PROCESSOR BRIDGE WITH POSTED WRITE BUFFER

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. The bridge also includes a memory subsystem and a bridge control mechanism. The bridge control mechanism is operable to monitor operation of the first and second processing sets in a combined, lockstep, operating mode and to be responsive to detection of a lockstep error to cause the bridge to be operable in an error mode in which write accesses initiated by the processor sets are buffered in a bridge buffer pending resolution of the error mode. A respective buffer region is provided for each processing set. In an initial error mode, any complete device write accesses initiated by the processing sets are stored in a posted write buffer. Where data is in transit through the bridge on entry to the error mode, the data is diverted to one or more disconnect registers. The bridge control mechanism is operable to permit read access to the posted write buffers and the disconnect registers by the processing sets to enable recovery from the error mode.

Description: TITLE: PROCESSOR BRIDGE WITH POSTED WRITE BUFFER BACKGROUND OF THE INVENTION This relates to fault tolerant computer systems and to mechanisms for enabling recovery from an error durmg operation of the computer system In particular, the mvention relates to a processor bπdge providing a posted write buffer for use in such a fault tolerant computer system In a fault tolerant computer system which provides detection of system errors, there can be some elapse of tune between detecting the error and taking an action to limit the impact of the error, or to effect recovery from the error Durmg this tune, I/O cycles can still be pendmg withm the system This is because the processors and bus controllers m the system can mclude I/O operations which have already been posted from the processors, and for which the processors will no longer have any record On detecting an error, it is possible to identify a bus fault to the issumg processors or processor sets which would allow the processor to re-issue the I/O access following resolution of the fault However, in the case of write accesses, the associated data would be lost if the accesses were simply bus errored Accordmgly, the aim of the present mvention is to provide a mechanism which facilitates the taking of action to limit the impact of an error or to completely recover from an error where pendmg I/O operations have already been mitiated by a processor or processor set SUMMARY OF THE flWENTION Particular and preferred aspects of the mvention are set out m the accompanymg independent and dependent claims Combinations of features from the dependent claims may be combmed with features of the mdependent claims as appropπate and not merely as explicitly set out m the claims In accordance with one aspect of the mvention, there is provided a bndge for a multi-processor system The bridge mcludes a bus mterfaces 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 The bridge also includes a memory subsystem and a bπdge control mechanism The bπdge control mechanism is operable to monitor operation of the first and second processing sets in a combined, lockstep, operating mode and to be responsive to detection of a lockstep eπor to cause the bπdge to be operable in an eπor mode m which write accesses initiated by the processor sets are buffered in a bridge buffer pendmg resolution of the eπor mode The provision of a bndge, including a bπdge control mechanism with a buffer for buffering write accesses initiated by processor sets, enables write commands and the associated data which are located in various queues withm the system to be stored pendmg resolution of an eπor Accordmgly, this stored information can be used at least to limit the impact of the eπor and preferably completely to recover from that eπor It can further be used to identify the originator of the eπor The buffer (posted write buffer) provides storage for I/O write accesses which occur after an eπor condition is detected To allow the accesses to be rerun, the address to which the transaction would have occuπed, any associated data, the type of wπte command and also any byte enable information required to identify which parts of the data are valid, can be stored for later use in the posted write buffer It should be noted that the bus mterfaces referenced above need not be separate components of the bridge, but may be incorporated in other components of the bridge, and may mdeed be sunply connections for the lmes of the buses concerned In an embodiment of the invention a respective buffer region is provided for each processmg set The bπdge control mechanism of an embodiment of the invention is operable, in an initial eπor mode to store in the posted write buffer any internal bridge wπte accesses issued by the processmg sets and to allow and to arbitrate any internal bridge read accesses initiated by the processmg sets, and to store m a posted wnte buffer any complete device write accesses mitiated by the processing sets and to abort any device bus read accesses mitiated by the processmg sets Moreover, the bndge control mechanism is operable m the initial eπor mode, where an address part of a device wnte access has already been issued to the device bus, to buffer burst data parts of the device write access in one or more disconnect registers The bndge control mechanism of an embodiment of the invention is operable, m a primary eπor mode in which a processmg set asserts itself as a primary processmg set to allow and to arbitrate any internal bridge write accesses initiated by the pπmary processmg set, to discard any internal bndge write accesses mitiated by any other processing set, and to allow and to arbitrate any internal bπdge read accesses mitiated by the processing sets, and to discard any device bus wπte accesses mitiated by the processmg sets and to abort any device bus read accesses mitiated by the processmg sets A controllable routing matrix connects the first processor bus interface, the second processor bus mterface, the device bus interface and the memory sub-system The bridge control mechanism is operable to control the routmg matrix selectively to interconnect the first processor bus mterface, the second processor bus mterface, the device bus mterface and the memory sub-system according to a cuπent mode of operation The bπdge control mechanism of an embodiment of the invention further comprises an address decode mechanism coupled to the first processor bus mterface, the second processor bus mterface and the device bus mterface, the address decode mechanism being operable to determine target addresses for wπte and read accesses The initiator of the write and read accesses will be determined by an arbitration mechanism Initiator and target controllers control the routing of a path from an input to an output of the routing matπx, the initiator and target controllers bemg responsive to the arbitration address decode mechanisms, respectively The bridge control mechanism of an embodiment of the mvention compπses a comparator connected to the first and second processor bus mterfaces The comparator is operable in the combined mode to detect differences between signals on the I/O buses of the first and second processmg sets as indicative of a lockstep eπor A bridge controller is connected to an output of the comparator, the bndge controller being operable, in response to a signal indicative of a lockstep eπor output from the comparator, to cause the bridge to cease operation in the combmed mode and instead to operate in the eπor mode The master and target controllers are responsive to an output of the bπdge controller to modify the path from the input to the output of the routing matrix In an embodiment of the invention a posted write buffer is configured in random access memory connected to or forming part of the bπdge Disconnect registers are configured as separate hardware registers The bπdge could mclude more than two processor bus interfaces for connection to respective I/O buses of further processmg sets. In accordance with another aspect of the mvention, there is provided a multi-processor system compnsmg a first processing set having an I/O bus, a second processmg set having an I/O bus, a device bus and a bridge as set out above. Each processing set can mclude at least one processor, memory and one or more processing set I/O bus controllers 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: momtormg operation of the first and second processing sets m a combmed, lockstep, operating