PROCESSOR BRIDGE WITH DISSIMILAR DATA REGISTERS

Abstract: A bridge for a multi-processor system includes bus interfaces for connection to an I/O bus of a first processing set, an I/O bus of a second processing set and a device bus. A bridge control mechanism is configured to compare address and data phases of I/O accesses by the first and second processing sets. At least one dissimilar data register is provided for each processing set. The bridge control mechanism is operable in response to an address phase of a dissimilar data register write access to disregard any differences in the data phase for the dissimilar data write access. Non-deterministic data (for example relating to a real time clock) can be output from the processing sets in a combined (lockstep comparison) node. A read destination address supplied in common by the first and second processing sets for a dissimilar data read access can cause data read from determined one of the dissimilar data registers to be supplied the first and second processing sets. In this manner, the processing sets may have the dissimilar dat replaced by the same data. The read destination address supplied in common by the first and second processing sets can determine the dissimilar data register from which data is read.

Description: TITLE: PROCESSOR BRIDGE WITH DISSIMILAR DATA REGISTERS BACKGROUND OF THE INVENTION This invention relates to a multi-processor computer system including first and second processing sets (each of which may comprise one or more processors) which communicate with an I O device bus. The application finds particular application to fault tolerant computer s> stems where two or more processor sets need to communicate with an I/O device bus in lockstep with provision for identifying lockstep errors in order to identify faulty operation of the system as a whole. It has been proposed to identify lockstep errors by comparing I/O operations output by the processing sets. This is a reliable method as long as the outputs from the processor sets are deterministic, that is as long as long as the outputs from the processing sets are going to be identical when the processors are operating correctly This will normally be the case when the processors are operating synchronously under the control of a common clock However, it can be that some events may not be wholly deterministic. An example of this is where the individual processing sets each include a real time clock which are not synchronized. If the real tune clocks are sampled by the respective processing sets, then a different sample may be generated in the respecuve processing sets. For example, if the sample is taken at just the time at which the clocks are incremented, one processmg set may take the sample just before the increment and the other may take the sample just after the increment. Where these different values are output by the processing sets and are evaluated by a comparator check for lockstep operation, this would be determined as a lockstep error. A further problem is that such values may be operated on by the individual processing sets which could lead to further divergence of the processing sets. These problems have had the effect of limiting the design of lockstep systems to avoid the danger of potentially non-deteiministic events. Accordmgly, an aim of the invention is to overcome or at least mitigate these problems. SUMMARY OF THE INVENTION Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Combinations of features from the dependent claims may be combined with features of the independent claims as appropπate and not merely as explicitly set out in the claims. In accordance with one aspect of the invention, there is provided a bridge for a multi-processor system The bridge compnses bus interfaces for connection to an I/O bus of a first processing set. an I/O bus of a second processing set and a device bus. A bridge control mechanism is configured to compare address and data phases of I/O accesses by the first and second processmg sets. At least one dissimilar data register is provided for each processing set. The bπdge control mechanism is operable m response to an address phase of a dissimilar data register write access to disregard any differences m the data phase for the dissimilar data write access. In an embodiment of the invention, therefore, non-deterministic data (for example relating to a real time clock) can be output from the processing sets in the combined (lockstep companson) mode In a subsequent phase, the processing sets can then read in common a selected one of the dissimilar data registers, whereby it is possible to transfer the dissimilar data from one processor to the other without causing a lockstep error to be detected The bπdge control mechanism can be operable to disregard the data and the associated data paπty for the DMA access It should be noted that the bus interfaces referenced above need not be separate components of the bridge, but may be incorporated m other components of the bridge, and may indeed be simply connections for the lmes of the buses concerned A comparator can be connected to the first and second processor bus interfaces and can be operable m the combined mode to detect differences between signals on the I/O buses of the first and second processing sets as indicative of a lockstep error However, in an embodiment of the invention, an address detector is responsive to a signal identifymg a wπte to a dissimilar data register duπng an address phase of an I O access by the first and second processmg sets to inhibit the comparator duπng the subsequent data phase of the I/O access The output of the comparator may be blocked or the operation of the comparator may be inhibited A read destmation address supplied in common by the first and second processmg sets for a dissimilar data read access can cause data read from a determined one of the dissimilar data registers to be supplied to the first and second processmg sets In this manner, the processmg sets may have the dissimilar data replaced by the same data The read destmation address supplied in common by the first and second processmg sets can determine the dissimilar data register from which data is read In an embodiment of the invention, an address decode mechanism is connected to the first processor bus interface, the second processor bus interface and the device bus interface The address decode mechanism is operable to detemune source and destmation addresses for wπte and read accesses and to respond to a common wπte destination address for a wπte to a dissimilar data register to modify the wπte destmation address dependent upon the source of the address The address decoder can thus deπve first and second dissimilar data register addresses for the first and second processmg sets, respectively, from a common wπte destmation address supplied by the first and second processmg sets A controllable routing matrix is connected between the first processor bus interface, the second processor bus interface, the device bus interface and the dissimilar data registers The bridge control mechanism is operable to control the routing matπx selectively to interconnect the first processor bus interface, the second processor bus interface, the device bus interface and the dissumlar data registers The address decode mechanism is responsive to a read destination address supplied m common by the first and second processmg sets for a dissimilar data read access to access one of the first and second dissimilar data registers from the common read destmation address The common wπte destmation address could different from the common read destmation address Alternatively, for example, it could be one of the possible read addresses Initiator and target controllers can be provided for determining a path from an input to an output of the routmg matπx, the initiator controller bemg responsive to determination of the master for an bus operation and the target controller bemg responsive to determination of a destmation address by the address decode mechanism The dissimilar data registers can be configured as a hardware registers in the bπdge Alternatively, they could be configured within random access memory in the bndge More than two processor bus interfaces may be provided for connection to an I/O bus of a further processmg set. In accordance with another aspect of the invention, there is provided a computer system compnsing a first processmg set havmg an I/O bus, a second processing set havmg an I/O bus. a device bus and a bπdge as defined above Each processmg set may comprises at least one processor, memory and a processmg set I O bus controller In accordance with a further aspect of the invention, there is provided a method of operatmg a multiprocessor system as set out above, the method compπsmg comparing address and data phases of I O accesses by the first and second processmg sets for determining lockstep operation of the processmg sets, detecting an address phase of a dissimilar data register wπte access, and disregardmg differences in the data phase for the dissimilar data wnte access, and storing the data of the data phase for the dissimilar data write access for the first and second processmg sets in first and second dissimilar data registers, respectively The method can compπses subsequently detecting a read destination address supplied m common by the first and second processmg sets for a dissimilar data read access to supply data read from a determmed one of the dissumlar data registers to the first and second processmg sets BRIEF DESCRIPTION OF THE DRAWINGS Exemplary embodiments of the present invention will be descπbed hereinafter, by way of example only, with reference to the accompanying drawings in which like reference signs relate to like elements and in which Figure 1 is a schematic overview of a fault tolerant computer system incorporating an embodiment of the invention, Figure 2 is a schematic overview of a specific implementation of a system based on that of Figure 1 Figure 3 is a schematic representation of one implementation of a processmg set Figure 4 is a schematic representation of another example of a processmg set. Figure 5 is a schematic representation of a further processmg set, Figure 6 is a schematic block diagram of an embodiment of a bπdge for the system of Figure 1, Figure 7 is a schematic block diagram of storage for the bπdge of Figure 6, Figure 8 is a schematic block diagram of control logic of the bndge of Figure 6 Figure 9 is a schematic representation of a routing matπx of the bndge of Figure 6, Figure 10 is an example implementation of the bπdge of Figure 6, Figure 11 is a state diagram illustrating operational states of the bπdge of Figure 6, Figure 12 is a flow diagram illustrating stages in the operation of the bπdge of Figure 6, Figure 13 is a detail of a stage of operation from Figure 12, Figure 14 illustrates the posting of I O cycles m the system of Figure 1 , Figure 15 illustrates the data stored m a posted wnte buffer, Figure 16 is a schematic representation of a slot response register, Figure 17 illustrates a dissimilar data wnte stage, Figure 18 illustrates a modification to Figure 17, Figure 19 illustrates a dissimilar data read stage, Figure 20 illustrates an alternative dissimilar data read stage; Figure 21 is a flow diagram summarizing the operation of a dissimilar data wnte mechanism; Figure 22 is a schematic block diagram explaining arbitration within the system of Figure 1 , Figure 23 is a state diagram illustrating the operation of a device bus arbiter: Figure 24 is a state diagram illustrating the operation of a bndge arbiter; Figure 25 is a timing diagram for PCI signals; Figure 26 is a schematic diagram illustrating the operation of the bndge of Figure 6 for direct memory access; Figure 27 is a flow diagram illustrating a direct memory access method in the bπdge of Figure 6; and Figure 28 is a flow diagram of a re-integration process including the monitoring of a dirty RAM. