. In this embodiment, data consisting of a value and an eπor detection/coπection code is input to a detector/coπector The detector/coπector checks the value against the eπor detection/coπection code to determine whether the value is coπect ("Coπect" is used herem to descπbe data for which the coπespondmg eπor detection/coπection code mdicates no eπors ) If the value is coπect, the value is output on a data lme while a "false" signal is asserted on a flag lme If the value is mcoπect but coπectable, the value is coπected and then output on the data lme while a "false" signal is asserted on the flag lme. If the value is mcoπect and cannot be coπected usmg the eπor detection/coπection code, hardware diagnosis is mitiated and a "true" signal is asserted on the flag lme When the "true" signal is asserted, the data lme may output either the uncoπectable value or a predetermined value (e.g some value descπbmg the detected eπor.) The circuit may mclude an ECC generator that produces an eπor detection coπection code coπespondmg to the value and the flag output by the circuit BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the invention will become apparent upon reading the following detailed descπpuon and upon reference to the accompanying drawings in which Fig. 1 is a block diagram of a first shared memory computing system Figs. 2a and 2b are block diagrams illustrating a seπes of subsystems. Fig. 3 is a flow diagram illustrating the operation of an interface for a subsystem m one embodiment. Fig. 4 is a flow diagram illustrating the operation of an interface for a subsystem in an alternate embodiment Fig. 5 is a flow diagram illustrating the operation of an interface for a subsystem in another alternate embodiment. Fig. 6 is a block diagram illustrating an interface circuit m accordance with one embodiment. Fig. 7 is a block diagram illustrating an interface circuit m accordance with an alternate embodiment. While the invention is susceptible to vaπous modifications and alternative forms, specific embodiments thereof are shown by way of example m the drawings and will herem be descπbed in detail. It should be understood, however, that the drawings and detailed descπption set forth below are not mtended to limit the invention to the particular form disclosed, but on the contrary, are mtended to cover all modifications, equivalents and alternatives falling within the spiπt and scope of the present invention as defined by the appended claims. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the invention is descπbed below It should be noted that this embodiment is presented as an example and that many other embodiments are possible In one embodiment, an interface is implemented between a transmitting subsystem and a receiving subsystem. The interface is configured to determme whether data transmitted from the transmitting subsystem to the receivmg subsystem contains unusable data and, if so, whether hardware fault diagnosis has already been mitiated to locate the source of the eπor that caused the data to be unusable If there is an mdication that diagnosis has already been mitiated for the detected eπor, no additional diagnosis is mitiated. If there is no such mdication, hardware diagnosis will be mitiated. For the purposes of this disclosure, "initiating" hardware diagnosis means taking an action that causes or is part of the diagnosis For example, one embodiment may initiate hardware diagnosis by producmg a signal which mdicates that diagnosis should be performed This signal may be transmitted to diagnosis circuitry to cause that circuitry to begin diagnosis, or it may be stored for later use m the diagnosis In another embodiment, diagnosis may be mitiated by stoπng an mdication of the eπor that necessitates the diagnosis or by stoπng status information related to the eπor or the hardware that detected the eπor. Hardware diagnosis may be performed by the same circuitry that initiates the diagnosis, or it may be performed by separate diagnosis circuitry Referring to Fig. 2a, a block diagram illustrating a seπes of subsystems is shown. Subsystem 121 is shown conveying data to subsystem 122. The data may be conveyed in response to a request from subsystem 122, or a request from another subsystem which will receive the data from subsystem 122 Subsystem 121 and other subsystems may also utilize an interface of the type employed m subsystem 122. Subsystem 121 is configured to transmit to subsystem 122 information including data, a coπespondmg flag, and an eπor detection/coπection code coπespondmg to the data and flag The flag is indicative of a previously mitiated diagnosis of the hardware fault which caused the data to be unusable In this embodiment, the eπor detection coπection code covers both the data and the flag, enablmg eπors m either the data or the flag to be detected. In other embodiments, the eπor detection/coπection code may cover only the data, only the flag, or neither The data transmitted by subsystem 121 is received by an interface within subsystem 122 The interface includes an eπor detection/coπection unit and a status capture / diagnosis control (status/diagnosis) unit The eπor detection coπection unit examines the information to determme whether the data and/or flag contain any eπors, as determined by the eπor detection/coπection code. (It is assumed in this instance that the eπor detection/coπection code transmitted by subsystem 121 has been generated based upon the data and flag, regardless of whether or not the data is usable.) If no eπors are detected, the interface simply passes the information through to the subsystem, along with the flag and eπor detection coπection code. If an eπor is detected, the eπor detection/coπection unit determines whether or not the eπor is coπectable. (For the purposes of this disclosure, "coπectable" data mcludes both data havmg coπectable eπors and data havmg no eπors at all.) If the eπor is coπectable, the eπor detection/coπection unit coπects it and passes the information through to the subsystem. If the eπor is not coπectable, several actions are taken by the status/diagnosis unit, regardless of whether or not the flag mdicates that the data is unusable. These actions mclude: stoπng status information; setting the flag to mdicate that the data is unusable; initiating hardware diagnosis; and generating a new eπor detection/coπection code based on the unusable data and the flag. The new eπor detection/coπection code is generated to enable the mterfaces of subsequent subsystems to determme whether eπors subsequent to this one have occuπed This new information is then passed through to the subsystem. If subsystem 122 is the end-user of the data (I e., it is not simply routing the data to another subsystem,) it must determme whether or not the data which has received is usable. If the flag is not set, the data is usable, and operation of the subsystem proceeds normally If the flag is set, the data is unusable, and the subsystem must take any steps which are necessary to handle the data eπor These steps may mclude, for example, notification of clients, eπor recovery procedures or termination of client processes. If subsystem 122 is not the end-user of the data (i.e., if its purpose m handling the data is merely to route the data to another subsystem,) it is not necessary for the subsystem to determme whether the data is usable The subsystem simply passes the information on to a subsequent subsystem. As illustrated by Fig. 2b, the first subsystem 131 may m some embodiments not be configured to transmit a flag coπespondmg to the transmitted data In such embodiments, the data will be handled by the second subsystem 132 m the same manner as if the data were accompanied by a flag that was not set. A flag is added by the interface (and set, if appropπate) before it is passed on to the subsystem The methods implemented by the mterfaces descπbed above are illustrated m Figs. 3 - 5 These figures are explamed m detail below. Figure 3 is a flow diagram illustrating the method embodied m the system descπbed above, in which a new eπor detection/coπection code is generated for unusable data which is detected and passed on by an interface (Other embodiments which may not generate a new eπor detection/coπection code are descπbed below.) When information is transmitted to the interface, it is examined to determme whether it contains uncoπectable eπors The presence of an uncoπectable eπor is determined usmg an eπor detection/coπection code coπespondmg to the information. If the information is coπectable, it is determined whether the information contains a coπectable eπor or no eπor at all. If there is no eπor at all, the information is transmitted from the interface to the subsystem. If there is an eπor which is coπectable, the eπor is coπected and the coπected information is transmitted to the subsystem If the information contains an uncoπectable eπor, the coπespondmg data is unusable. Further, because the eπor detection/coπection code is assumed m this embodiment to have been generated by the subsystem from which the information was received (or an interface coπespondmg thereto,) diagnosis of the hardware fault which caused the uncoπectable eπor is required In other words, smce the eπor detection/coπection code transmitted by the previous subsystem is assumed to match the data, any eπors must necessaπly have occuπed after any hardware diagnosis could have been performed by that subsystem The interface therefore performs several functions, including stoπng status information associated with the eπor, setting the flag to mdicate that the data is unusable and initiating hardware fault diagnosis The unusable data is then passed to the subsystem (with the flag set to mdicate that it is unusable ) The unusable data and the flag are transmitted with a new eπor detection/coπection code The new eπor detection/coπection code is generated based upon the unusable data and the flag so that any further eπors in this information (which would mdicate further hardware faults) can be detected The descπption m the preceding paragraphs is directed to an embodiment m which, after detection of uncoπectable eπors, a new eπor detection/coπection code is generated for the unusable data and the coπespondmg