mode, and on detection of a lockstep eπor, buffering write accesses mitiated by the processor sets in a buffer pending resolution of the eπor. BRIEF DESCRIPTION OF THE DRAWINGS Exemplary embodiments of the present mvention will be descπbed hereinafter, by way of example only, with reference to the accompanymg drawmgs m which like reference signs relate to like elements and m which. Figure 1 is a schematic overview of a fault tolerant computer system incorporating an embodiment of the mvention; 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 bndge 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 routmg matnx of the bπdge of Figure 6, Figure 10 is an example implementation of the bndge of Figure 6; Figure 11 is a state diagram illustrating operational states of the bndge of Figure 6; Figure 12 is a flow diagram illustrating stages m the operation of the bndge 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 in 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 summansmg the operation of a dissimilar data wnte mechamsm; Figure 22 is a schematic block diagram explaining arbitration withm 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 bridge 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 m the bπdge of Figure 6, and Figure 28 is a flow diagram of a re-integration process mcludmg the momtormg of a dirty RAM DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 is a schematic overview of a fault tolerant computmg system 10 comprising 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 processing sets The bridge 12 forms an mterface between the processmg sets and I/O devices such as devices 28, 29, 30, 31 and 32 In this document, the term "processing set" is used to denote a group of one or more processors, possibly mcludmg memory, which output and receive common outputs and mputs It should be noted that the alternative term mentioned above, "CPUset", could be used instead, and that these terms could be used mterchangeably throughout this document Also, it should be noted that the term "bπdge" is used to denote any device, apparatus or aπangement 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 Penpheral Component Interconnect (PCI) bus. The second processmg set 16 is connected to the bndge 12 via a second processing 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 bridge 12 via a device I/O bus (D bus) 22, in the present mstance also a PCI bus Although, in the particular example described, the buses 22, 24 and 26 are all PCI buses, this is merely by way of example, and in 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 bridge 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 mcludmg an Ethernet (E-NET) interface 28 and a Small Computer System Interface (SCSI) interface 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) switching can be provided for the slots 33, 34 and 35 to enable hot insertability 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, reducing 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 according to a particular implementation in accordance with specific design requirements Figure 2 is a schematic overview of a particular implementation of a fault tolerant computer employing a bπdge structure of the type illustrated in Figure 1 In Figure 2, the fault tolerant computer system includes a plurality (here four) of bndges 12 on first and second I/O motherboards (MB 40 and MB 42) order to increase the number of I/O devices which may be connected and also to improve reliability and redundancy. Thus, m 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 processmg set boards 44 and 46 'bridging' 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 tn Figure 2) First and second bndges 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 processing sets 14 and 16 by P buses 24.2 and 26.2, respectively. The bπdge 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 bndges 12.3 and 12.4 are mounted on the second I/O motherboard 42. The bridge 12.3 is connected to the processmg sets 14 and 16 by P buses 24 3 and 26.3, respectively. Similarly, the bridge 4 is connected to the processmg sets 14 and 16 by P buses 24 4 and 26.4, respectively. The bridge 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 aπangement shown m 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 increasing 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 processing set, such as the processing set 14 of Figure 1. The processing 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 processing set bus controller 50. As shown in Figure 3, one or more processing set output buses 24 are connected to the processmg set bus controller 50, each processmg set output bus 24 being connected to a respective bridge 12. For example, in the aπangement of Figure 1, only one processmg set I/O bus (P bus) 24 would be provided, whereas in the aπangement of Figure 2, four such processmg set I/O buses (P buses) 24 would be provided In the processing set 14 shown in Figure 3, mdividual processors operate using 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 interface 65 connects the internal group bus 63 to the common internal bus 64 Accordmgly, in the aπangement shown in Figure 4, individual processmg groups, with 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 mterfaces 65 enable a processor 62 of one processing group to operate not only on the data in its local memory 66, but also in the memory of another processing group 61 within the processing set 14 The processing set bus controller 60 provides a common interface between the common internal bus 64 and the processing set I/O bus(es) (P bus(es)) 24 connected to the bπdge(s) 12 It should be noted that although only two processmg groups 61 are shown in Figure 4, it will be appreciated that such a structure is not limited to this number of processmg 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 single processor 72 and associated memory 76 connected via a common bus 74 to a processing set bus controller 70 The processing 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 bπdge(s) 12. Accordmgly, it will be appreciated from Figures 3, 4 and 5 that the processing set may have many different forms and that the particular choice of a particular processing set structure can be made on the basis of the processing requirement of a particular application and the degree of redundancy required. In the following descnption, it is assumed that the processing sets 14 and 16 refeπed to have a structure as shown m Figure 3, although it will be appreciated that another form of processing set could be provided. The bndge(s) 12 are operable in a number of operatmg modes. These modes of operation will be descnbed m more detail later. However, to assist in a general understandmg of the structure of the bπdge, the two operatmg modes will be bπefly summarized here. In a first, combmed mode, a bndge 12 is operable to route addresses and data between the processing 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 coπectly. Comparison failures force the bridge 12 mto an eπor limiting mode (EState) in 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 processing sets 14 and 16 are not synchronized and no I/O compaπsons are made. DMA operations are also permitted m both modes. As mentioned above, the different modes of operation, mcludmg the combined and split modes, will be described in more detail later. However, there now follows a description of the basic structure of an example of the bridge 12. Figure 6 is a schematic functional overview of the bridge 12 of Figure 1 First and second processing set I/O bus interfaces, PA bus mterface 84 and PB bus mterface 86, are connected to the PA and PB buses 24 and 26, respectively. A device I/O bus interface, D bus interface 82, is connected to the D bus 22. It should be noted that the PA, PB and D bus interfaces need not be configured as separate elements but could be incorporated in other elements of the bridge. Accordingly, 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 bridge to connect to the bus concerned, for example by means of physical or logical bridge connections for the lmes of the buses concerned. Routmg (heremafter termed a routing matnx) 80 is connected via a first internal path 94 to the PA bus mterface 84 and via a second internal path 96 to the PB bus interface 86. The routing matnx 80 is further connected via a third internal path 92 to the D bus interface 82. The routmg matnx 80 is thereby able to provide I/O bus transaction routmg in both directions between the PA and PB bus