DIRECT MEMORY ACCESS IN A BRIDGE FOR A MULTI-PROCESSOR SYSTEM

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 provide geographic addressing for devices on the device bus and to be responsive to a request from a device on the device bus for direct access to a resource of a processing set to verify that an address supplied by the device falls within a correct geographic range. A different geographic address range can be allocated to each of a plurality of device slots on the device bus. A different geographic address range can also be allocated to the processor set resources (e.g., processor set memory). An address decoding mechanism maintains geographic address mappings, and verifies geographic addresses for direct memory access. The geographic address mappings can be configured in random access memory of the bridge. A slot response register is associated with each slot on the device bus. The slot response register records ownership of a device by the processing sets. The bridge control mechanism responds to a direct memory access request from a device on the device bus to access the slot response register for the slot for the requesting device for identifying the owning processor set, for enabling access to the memory of the owning processor set. The slot response registers can be configured in random access memory in the bridge.

Description: TITLE: DIRECT MEMORY ACCESS IN A BRIDGE FOR A MULTI-PROCESSOR SYSTEM BACKGROUND OF THE INVENTION This mvention relates to a multi-processoi computer system first and second processing sets (each of which 5 may compnse one or more processors) communicate with an I/O device bus In a system with several I/O cards which can carry out DMA transfers to mam memory, a mis-programmed or broken I/O card can corrupt the mam memory buffers belonging to another I/O card This is particularly undesirable m a fault-tolerant or high availability system Accordmgly, the mvention finds particular, but not exclusive application to fault tolerant computer systems 0 An aim of the present mvention is to provide avoid or mitigate the problems mennoned above which can result from a faulty or mis-programmed I/O device making faulty DMA requests SUMMARY OF THE INVENTION Particular and preferred aspects of the mvention are set out in the accompanying mdependent and dependent ^ claims Combinations of features trom the dependent claims may be combmed with features of the independent claims as appropπate and not merelv 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 comprises bus interfaces tor 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 provide geographic addressing for0 devices on the device bus and to be responsive to a request from a device on the device bus for direct access to a resource of a processing set to verify that an address supplied by the device falls within a correct geographic range In an embodiment of the invention, therefore, it is possible to check Direct Memory Access (DMA) and Direct Virtual Memory Access (DVMA) addresses to ensure correct operation of the device DMA operations It should be noted that the bus interfaces referenced above need not be separate components of the bridge,5 but mav be incorporated m other components of the bridge, and mav indeed be simply connections for the lines of the buses concerned A different geographic address range can be allocated to each of a plurality of device slots on the device bus A different geographic address range can also be allocated to the processor set resources (e g , processor set memory) 0 In an embodiment of the invention, therefore, the bndge control mechanism is responsive to the request for direct memory access from the device on the device bus to verify that an address supplied by a device falls within the correct geographic range for the slot in which the device is located The bridge control mechanism mcludes an address decoding mechanism configured to be operable to maintain geographic address mappings, and to venfy geographic addresses for direct memory access The geographic address mappings can be configured in random5 access memory or m a register in the bridge In the embodiment of the invention, a slot response register is associated with each slot on the device bus, wherein the slot response register records ownership of a device by the first processing set, the second processing set or neither processing set The bridge control mechanism can be configured to be responsive to a direct memory access request from a device on the device bus to access the slot response register for the slot of the requesting device for identifying the owning processor set and for enabling access to the memory of the owning processor set The slot response registers can be configured in random access memory or in a register in the bndge There can be more than two processor bus interfaces for connection to an I/O bus of a respective processing set In accordance with another aspect of the invention, there is provided a computer system compnsmg a first processing set having an I/O bus, a second processing set havmg an I/O bus, a device bus and a bndge, the bndge compnsmg a first processor bus interface for connection to the I/O bus of the first processing set, a second processor bus interface for connection to the I/O bus of the second processmg set, a device bus mterface for connection to the device bus, at least one device on the device bus, and a bndge as set out above Each processing set may comprise at least one processor, memory and a processing set I/O bus controller A plurality of switchable device slots may be provided on the device bus, each device slot having a respective geographic address range associated therewith 