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 is a schematic overview of a fault tolerant computing system 10 compπsmg a plurality of CPUsets (processmg sets) 14 and 16 and a bπdge 12 As shown m Figure 1, there are two processmg sets 14 and 16, although m other embodiments there may be three or more processmg sets. The bπdge 12 forms an interface between the processmg sets and I/O devices such as devices 28, 29, 30, 31 and 32. In this document, the term "processmg set" is used to denote a group of one or more processors, possibly including memory, which output and receive common outputs and mputs It should be noted that the alternative term mentioned above, "CPUset", could be used mstead, and that these terms could be used mterchangeably throughout this document Also, it should be noted that the term "bndge" is used to denote any device, apparatus or anangement suitable for interconnecting two or more buses of the same or different types The first processmg set 14 is connected to the bndge 12 via a first processmg set I/O bus (PA bus) 24, in the present instance a Peripheral Component Interconnect (PCI) bus The second processmg set 16 is connected to the bndge 12 via a second processmg set I O bus (PB bus) 26 of the same type as the PA bus 24 (I e here a PCI bus). The I/O devices are connected to the bπdge 12 via a device I O bus (D bus) 22, in the present instance also a PCI bus. Although, m the particular example descnbed, the buses 22, 24 and 26 are all PCI buses, this is merely by way of example, and m other embodiments other bus protocols may be used and the D-bus 22 may have a different protocol from that of the PA bus and the PB bus (P buses) 24 and 26 The processmg sets 14 and 16 and the bndge 12 are operable in synchronism under the control of a common clock 20, which is connected thereto by clock signal lmes 21 Some of the devices including an Ethernet (E-NET) mterface 28 and a Small Computer System Interface (SCSI) mterface 29 are permanently connected to the device bus 22, but other I/O devices such as I/O devices 30, 31 and 32 can be hot insertable into individual switched slots 33, 34 and 35 Dynamic field effect transistor (FET) switchmg can be provided for the slots 33, 34 and 35 to enable hot lnsertability of the devices such as devices 30, 31 and 32. The provision of the FETs enables an mcrease m the length of the D bus 22 as only those devices which are active are switched on, reducmg the effective total bus length It will be appreciated that the number of I/O devices which may be connected to the D bus 22. and the number of slots provided for them, can be adjusted accordmg to a particular implementation m accordance with specific design lequirements. Figure 2 is a schematic overview of a particular implementation of a fault tolerant computer employmg a bndge structure of the type illustrated in Figure 1. In Figure 2, the fault tolerant computer system mcludes a plurality (here four) of bπdges 12 on first and second I/O motherboards (MB 40 and MB 42) order to mcrease the number of I/O devices which may be connected and also to improve reliability and redundancy. Thus, in the embodiment shown in Figure 2, two processmg sets 14 and 16 are each provided on a respective processmg set board 44 and 46, with the processing set boards 44 and 46 'bndging' the I/O motherboards MB 40 and MB 42. A first, master clock source 20A is mounted on the first motherboard 40 and a second, slave clock source 20B is mounted on the second motherboard 42. Clock signals are supplied to the processmg set boards 44 and 46 via respective connections (not shown in Figure 2). First and second bπdges 12.1 and 12.2 are mounted on the first I/O motherboard 40. The first bπdge 12.1 is connected to the processmg sets 14 and 16 by P buses 24.1 and 26.1, respectively. Similarly, the second bridge 12.2 is connected to the processmg sets 14 and 16 by P buses 24.2 and 26 2, respectively. The bndge 12.1 is connected to an I/O databus (D bus) 22.1 and the bndge 12.2 is connected to an I/O databus (D bus) 22.2. Third and fourth bπdges 12.3 and 12.4 are mounted on the second I/O motherboard 42. The bπdge 12.3 is connected to the processmg sets 14 and 16 by P buses 24.3 and 26.3, respectively. Similarly, the bπdge 4 is connected to the processing sets 14 and 16 by P buses 24.4 and 26.4, respectively. The bπdge 12.3 is connected to an I/O databus (D bus) 22.3 and the bndge 12.4 is connected to an I/O databus (D bus) 22.4. It can be seen that the arrangement shown in Figure 2 can enable a large number of I/O devices to be connected to the two processmg sets 14 and 16 via the D buses 22.1, 22.2, 22.3 and 22 4 for either increasmg the range of I/O devices available, or providing a higher degree of redundancy, or both Figure 3 is a schematic overview of one possible configuration of a processmg set, such as the processing set 14 of Figure 1. The processmg set 16 could have the same configuration In Figure 3, a plurality of processors (here four) 52 are connected by one or more buses 54 to a processmg set bus controller 50. As shown m Figure 3, one or more processmg set output buses 24 are connected to the processmg set bus controller 50. each processmg set output bus 24 bemg connected to a respective bndge 12. For example, in the anangement of Figure 1, only one processmg set I/O bus (P bus) 24 would be provided, whereas m the anangement of Figure 2, four such processmg set I/O buses (P buses) 24 would be provided. In the processmg set 14 shown m Figure 3, individual processors operate usmg the common memory 56. and receive inputs and provide outputs on the common P bus(es) 24 Figure 4 is an alternative configuration of a processmg set, such as the processmg set 14 of Figure 1 Here a plurality of processor/memory groups 61 are connected to a common internal bus 64 Each processor/memory group 61 includes one or more processors 62 and associated memory 66 connected to a internal group bus 63 An mterface 65 connects the internal group bus 63 to the common internal bus 64 Accordmgly, m the anangement shown m Figure 4, individual processmg groups each of the processors 62 and associated memory 66 are connected via a common internal bus 64 to a processmg set bus controller 60 The interfaces 65 enable a processor 62 of one processmg group to operate not only on the data m its local memory 66, but also m the memory of another processmg group 61 within the processmg set 14 The processmg set bus controller 60 provides a common mterface between the common internal bus 64 and the processmg set I/O bus(es) (P bus(es)) 24 connected to the bndge(s) 12 It should be noted that although only two processmg groups 61 are shown m Figure 4, it will be appreciated that such a structure is not limited to this number of processing groups Figure 5 illustrates an alternative configuration of a processmg set, such as the processmg set 14 of Figure 1 Here a simple processmg set includes a smgle processor 72 and associated memory 76 connected via a common bus 74 to a processmg set bus controller 70 The processmg set bus controller 70 provides an interface between the internal bus 74 and the processmg set I/O bus(es) (P bus(es)) 24 for connection to the bndge(s) 12 Accordmgly, it will be appreciated from Figures 3, 4 and 5 that the processmg set may have many different forms and that the particular choice of a particular processmg set structure can be made on the basis of the processmg requirement of a particular application and the degree of redundanc) required In the following descπption, it is assumed that the processmg sets 14 and 16 refeπed to have a structure as shown m Figure 3, although it will be appreciated that another form of processmg set could be provided The bπdge(s) 12 are operable m a number of operating modes These modes of operation will be descnbed m more detail later However, to assist in a general understandmg of the structure of the bndge, the two operating modes will be bπefly summarized here In a first, combmed mode, a bπdge 12 is operable to route addresses and data between the processmg sets 14 and 16 (via the PA and PB buses 24 and 26, respectively) and the devices (via the D bus 22) In this combmed mode, I/O cycles generated by the processing sets 14 and 16 are compared to ensure that both processing sets are operatmg correctly Companson failures force the bπdge 12 into an enor limiting mode (EState) m which device I/O is prevented and diagnostic information is collected In the second, split mode, the bridge 12 routes and arbitrates addresses and data from one of the processmg sets 14 and 16 onto the D bus 22 and/or onto the other one of the processing sets 16 and 14, respectively In this mode of operation, the processmg sets 14 and 16 are not synchronized and no I/O comparisons are made DMA operations are also permitted m both modes As mentioned above, the different modes of operation, mcludmg the combmed and split modes, will be descnbed m more detail later However, there now follows a descπption of the basic structure of an example of the bndge 12 Figure 6 is a schematic functional overview of the bπdge 12 of Figure 1 First and second processmg set I O bus interfaces, PA bus interface 84 and PB bus mterface 86, are connected to the PA and PB buses 24 and 26, respectively A device I/O bus mterface, D bus mterface 82, is connected to the D bus 22 It should be noted that the PA, PB and D bus mterfaces need not be configured as separate elements but could be incorporated m other elements of the bndge Accordmgly, withm the context of this document, where a references is made to a bus interface, this does not require the presence of a specific separate component, but rather the capability of the bπdge to connect to the bus concerned, for example by means of physical or logical bπdge connections for the lmes of the buses concerned Routmg (heremafter termed a routing matπx) 80 is connected via a first internal path 94 to the PA bus interface 84 and via a second internal path 96 to the PB bus mterface 86 The routing matπx 80 is further connected via a third internal path 92 to the D bus mterface 82. The routing matπx 80 is thereby able to provide 170 bus transaction routmg m both directions between the PA and PB bus mterfaces 84 and 86 It is also able to provide routing m both