interfaces 84 and 86. It is also able to provide routing in both directions between one or both of the PA and PB bus mterfaces and the D bus interface 82. The routing matnx 80 is connected via a further internal path 100 to storage control logic 90 The storage control logic 90 controls access to bridge registers 110 and to a random access memory (SRAM) 126 The routmg matrix 80 is therefore also operable to provide routing in 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 bπdge control logic 88 over control paths 98 and 99 The bridge 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 mvention, 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 Accordingly, 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 memories and registers of the bridge m the storage subsystem 90 The bus mterfaces 84 and 86 can provide diagnostic mformation to internal bπdge status registers in the storage subsystem 90 on transition of the bridge to an eπor state (EState) or on detection of an I/O eπor The device bus mterface 82 performs all the functionality required for a PCI compliant master and slave interface for transfeπing data to and from one of the PA and PB buses 84 and 86 The D bus 82 is operable durmg direct memory access (DMA) transfers to provide diagnostic mformation to internal status registers in the storage subsystem 90 of the bridge on transition to an EState or on detection of an I/O eπor Figure 7 illustrates m more detail the bridge 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 m which a posted write 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 wπte buffer 122 and the dirty 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 various operating modes of the bridge, allow the capture of diagnostic mformation for an EState and for I/O eπors, and control processmg set access to PCI slots and devices connected to the D bus 22 These registers are set by signals from the routing matπx 80 Dissimilar data registers (DDRs) 116 provide locations for containing 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 Bπdge decode logic enables a common write to disable a data comparator and allow writes 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 appropriate processing 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 cycle which is aborted while data is in the bridge on the way to another bus The disconnect registers 120 receive all data queued m the bndge when a target device disconnects a transaction, or as the EState is detected. These registers are connected to the routmg matnx 80 The routing matnx can queue up to three data words and byte enables. Provided the initial addresses are voted as bemg equal, address target controllers denve addresses which mcrement as data is exchanged between the bπdge and the destmation (or target) Where a wnter (for example a processor I/O write, or a DVMA (D bus to P bus access)) is wnπng data to a target, this data can be caught m the bπdge when an eπor occurs. Accordmgly, this data is stored in the disconnect registers 120 when an eπor occurs. These disconnect registers can then be accessed on recovery from an EState to recover the data associated with the wnte or read cycle which was in progress when the EState was mitiated. Although shown separately, the DDRs 116, the SRRs 118 and the disconnect registers may form an mtegral part of the CSRs 114. EState and eπor CSRs 114 provided for the capture of a failing cycle on the P buses 24 and 26, with an indication of the failing datum. Following a move to an EState, all of the writes mitiated to the P buses are logged in the posted write buffer 122 These may be other writes that have been posted m the processing set bus controllers 50, or which may be initiated by software before an EState mterrupt causes the processors to stop carrying out wπtes to the P buses 24 and 26 A dirty RAM 124 is used to indicate 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 single bit in the dirty RAM 124 which is set when a DMA write occurs and can be cleared by a read and clear cycle initiated on the dirty RAM 124 by a processor 52 of a processing set 14 and 16. The dirty RAM 124 and the posted write buffer 118 may both be mapped mto the memory 124 in the bndge 12. This memory space can be accessed during normal read and wnte 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 geographically. Accordmgly, the bridge caπies out decoding necessary to enable the isolating FETs for each slot before an access to those slots is mitiated. 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 in the combined mode) by one processmg set (e.g. processing set 14 of Figure 1) to the other processing set (e.g. processing 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 m the split mode, or both processmg sets 14 and 16 in the combined mode to an I O device on the D bus 22, in which case the access is routed via the D bus mterface 82; - a DMA access by a device on the D bus 22 to one or both of the processing sets 14 and 16, which would be directed to both processing sets 14 and 16 m the combmed 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 addressmg is employed Thus, for example, slot 0 on motherboard A has the same address when refeπed to by processmg set 14 or by processmg set 16. Geographic addressing is used in combination with the PCI slot FET switchmg Durmg 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 coπect card. Separate FET switch lmes are provided to each slot for separately switching 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 in a number of different roles which will be descπbed m more detail later. However, some of the roles are summarized here In a combmed mode, each slot may be disabled so that writes are simply acknowledged without any transaction occurring on the device bus 22, whereby the data is lost Reads will return meanmgless data, once agam 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 processing set 14 or 16 making an access (this includes not owned or unowned slots) cannot be accessed. Accordingly, such an access is aborted When a processing set 14 or 16 is powered off, all slots owned by it move 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 powering off the other processmg set, or by getting the other processmg set to relinquish ownership The ownership bits are assessable and settable while m the combined 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 determined by the PCI slot Where a device caπies out DMA, the bridge is able to check that the device is using the coπect address because a D bus arbiter mforms the bπdge which device is usmg the bus at a particular time. If a device access is a processing 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 in the processing set bus controller 50 to an actual memory address. The addresses output by the address decoders are passed via the initiator and target controllers 138 and 140 to the routmg matπx 80 via the lines 98 under control of a bridge controller 132 and an arbiter 134 An arbiter 134 is operable m vaπous different modes to arbitrate for use of the bridge on a first-come- first-served basis using conventional PCI bus signals on the P and D buses In a combined mode, the arbiter 134 is operable to arbitrate between the ln-sync processing sets 14 and 16 and any initiators on the device bus 22 for use of the bndge 12 Possible scenanos are - processmg set access to the device bus 22, - processmg set access to internal registers m the bridge 12, - Device access to the processmg set memory 56 In split mode, both processmg sets 14 and 16 must arbitrate the use of the bπdge and thus access to the device bus 22 and internal bridge registers (e g CSR registers 114) The bndge 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 eπor, the arbitration enable bit for that device is automatically reset by the bridge 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 bridge 12 cannot directly control access to the PA and PB buses 24 and 26 The bπdge 12 competes for access to the PA or PB bus with the processing set on the bus concerned under the control of the bus controller 50 on the bus concerned. Also shown in 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 determme any 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 matnx 80 comprises a multiplexer 143 which is responsive to initiator control signals 98 from the initiator controller 138 of Figure 8 to select one of the PA bus path 94 , PB bus path 96, D bus path 92 or internal bus path 100 as the cuπent input to the routing matrix 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 