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 comprising maintaining respective geographic address ranges for devices on the dev ice bus, and responding to a request from a device on the device bus for direct memory access to a memory of a processing set to verify that an address supplied by a device falls within a conect geographic range Direct memory access can be permitted to proceed where the address falls within the correct geographic range, otherwise a direct memory access request is terminated BRIEF DESCRIPTION OF THE DRAWINGS Exemplary embodiments of the present mvention will be descnbed hereinafter, by way of example only, with reference to the accompanying drawmgs m which like reference signs relate to like elements and in which Figure 1 is a schemanc 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 bndge 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 routing matrix of the bndge 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 in 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 m the system of Figure 1 , Figure 15 illustrates the data stored in 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 summansing 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 operatton 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 operatton of the bndge of Figure 6 for direct memory access; Figure 27 is a flow diagram illustrating a direct memory access method m the bndge of Figure 6, and Figure 28 is a flow diagram of a re-integration process mcludmg the monitoring of a dirty RAM DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 is a schematic overview of a fault tolerant computing system 10 compnsmg a plurality of CPUsets (processing sets) 14 and 16 and a bridge 12 As shown in Figure 1 there are two processmg sets 14 and 16, although in other embodiments there may be three or more processing sets The bridge 12 forms an mterface between the processing 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 including memory, which output and receive common outputs and inputs It should be noted that the alternative term mentioned above, "CPUset", could be used instead, and that these terms could be used interchangeably throughout this document Also, it should be noted that the term "bridge" is used to denote any device, apparatus or arrangement suitable for mterconnectmg two or more buses of the same or different types The first processing set 14 is connected to the bndge 12 v ia a first processing set I/O bus (PA bus) 24, m the present instance a Peripheral 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 descnbed, 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 bndge 12 are operable m synchronism under the control of a common clock 20, which is connected thereto by clock signal lines 21 Some of the devices including an Ethernet (E-NET) interface 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 msertable 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 increase in 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 according to a particular implementation m accordance with specific design requirements Figure 2 is a schematic overview of a particular implementation of a fault tolerant computer employmg a bridge structure of the type illustrated in Figure 1 In Figure 2, the fault tolerant computer system mcludes 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 m Figure 2, two processmg sets 14 and 16 are each provided on a respective processing 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 processing set boards 44 and 46 via respective connections (not shown in Figure 2) First and second bridges 12 1 and 12 2 are mounted on the first I/O motherboard 40 The first bndge 12 1 is connected to the processing sets 14 and 16 by P buses 24 1 and 26 1, respectively Similarly, the second bndge 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 bridge 12 2 is connected to an I/O databus (D bus) 22 2 Third and fourth bridges 12 3 and 12 4 are mounted on the second I/O motherboard 42 The bndge 12 3 is connected to the processing sets 14 and 16 by P buses 24 3 and 26 3 respectively Similarly, the bridge 4 is connected to the processing sets 14 and 16 by P buses 24 4 and 26 4, respectively The bndge 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 m Figure 2 can enable a large number of I/O devices to be connected to the two processing sets 14 and 16 via the D buses 22 1, 22 2, 22 3 and 22 4 for either mcreasmg 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 m Figure 3, one or more processing set output buses 24 are connected to the processing 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 processing set I/O bus (P bus) 24 would be provided, whereas in the arrangement 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 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 processing 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 arrangement shown in Figure 4, individual processing 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 interfaces 65 enable a processor 62 of one processmg group to operate not only on the data in its local memory 66, but also in the memory of another processing group 61 withm the processing set 14 The processmg set bus controller 60 provides a common interface between the common internal bus 64 and the processmg set I/O bus(es) (P bus(es)) 24 connected to the bπdge(s) 12 It should be noted that although only two processing 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 processing 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 processing set I/O bus(es) (P bus(es)) 24 for connection to the bndge(s) 12. Accordingly, 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 redundancy required. In the following descnption, it is