directions between one or both of the PA and PB bus mterfaces and the D bus mterface 82 The routmg matnx 80 is connected via a further internal path 100 to storage control logic 90. The storage control logic 90 controls access to bndge registers 110 and to a random access memory (SRAM) 126. The routmg matnx 80 is therefore also operable to provide routmg m both directions between the PA, PB and D bus interfaces 84, 86 and 82 and the storage control logic 90 The routing matnx 80 is controlled by bridge control logic 88 over control paths 98 and 99. The bπdge control logic 88 is responsive to control signals, data and addresses on internal paths 93, 95 and 97, and also to clock signals on the clock lme(s) 21. In the embodiment of the invention, each of the P buses (PA bus 24 and PB bus 26) operates under a PCI protocol. The processmg set bus controllers 50 (see Figure 3) also operate under the PCI protocol Accordmgly, the PA and PB bus mterfaces 84 and 86 each provide all the functionality required for a compatible mterface providmg both master and slave operation for data transfeπed to and from the D bus 22 or internal memoπes and registers of the bπdge m the storage subsystem 90 The bus mterfaces 84 and 86 can provide diagnostic information to internal bndge status registers m the storage subsystem 90 on transition of the bndge to an enor state (EState) or on detection of an I/O enor. The device bus mterface 82 performs all the functionality required for a PCI compliant master and slave interface for transferring data to and from one of the PA and PB buses 84 and 86. The D bus 82 is operable during direct memory access (DMA) transfers to provide diagnostic information to internal status registers m the storage subsystem 90 of the bndge on transition to an EState or on detection of an I/O enor. Figure 7 illustrates m more detail the bndge registers 110 and the SRAM 124 The storage control logic 110 is connected via a path (e.g a bus) 112 to a number of register components 114, 116, 118, 120 The storage control logic is also connected via a path (e g a bus) 128 to the SRAM 126 in which a posted wπte buffer component 122 and a dirty RAM component 124 are mapped Although a particular configuration of the components 114, 116, 118, 120, 122 and 124 is shown in Figure 7, these components may be configured in other ways, with other components defined as regions of a common memory (e.g a random access memory such as the SRAM 126, with the path 112/128 being formed by the internal addressing of the regions of memory) As shown m Figure 7, the posted write buffer 122 and the duty RAM 124 are mapped to different regions of the SRAM memory 126, whereas the registers 114, 116, 118 and 120 are configured as separate from the SRAM memory Control and status registers (CSRs) 114 form internal registers which allow the control of vanous operating modes of the bridge, allow the capture of diagnostic information for an EState and for I/O enors, and control processmg set access to PCI slots and devices connected to the D bus 22 These registers are set by signals from the routmg matπx 80 Dissimilar data registers (DDRs) 116 provide locations for containmg dissimilar data for different processmg sets to enable non-deterministic data events to be handled These registers are set by signals from the PA and PB buses Bridge decode logic enables a common write to disable a data comparator and allow wntes to two DDRs 116, one for each processmg set 14 and 16 A selected one of the DDRs can then be read ui-sync by the processmg sets 14 and 16 The DDRs thus provide a mechanism enablmg a location to be reflected from one processmg set (14 16) to another (16/14) Slot response registers (SRRs) 118 determine ownership of device slots on the D bus 22 and to allow DMA to be routed to the appropπate processmg set(s) These registers are linked to address decode logic Disconnect registers 120 are used for the storage of data phases of an I/O c\cle which is aborted while data is m the bridge on the way to another bus The disconnect registers 120 receπ, e all data queued m the bπdge when a target device disconnects a transaction, or as the EState is detected These registers are connected to the routing matnx 80 The routing matnx can queue up to three data words and byte enables Provided the mitial addresses are voted as bemg equal, address target controllers denve addresses which mcrement as data is exchanged between the bndge and the destination (or target) Where a wnter (for example a processor I/O wπte, or a DVMA (D bus to P bus access)) is writing data to a target, this data can be caught in the bndge when an error occurs Accordmgly, this data is stored m the disconnect registers 120 when an error occurs These disconnect registers can then be accessed on recovery from an EState to recover the data associated with the wπte or read cycle which was m progress when the EState was initiated Although shown separately, the DDRs 116, the SRRs 118 and the disconnect registers may form an integral part of the CSRs 114 EState and enor CSRs 114 provided for the capture of a failmg cycle on the P buses 24 and 26, with an mdication of the failmg datum Following a move to an EState, all of the wntes initiated to the P buses are logged m the posted wnte buffer 122 These may be other wntes that have been posted m the processing set bus controllers 50, or which may be initiated by software before an EState interrupt causes the processors to stop carrying out wntes to the P buses 24 and 26 A duty RAM 124 is used to mdicate which pages of the mam memory 56 of the processmg sets 14 and 16 have been modified by direct memory access (DMA) transactions from one or more devices on the D bus 22 Each page (e g each 8K page) is marked by a smgle bit m the duty RAM 124 which is set when a DMA wπte occurs and can be cleared by a read and clear cycle initiated on the duty RAM 124 by a processor 52 of a processing set 14 and 16 The dirty RAM 124 and the posted wπte buffer 118 may both be mapped into the memory 124 in the bridge 12 This memory space can be accessed during normal read and write cycles for testmg purposes Figure 8 is a schematic functional overview of the bπdge control logic 88 shown m Figure 6 All of the devices connected to the D bus 22 are addressed geographicallv Accordmgly, the bridge cames out decodmg necessary to enable the isolating FETs for each slot before an access to those slots is initiated The address decodmg performed by the address decode logic 136 and 138 essentially permits four basic access types - an out-of-sync access (I e not m the combmed mode) by one processmg set (e g processmg set 14 of Figure 1) to the other processmg set (e g processmg set 16 of Figure 1), m which case the access is routed from the PA bus interface 84 to the PB bus mterface 86, - an access by one of the processing sets 14 and 16 in the split mode, or both processmg sets 14 and 16 in the combmed mode to an I/O device on the D bus 22, in which case the access is routed via the D bus interface 82, - a DMA access by a device on the D bus 22 to one or both of the processmg sets 14 and 16, which would be directed to both processmg sets 14 and 16 m the combined mode, or to the relevant processing set 14 or 16 if out-of-sync, and if m a split mode to a processmg set 14 or 16 which owns a slot m which the device is located; and - a PCI configuration access to devices m I/O slots. As mentioned above, geographic addressing is employed. Thus, for example, slot 0 on motherboard A has the same address when refened to by processmg set 14 or by processmg set 16 Geographic addressing is used in combination with the PCI slot FET switchmg During a configuration access mentioned above, separate device select signals are provided for devices which are not FET isolated A smgle device select signal can be provided for the switched PCI slots as the FET signals can be used to enable a conect card. Separate FET switch lmes are provided to each slot for separately switchmg the FETs for the slots. The SRRs 118, which could be incorporated m the CSR registers 114, are associated with the address decode functions. The SRRs 118 serve m a number of different roles which will be descnbed in more detail later. However, some of the roles are summarized here. In a combmed mode, each slot may be disabled so that wntes are simply acknowledged without any transaction occumng on the device bus 22, whereby the data is lost Reads will return meanmgless data, once again without causmg a transaction on the device board. In the split mode, each slot can be m one of three states. The states are- - Not owned; - Owned by processmg set A 14, - Owned by processmg set B 16 A slot that is not owned by a processmg set 14 or 16 makmg an access (this includes not owned or unowned slots) cannot be accessed. Accordmgly, such an access is aborted When a processmg set 14 or 16 is powered off, all slots owned by it mo\ e to the un-owned state A processmg set 14 or 16 can only claim an un-owned slot, it cannot wrest ownership away from another processmg set This can only be done by powermg off the other processmg set. or by gettmg the other processmg set to relinquish ownership. The ownership bits are assessable and settable while in the combmed mode, but have no effect until a split state is entered This allows the configuration of a split system to be determined while still in the combined mode Each PCI device is allocated an area of the processing set address map The top bits of the address are determmed by the PCI slot Where a device carnes out DMA, the bndge is able to check that the device is using the conect address because a D bus arbiter informs the bridge which device is using the bus at a particular tune If a device access is a processmg set address which is not valid for it, then the device access will be ignored It should be noted that an address presented by a device will be a virtual address which would be translated by an I/O memory management unit m the processmg set bus controller 50 to an actual memory address The addresses output by the address decoders are passed via the mitiator and target controllers 138 and 140 to the routing matπx 80 via the lmes 98 under control of a bπdge controller 132 and an arbiter 134 An arbiter 134 is operable m vanous different modes to arbitrate for use ot the bndge on a first-come- first-served basis usmg conventional PCI bus signals on the P and D buses In a combmed mode, the arbiter 134 is operable to arbitrate between the ln-svnc processmg sets 14 and 16 and any initiators on the device bus 22 for use of the bridge 12 Possible scenanos are - processmg set access to the device bus 22, - processmg set access to internal registers m the bndge 12, - Device access to the processmg set memory 56 In split mode, both processmg sets 14 and 16 must arbitrate the use of the bndge and thus access to the device bus 22 and mtemal bndge registers (e g CSR registers 114) The bπdge 12 must also contend with initiators on the device bus 22 for use of that device bus 22 Each slot on the device bus has an arbitration enable bit associated with it These arbitration enable bits