being 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 in the pipeline represented by the multiplexer 143, the buffer 149 and the buffers 144 In Figures 6 to 9 a functional description of elements of the bπdge has been given Figure 10 is a schematic representation of a physical configuration of the bπdge m which the bridge control logic 88, the storage control logic 90 and the bridge registers 110 are implemented m a first field programmable gate aπay (FPGA) 89, the routing matnx 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 127 The bus mterfaces 82, 84 and 86 shown m 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 single FPGA could be employed for the routing matnx 80 where the necessary logic can be accommodated withm the device. Indeed, where a FPGA of sufficient capacity is available, the bndge control logic, storage control logic and the bndge registers could be incorporated m the same FPGA as the routmg matrix. Indeed many other configurations may be envisaged, and mdeed 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 internal bus paths 85 and path control lmes 87 Figure 11 is a transition diagram illustrating m more detail the various operatmg modes of the bridge The bridge operation can be divided into three basic modes, namely an eπor 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 initial resetting on powering up the bndge, or following an out-of sync event, the bridge is in this initial EState 152. In this state, all wntes are stored m the posted wπte buffer 120 and reads from the internal bridge registers (e.g., the CSR registers 116) are allowed, and all other reads are treated as eπors (i.e. they are aborted). In this state, the individual processing sets 14 and 16 perform evaluations for determining a restart time. Each processmg set 14 and 16 will determme its own restart timer timing. The tuner 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 eπor sets a long tune for the timer. A processmg set which thinks it unlikely to have caused the eπor sets a short time for the timer. The first processmg set 14 and 16 which tunes out, becomes a primary processing set Accordmgly, when this is determined, the bndge moves (153) to the pnmary EState 154. When either processmg set 14/16 has become the pnmary processmg set, the bndge is then operating in the pnmary EState 154. This state allows the primary processmg set to wnte to bndge registers (specifically the SRRs 118) Other writes are no longer stored in 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 bndge 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 bndge storage is simply arbitrated. The pnmary status of the processmg sets 14 and 16 is ignored. Transition to a combined operation is achieved by means of a sync_reset (157) After issue of the sync_reset operation, the bridge is then operable in the combmed state 158, whereby all read and write 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 in the comparator 130. Detection of a mismatch between any read and write cycles (with an exception of specific dissimilar data I/O cycles) cause a transition 151 to the EState 150 The vanous states descnbed are controlled by the bridge controller 132. The role of the comparator 130 is to monitor and compare I/O opeiations on the PA and PB buses in the combined 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 eπor state 150. The I/O operations can mclude all I/O operations initiated by the processing sets, as well as DMA transfers m respect of DMA mitiated by a device on the device bus. Table 1 below summarizes the various access operations which are allowed m each of the operational states TABLE 1 D Bus - Read D Bus- Write E State Master Abort Stored m Post Wπte Buffer Primary EState Master Abort Lost Split Controlled by SRR bits Controlled by SRR bits and arbitrated and arbitrated Combined Allowed and compared Allowed and compared As descnbed above, after an initial reset, the system is in the initial 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 processing set 16 or 14 The internal bridge registers 116 of the bridge are accessible, but are read only A system running m the combined mode 158 transitions to the EState 150 where there is a comparison failure detected in this bridge, or alternatively a companson failure is detected in another bridge in a multi- bridge system as shown, for example, m Figure 2 Also, transitions to an EState 150 can occur in 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 mterrupt is signaled to all or a subset of the processors of the processmg sets via an mterrupt line 95 Following this, all I/O cycles generated on a P bus 24 or 26 result m reads being returned with an exception and wπtes bemg recorded m the posted write buffer The operation of the comparator 130 will now be descnbed m more detail The comparator is connected to paths 94, 95, 96 and 97 for comparing address, data and selected control signals from the PA and PB bus interfaces 84 and 86 A failed comparison of m-sync accesses to device I/O bus 22 devices causes a move from the combmed state 158 to the EState 150 For processing set I/O read cycles, the address, command, address panty, byte enables and parity eπor parameters are compared If the companson fails dunng the address phase, the bπdge asserts a retry to the processing set bus controllers 50, which prevents data leaving the I/O bus controllers 50 No activity occurs in this case on the device I/O bus 22 On the processor(s) retrying, no eπor is returned If the comparison fails durmg a data phase (only control signals and byte enables are checked), the bπdge signals a target-abort to the processmg set bus controllers 50 An eπor is returned to the processors In the case of processing set I/O bus wπte cycles, the address, command, parity, byte enables and data parameters are compared If the comparison fails during the address phase, the bridge asserts a retry to the processmg set bus controllers 50, which results in the processmg 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 processmg sets 14 and 16 are stored in the disconnect registers 122, and any subsequent posted write cycles are recorded in the posted wnte buffer 118 In the case of direct virtual memory access (DVMA) reads, the data control and panty 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 eπor signals are checked for coπectness. Other signals in addition to those specifically mentioned above can be compared to give an indication of divergence of the processmg sets Examples of these are bus grants and various specific signals during processmg set transfers and during DMA transfers Eπors fall roughly mto 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 mterrupt from the bridge 12. Accordmgly, the bridge is operable to capture eπors reported m connection with processing set read and wπte cycles, and DMA reads and wntes. Clock control for the bndge is performed by the bridge controller 132 in response to the clock signals from the clock lme 21. Individual control lmes from the controller 132 to the vaπous elements of the bridge are not shown in Figures 6 to 10. Figure 12 is a flow diagram illustrating a possible sequence of operatmg stages where lockstep eπors are detected during a combmed mode of operation. Stage SI represents the combined mode of operation where lockstep eπor checking is performed by the comparator 130 shown in Figure 8. In Stage S2, a lockstep eπor is assumed to have been detected by the comparator 130. In Stage S3, the cuπent state is saved in the CSR registers 114 and posted wπtes are saved in the posted wπte 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 eπor notified by the comparator 130 has occuπed durmg a data phase m which it is possible to pass data to the device bus 22. In this case, in Stage S32, the bus cycle is terminated. Then, m Stage S33 the data phases are stored m 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 eπor did not occur during a data phase, the address and data phases for any posted write I/O cycles are stored m the posted write buffer 122 At Stage S34, if there are any further posted wπte I/O operations pendmg, these are also stored in the posted write buffer 122. Stage S3 is performed at the initiation of the initial eπor state 152 shown m Figure 1 1. In this state, the first and second processing sets arbitrate for access to the bridge. Accordmgly, m Stage S31-S35, the posted write address and data phases for each of the processing sets 14 and 16 are stored m separate portions of the posted write buffer 122, and/or m