flag. Because a new eπor detection/coπection code is generated, additional eπors can be detected and diagnosis of the hardware faults that caused the additional eπors can be mitiated. In an alternate embodiment, a new eπor detection/coπection code may not be generated Such an embodiment implements a slightly different method than descπbed above This method is illustrated in Fig 4 In this embodiment, when information is received by the interface, it is first examined to determme whether the flag is set. If the flag is set, no further action needs to be taken before the information is passed to the subsystem. If the flag is not set, the information is examined to determme whether the data is coπectable. If the data is coπectable, it is determined whether the data is coπect or whether it contains coπectable eπors If the data is coπect, the information is passed onto the subsystem without further action. If the data contains coπectable eπors, the eπors are coπected and the information is then passed on to the subsystem. If the data contains uncoπectable errors, procedures associated with hardware fault diagnosis must be undertaken (Diagnosis must be performed because the flag m this instance was not set, indicating that diagnosis had not previously been performed.) The procedures which must be performed mclude stoπng status information, setting the flag to mdicate that diagnosis has been mitiated, and actually initiating the diagnosis. The unusable data, accompanied by the old eπor detection/coπection code and the flag which has now been set, is then transmitted to the subsystem. In yet another embodiment, the data may be covered by a first eπor detection/coπection code, while the flag that mdicates whether the data is unusable is covered by a second eπor detection/coπection code. The method used m this embodiment vanes somewhat from the method descπbed m the precedmg paragraphs, as explamed below and illustrated m Fig 5 (Fig. 5 actually depicts only the initial steps of the method — if the flag is coπect or coπectable, the method continues as illustrated m Fig. 4 ) In this embodiment, when information is received by the subsystem's mterface, the flag and the eπor detection/coπection code coπespondmg to the flag are exammed first to determme whether the flag is coπectable If the flag is coπect, the method proceeds with the method illustrated m Fig. 4 (i.e., the flag is exammed to determme whether it is set, and so on.) If the flag contains coπectable eπors, the eπors are coπected and then the method continues as illustrated m Fig 4 If, when the information is received by the mterface, it is determmed that the flag contains an uncoπectable eπor, steps will be taken to determme the cause of the eπor. These steps mclude stoπng status information, initiating the diagnosis and setting the flag to mdicate that diagnosis has been mitiated The data, flag and coπespondmg eπor detection/coπection codes are then transmitted to the subsystem. As mdicated above, the information received by the mterface is processed by the mterface and is then passed on to the subsystem. If the data passed from the mterface to the subsystem is usable, the subsystem processes the information normally For example, it may perform operations usmg the data, or it may simply route the data to another subsystem. If the data is unusable (i.e., the flag is set) and the subsystem requested the data, the subsystem will initiate data eπor handling procedures which may mclude notifying client processes, terminating client processes, requesting the information agam, or other actions. If the subsystem did not request the data, but is only routing the data to another subsystem, it typically will not initiate procedures to handle the data eπor. It will simply route the data. The eπor handlmg procedures will be left to the subsystem that requested the data. It should be noted that, although the foregomg descπption refers to systems and methods which mclude eπor coπection mechanisms, it is not necessary to mclude these mechanisms. For example, an mterface for a subsystem may be configured to receive information which mcludes data, a paπty bit and a flag. If the paπty bit covers the data, single-bit eπors can be detected m the data. When an eπor is detected, the flag can be exammed to determme whether hardware fault diagnosis has already been mitiated for this data. If the flag has been set, the data eπor can be handled without initiating further hardware diagnosis. If the flag has not been set, both hardware diagnosis and data eπor handlmg can be mitiated and the flag can be set to mdicate that this has occuπed. Figs. 6 and 7 depict two embodiments of mterface circuits which implement the methods descπbed above Fig. 6 illustrates an mterface circuit in accordance with one embodiment. In this embodiment, the mterface is configured to accept information compπsmg data, a coπespondmg flag and an eπor detection/coπection code which covers the data, the flag and the eπor detection/coπection code itself. If no eπors are detected m the information, or if coπectable eπors are detected, coπect or coπected information is