assumed that the processing sets 14 and 16 referred 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 in a number of operatmg modes These modes of operation will be descnbed in more detail later However, to assist in a general understanding of the structure of the bndge, the two operating modes will be briefly summarized here In a first, combined 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 combined mode, I/O cycles generated by the processmg sets 14 and 16 are compared to ensure that both processing sets are operatmg correctly Comparison failures force the bndge 12 mto an error limiting mode (EState) in which device I/O is prevented and diagnostic mformation 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 comparisons are made DMA operations are also permitted in both modes As mentioned above, the different modes of operation, including the combmed and split modes, will be described in more detail later However, there now follows a descnption 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 processmg set I/O bus mterfaces, 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 interface, D bus mterface 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 mcorporated 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 bndge to connect to the bus concerned, for example by means of physical or logical bndge connections for the lmes of the buses concerned Routmg (heremafter termed a routmg matnx) 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 interface 86 The routmg matnx 80 is further connected via a third internal path 92 to the D bus interface 82 The routing matrix 80 is thereby able to provide I/O bus transaction routing in both directions between the PA and PB bus interfaces 84 and 86 It is also able to provide routmg in both directions between one or both of the PA and PB bus interfaces and the D bus interface 82 The routmg matrix 80 is connected via a further internal path 100 to storage control logic 90 The storage control logic 90 controls access to bridge registers 1 10 and to a random access memory (SRAM) 126 The routmg matnx 80 is therefore also operable to provide routmg in both directions between the PA, PB and D bus mterfaces 84, 86 and 82 and the storage control logic 90 The routing matrix 80 is controlled by bridge 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 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 interfaces 84 and 86 each provide all the functionality required for a compatible mterface providing both master and slave operation for data transfened to and from the D bus 22 or internal memoπes and registers of the bridge m the storage subsystem 90 The bus interfaces 84 and 86 can provide diagnostic information to internal bridge status registers m the storage subsystem 90 on transition of the bndge to an error state (EState) or on detection of an I/O enor The device bus interface 82 performs all the functionality required for a PCI compliant master and slave interface for transfemng 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 bridge on transition to an EState or on detection of an I/O error Figure 7 illustrates in more detail the bndge registers 110 and the SRAM 124 The storage control logic 110 is connected via a path (e g a bus) 1 12 to a number of register components 1 14, 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 wnte 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 m Figure 7, these components may be configured m 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 in Figure 7, the posted write 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 vanous operatmg modes of the bridge, allow the capture of diagnostic information for an EState and for I O errors, 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 matnx 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 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 m-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 routing 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 bndge and the destination (or target) Where a wnter (for example a processor I/O wnte, or a DVMA (D bus to P bus access)) is writing data to a target, this data can be caught m 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 wnte or read cycle which was in progress when the EState was initiated Although shown separately, the DDRs 116, the SRRs 1 18 and the disconnect registers may form an integral part of the CSRs 114 EState and error 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 initiated to the P buses are logged in the posted write buffer 122 These may be other wntes that have been posted in the processmg set bus controllers 50, or which may be initiated by software before an EState interrupt causes the processors to stop carrying out writes to the P buses 24 and 26 A dirty RAM 124 is used to mdicate which pages of the mam memory 56 of the processing 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 m the dirty RAM 124 which is set when a DMA wnte occurs and can be cleared by a read and clear cycle initiated on the dirty RAM 124 by a processor 52 of a processmg set 14 and 16 The dirty RAM 124 and the posted write buffer 118 may both be mapped into the memory 124 in the bridge 12 This memory space can be accessed durmg normal read and wnte cycles for testmg purposes Figure 8 is a schematic functional overview of the bndge control logic 88 shown m Figure 6 All of the devices connected to the D bus 22 are addressed geographically Accordingly, the bndge carnes out decoding 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 processing set 14 of Figure 1) to the other processmg set (e g processing set 16 of Figure 1), in which case the access is routed from the PA bus mterface 84 to