the single set of disconnect registers as described above. Figure 14 illustrates the source of the posted wnte I/O cycles which need to be stored m the posted wnte buffer 122. During normal operation of the processing sets 14 and 16, output buffers 162 in the individual processors contam I/O cycles which have been posted for transfer via the processing set bus controllers 50 to the bπdge 12 and eventually to the device bus 22. Similarly, buffers 160 in the processmg set controllers 50 also contain posted I/O cycles for transfer over the buses 24 and 26 to the bridge 12 and eventually to the device bus 22. Accordmgly, it can be seen that when an eπor state occurs, I/O write cycles may already have been posted by the processors 52, either in their own buffers 162, or already transfeπed to the buffers 160 of the processmg set bus controllers 50 It is the I/O write cycles in the buffers 162 and 160 which gradually propagate through and need to be stored in the posted wπte buffer 122 As shown m Figure 15, a wnte cycle 164 posted to the posted wnte buffer 122 can comprise an address field 165 mcludmg an address and an address type, and between one and 16 data fields 166 including a byte enable field and the data itself The data is wπtten into the posted write buffer 122 m the EState unless the initiating processing set has been designated as a pnmary CPU set At that tune, non-primary writes m an EState still go to the posted wπte buffer even after one of the CPU sets has become a prunary processmg set An address pomter m the CSR registers 114 pomts to the next available posted write buffer address, and also provides an overflow bit which is set when the bπdge attempts to write 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 in each buffer Attempts to write beyond the top of the posted wπte buffer are ignored The value of the posted wπte buffer pomter can be cleared at reset, or by software usmg a write under the control of a pnmary processmg set Returning to Figure 12, after saving the status and posted wntes, at Stage S4 the mdividual processmg sets independently seek to evaluate the eπor state and to determine whether one of the processmg sets is faulty This determination is made by the mdividual processors m an eπor state in which they individually read status from the control state and EState registers 114 Durmg this eπor mode, the arbiter 134 arbitrates for access to the bπdge 12 In Stage S5, one of the processmg sets 14 and 16 establishes itself as the pnmary processing set This is determined by each of the processing sets identifying a time factor based on the estimated degree of responsibility for the eπor, whereby the first processing set to time out becomes the primary processing set In Stage S5, the status is recovered for that processing set and is copied to the other processing set The primary processing is able to access the posted wπte buffer 122 and the disconnect registers 120 In Stage S6, the bridge 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 processing sets m the combmed mode at Stage SI However, it may not be possible to re-establish an equivalent state until a faulty processing set is replaced Accordingly the system will stay in the Split mode of Stage S6 in order to continued operation based on a single processing set After replacing the faulty processing 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 combmed mode to compare the I/O operations output by the first and second processing 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 Any deviation from this will be mterpreted by the comparator 130 as a loss of lockstep This is in pnnciple coπect as even a mmor deviation from identical outputs, if not trapped by the comparator 130, could lead to the processing sets diverging further from each other as the individual processing sets act on the deviating outputs However, a stnct application of this puts significant constraints on the design of the individual processing sets An example of this is that it would not be possible to have independent time of day clocks in the individual processing sets operatmg under then own clocks This is because it is impossible to obtam two crystals which are 100% identical in operation Even small differences in 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 processing sets Accordmgly, a solution to this problem employs the dissimilar data registers (DDR) 116 mentioned earlier The solution is to wπte data from the processing sets mto respective DDRs m the bndge while disabling the comparison 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 processing sets is able to act on the same data Figure 17 is a schematic representation of details of the bndge of Figures 6 to 10 It will be noted that details of the bridge not shown in Figure 6 to 8 are shown in Figure 17, whereas other details of the bridge shown in Figures 6 to 8 are not shown in Figure 17, for reasons of claπty The DDRs 116 are provided in the bπdge registers 110 of Figure 7, but could be provided elsewhere in the bπdge 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 processing sets 14 and 16, respectively Figure 17 represents a dissimilar data write stage The addressmg 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 Durmg an address phase of a dissimilar data I/O write operation each of the processing sets 14 and 16 outputs the same predetermined address DDR-W which is separately interpreted by the respective first and second decoding sections 136A and 136B as addressmg 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 mterpreted by the comparator 130 as a lockstep eπor The decodmg section 136A, or the decoding section 136B, or both are aπanged to further output a disable signal 137 in response to the predetermined write address supplied by the first and second processing sets 14 and 16 This disable signal is supplied to the comparator 130 and is operative dunng the data phase of the write operation to disable the comparator As a result, the data output by the first processing set can be stored m the first DDR 116A and the data output by the second processing set can be stored in the second DDR 116B without the comparator being operative to detect a difference, even if the data from the first and second processing sets is different The first decoding section is operable to cause the routing matnx to store the data from the first processmg set 14 in the first DDR 116A and the second decodmg section is operable to cause the routmg matnx to store the data from the second processing set 16 m the second DDR 116B At the end of the data phase the comparator 130 is once again enabled to detect any differences between I/O address and/or data phases as indicative of a lockstep eπor 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 aπangement where the disable signal 137 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 reading of the first DDR 116A in a subsequent dissimilar data read stage. As illustrated m Figure 19, each of the processing sets 14 and 16 outputs the same predetermmed address DDR-RA which is separately interpreted by the respective first and second decoding 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 enabling those processmg sets to receive the same data. This enables the two processmg sets 14 and 16 to achieve determmistic behavior, even if the source of the data wntten mto the DDRs 116 by the processmg sets 14 and 16 was not deterministic. As an alternative, the processmg sets could each read the data from the second DDR 116B Figure 20 illustrates the reading of the second DDR 116B in a dissimilar data read stage following the dissimilar data wπte stage of Figure 15. As illustrated in Figure 20, each of the processing sets 14 and 16 outputs the same predetermined address DDR-RB which is separately interpreted by the respective first and second decodmg sections 136A and 136B as addressmg 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 processing sets to receive the same data. As with the dissimilar data read stage of Figure 16, this enables the two processing sets 14 and 16 to achieve determmistic behavior, even if the source of the data written mto the DDRs 116 by the processing 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 determined in any appropπate manner by the software operating 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 randomly or m any other manner as long as the same choice of DDR is made by both or all of the processmg sets. Figure 21 is a flow diagram summarizing the various stages of operation of the DDR mechanism described