output from the mterface. If uncoπectable eπors are detected, the mterface produces diagnosis-related information at its output. In the depicted circuit, this diagnosis information consists of predetermmed data, mcludmg a seπes of zeros (m place of the data,) a flag which is set to 1 to mdicate that the data is unusable, and a fixed eπor detection/coπection code In Fig. 6, the data, flag and eπor detection/coπection code are mput to both eπor detector 41 and exclusive OR gate 42. (It should be noted that exclusive OR gate 42 may be implemented usmg 73 two-input exclusive OR gates ) Eπor detector 41 generates a syndrome code which is transmitted to syndrome-to-bit converter 43. If the information mput to eπor detector 41 contains coπectable eπors, syndrome-to-bit converter 43 uses the syndrome code to generate a set of coπection bits which are also mput to exclusive OR gate 42. As mdicated m the figure, the information and coπection bits are each 73 bits wide m this embodiment (64 bits of data, one bit of flag and eight bits of eπor detection/coπection code.) The coπection bits mclude 0's coπespondmg to each coπect bit m the information and l's coπespondmg to each mcoπect bit (If the information contains no eπors, all the coπection bits are 0's ) Thus, if the information mput to eπor detector 41 is coπect, the output of exclusive OR gate 42 is identical to the mput information If the information is coπectable, the mcoπect data bits are inverted by exclusive OR gate 42, thus creating coπect information If the information mput to eπor detector 41 contains uncoπectable eπors, an mdication of the eπors (i.e., 1) is mput to OR gate 44 OR gate 44 also accepts as an mput an mdication of whether a request for the information has timed out. Thus, OR gate 44 outputs a 1 if the data is unusable (i.e., it contains uncoπectable eπors or is missing) and a 0 if the data is coπect or coπectable This output signal of OR gate 44 transmitted to multiplexer 45 and is used as a select signal. Multiplexer 45 accepts two other sets of inputs: the coπected data from exclusive OR gate 42; and the predetermmed data descπbed above (i.e., zeros, a set flag and a fixed eπor detection/coπection code.) This predetermmed data may be stored m registers (not shown) which are hard- wired to multiplexer 45. If the select mput is a 0, the coπect/coπected information is output from multiplexer 45. If the select mput is a 1, the predetermmed data is output. In the circuit of Fig. 6, the predetermmed data selected by multiplexer 45 provides information which is used m the diagnosis of the hardware fault that caused the eπor. The selection of this predetermmed data can be considered initiation of hardware diagnosis. In other embodiments, the output of OR gate 44, which controls selection of the predetermmed data, can be transmitted to external diagnosis circuitry to tπgger diagnosis. Although the latter embodiment employs a more obvious means of initiating diagnosis, both tπggeπng external circuitry and selecting predetermmed data are considered to initiate diagnosis for the purposes of this disclosure. It should be noted that, although the purpose of the mterface descπbed herein is to mcrease the system's abihty to diagnose and recover from eπors, the additional circuitry can also have the paradoxical effect of mak g the system more vulnerable to undetected data corruption. The mterface circuit descπbed m the precedmg paragraphs, however, is designed to minimize, if not eliminate, this problem. The circuit descπbed above and illustrated m figure 6 can be refeπed to as an "honest" circuit because, m contrast to some of the other implementations descπbed above, it does not generate a new eπor detection/coπection code each time information is passed through the circuit. Instead, it merely makes coπections to the existing data, flag and eπor detection coπection code (or outputs predetermmed data, flag and eπor detection/coπection code.) If the circuit were to introduce one or more eπors mto the information it is highly unlikely that the resulting eπor detection/coπection code would match the data Thus, rather than hiding any eπors behind a newly generated eπor detection/coπection code, the eπors should be easily identifiable by subsequent mterfaces The eπor can therefore be considered an "honest" mistake Fig. 7 illustrates an mterface circuit m another embodiment The circuit depicted m this figure is very similar to the circuit of Fig. 6, but rather than producing a predetermmed data value, flag and eπor detection/coπection code at the output of the multiplexer when the mput information is unusable, new information is generated which may be more useful m the diagnosis of detected uncoπectable eπors. Information compπsmg data, a flag and a coπespondmg eπor detection/coπection code is mput to eπor detector 51 and exclusive OR gate 52 (Similar to exclusive OR gate 42 of Fig. 6, exclusive OR gate 52 may be implemented usmg 73 two-input exclusive OR gates.) Eπor detector 51 generates a syndrome code which is mput to syndrome-to-bit converter 53. Syndrome-to-bit converter 53 converts the syndrome code to a