the PB bus mterface 86, - an access by one of the processmg sets 14 and 16 m the split mode, or both processing sets 14 and 16 m the combmed 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 in the combmed mode, or to the relevant processmg set 14 or 16 if out-of-sync, and if in a split mode to a processing set 14 or 16 which owns a slot in 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 referred to by processing set 14 or by processing set 16 Geographic addressing is used m combmation 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 correct card Separate FET switch lines are provided to each slot for separately switchmg the FETs for the slots The SRRs 118, which could be incorporated in the CSR registers 114, are associated with the address decode functions The SRRs 118 serve in a number of different roles which will be described in 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 meaningless data, once agam without causing 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 processing set A 14, - Owned by processmg set B 16 A slot that is not owned by a processmg 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 processing 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 processing set, or by getting the other processmg set to relinquish ownership The ownership bits are assessable and settable while in the combined mode, but e no effect until a split state is entered This allows the configuration of a split system to be determined while still in the combmed mode Each PCI device is allocated an area of the processmg set address map The top bits of the address are determmed by the PCI slot Where a device carnes out DMA, the bridge is able to check that the device is usmg the correct address because a D bus arbiter mforms the bndge which device is usmg the bus at a particular time 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 in the processmg 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 matnx 80 via the lmes 98 under control of a bndge controller 132 and an arbiter 134 An arbiter 134 is operable in various different modes to arbitrate for use of 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 m-sync processmg sets 14 and 16 and any initiators on the device bus 22 for use of the bridge 12 Possible scenarios are - processing set access to the device bus 22, - processing set access to internal registers in the bridge 12, - Device access to the processing set memory 56 In split mode, both processing sets 14 and 16 must arbitrate the use of the bndge and thus access to the device bus 22 and internal bridge registers (e g CSR registers 1 14) The bridge 12 must also contend with initiators on the device bus 22 for use of that device bus 22 Each slot on the ice 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 bndge. A PCI bus interface m 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 bndge 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 m Figure 8 is a comparator 130 and a bndge controller 132 The comparator 130 is operable to compare I/O cycles from the processing sets 14 and 16 to determine any out-of-sync events On determmmg an out-of-sync event the comparator 130 is operable to cause the bridge controller 132 to activate an EState for analysis of the out-of-svnc event and possible recovery therefrom Figure 9 is a schematic tunctional overview of the routmg matnx 80 The routmg matrix 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 cunent input to the routing 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 in 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 bridge has been given Figure 10 is a schematic representation of a physical configuration of the bndge in which the bndge control logic 88, the storage control logic 90 and the bridge registers 110 are implemented m a first field programmable gate anay (FPGA) 89, the routmg matrix 80 is implemented in further FPGAs 80 1 and 80 2 and the SRAM 126 is implemented as one or more separate SRAMs addressed bv a address control lines 127 The bus interfaces 82, 84 and 86 shown m Figure 6 are not separate elements, but are integrated 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 matnx 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 bridge registers could be mcorporated in the same FPGA as the routmg matnx 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 internal bus paths 85 and path control lmes 87 Figure 11 is a transition diagram illustrating in more detail the various operatmg modes of the bndge The bridge operation can be divided into three basic modes, namely an error state (EState) mode 150, a split state mode 156 and a combined state mode 158 The EState mode 150 can be further divided into 2 states After initial resetting on powering up the bridge, or following an out-of sync event, the bndge is in this initial EState 152 In this state, all wntes are stored m the posted wnte buffer 120 and reads from the internal bndge registers (e g., the CSR registers 1 16) are allowed, and all other reads are treated as errors (i.e. they are aborted) In this state, the mdividual processmg sets 14 and 16 perform evaluations for determmmg a restart time. 