above In stage S10, a DDR write address DDR-W is received and decoded by the address decoders sections 136A and 136B durmg the address phase of the DDR write operation In stage SI 1, the comparator 130 is disabled In stage S12, the data received from the processing sets 14 and 16 dunng 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 m stage SI 5 the content of that DDR 116B is read by both of the processing 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 processing set bus controllers 50 m the respective processmg sets 14 and 16 includes a conventional PCI master bus arbiter 180 for providing 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 bndge 12 on respective request (REQ) lines 181 and 182 The master arbiters 180 allocate access to the bus on a first-come-first-served basis, issuing a grant (GNT) signal to the winning party on an appropπate grants line 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 contendmg devices, mcludmg the bridge and the devices 30, 31, etc. connected to the device bus 22. Respective request lines 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 lines 189, 190, 192, etc. Figure 23 is a state diagram summansmg 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 bπdge itself On a transition mto the GRANT state, these are sorted by a priority 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 priorities m a priority 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 bridge 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 withm 16 cycles it may lose GNT# via the BACKOFF state. BACKOFF is required as, under PCI rules, 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 in the EState. A new GNT# is produced at the times 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 against a DMA address provided by the device. Turning now to the bridge arbiter 134, this allows access to the bridge for the first device which asserts the PCI FRAME# signal mdicatmg an address phase. Figure 24 is a state diagram summansmg 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 retried which forces them to give up the bus Under PCI rules 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 hoggmg the bridge, retried interfaces are remembered and assigned a higher priority. These remembered retries are pnoπtised m the same way as address phases. However as a precaution this mechanism is timed out so as not to get stuck waiting for a faulty or dead device. The algorithm employed prevents a device which hasn't yet been retried, but which would be a higher pnonty retry than a device cuπently waiting for, from being retπed at the first attempt. In combined operations a PA or PB bus input selects which P bus interface will win a bndge access. Both are informed they won. Allowed selection enables latent fault checkmg during normal operation EState prevents the D bus from winning. The bπdge arbiter 134 is responsive to standard PCI signals provided on standard PCI control lmes 22, 24 and 25 to control access to the bridge 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 appropriate command (write/read) signals (C) will be available on the command bus (CMD BUS) Shortly after the frame signal bemg asserted low, the initiator 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 bπdge resources and thereby to negotiate allocation of a target bus m response to the FRAME# bemg asserted low for the initiator bus concerned Accordmgly, the bridge arbiter 134 is operable to allocate access to the bndge resources and/or to a target bus on a first-come- first-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 tune Alternatively, a simple priority can be allocated to the vanous requesters, whereby, m 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 comprises a four bit register However, it will be appreciated that a larger register will be required to determme ownership between more than two processing sets For example, if three processmg sets are provided, then a five bit register will be required for each slot Figure 16 illustrates schematically one such four bit register 600 As shown in Figure 16, a first bit 602 is identified as SRR[0], a second bit 604 is identified as SRR[1], a thud 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 writes 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 processing set 14 and the device slot When this bit is set, the first processing 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 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 processing 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 processing 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 return indeterminate data without causing a transaction on the device bus 22. In the event of an I/O eπor the fake bit SRR[0] of the SRR 188 coπesponding to the device which caused the eπor is set by the hardware configuration of the bridge to disable further access to the device slot concerned. An interrupt may also be generated by the bridge to inform the software which originated the access leading to the I/O eπor that the eπor has occuπed. The fake bit has an effect whether the system is in the split or the combmed mode of operation. The ownership bits only have effect, however, in the split system mode of operation. In this mode, each slot can be in three states: Not-owned; Owned by processing set 14; and Owned by processing set 16 This is determined by the two SRR bits SRR[1] and SRR[2], with SRR[1] being set when the slot is owned by processing set 14 and SRR[2] being set when the slot is owned by processing 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 processing set making the access (this includes un-owned slots) cannot be accessed and results in an abort. A processing set can only claim an un-owned slot; it cannot wrest ownership away from another processing set. This can only be done by powering-off the other processing set. When a processing set is powered off, all slots owned by it move to the un-owned state. Whilst it is not possible for a processing set to wrest ownership from another processing set, it is possible for a processing set to give ownership to another processing set. The owned bits can be altered when in the combined mode of operation state but they have no effect until the split mode is entered. Table 2 below summarizes the access rights as determined by an SRR 118. From Table 2, it can be seen that when the 4-bit SRR for a given device is set to 1100, for example, then the slot is owned by processing set B (i.e. SRR[2] is logic high) and processing set A may not read from or write to the device (i.e. SRR[1] is logic low), although it may read from or write to the bridge. "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 operating coπectly. TABLE 2 SRR PA BUS PB BUS Device Interface [3[2][1][0] 0000 xOOx Read/Write bridge SRR Read/Write bπdge SRR Access denied 0010 Read/Write bπdge Read/Write bndge Access Denied because Owned D Slot No access to D Slot arbitration bit is off 0100 Read Wnte bπdge Read/write bridge Access Denied because No access to D Slot Access to D Slot arbitration bit is off 1010 Read/Wπte bπdge, Read/Wnte Bridge Access to CPU B Denied Owned D Slot No access to D Slot Access to CPU A OK 1100 Read/Wnte bndge, Read/Wnte bπdge Access to CPU A Denied No access to D Slot Access to D Slot Access to CPU B OK 0011 Read/Write bridge, Read/Write bndge Access Denied because Bπdge discard writes No access to D Slot Arbitration bit is off 0101 Read/Wnte bndge, Read/Wnte bndge Access Denied because No access to D slot Bndge discards wntes Arbitration bit is off 1011 Read/Write bndge, Read/Write bridge Access to CPU B Denied Bridge discard wntes No access to D Slot Access to CPU A OK 1101 Read/Write bridge, Read/Wnte bπdge Access to CPU B Denied No access to D slot Bridge 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 processing set B However, as the device is malfunctioning, SRR[3] is set logic low and the device is not allowed access to the processing set SRR[0] is set high so that any writes to the device are ignored and reads therefrom return indeterminate data In this way, the malfunctioning device is effectively isolated from the processing set, and provides indeterminate 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 bridge 12 for direct memory access by a device such as one of the devices 28, 29, 30, 31 and 32 to the memory 56 of the processing 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 in 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 in the bridge, 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 