are cleared after reset and must be set to allow a slot to request a bus When a device on the device bus 22 is suspected of providmg an I/O enor, the arbitration enable bit for that device is automatically reset by the bπdge A PCI bus mterface in the processmg set bus controller(s) 50 expects to be the master bus controller for the P bus concerned, that is it contams the PCI bus arbiter for the PA or PB bus to which it is connected The bndge 12 cannot directly control access to the PA and PB buses 24 and 26 The bndge 12 competes for access to the PA or PB bus with the processmg set on the bus concerned under the control of the bus controller 50 on the bus concerned Also shown m Figure 8 is a comparator 130 and a bπdge controller 132 The comparator 130 is operable to compare I/O cycles from the processmg sets 14 and 16 to determine anv out-of-sync events On determining an out-of-sync event, the comparator 130 is operable to cause the bndge controller 132 to activate an EState for analysis of the out-of-sync event and possible recovery therefrom Figure 9 is a schematic functional overview of the routing matπx 80 The routmg matrix 80 compnses a multiplexer 143 which is responsive to mitiator control signals 98 from the mitiator controller 138 of Figure 8 to select one of the PA bus path 94 , PB bus path 96, D bus path 92 or mtemal bus path 100 as the cunent mput to the routmg matnx Separate output buffers 144, 145, 146 and 147 are provided for output to each of the paths 94, 96, 92 and 100, with those buffers bemg selectively enabled by signals 99 from the target controller 140 of Figure 8 Between the multiplexer and the buffers 144-147 signals are held m a buffer 149 In the present embodiment three cycles of data for an I/O cycle will be held m the pipeline represented by the multiplexer 143, the buffer 149 and the buffers 144 In Figures 6 to 9 a functional descnption of elements of the bπdge has been given Figure 10 is a schematic representation of a physical configuration of the bridge m which the bndge control logic 88, the storage control logic 90 and the bndge registers 110 are implemented in a first field programmable gate anay (FPGA) 89 the routing matπx 80 is implemented in further FPGAs 80 1 and 80 2 and the SRAM 126 is implemented as one or more separate SRAMs addressed by a address control lmes ι 7 The bus mterfaces 82, 84 and 86 shown in Figure 6 are not separate elements, but are integrated m the FPGAs 80.1, 80.2 and 89 Two FPGAs 80 1 and 80.2 are used for the upper 32 bits 32-63 of a 64 bit PCI bus and the lower 32 bits 0-31 of the 64 bit PCI bus. It will be appreciated that a smgle FPGA could be employed for the routing matπx 80 where the necessary logic can be accommodated within the device Indeed, where a FPGA of sufficient capacity is available, the bndge control logic, storage control logic and the bπdge registers could be incorporated m the same FPGA as the routing matrix. Indeed many other configurations may be envisaged, and indeed technology other than FPGAs, for example one or more Application Specific Integrated Circuits (ASICs) may be employed As shown m Figure 10, the FPGAs 89, 80 1 and 80.2 and the SRAM 126 are connected via mtemal bus paths 85 and path control lmes 87. Figure 11 is a transition diagram illustrating in more detail the vaπous operating modes of the bndge The bndge operation can be divided into three basic modes, namely an enor state (EState) mode 150. a split state mode 156 and a combined state mode 158. The EState mode 150 can be further divided mto 2 states After mitial resetting on powermg up the bndge, or following an out-of sync event, the bndge is m this initial EState 152. In this state, all wntes are stored m the posted wnte buffer 120 and reads from the mtemal bridge registers (e.g., the CSR registers 116) are allowed, and all other reads are treated as enors (i.e. they are aborted). In this state, the individual processmg sets 14 and 16 perform evaluations for determining a restart tune. Each processmg set 14 and 16 will determine its own restart timer timing. The timer setting depends on a "blame" factor for the transition to the EState. A processmg set which determines that it is likely to have caused the enor sets a long time for the timer. A processmg set which thinks it unlikely to have caused the error sets a short time for the timer. The first processmg set 14 and 16 which times out, becomes a primary processmg set Accordmgly, when this is determmed, the bπdge moves (153) to the primary EState 154. When either processmg set 14/16 has become the primary processmg set, the bπdge is then operating in the primary EState 154. This state allows the primary processmg set to wnte to bndge registers (specifically the SRRs 118) Other wntes are no longer stored m the posted wπte buffer, but are simply lost Device bus reads are still aborted m the primary EState 154 Once the EState condition is removed, the bπdge then moves (155) to the split state 156 In the split state 156, access to the device bus 22 is controlled by the SRR registers 118 while access to the bridge storage is simply arbitrated. The primary status of the processmg sets 14 and 16 is ignored Transition to a combmed operation is achieved by means of a sync_reset (157). After issue of the sync_reset operation, the bπdge is then operable m the combmed state 158, whereby all read and wπte accesses on the D bus 22 and the PA and PB buses 24 and 26 are allowed All such accesses on the PA and PB buses 24 and 26 are compared m the comparator 130. Detection of a mismatch between any read and wπte cycles (with an exception of specific dissimilar data I O cycles) cause a transition 151 to the EState 150 The vaπous states descnbed are controlled by the bndge controller 132 The role of the comparator 130 is to monitor and compare I/O operations on the PA and PB buses in the combmed state 151 and, in response to a mismatched signal, to notify the bndge controller 132, whereby the bπdge controller 132 causes the transition 152 to the enor state 150 The I/O operations can mclude all I O operations mitiated by the processmg sets, as well as DMA transfers m respect of DMA mitiated by a device on the device bus Table 1 below summarizes the vaπous access operations which are allowed m each of the operational states TABLE 1 D Bus - Read D Bus- Wnte E State Master Abort Stored m Post Wnte Buffer Primary EState Master Abort Lost Split Controlled by SRR bits Controlled by SRR bits and arbitrated and arbitrated Combmed Allowed and compared Allowed and compared As described above, after an mitial reset, the system is m the mitial EState 152 In this state, neither processmg sets 14 or 16 can access the D bus 22 or the P bus 26 or 24 of the other processmg set 16 or 14 The internal bndge registers 116 of the bndge are accessible, but are read only A system running m the combmed mode 158 transitions to the EState 150 where there is a compaπson failure detected in this bπdge, or alternatively a compaπson failure is detected m another bπdge m a multi- bndge system as shown, for example, in Figure 2 Also, transitions to an EState 150 can occur m other situations, for example m the case of a software controlled event forming part of a self test operation On moving to the EState 150, an interrupt is signaled to all or a subset of the processors of the processmg sets via an interrupt lme 95 Following this, all I/O cycles generated on a P bus 24 or 26 result in reads being returned with an exception and wntes bemg recorded in the posted wnte buffer The operation of the comparator 130 will now be descnbed in more detail The comparator is connected to paths 94, 95, 96 and 97 for comparmg address, data and selected control signals from the PA and PB bus mterfaces 84 and 86 A failed companson of ui-sync accesses to device I O bus 22 devices causes a move from the combmed state 158 to the EState 150 For processmg set I/O read cycles, the address, command, address paπty, byte enables and parity enor parameters are compared If the compaπson fails durmg the address phase, the bπdge asserts a retry to the processing set bus controllers 50, which prevents data leavmg the I/O bus controllers 50 No activity occurs m this case on the device I/O bus 22 On the processor(s) retrying, no enor is returned If the compaπson fails duπng a data phase (only control signals and byte enables are checked), the bridge signals a target-abort to the processmg set bus controllers 50 An enor is returned to the processors In the case of processing set I/O bus wnte cycles, the address, command, panty, byte enables and data parameters are compared If the comparison fails during the address phase, the bridge asserts a retry to the processing set bus controllers 50, which results in the processing set bus controllers 50 retrying the cycle again. The posted write buffer 122 is then active. No activity occurs on the device I O bus 22. If the comparison fails during the data phase of a write operation, no data is passed to the D bus 22. The failing data and any other transfer attributes from both processing sets 14 and 16 are stored in the disconnect registers 122, and any subsequent posted write cycles are recorded in the posted write buffer 118. In the case of direct virtual memory access (DVMA) reads, the data control and parity are checked for each datum. If the data does not match, the bridge 12 terminates the transfer on the P bus. In the case of DVMA writes, control and parity enor signals are checked for conectness. Other signals in addition to those specifically mentioned above can be compared to give an indication of divergence of the processing sets. Examples of these are bus grants and various specific signals during processing set transfers ancTduring DMA transfers. Enors fall roughly into two types, those which are made visible to the software by the processing set bus controller 50 and those which are not made visible by the processing set bus controller 50 and hence need to be made visible by an interrupt from the bridge 12. Accordingly, the bridge is operable to capture enors reported in connection with processing set read and write cycles, and DMA reads and writes. Clock control for the bridge is performed by the bridge controller 132 in response to the clock signals from the clock line 21. Individual control lines from the controller 132 to the various elements of the bridge are not shown in Figures 6 to 10. Figure 12 is a flow diagram illustrating a possible sequence of operating stages where lockstep enors are detected during a combined mode of operation. Stage SI represents the combined mode of operation where lockstep enor checking is performed by the comparator 130 shown in Figure 8. In Stage S2, a lockstep enor is assumed to have been detected by the comparator 130. In Stage S3, the current state is saved in the CSR registers 114 and posted writes are saved in the posted write buffer 122 and/or in the disconnect registers 120. Figure 13 illustrates Stage S3 in more detail. Accordingly, in Stage S31. the bridge controller 132 detects whether the lockstep enor notified by the comparator 130 has occuned during a data phase