set of coπection bits which are mput to exclusive OR gate 52 The syndrome code is also transmitted to status capture unit 56, which will be descπbed m more detail below As with the circuit of Fig 6, OR gate 54 receives two mput signals- the first indicating whether the information mput to eπor detector 51 contains uncoπectable eπors; and the second indicating whether the request for the information has timed out If either of these signals is asserted (i.e., the signal is a 1,) the output of OR gate 54 will be asserted. The signal output by OR gate 54 is transmitted to multiplexer 55, status capture unit 56 and counter 57. Multiplexer 55 receives the output of OR gate 54 as a select signal. If the signal is a 0, the coπect/coπected information generated by exclusive OR gate 52 is selected to be output by multiplexer 55. (Exclusive OR gate 52 functions m the same manner as exclusive OR gate 42 descπbed above with reference to Fig. 6.) If the signal is a 1, diagnosis information (which m this embodiment compπses status information) is selected to be output by multiplexer 55. This status information m this embodiment identifies whether the data was missing or uncoπectable and identifies the associated syndrome code, mterface number, event number and any other additional information which may be useful m performing hardware diagnosis. It should be noted that the status information captured m other embodiments may not mclude all of the data captured m the embodiment of The status information which is mput to multiplexer 55 is collected by status capture unit 56. Inputs to status capture unit 56 mclude the timeout and uncoπectable eπor signals which are mput to OR gate 54, the syndrome code conveyed from the eπor detector 51 to syndrome-to-bit converter 53, an mterface identifier signal (which is fixed) and an event number signal from counter 57 As shown m the figure, counter 57 is clocked by the output of OR gate 54 so that it is mcremented each time an uncoπectable eπor or a timeout is detected. Fig. 7 also depicts an mput for additional information to show that any information which might be useful m diagnosis of hardware faults may be mcluded m the status information. If the data mput to eπor detector 51 is unusable, the status information is output by multiplexer 55 in the data field of the output information. Although m this embodiment, the status information is transmitted m the data field, it should be noted that the status information may be accessed m other ways m other embodiments. For example, m one embodiment, the status information is stored m one or more status registers of status capture unit 56. These status registers may be read out m a completely separate operation from the oπgmal data transmission, such as a read cycle addressing the status registers. This separate operation may use the same bus as the data transmission, or it may use a completely separate data transmission mechanism The circuit m Fig. 7 also mcludes an ECC generator 58. ECC generator 58 produces an eπor detection/coπection code coπespondmg to the status information generated by status capture unit 56 and the unusable data flag ("1 ") Even though a new eπor detection/coπection code is generated, however, this circuit can still be considered an "honest" circuit because the newly generated eπor detection/coπection code is output by multiplexer 55 only if the information mput to eπor detector 51 is unusable. If the information is coπect or contains coπectable eπors, syndrome-to-bit converter 53 and exclusive OR gate 52 only coπect the eπoneous bits As explamed above, it is highly unlikely that eπors generated by the mterface itself would result in an eπor detection coπection code that matched the output data "Honest" mistakes made by the mterface should therefore be easily detectable and coπectable by subsequent mterfaces. (It should also be noted that eπors m the status information may go undetected, but while this might hmder the diagnosis of a particular eπor, it will not affect normal processing of eπor-free data.) As mdicated above, Fig. 6 does not mclude circuitry which is explicitly refeπed to as a diagnosis umt. Likewise, the circuit of Fig 7 does not explicitly show diagnosis circuitry. It is contemplated that the signals produced by these circuits (e.g , the outputs of OR gates 44 or 54, or the outputs of multiplexers 45 or 55) may be transmitted to diagnosis circuitry upon detection of unusable data. Nevertheless, status capture umt 56, counter 57 and ECC generator 58 m the circuit of Fig 7 perform functions that are considered initial steps in the diagnosis of hardware faults, and activation of these units is considered for the purposes of this disclosure to initiate diagnosis. While the present invention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrative and that the invention scope is not limited to these embodiments. Many vanations, modifications, additions and improvements to the embodiments described are possible. These variations, modifications, additions and improvements may fall within the scope of the invention as detailed within the following claims.