in the bπdge memory 126, along with the posted write buffer 122 and the dirty RAM 124. Alternatively, it could be held as a table in 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 verify the coπectness of the DMA addresses supplied by the device 30. In one embodiment of the invention, this is achieved by comparing four significant address bits of the address supplied by the device 30 with the coπesponding four address bits of the address held in 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 determine whether the address supplied is within the coπect address range. In this specific example, 32 bit PCI bus addresses are used, with bits 31 and 30 always being set to 1, bit 29 being allocated to identify which of two bridges on a motherboard is being 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 comparing bits 29-26, it is possible to identify whether the address(es) supplied fall(s) within the appropriate 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 in that entry with the address(es) received with the DMA request as described above. Alternatively, the address decode logic 142 could be aπanged to use the address(es) received with the DMA address to address a relational geographic address map and to determine a slot number therefrom, which could be compared to the slot for which the bus grant signal 194 is intended and thereby to determine whether the addresses fall within the address range appropriate 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 aπanged 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 processing set(s) 14/16. If the bridge is in the combined mode, the DMA access will automatically be allocated to all of the in-sync processing sets 14/16. The address decode logic 142 will be aware that the bridge is in the combined mode as it is under the control of the bridge controller 132 (see Figure 8). However, where the bridge is in the split mode, a decision will need to be made as to which, if any, of the processing sets the DMA request is to be sent. When the system is in split mode, the access will be directed to a processing 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 originating the DMA request by accessing the SRR 118 for the slot concerned. The appropriate 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 processing set 14 is the owner and the DMA request is passed to the first processing set. If bit SRR[2] is set, the second processing set 16 is the owner and the DMA request is passed to the second processing 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 processing sets 14 and 16. Figure 27 is a flow diagram summarizing the DMA verification 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 verifies the DMA addresses supplied with the DMA request by accessing 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 bridge is in the combined or the split mode. If the bridge is in the combined mode, then in stage S24 the address decoder controls the target controller 140 (see Figure 8) to cause the routing matrix 80 (see Figure 6) to pass the DMA request to both processing sets 14 and 16. If the bridge is in 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 in stage S25. If the slot is allocated to the first processing set 14 (i.e. the SRR[1] bit is set), then in stage S26 the address decoder 142 controls the target controller 140 (see Figure 8) to cause the routing matrix 80 (see Figure 6) to pass the DMA request to first processing set 14. If the slot is allocated to the second processing set 16 (i.e. the SRR[2] bit is set), then in stage S27 the address decoder 142 controls the target controller 140 (see Figure 8) to cause the routing matrix 80 (see Figure 6) to pass the DMA request to the second processing set 16. If the slot is unallocated (i.e. neither the SRR[1] bit nor the SRR[2] bit is set), then in step SI 8 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 appropriate) to be effected on the processing set memory. There now follows a description of an example of a mechanism for enabling automatic recovery from an EState (see Figure 11). The automatic recovery process includes reintegration of the state of the processing sets to a common status in order to attempt a restart in lockstep. To achieve this, the processing set which asserts itself as the primary processing set as described above copies its complete state to the other processing set. This involves ensuring that the content of the memory of both processors is the same before trying a restart in lockstep mode. However, a problem with the copying of the content of the memory from one processing set to the other is that during 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 processing set. If DMA is enabled, then a write 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 being the same. In principle, it would be possible to inhibit DMA for the whole of the copy process. However, this would be undesirable, bearing m mind that it is desirable to minimise 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 occuπed dunng the penod of the copy. However, it is likely that further DMA operations would be performed dunng the copy retry, and accordmgly this is not a good option either. Accordmgly, m the present system, a duty RAM 124 is provided m the bndge. As descnbed earlier the dirty RAM 124 is configured as part of the bπdge SRAM memory 126 The duty RAM 124 compnses a bit map havmg a duly mdicator, 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 mvention 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 mstructions at the beginning of a copy pass, so that it can start to record pages which are dirtied since a given tune. 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 optimise the readmg and resetting of the duty RAM 124. Accordmgly, at the end of the copy pass the bits m the duty RAM 124 will mdicate those pages of processmg set memory which have been changed (or dirtied) by DMA wπtes dunng 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 tune 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 m 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. Eπoneous accesses which present illegal combmations of the address bits 29 to 31 are mapped into the duty RAM 124 and a bit is dirtied on a wπte, even though the bridge will not pass these transactions to the processing sets. When readmg the duty RAM 124, the bndge defines the whole area from 0x00008000 to OxOOOOffff as duty RAM and will clear the contents of any location m this range on a read. As an alternative to providing a single duty 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 being wntten to while another is read. Figure 28 is a flow diagram summansmg the operation of the duty RAM 124. In stage S41, the prunary processmg set reads the duty RAM 124 which has the effect of resetting the duty RAM 124. In stage S42, the pnmary 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 determmes whether less than a predetermmed number of bits have been wntten m the duty RAM 124 If more than the predetermmed number of bits have been set, then the processor m stage S45 copies those pages of its memory 56 which have been dirtied, as indicated by the duty bits read from the duty RAM 124 in stage S43, to the memory 56 of the other processmg set Control then passes back to stage S43 If, m stage S44, it is determined less than the predetermmed number of bits have been wπtten m the duty RAM 124, then m 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 clearmg the arbitration enable bit for each of the device slots, thereby denying access of the DMA devices to the D bus 22 Alternatively, the address decoder 142 could be configured to ignore DMA requests under mstructions from the pnmary processor Dunng the period m which DMA accesses are prevented, the pnmary processor then makes a final copy pass from its memory to the memory 56 of the other processor for those memory pages coπespondmg to the bits set in the dirty RAM 124 In stage S47 the pnmary 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 withm the spurt and scope of the present mvention as defined m 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 bπdge for a multi-processor system, the bridge comprising a first processor bus mterface for connection to an I/O bus of a first processing set, a second processor bus interface for connection to an I/O bus of a second processmg set, a device bus interface for connection to a device bus and a bπdge control mechanism configured to be operable to monitor operation of the first and second processing sets m a combined, lockstep, operating mode and to respond to detection of a lockstep eπor to cause the bπdge to be operable m an eπor mode in which wnte accesses initiated by the processmg sets are stored in a bridge buffer pendmg resolution of the eπor mode. 2. The bridge of claim 1, wherein a respective buffer region is provided for each processmg set. 3 The bridge of claim 1, wherein the bridge control mechanism is configured to be operable in an initial eπor mode: to store m the posted wπte buffer any internal bridge write accesses issued by the processing sets and to allow and to arbitrate any internal bridge read accesses initiated by the processing sets; and to store m a posted write buffer any complete device write accesses mitiated by the processmg sets and to abort any device bus read accesses initiated by the processing sets 4 The bπdge of claim 3, wherein the bridge control mechanism is operable in the initial eπor mode, where data is in transit through the bπdge to divert the data to one or more disconnect registers. 