in which it is possible to pass data to the device bus 22. In this case, in Stage S32, the bus cycle is terminated. Then, in Stage S33 the data phases are stored in the disconnect registers 120 and control then passes to Stage S35 where an evaluation is made as to whether a further I/O cycle needs to be stored. Alternatively, if at Stage S31, it is determined that the lockstep enor did not occur during a data phase, the address and data phases for any posted write I/O cycles are stored in the posted write buffer 122. At Stage S34, if there are any further posted write I O operations pending, these are also stored in the posted write buffer 122. Stage S3 is performed at the initiation of the initial enor state 152 shown in Figure 11. In this state, the first and second processing sets arbitrate for access to the bridge. Accordingly, in Stage S31-S35, the posted write address and data phases for each of the processing sets 14 and 16 are stored in separate portions of the posted write buffer 122, and/or in the single set of disconnect registers as described above. Figure 14 illustrates the source of the posted wπte I/O cycles which neeα to be stored in the posted wπte buffer 122 Durmg normal operation of the processmg sets 14 and 16, output buffers 162 m the individual processors contain I O cycles which have been posted for transfer via the processmg set bus controllers 50 to the bπdge 12 and eventually to the device bus 22 Similarly, buffers 160 m the processmg set controllers 50 also contain posted I/O cycles for transfer over the buses 24 and 26 to the bπdge 12 and eventually to the device bus 22 Accordmgly, it can be seen that when an enor state occurs, I/O wπte may already have been posted by the processors 52, either in their own buffers 162, or already transfened to the buffers 160 of the processmg set bus controllers 50 It is the I/O wnte cycles m the buffers 162 and 160 which gradually propagate through and need to be stored m the posted wnte buffer 122 As shown m Figure 15, a wnte cycle 164 posted to the posted wπte buffer 122 can compπse an address field 165 mcludmg an address and an address type, and between one and 16 data fields 166 mcludmg a byte enable field and the data itself The data is wπtten to the posted write buffer 122 m the EState unless the initiating processing set has been designated as a primary CPU set. At that time, non-pnmary wntes m an EState still go to the posted write buffer even after one of the CPU sets has become a primary processmg set An address pointer in the CSR registers 114 points to the next available posted wπte buffer address, and also provides an overflow bit which is set when the bπdge attempts to wπte past of the top of the posted wnte buffer for any one of the processmg sets 14 and 16. Indeed, m the present implementation, only the first 16 K of data is recorded m each buffer. Attempts to wnte beyond the top of the posted wnte buffer are ignored. The value of the posted wnte buffer pomter can be cleared at reset, or by software usmg a wnte under the control of a primary processmg set Returning to Figure 12, after savmg the status and posted wntes, at Stage S4 the individual processmg sets independently seek to evaluate the enor state and to determine whether one of the processmg sets is faulty This determination is made by the individual processors m an enor state m which they individually read status trom the control state and EState registers 114 Durmg this enor mode, the arbiter 134 arbitrates for access to the bridge 12 In Stage S5, one of the processmg sets 14 and 16 establishes itself as the primary processing set This is determmed by each of the processmg sets identifymg a time factor based on the estimated degree of responsibility for the enor, whereby the first processmg set to time out becomes the primary processing set In Stage S5, the states is recovered for that processing set and is copied to the other processmg set The primary processing is able to access the posted write buffer 122 and the disconnect registers 120 In Stage S6, the bndge is operable in a split mode If it is possible to re-establish an equivalent status for the first and second processmg sets, then a reset is issued at Stage S7 to put the processmg sets in the combmed mode at Stage SI However, it may not be possible to re-establish an equivalent state until a faulty processmg set is replaced. Accordmgly the system will stay in the Split mode of Stage S6 in order to contmued operation based on a smgle processmg set After replacing the faulty processmg set the system could then establish an equivalent state and move via Stage S7 to Stage SI As described above, the comparator 130 is operable m the combined mode to compare the I/O operations output by the first and second processmg sets 14 and 16 This is fine as long as all of the I/O operations of the first and second processing sets 14 and 16 are fully synchronized and deterministic Anv deviation from this will be interpreted by the comparator 130 as a loss of lockstep This is in principle conect as even a minor deviation from identical outputs, if not trapped by the comparator 130, could lead to the processmg sets divergmg further from each other as the individual processmg sets act on the deviating outputs However, a strict application of this puts significant constraints on the design of the individual processmg sets An example of this is that it would not be possible to have mdependent tune of day clocks in the individual processmg sets operatmg under their own clocks This is because it is impossible to obtain two crystals which are 100% identical m operation Even small differences m the phase of the clocks could be critical as to whether the same sample is taken at any one time, for example either side of a clock transition for the respective processmg sets Accordmgly, a solution to this problem employs the dissimilar data registers (DDR) 116 mentioned earlier The solution is to wπte data from the processmg sets mto respective DDRs in the bndge while disabling the companson of the data phases of the write operations and then to read a selected one of the DDRs back to each processmg set, whereby each of the processmg sets is able to act on the same data Figure 17 is a schematic representation of details of the bπdge of Figures 6 to 10 It will be noted that details of the bndge not shown m Figure 6 to 8 are shown m Figure 17, whereas other details of the bndge shown m Figures 6 to 8 are not shown in Figure 17, for reasons of clanty The DDRs 116 are provided in the bπdge registers 110 of Figure 7, but could be provided elsewhere m the bndge in other embodiments One DDR 116 is provided for each processmg set In the example of the multi-processor system of Figure 1 where two processmg sets 14 and 16 are provided, two DDRs 116A and 116B are provided, one for each of the first and second processmg sets 14 and 16, respectively Figure 17 represents a dissimilar data wnte stage The addressing logic 136 is shown schematically to compnse two decoder sections, one decoder section 136A for the first processmg set and one decoder section 136B for the second processmg set 16 Duπng an address phase of a dissimilar data LO wnte operation each of the processmg sets 14 and 16 outputs the same predetermmed address DDR-W which is separately interpreted by the respective first and second decodmg sections 136A and 136B as addressing the respective first and second respective DDRs 116A and 116B As the same address is output by the first and second processing sets 14 and 16, this is not interpreted by the comparator 130 as a lockstep enor The decodmg section 136A, or the decodmg section 136B, or both are ananged to further output a disable signal 137 in response to the predetermmed wπte address supplied by the first and second processing sets 14 and 16 This disable signal is supplied to the comparator 130 and is operative durmg the data phase of the write operation to disable the comparator As a result, the data output by the first processmg set can be stored m the first DDR 116A and the data output by the second processmg set can be stored in the second DDR 116B without the comparator bemg operative to detect a difference, even if the data from the first and second processmg sets is different The first decoding section is operable to cause the routmg matrix to store the data trom the first processing set 14 m the first DDR 116A and the second decodmg section is operable to cause the routmg matrix to store the data from the second processmg set 16 m the second DDR 116B At the end of the data phase the comparator 130 is once agam enabled to detect any differences between I/O address and/or data phases as indicative of a lockstep enor Following the writing of the dissimilar data to the first and second DDRs 116A and 116B. the processing sets are then operable to read the data from a selected one of the DDRs 116A/116B Figure 18 illustrates an alternative anangement where the disable signal 1 7 is negated and is used to control a gate 131 at the output of the comparator 130 When the disable signal is active the output of the comparator is disabled, whereas when the disable signal is inactive the output of the comparator is enabled Figure 19 illustrates the readmg of the first DDR 116A m a subsequent dissimilar data read stage As illustrated m Figure 19, each of the processmg sets 14 and 16 outputs the same predetermmed address DDR-RA which is separately interpreted by the respective first and second decodmg sections 136A and 136B as addressing the same DDR, namely the first DDR 116A As a result, the content of the first DDR 116A is read by both of the processmg sets 14 and 16, thereby enablmg those processmg sets to receive the same data This enables the two processmg sets 14 and 16 to achieve determmistic behavior, if the source of the data wntten into the DDRs 116 by the processmg sets 14 and 16 was not determmistic As an alternative, the processmg sets could each read the data from the second DDR 116B Figure 20 illustrates the readmg of the second DDR 116B in a dissimilar data read stage following the dissumlar data wπte stage of Figure 15 As illustrated m Figure 20, each of the processmg sets 14 and 16 outputs the same predetermmed address DDR-RB which is separately interpreted by the respective first and second decodmg sections 136A and 136B as addressing the same DDR, namely the second DDR 116B As a result, the content of the second DDR 116B is read by both of the processmg sets 14 and 16, thereby enablmg those processmg sets to receive the same data. As with the dissimilar data read stage of Figure 16, this enables the two processmg sets 14 and 16 to achieve determmistic behavior, even if the source of the data wπtten mto the DDRs 116 by the processmg sets 14 and 16 was not determmistic. The selection of which of the first and second DDRs 116A and 116B to be read can be determmed m any appropriate manner by the software operatmg on the processmg modules This could be done on the basis of a simple selection of one or the other DDRs, or on a statistical basis or randomlv or m any other manner as long as the same choice of DDR is