Claims: WHAT IS CLAIMED IS: 1. A system compπsmg: a first subsystem configured to provide as an output first information; and a second subsystem coupled to said first subsystem to receive said first information, wherem said second subsystem is configured to produce as an output second information; and wherem said first information mcludes an mput data value; and wherem said second subsystem is configured to determme whether said mput data value is unusable and to produce an output flag mdicative of whether said mput data is unusable, wherem said output flag is contained m said second information. 2. The system of claim 1 wherem if said mput data value is unusable, said second subsystem is configured to inhibit hardware diagnosis if said first information contams an mput flag which is set to mdicate that said mput data value is unusable 3. The system of claim 1 wherem said first information contains an mput eπor detection code coπespondmg to said mput data value 4. The system of claim 3 wherem if said mput data value is unusable and said first information does not mclude an mput flag mdicative of whether said mput data value is unusable, said second subsystem is configured to initiate hardware diagnosis 5. The system of claim 3 wherem said eπor detection code compπses an eπor coπection code. 6. The system of claim 5 wherem said second information further mcludes an output data value and an output eπor detection code coπesponding to said output data value; wherem if said first information does not contain an uncoπectable eπor, said second subsystem is configured to coπect said mput data value and provide said first information as said second information; and wherem if said first information contams an uncoπectable eπor, said second subsystem is configured to provide predetermmed information as said second information. 7. The system of claim 5 wherem said second information further mcludes an output data value and an output eπor detection code coπespondmg to said output data value, wherem if said first information does not contain an uncoπectable eπor, said second subsystem is configured to coπect said mput data value and provide said first information as said second information; and wherem if said first information contams an uncoπectable eπor, said second subsystem is configured to provide status information as said second information 8 The system of claim 1 wherem said first information mcludes an mput flag mdicative of whether said mput data value is unusable and wherem said second subsystem is configured to inhibit hardware diagnosis if said mput flag mdicates that said mput data is unusable 9 The system of claim 1 wherem initiating hardware diagnosis compπses stoπng status information 10 The system of claim 9 wherem said second subsystem is configured to store said status information m one or more registers within said second subsystem 11 The system of claim 9 wherem said second subsystem is configured to provide said status information as part of said second information if said first information is unusable 12 A method compπsmg receivmg information at an mterface, determining whether said information is unusable, if said information is unusable, initiating hardware diagnosis unless said information mcludes a flag which mdicates that hardware diagnosis has been performed, and transmitting said information to a subsystem 13 The method of claim 12 further compπsmg if said information is unusable, setting a flag associated with said information and transmitting said flag to said subsystem with said information, wherem said flag mdicates that said information is unusable 14 The method of claim 12 wherem determining whether said information is unusable compπses determining whether said information contams uncoπectable eπors 15 The method of claim 12 further compπsmg stoπng status information associated with detection of said unusable data 16 The method of claim 12 further compπsmg setting said flag to mdicate that hardware diagnosis has been performed 17 A method compπsmg receivmg information at an mterface determining whether said information is unusable, if said information is unusable, determining whether hardware diagnosis has been mitiated, if said information is unusable and hardware diagnosis has not been mitiated, initiating hardware diagnosis and setting a flag which mdicates that hardware diagnosis has been mitiated; and transmitting said mformation to a subsystem. 18. The method of claim 17 wherem determining whether said information is unusable compπses determining whether said information contains uncoπectable eπors based upon an eπor detection code contained m said information. 