5. The bridge of claim 1 , wherein the bndge control mechanism is configured to be operable, m a primary eπor mode in which a processing set asserts itself as a primary processing set: to allow and to arbitrate any internal bridge wπte accesses initiated by the pnmary processing set, to discard any internal bndge write accesses mitiated by any other processmg set, and to allow and to arbitrate any internal bπdge read accesses initiated by the processing sets; and to discard any device bus write accesses initiated by the processing sets and to abort any device bus read accesses initiated by the processmg sets 6. The bπdge of claim 1, comprising a controllable routing matnx coupled between the first processor bus mterface, the second processor bus interface, the device bus interface and the memory sub-system, the bridge control mechanism bemg configured to be operable to control the routing matrix selectively to interconnect the first processor bus interface, the second processor bus mterface, the device bus interface and the memory sub- system accordmg to a cuπent mode of operation. 7 The bridge of claim 6, wherein the bridge control mechanism comprises: an address decode mechanism configured to be operable to determine destination addresses for write and read accesses; and initiator and target controllers for determining a path from an mput to an output of the routmg matrix, the initiator controller bemg responsive to the determination of a master for a transfer m progress and the target controller being responsive to decoded destmation addresses from the address decode mechanism 8 The bridge of claim 7, wherem the bridge control mechanism compnses a comparator configured to be operable in the combmed mode to detect differences between signals on the I/O buses of the first and second processing sets as indicative of a lockstep eπor 9 The bridge of claim 8, wherein the bridge control mechanism further comprises a bridge controller connected to an output of the comparator, the bridge controller being configured to be operable, in response to a signal indicative of a lockstep eπor output from the comparator, to cause the bridge to cease operation in the combined mode and instead to operate in the eπor mode 10 The bridge of claim 9, wherem the master and target controllers are further responsive to an output of the bridge controller to modify the path from the input to the output of the routing matπx 11 The bridge of claim 3, wherem the posted write buffer is configured m random access memory 12 The bπdge of claim 4, wherein the at least one disconnect register is a hardware register 13 The bridge of claim 1, comprising at least one further processor bus mterface for connection to an I/O bus of a further processing set 14 A bπdge for a multi-processor system, the bridge comprising means for interfacing with an I/O bus for each of first and second processing sets and with a device bus, a posted write buffer, means for monitoring operation of the first and second processmg sets in a lockstep mode and means responsive to detection of a lockstep eπor to store write operations posted by the processmg sets m the posted wnte buffer 15 A computer system comprising a first processing set having an I/O bus, a second processing set having an I/O bus, a device bus and a bridge, the bridge being connected to the I/O bus of the first processing set, the I/O bus of the second processing set and the device bus and compπsmg a bridge control mechanism configured to be operable to monitor operation of the first and second processmg sets m a combined, lockstep, operating mode and to respond to detection of a lockstep eπor to cause the bπdge to be operable in an eπor mode in which write accesses initiated by the processing sets are stored in a bridge buffer pending resolution of the eπor mode 16 A computer system according to claim 15, wherein each processing set compπses at least one processor, memory and a processing set I/O bus controller 17 The computer system of claim 15, further comprising at least one further processing set. 18 A method of operating a multi-processor system compπsmg a first processmg set havmg an I/O bus, a second processmg set havmg an I/O bus, a device bus and a bndge, the bridge being connected to the I/O bus of the first processmg set, the I/O bus of the second processing set and the device bus and comprising a bridge buffer, the method compπsmg: momtonng operation of the first and second processing sets m a combmed, lockstep, operatmg mode; and on detection of a lockstep eπor, storing write accesses initiated by the processing sets in the bπdge pending resolution of the eπor. 19 The method of claim 18, wherein write accesses for each processmg set are buffered in respective buffer regions for each processing set 20 The method of claim 18, wherein, in an initial eπor mode. any internal bridge write accesses issued by the processing sets are buffered in the posted write buffer and any internal bndge read accesses mitiated by the processmg sets are allowed and arbitrated; and any complete device write accesses initiated by the processing sets are stored m a posted write buffer and any device bus read accesses mitiated by the processmg sets are aborted. 21 The method of claim 20, wherein: in the initial eπor mode, where data is in transit through the bπdge, the data is diverted to one or more disconnect registers. 22 The method of claim 18, wherein, in a primary eπor mode in which a processmg set asserts itself as a primary processmg set: any internal bridge write accesses initiated by the primary processmg set are allowed and arbitrated, any internal bridge wπte accesses initiated by any other processing set are discarded, and any internal bridge read accesses mitiated by the processing sets are allowed and arbitrated; and any device bus write accesses initiated by the processing sets are discarded and any device bus read accesses initiated by the processing sets are discarded. 23 The method of claim 17, wherem a controllable routmg matrix is connected between the I/O buses, the data buses and bridge memory, the routing matrix being controlled selectively to interconnect the first processor bus mterface, the second processor bus interface, the device bus mterface and the bridge memory according to a cuπent mode of operation. 24. The method of claim 22, wherein: an address decode mechanism connected to the first processor bus interface, the second processor bus interface and the device bus interface, and is operable to determine source and destination addresses for write and read accesses; and initiator and target controllers for determining a path from an input to an output of the routing matrix are responsive to identification of a master and to decoded destination addresses, respectively. 25. The method of claim 23, wherein: a comparator connected to the first and second processor bus interfaces is operable in 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 eπor. 26. The method of claim 24, wherein: a bridge controller connected to an output of the comparator is operable, in response to a signal indicative of a lockstep eπor output from the comparator, to cause the bridge to cease operation in the combined mode and instead to operate in the eπor mode. 27. The method of claim 25, wherein the master and target controllers are further responsive to an output of the bridge controller to modify the path from the input to the output of the routing matrix.

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