made by both or all of the processing sets Figure 21 is a flow diagram summarizing the vaπous stages of operation of the DDR mechanism descnbed above In stage S10, a DDR wnte address DDR-W is received and decoded by the address decoders sections 136A and 136B during the address phase of the DDR wπte operation In stage SI 1, the comparator 130 is disabled In stage S12, the data received from the processing sets 14 and 16 during the data phase of the DDR write operation is stored m the first and second DDRs 116A and 116B, respectively, as selected by the first and second decode sections 136A and 136B, respectively In stage SI 3, a DDR read address is received from the first and second processing sets and is decoded by the decode sections 136A and 136B, respectively If the received address DDR-RA is for the first DDR 116A, then m stage S 14 the content of that DDR 116A is read by both of the processmg sets 14 and 16 Alternatively, 116A if the received address DDR-RB is for the second DDR 116B, then in stage SI 5 the content of that DDR 116B is read by both of the processmg sets 14 and 16 Figure 22 is a schematic representation of the arbitration performed on the respective buses 22, 24 and 26. and the arbitration for the bπdge itself Each of the processmg set bus controllers 50 m the respective processmg sets 14 and 16 includes a conventional PCI master bus arbiter 180 for providmg arbitration to the respective buses 24 and 26. Each of the master arbiters 180 is responsive to request signals from the associated processmg set bus controller 50 and the bridge 12 on respective request (REQ) lmes 181 and 182 The master arbiters 180 allocate access to the bus on a first-come-frrst-served basis, issumg a grant (GNT) signal to the winning party on an appropπate grants lme 183 or 184. A conventional PCI bus arbiter 185 provides arbitration on the D bus 22. The D bus arbiter 185 can be configured as part of the D bus mterface 82 of Figure 6 or could be separate therefrom. As with the P bus master arbiters 180, the D bus arbiter is responsive to request signals from the contending devices, mcludmg the bπdge and the devices 30, 31, etc. connected to the device bus 22. Respective request lmes 186, 187, 188, etc for each of the entities competing for access to the D bus 22 are provided for the request signals (REQ). The D bus arbiter 185 allocates access to the D bus on a first-come-first-served basis, issumg a grant (GNT) signal to the winning entity via respective grant lmes 189, 190, 192, etc. Figure 23 is a state diagram summarizing the operation of the D bus arbiter 185. In a particular embodiment up to six request signals may be produced by respective D bus devices and one by the bndge itself On a transition mto the GRANT state, these are sorted by a pnonty encoder and a request signal (REQ#) with the highest pnonty is registered as the winner and gets a grant (GNT#) signal. Each winner which is selected modifies the pnonties m a pnonty encoder so that given the same REQ# signals on the next move to grant. A different device has the highest pnonty, hence each device has a "fair" chance of accessmg DEVs. The bndge REQ# has a higher weighting than D bus devices and will, under very busy conditions, get the bus for every second device. If a device requesting the bus fails to perform a transaction within 16 cycles it may lose GNT# via the BACKOFF state BACKOFF is required as, under PCI mles, a device may access the bus one cycle after GNT# is removed Devices may only be granted access to D bus if the bndge is not in the not m the EState. A new GNT# is produced at the tunes when the bus is idle. In the GRANT and BUSY states, the FETs are enabled and an accessmg device is known and forwarded to the D bus address decode logic for checkmg agamst a DMA address provided by the device. Turning now to the bndge arbiter 134, this allows access to the bndge for the first device which asserts the PCI FRAME# signal indicating an address phase Figure 24 is a state diagram summarizing the operation of the bridge arbiter 134 As with the D bus arbiter, a pnonty encoder can be provided to resolve access attempts which collide In this case 'a collision" the loser/losers are retned which forces them to give up the bus Under PCI mles retried devices must try repeatedly to access the bridge and this can be expected to happen To prevent devices which are very quick with their retry attempt from hogging the bndge, retned interfaces are remembered and assigned a higher pnonty These remembered retnes are pπoπtized m the same way as address phases However as a precaution this mechanism is timed out so as not to get stuck waitmg for a faulty or dead device The algorithm employed prevents a device which hasn t \ et been retned, but which would be a higher pnonty retry than a device cunently waitmg for, from bemg retned at the first attempt In combmed operations a PA or PB bus mput selects which P bus mterface will win a bndge access Both are informed they won Allowed selection enables latent fault checkmg durmg normal operation EState prevents the D bus from winning The bndge arbiter 134 is responsive to standard PCI signals provided on standard PCI control lmes 22, 24 and 25 to control access to the bπdge 12 Figure 25 illustrates signals associated with an I/O operation cycle on the PCI bus A PCI frame signal (FRAME#) is initially asserted At the same time, address (A) signals will be available on the DATA BUS and the appropπate command (wπte/read) signals (C) will be available on the command bus (CMD BUS) Shortly after the frame signal bemg asserted low, the mitiator ready signal (IRDY#) will also be asserted low When the device responds, a device selected signal (DEVSEL#) will be asserted low When a target ready signal is asserted low (TRDY#), data transfer (D) can occur on the data bus The bndge is operable to allocate access to the bndge resources and thereb\ to negotiate allocation of a target bus m response to the FRAME# bemg asserted low for the mitiator bus concerned Accordmgly, the bndge arbiter 134 is operable to allocate access to the bndge resources and/or to a target bus on a first-come- frrst-served basis m response to the FRAME# bemg asserted low As well as the simple first-come-first-served basis, the arbiters may be additionally provided with a mechanism for loggmg the arbitration requests, and can imply a conflict resolution based on the request and allocation history where two requests are received at an identical time Alternatively, a simple pnonty can be allocated to the vanous requesters, whereby, in the case of identically timed requests, a particular requester always wms the allocation process Each of the slots on the device bus 22 has a slot response register (SRR) 118, as well as other devices connected to the bus, such as a SCSI mterface Each of the SRRs 118 contams bits defining the ownership of the slots, or the devices connected to the slots on the direct memory access bus In this embodiment, and for reasons to be elaborated below, each SRR 118 compnses a four bit register However, it will be appreciated that a larger register will be required to detemune ownership between more than two processmg sets For example, if three processmg sets are provided, then a five bit register will be requued for each slot Figure 16 illustrates schematically one such four bit register 600 As shown m Figure 16, a first bit 602 is identified as SRR[0], a second bit 604 is identified as SRR[1], a third bit 606 is identified as SRR[2] and a fourth bit 608 is identified as SRR[3] Bit SRR[0] is a bit which is set when wntes for valid transactions are to be suppressed Bit SRR[1] is set when the device slot is owned by the first processmg set 14 This defines the access route between the first processmg set 14 and the device slot When this bit is set, the first processmg set 14 can always be master of a device slot 22, while the ability for the device slot to be master depends on whether bit SRR[3] is set Bit SRR[2] is set when the device slot is owned by the second processmg set 16 This defines the access route between the second processing set 16 and the device slot When this bit is set, the second processmg set 16 can always be master of the device slot or bus 22, while the ability for the device slot to be master depends on whether bit SRR[3 ] is set Bit SRR[3] is an arbitration bit which gives the device slot the ability to become master of the device bus 22, but only if it is owned by one of the processmg sets 14 and 16, that is if one of the SRR [1] and SRR[2] When the fake bit (SRR[0]) of an SRR 118 is set, wntes to the device for that slot are ignored and do not appear on the device bus 22. Reads re m mdetermmate data without causmg a transaction on the device bus 22. In the event of an I/O enor the fake bit SRR[0] of the SRR 188 conespondmg to the device which caused the enor is set by the hardware configuration of the bndge to disable further access to the device slot concerned. An interrupt may also be generated by the bndge to inform the software which ongmated the access leadmg to the I/O enor that the enor has occuned. The fake bit has an effect whether the system is m the split or the combmed mode of operation. The ownership bits only have effect, however, m the split system mode of operation. In this mode, each slot can be m three states: Not-owned; Owned by processmg set 14; and Owned by processmg set 16 This is determmed by the two SRR bits SRR[1] and SRR[2], with SRR[1] bemg set when the slot is owned by processmg set 14 and SRR[2] bemg set when the slot is owned by processmg set B. If the slot is unowned, then neither bit is set (both bits set is an illegal condition and is prevented by the hardware). A slot which is not owned by the processmg set making the access (this includes un-owned slots) cannot be accessed and results m an abort. A processmg set can only claim an un-owned slot; it cannot wrest ownership away from another processmg set. This can only be done by powenng-off the other processmg set. When a processmg set is powered off, all slots owned by it move to the un-owned state. Whilst it is not possible for a processmg set to wrest ownership from another processmg set, it is possible for a processmg set to give ownership to another processmg set. The owned bits can be altered when m the combmed mode of operation state but they have no effect until the split mode is entered. Table 2 below summarizes the access rights as determmed by an SRR 118. From Table 2, it can be seen that when the 4-bit SRR foi a given device is set to 1100, for example, then the slot is owned by processmg set B (i.e. SRR[2] is logic high) and processmg set A may not read from or write to the device (i.e. SRR[1] is logic low), although it may read from or wnte to the bπdge. "FAKE_AT" is set logic low (i.e. SRR[0] is logic low) indicating that access to the device bus is allowed as there are no faults on the bus. As "ARB_EN" is set logic high (i.e SRR[3] is logic high), the device with which the register is associated can become master of the D bus. This example demonstrates the operation of the register when the bus