19. The method of claim 17 further compπsmg stoπng status mformation associated with detection of said unusable data. 20. An mterface compπsmg' a set of mput lmes; a set of output lmes; and an eπor detector coupled to said mput lines and said output lmes, wherem said eπor detector is configured to determme whether mput information received on said mput lmes is unusable; and wherem if said mput information is unusable and if said mput information does not mclude a flag which mdicates that said mput mformation is unusable, the mterface is configured to initiate hardware diagnosis wherem if said mput mformation is unusable and if said mput mformation mcludes a flag which mdicates that said mput mformation is unusable, the mterface is configured to inhibit hardware diagnosis. 21. The mterface of claim 20 further compπsmg a multiplexer, wherem said mput lmes are coupled to a first set of mputs of said multiplexer, wherem said output lmes are coupled to a set of outputs of said multiplexer, wherem a second set of mputs of said multiplexer are coupled to receive diagnosis mformation, wherem if said mput mformation is usable, said multiplexer is configured to output said mput information, and wherem if said mput mformation is unusable, said multiplexer is configured to output said predetermmed output information. 22. The mterface of claim 21 further compπsmg a status capture umt coupled to said second set of mputs of said multiplexer and configured to provide status mformation to said multiplexer. 23. The mterface of claim 21 further compπsmg a set of registers coupled to said second set of mputs of said multiplexer and configured to provide predetermmed diagnosis mformation to said multiplexer. 24. The mterface of claim 21 wherem if said mput mformation is unusable, said multiplexer is configured to inhibit hardware diagnosis if said mput mformation mcludes a flag which mdicates that said mput mformation is unusable. 25 The mterface of claim 21 wherem the mterface further compπses an eπor coπector which is configured to coπect coπectable eπors m said mput information 26 The mterface of claim 25 wherem said eπor coπector compπses a syndrome-to-bit converter configured to receive a syndrome code from said eπor detector and to generate one or more coπection bits coπespondmg to eπoneous bits m said mput information 27 The mterface of claim 21 further compπsmg a status capture umt, wherem if said mput mformation is unusable and if said mput information does not mclude a flag which mdicates that said mput information is unusable, said status capture umt is configured to capture stams mformation associated with said mput mformation 28 The mterface of claim 21 further compπsmg one or more registers coupled to said multiplexer, wherem said registers are configured to store said predetermmed output mformation 29 The mterface of claim 28 wherem said predetermmed output information stored m said registers compπses a data field containing 0's, a flag field containing a 1 and a predetermmed eπor detection/coπection code coπespondmg to said data field and said flag field 30 An mterface compπsmg an eπor detector, a signal lme coupled to said eπor detector, an mput lme coupled to said eπor detector, and an output lme coupled to said eπor detector, wherem said eπor detector is configured to receive mput mformation on said mput lme and to determme whether said first information contains an uncoπectable eπor, wherem if said mput information contams said uncoπectable eπor, said eπor detector is configured to provide a hardware diagnosis signal on said signal lme, wherem said hardware diagnosis signal initiates hardware diagnosis, and wherem said eπor detector is configured to provide output mformation coπespondmg to said mput information on said output lme, wherem said output mformation contams a flag mdicative of whether said hardware diagnosis has been mitiated 31 The mterface of claim 30 further compπsmg hardware diagnosis circuitry coupled to said signal lme, wherem said hardware diagnosis circuitry is configured to perform hardware diagnosis m response to receivmg said hardware diagnosis signal 32 The mterface of claim 30 further compπsmg an eπor coπector, wherem said eπor coπector is configured to coπect coπectable eπors m said mput mformation, a multiplexer configured to receive said mput mformation and predetermmed information, wherem if said mput mformation does not contam said uncoπectable eπor, said multiplexer is configured to produce said mput mformation at said output lme, and wherem if said mput mformation contains said uncoπectable eπor, said multiplexer is configured to produce said predetermmed information at said output lme

Inventors: CYPHER ROBERT, WILLIAMS EMRYS, CYPHER, ROBERT, WILLIAMS, EMRYS
Applicants: SUN MICROSYSTEMS INC, SUN MICROSYSTEMS, INC.
Application year: 2000
Main classification: G06F11
IPC codes: G06F11