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 error 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. Accordingly, when this is determined, the bridge moves (153) to the primary EState 154 When either processing set 14/16 has become the primary processmg set, the bridge is then operating in the primary EState 154 This state allows the primary processing set to wnte to bndge registers (specifically the SRRs 118) Other writes are no longer stored in the posted write buffer, but are simply lost Device bus reads are still aborted in the primary EState 154 Once the EState condition is removed, the bridge 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 primary status of the processing 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 bridge is then operable m the combined 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 various 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 m the combined state 151 and, in response to a mismatched signal, to notify the bndge controller 132, whereby the bndge controller 132 causes the transition 152 to the enor 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 initiated by a device on the device bus Table 1 below summarizes the vanous access operations which are allowed in each of the operational states TABLE 1 D Bus - Read D Bus- Wnte E State Master Abort Stored m Post Wnte Buffer Pnmary 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 initial reset, the system is in the initial EState 152 In this state, neither processing 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 bndge registers 1 16 of the bridge are accessible, but are read only A system running in the combined mode 158 transitions to the EState 150 where there is a companson failure detected in this bridge, or alternatively a comparison failure is detected in another bridge 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 in 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 line 95 Following this, all I/O cycles generated on a P bus 24 or 26 result m reads bemg returned with an exception and writes being recorded in the posted write 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 companng address, data and selected control signals from the PA and PB bus interfaces 84 and 86 A failed companson of m-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 parity, byte enables and panty enor parameters are compared If the comparison fails durmg the address phase, the bridge asserts a retry to the processmg set bus controllers 50, which prevents data leaving the I/O bus controllers 50 \o activity occurs in this case on the device I/O bus 22 On the processor(s) retrying, no enor is returned If the comparison fails durmg a data phase (only control signals and byte enables are checked), the bndge signals a target-abort to the processing set bus controllers 50 An error is returned to the processors In the case of processing set O bus write 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 processmg set bus controllers 50, which results in the processmg set bus controllers 50 retrying the cycle agam The posted wnte buffer 122 is then active No activity occurs on the device I/O bus 22 If the comparison fails durmg 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 m 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 wntes, control and parity error signals are checked for conectness Other signals 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 durmg processmg set transfers and during DMA transfers Enors fall roughly into two types, those which are made visible to the software by the processmg 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 bndge 12 Accordmgly, the bridge is operable to capture enors reported m 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 bndge are not shown in Figures 6 to 10 Figure 12 is a flow diagram illustrating a possible sequence of operatmg stages where lockstep enors are detected during a combined mode of operation Stage S 1 represents the combined mode of operation where lockstep enor checkmg is performed by the comparator 130 shown m Figure 8 In Stage S2, a lockstep enor is assumed to have been detected by the comparator 130 In Stage S3, the cunent state is saved m the CSR registers 1 14 and posted writes are saved m 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 bndge 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, m Stage S32, the bus cycle is terminated Then, in 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 determmed 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 pendmg, these are also stored m 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 processmg sets arbitrate for access to the bndge Accordmgly, in Stage S31-S35, the posted wnte address and data phases for each of the processing sets 14 and 16 are stored in separate portions of the posted wnte buffer 122, and/or in the single set of disconnect registers as descnbed above Figure 14 illustrates the source of the posted write I'O cycles which need to be stored in the posted wnte buffer 122 During normal operation of the processmg sets 14 and 16, output buffers 162 in the mdividual processors contain I/O cycles which have been posted for transfer via the processing set bus controllers 50 to the bridge 12 and eventually to the device bus 22 Similarly, buffers 160 in the processing set controllers 50 also contain posted I/O cycles for transfer over the buses 24 and 26 to the bndge 12 and eventually to the device bus 22 Accordingly, it can be seen that when an enor state occurs, I/O write cycles 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 write cycles in the buffers 162 and 160 which gradually propagate through and need to be stored m the posted write buffer 122 As shown m Figure 15, a write cycle 164 posted to the posted wnte buffer 122 can comprise an address field 165 including 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 written into the posted wnte buffer 122 in the EState unless the initiating processmg set has been designated as a primary CPU set At that time, non-primary writes m an EState still go to the posted wnte buffer even after one of the CPU sets has become a pnmary processing set An address pointer in the CSR registers 114 points to the next available posted write