and associated devices are operatmg conectly TABLE 2 SRR PA BUS PB BUS Device Interface [3[2][1][0] 0000 xOOx Read/Wnte bridge SRR Read/Wnte bπdge SRR Access demed 0010 Read/Write bndge Read Wnte bndge Access Demed because Owned D Slot No access to D Slot arbitration bit is off 0100 Read Wnte bndge Read/wnte bπdge Access Demed because No access to D Slot Access to D Slot arbitration bit is off 1010 Read/Write bndge, Read/Wnte Bπdge Access to CPU B Denied Owned D Slot No access to D Slot Access to CPU A OK 1100 Read/Write bndge, Read/Wnte bndge Access to CPU A Denied No access to D Slot Access to D Slot Access to CPU B OK 0011 Read/Wnte bndge, Read/Wnte bπdge Access Demed because Bndge discard wntes No access to D Slot Arbitration bit is off 0101 Read/Wnte bndge, Read/Wnte bndge Access Demed because No access to D slot Bndge discards wntes Arbitration bit is off 1011 Read/Wnte bndge, Read/Wnte bndge Access to CPU B Demed Bndge discard wntes No access to D Slot Access to CPU A OK 1101 Read/Write bndge, Read/Wnte bndge Access to CPU B Demed No access to D slot Bndge discards wntes Access to CPU A OK In an alternative example, where the SRR for the device is set to 0101, the setting of SRR[2] logic high indicates that the device is owned by processmg set B However, as the device is malfunctionmg, SRR[3] is set logic low and the device is not allowed access to the processmg set SRR[0] is set high so that any writes to the device are ignored and reads therefrom return mdetermmate data In this way, the malfunctionmg device is effectively isolated from the processmg set, and provides mdetermmate data to satisfy any device drivers, for example, that might be looking for a response from the device Figure 26 illustrates the operation of the bπdge 12 for direct memory access bv a device such as one of the devices 28, 29, 30, 31 and 32 to the memory 56 of the processmg sets 14 and 16 When the D bus arbiter 185 receives a direct memory access (DMA) request 193 from a device (e g , device 30 m slot 33) on the device bus, the D bus arbiter determines whether to allocate the bus to that slot As a result of this granting procedure, the D-bus arbiter knows the slot which has made the DMA request 193 The DMA request is supplied to the address decoder 142 m the bndge, where the addresses associated with the request are decoded The address decoder is responsive to the D bus grant signal 194 for the slot concerned to identify the slot which has been granted access to the D bus for the DMA request The address decode logic 142 holds or has access to a geographic address map 196, which identifies the relationship between the processor address space and the slots as a result of the geographic address employed This geographic address map 196 could be held as a table m the bndge memory 126, along with the posted write buffer 122 and the duty RAM 124 Alternatively, it could be held as a table m a separate memory element, possibly forming part of the address decoder 142 itself The map 182 could be configured in a form other than a table The address decode logic 142 is configured to venfy the conectness of the DMA addresses supplied by the device 30 In one embodiment of the invention, this is achieved by comparmg four significant address bits of the address supplied by the device 30 with the conespondmg four address bits of the address held m the geographic addressing map 196 for the slot identified by the D bus grant signal for the DMA request In this example, four address bits are sufficient to detemune whether the address supplied is within the conect address range In this specific example, 32 bit PCI bus addresses are used, with bits 31 and 30 always bemg set to 1, bit 29 being allocated to identify which of two bndges on a motherboard is bemg addressed (see Figure 2) and bits 28 to 26 identifying a PCI device. Bits 25-0 define an offset from the base address for the address range for each slot Accordingly, by comparmg bits 29-26, it is possible to identify whether the address(es) supplied fall(s) within the appropnate address range for the slot concerned It will be appreciated that in other embodiments a different number of bits may need to be compared to make this determination depending upon the allocation of the addresses The address decode logic 142 could be aπanged to use the bus grant signal 184 for the slot concerned to identify a table entry for the slot concerned and then to compare the address m that entry with the address(es) received with the DMA request as descnbed above Alternatively, the address decode logic 142 could be ananged to use the address(es) received with the DMA address to address a relational geographic address map and to detemune a slot number therefrom, which could be compared to the slot for which the bus grant signal 194 is intended and thereby to detemune whether the addresses fall within the address range appropnate for the slot concerned Either way, the address decode logic 142 is aπanged to permit DMA to proceed if the DMA addresses fall within the expected address space for the slot concerned Otherwise, the address decoder is ananged to ignore the slots and the physical addresses The address decode logic 142 is further operable to control the routing of the DMA request to the appropriate processmg set(s) 14/16 If the bndge is in the combmed mode, the DMA access will automatically be allocated to all of the m-sync processmg sets 14/16 The address decode logic 142 will be aware that the bridge is m the combmed mode as it is under the control of the bndge controller 132 (see Figure 8) However, where the bndge is m the split mode, a decision will need to be made as to which, if any, of the processmg sets the DMA request is to be sent When the system is in split mode, the access will be directed to a processmg set 14 or 16 which owns the slot concerned If the slot is un-owned, then the bridge does not respond to the DMA request In the split mode, the address decode logic 142 is operable to determine the ownership of the device ongmatmg the DMA lequest by accessmg the SRR 118 for the slot concerned The appropπate slot can be identified by the D bus grant signal The address decode logic 142 is operable to control the target controller 140 (see Figure 8) to pass the DMA request to the appropriate processing set(s) 14/16 based on the ownership bits SRR[1] and SRR[2] If bit SRR[1] is set, the first processmg set 14 is the owner and the DMA request is passed to the first processing set If bit SRR[2] is set, the second processmg set 16 is the owner and the DMA request is passed to the second processmg set. If neither of the bit SRR[1] and SRR[2] is set, then the DMA request is ignored by the address decoder and is not passed to either of the processmg sets 14 and 16 Figure 27 is a flow diagram summarizing the DMA veπfication process as illustrated with reference to Figure 24 In stage S20, the D-bus arbiter 160 arbitrates for access to the D bus 22. In stage S21, the address decoder 142 venfies the DMA addresses supplied with the DMA request by accessmg the geographic address map. In stage S22, the address decoder ignores the DMA access where the address falls outside the expected range for the slot concerned. Alternatively, as represented by stage S23, the actions of the address decoder are dependent upon whether the bndge is m the combmed or the split mode. If the bndge is m the combmed mode, then in stage S24 the address decoder controls the target controller 140 (see Figure 8) to cause the routing matnx 80 (see Figure 6) to pass the DMA request to both processmg sets 14 and 16. If the bndge is m the split mode, the address decoder is operative to verify the ownership of the slot concerned by reference to the SRR 118 for that slot m stage S25. If the slot is allocated to the first processmg set 14 (i.e. the SRR[1] bit is set), then m stage S26 the address decoder 142 controls the target controller 140 (see Figure 8) to cause the routing matnx 80 (see Figure 6) to pass the DMA request to first processmg set 14. If the slot is allocated to the second processmg set 16 (l e. the SRR[2] bit is set), then m stage S27 the address decoder 142 controls the target controller 140 (see Figure 8) to cause the routing matnx 80 (see Figure 6) to pass the DMA request to the second processmg set 16 If the slot is unallocated (i.e. neither the SRR[1] bit nor the SRR[2] bit is set), then in step S18 the address decoder 142 ignores or discards the DMA request and the DMA request is not passed to the processing sets 14 and 16 A DMA, or direct vector memory access (DVMA), request sent to one or more of the processing sets causes the necessary memory operations (read or write as appropnate) to be effected on the processing set memory There now follows a description of an example of a mechanism for enablmg automatic recovery from an EState (see Figure 11). The automatic recovery process mcludes reintegration of the state of the processmg sets to a common status in order to attempt a restart m lockstep To achieve this, the processmg set which asserts itself as the primary processmg set as descnbed above copies its complete state to the other processmg set This involves ensuring that the content of the memory of both processors is the same before trying a restart m lockstep mode However, a problem with the copying of the content of the memory from one processmg set to the other is that duπng this copying process a device connected to the D bus 22 might attempt to make a direct memory access (DMA) request for access to the memory of the primary processmg set If DMA is enabled, then a wnte made to an area of memory which has already been copied would result in the memory state of the two processors at the end of the copy not bemg the same. In principle, it would be possible to inhibit DMA for the whole of the copy piocess However, this would be undesnable, bearing in mind that it is desirable to minimize the time that the system or the resources of the system are unavailable As an alternative, it would be possible to retry the whole copy operation when a DMA operation has occurred during the penod of the However, it is likely that further DMA operations would be performed duπng the copy retry, and accordmgly this is not a good option eithei Accordmgly, m the present system, a duty RAM 124 is provided m the bπdge As descnbed earlier the duty RAM 124 is configured as part of the bπdge SRAM memory 126 The duty RAM 124 compnses a bit map havmg a duty indicator, for example a duty bit, for each block, or page, of memory The bit for a page of memory is set when a wnte access to the area of memory concerned is made In an embodiment of the invention one bit is provided for every 8K page of mam processmg set memory The bit for a page of processmg set memory is set automatically by the address decoder 142 when this decodes a DMA request for that page of memory for either of the processmg sets 14 or 16 from a device connected to the D bus 22 The duty RAM can be reset, or cleared when it is read by a processmg set, for example by means of read and clear instructions at the beginning of a copy pass, so that it can start to record