buffer address, and also provides an overflow bit which is set when the bridge attempts to wnte past of the top of the posted write buffer for any one of the processing sets 14 and 16 Indeed, in the present implementation, only the first 16 K of data is recorded m each buffer. Attempts to wnte beyond the top of the posted write buffer are ignored. The value of the posted wnte buffer pointer can be cleared at reset, or by software usmg a write under the control of a primary processing set. Returning to Figure 12, after saving the status and posted writes, at Stage S4 the individual processmg sets independently seek to evaluate the enor state and to determine whether one of the processing sets is faulty. This determination is made by the individual processors m an enor state in which they individually read status from the control state and EState registers 1 14 During this enor mode, the arbiter 134 arbitrates for access to the bridge 12. In Stage S5, one of the processing sets 14 and 16 establishes itself as the primary processing set. This is determined by each of the processing sets identifying a time factor based on the estimated degree of responsibility for the enor, whereby the first processmg set to time out becomes the primary processmg set. In Stage S5, the status is recovered for that processing set and is copied to the other processing set. The pnmary processmg is able to access the posted write 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 processing sets, then a reset is issued at Stage S7 to put the processing sets in the combined mode at Stage S 1 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 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 in the combined 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 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 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 processmg sets An example of this is that it would not be possible to have independent time of day clocks in the individual processing sets operating under their own clocks. This is because it is impossible to obtain 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 processmg sets. Accordmgly, a solution to this problem employs the dissimilar data registers (DDR) 116 mentioned earlier. The solution is to write data from the processmg sets into respective DDRs in 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 processing 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 m Figure 17, whereas other details of the bndge shown in Figures 6 to 8 are not shown in Figure 17, for reasons of clanty. The DDRs 116 are provided in the bndge registers 110 of Figure 7, but could be provided elsewhere m the bndge in other embodiments One DDR 1 16 is provided for each processing 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 wnte stage The addressing logic 136 is shown schematically to comprise two decoder sections, one decoder section 136A for the first processing set and one decoder section 136B for the second processing set 16 During 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 addressing the respective first and second respective DDRs 116A and 1 16B 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 error The decoding section 136A, or the decoding section 136B, or both are ananged to further output a disable signal 137 in response to the predetermined write address supplied by the first and second processmg sets 14 and 16 This disable signal is supplied to the comparator 130 and is operative dunng the data phase of the wnte operation to disable the comparator As a result, the data output by the first processing set can be stored m the first DDR 1 16A and the data output by the second processmg set can be stored m 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 routing matrix to store the data from the first processing set 14 in the first DDR 116A and the second decoding section is operable to cause the routing matnx to store the data from the second processing set 16 in 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 enor Following the writing of the dissimilar data to the first and second DDRs 1 16A and 116B, the processmg 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 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 readmg of the first DDR 116A in a subsequent dissimilar data read stage As illustrated in Figure 19, each of the processing sets 14 and 16 outputs the same predetermined 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 processing sets to receive the same data This enables the two processmg sets 14 and 16 to achieve deterministic behavior, even if the source of the data written mto the DDRs 116 by the processmg sets 14 and 16 was not deterministic As an alternative, the processing 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 wnte stage of Figure 15 Λs 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 decoding 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 processing sets 14 and 16, thereby enabling those processing 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 deterministic behavior, even if the source of the data written into the DDRs 116 by the processmg sets 14 and 16 was not deterministic The selection of which of the first and second DDRs 1 16A and 1 16B to be read can be determmed in any appropriate manner by the software operatmg on the processing 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 in 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 descnbed above In stage S10, a DDR write address DDR-W is received and decoded by the address decoders sections 136A and 136B during the address phase of the DDR write operation In stage SI 1, the comparator 130 is disabled In stage SI 2, the