pages which are dirtied smce a given time The duty RAM 124 can be read word by word If a large word size is chosen for readmg the duty RAM 124, this will optimize the readmg and resetting of the dirty RAM 124 Accordmgly, at the end of the copy pass the bits m the duty RAM 124 will indicate those pages of processmg set memory which have been changed (or dirtied) by DMA wntes during the penod of the copy A further copy pass can then be performed for only those pages of memory which have been dirtied This will take less time that a full copy of the memory Accordmgly, there are typically less pages marked as duty at the end of the next copy pass and, as a result, the copy passes can become shorter and shorter As some time it is necessary to decide to inhibit DMA wntes for a short penod for a final, short, copy pass, at the end of which the memones of the two processmg sets will be the same and the primary processmg set can issue a reset operation to restart the combmed mode The duty RAM 124 is set and cleared in both the combmed and split modes This means that m split mode the duty RAM 124 may be cleared by either processmg set The duty RAM 124 address is decoded from bits 13 to 28 of the PCI address presented by the D bus device Enoneous accesses which present illegal combmations of the address bits 29 to 31 are mapped into the dirty RAM 124 and a bit is dirtied on a wπte, even though the bndge will not pass these transactions to the piocessmg sets When readmg the dirty RAM 124, the bndge defines the whole area from 0x00008000 to OxOOOOffff as dirty RAM and will clear the contents of any location m this range on a read As an alternative to providmg a smgle dirty RAM 124 which is cleared on bemg read, another alternative would be to provide two duty RAMs which are used m a toggle mode, with one bemg written to while another is read Figure 28 is a flow diagram summarizing the operation of the duty RAM 124 In stage S41. the primary processmg set reads the dirty RAM 124 which has the effect of resetting the duty RAM 124 In stage S42. the primary processor (e g. processmg set 14) copies the whole of its memory 56 to the memory 56 of the other processmg set (e.g processmg set 16). In stage S43, the primary processmg set reads the duty RAM 124 which has the effect of resetting the duty RAM 124 In stage S44, the primary processor determines whether less than a predetermmed number of bits have been written in the duty RAM 124. If more than the predetermmed number of bits have been set, then the processor in stage S45 copies those pages of its memory 56 which have been dutied, as mdicated by the duty bits read from the dirty RAM 124 m stage S43, to the memory 56 of the other processmg set Control then passes back to stage S43. If, in stage S44, it is determmed less than the predetermmed number of bits have been wπtten m the duty RAM 124, then in Stage S45 the primary processor causes the bndge to inhibit DMA requests from the devices connected to the D bus 22 This could, for example, be achieved by clearing the arbitration enable bit for each of the device slots, thereby denymg access of the DMA devices to the D bus 22. Alternatively, the address decoder 142 could be configured to ignore DMA requests under instructions from the primary processor. Duπng the penod in which DMA accesses are prevented, the primary processor then makes a final copy pass from its memory to the memory 56 of the other processor for those memory pages coπesponding to the bits set in the duty RAM 124 In stage S47 the primary processor can issue a reset operation for initiating a combmed mode. In stage S48, DMA accesses are once more permitted. It will be appreciated that although particular embodiments of the mvention have been descnbed, many modifications/additions and/or substitutions may be made within the spint and scope of the present mvention as defined in the appended claims. For example, although in the specific descnption two processmg sets are provided, it will be appreciated that the specifically descnbed features may be modified to provide for three or more processmg sets

Claims: WHAT IS CLAIMED 1 A bridge for a multi-processor system, the bndge compπsmg a first I/O bus mterface for connection to a first I/O bus for a first processmg set, a second I/O bus mterface for connection to a second I/O bus for a second processing set, a device bus mterface for connection to a device bus, a bπdge control mechanism configured to be operable to compare address and data phases of I/O accesses by the first and second processmg sets and at least one dissimilar data register for each processmg set, wherem the bndge control mechanism is configured to be operable m response to an address phase of a dissimilar data register wnte access to disregard any differences m data for the dissimilar data write access 2 The bndge of claim 1, wherem the bndge control mechanism compnses a comparator configured to be operable m the combmed mode to detect differences between signals on the first and second I/O buses as indicative of a lockstep enor 3 The bridge of claim 2, wherem the bπdge control mechamsm compnses an address detector responsive to a signal identifymg a wnte to a dissumlar data register durmg an address phase of an I/O access by the first and second processmg sets to inhibit the comparator duπng the subsequent data phase of the I/O access 4 The bndge of claim 3, wherem the bndge control mechanism is operable to block the output of the comparator 5 The bπdge of claim 3, wherem the bndge control mechamsm is operable to inhibit the operation of the comparator 6 The bndge of claim 3, wherem the bndge control mechanism is configured to be operable to respond to a read address supplied in common by the first and second processmg sets for a dissimilar data read access to supply data read from a determmed one of the dissumlar data registers to the first and second processmg sets 7 The bridge of claim 6, wherem the read address supplied in common by the first and second processmg sets determines the dissimilar data register from which data is read 8 The bndge of claim 2, compnsmg an address decode mechanism configured to be operable to determine target addresses for write and read accesses, the address decode mechamsm bemg configured to be operable to be responsive to a common write address from the first and second processmg sets for a write to a dissimilar data register to derive first and second dissimilar data register addresses for the first and second processing sets, respectively 9 The bπdge of claim 8, wherein the address decode mechanism is responsn e to a read address supplied in common by the first and second processmg sets for a dissimilar data read access to access a selected one of the first and second dissimilar data registers. 10 The bndge of claim 9, wherem the common wπte address is different from the common read address 11 The bridge of claim 9, wherem the common wπte address is one of a set of possible common read addresses 12 The bndge of claim 1 , compnsmg a controllable routmg matπx coupled betw een the first processor bus mterface, the second processor bus mterface, the device bus mterface and the dissimilar data registers, the bndge control mechanism bemg configured to be operable to control the routing matnx selectively to interconnect the first processor bus mterface, the second processor bus mterface, the device bus mterface and the dissimilar data registers. 13 The bndge of claim 12, compπsmg mitiator and target controllers for determining a path from an input to an output of the routmg matπx, the mitiator controller bemg responsive to determination of the master for an bus operation and the target controller bemg responsive to detemunation of a destination address by the address decode mechamsm. 14 The bπdge of claim 13, wherem the master and target controllers are further responsive to an output of the address detector to modify the path from the mput to the output of the routing matπx. 15 The bπdge of claim 1, wherem the dissimilar data registers are configured as separate registers in the bridge 16 The bπdge of claim 1, comprising at least one further processor bus interface for connection to an I/O bus of a further processmg set 17 A bπdge for a multi-processor system, the bπdge compnsmg means for interfacing to a first I/O bus for a first processmg set, a second I/O bus for a second processmg set and a device bus, means for comparmg address and data phases of I/O accesses by the first and second processmg sets, at least one dissimilar data register for each processmg set and means for respondmg to an address phase of a dissimilar data register write access to cause the comparmg means to disregard any differences m data for the dissimilar data wnte access 18 A computer system compnsmg a first processmg set havmg a first I/O bus, a second processmg set having a second I/O bus, a device bus and a bndge, the bndge bemg connected to the first I/O bus, the second I/O bus and a device bus and compπsmg a bridge control mechanism configured to be operable to compare address and data phases of I O accesses by the first and second processmg sets and at least one dissimilar data legister tor each processmg set, wherem the bridge control mechanism is configured to be operable m response to an address phase of a dissimilar data register wnte access to disregard differences m data for the dissimilar data write access 19 The system of claim 18, wherem each processmg set compnses at least one processor, memory and a processing set I/O bus controller 20 The system of claim 16, further compnsmg at least one further processmg set 21 A method of operating a multi-processor system compπsmg a first processmg set havmg a first I/O bus, a second processmg set havmg a second I/O bus, a device bus and a bndge, the bndge bemg connected to the first O bus, the second I/O bus and the device bus, the method compnsmg comparing address and data phases of I/O accesses by the first and second processmg sets for determining lockstep operation of the processmg sets, detectmg an address phase of a dissimilar data register wnte access, and disregardmg differences m the data for the dissimilar data write access, and storing the data of the data phase for the dissumlar data wπte access for the first and second processmg sets in first and second dissimilar data registers, respectively 22 The method of claim 21, compπsmg subsequently detecting a read destination address supplied m common by the first and second processmg sets for a dissimilar data read access to supply data read from a determined one of the dissumlar data registers to the first and second processmg sets 23 The method of claim 21, wherem the read destination address supplied in common by the first and second processing sets determines the dissimilar data register from which data is read

Inventors: GARNETT PAUL J, OYELAKIN FEMI A, ROWLINSON STEPHEN, WILLIAMS EMRYS J, GARNETT, PAUL, J., OYELAKIN, FEMI, A., ROWLINSON, STEPHEN, WILLIAMS, EMRYS, J.
Applicants: SUN MICROSYSTEMS INC, SUN MICROSYSTEMS, INC.
Application year: 1999
Main classification: G06F11
IPC codes: G06F11, G06F13