data received from the processmg sets 14 and 16 during the data phase of the DDR wnte 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 1 16 A., then in stage S14 the content of that DDR 116A is read by both of the processing sets 14 and 16 Alternatively, 116A if the received address DDR-RB is for the second DDR 1 16B, then m stage S15 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 bridge itself Each of the processing set bus controllers 50 in the respective processing 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 processing set bus controller 50 and the bridge 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 appropriate 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 interface 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 bndge and the devices 30, 31, etc. connected to the device bus 22 Respective request lmes 186, 187, 188, etc. for each of the entities competmg 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, issuing a grant (GNT) signal to the winning entity via respective grant lines 189, 190, 192, etc Figure 23 is a state diagram summarising 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 bridge itself On a transition into the GRANT state, these are sorted by a priority encoder and a request signal (REQ#) with the highest priority is registered as the winner and gets a grant (GNT#) signal Each winner which is selected modifies the priorities in a priority encoder so that given the same REQ# signals on the next move to grant A different device has the highest priority, hence each device has a "fair" chance of accessing 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 bridge is not m the not m 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 accessing device is known and forwarded to the D bus address decode logic for checking agamst 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 indicating an address phase Figure 24 is a state diagram summarising the operation of the bndge arbiter 134 As with the D bus arbiter, a pπonty 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 rules retned 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 priority These remembered retries are prioritised in 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 pπority retry than a device cunently waiting for, from bemg retned at the first attempt In combined operations a PA or PB bus input selects which P bus interface will win a bridge access Both are informed they won Allowed selection enables latent fault checking 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 lines 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 being 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 bridge is operable to allocate access to the bndge resources and thereby to negotiate allocation of a target bus in response to the FRAME# being asserted low for the initiator bus concerned Accordmgly, the bndge arbiter 134 is operable to allocate access to the bridge resources and/or to a target bus on a first-come-first-served basis m response to the FRAME# being asserted low As well as the simple first-come-first-served basis, the arbiters may be additionally provided with a mechanism for logging 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 priority can be allocated to the various requesters whereby, in the case of identically timed requests, a particular requester always wins 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 1 18 contains 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 1 18 comprises a four bit register However, it will be appreciated that a larger register will be required to determine ownership between more than two processmg sets For example, if three processing 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 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 processing set 14 This defines the access route between the first processing 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] bits is set When the fake bit (SRRfO]) 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 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 bridge to disable further access to the device slot concerned An interrupt may also be generated by the bridge 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 in three states Not-owned, Owned by processing set 14, and Owned by processmg set 16 This is determined 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] being 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 makmg 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 poweπng-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 1 18. From Table 2, it can be seen that when the 4-bit SRR for a given device is set to 1 100, 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. SRRfO] 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 conectly. TABLE 2 SRR PA BUS PB BUS Device Interface [3[2][1][0] 0000 xOOx Read/Write bridge SRR Read/Write bridge SRR Access denied 0010 Read/Write bridge Read/Write bridge Access Denied because Owned D Slot No access to D Slot arbitration bit is off 0100 Read/Write bridge Read/write bridge Access Denied because No access to D Slot Access to D Slot arbitration bit is off 1010 Read/Write bridge, Read/Write Bridge Access to CPU B Denied Owned D Slot No access to D Slot Access to CPU A OK 1 100 Read/Write bridge, Read/Write bridge Access to CPU A Denied No access to D Slot Access to D Slot Access to CPU B OK 001 1 Read/Write bridge, Read/Write bridge Access Denied because Bridge discard writes No access to D Slot Arbitration bit is off 0101 Read/Write bridge, Read/Write bridge Access Denied because No access to D slot Bridge discards writes Arbitration bit is off 1011 Read/Write bridge, Read/Write bridge Access to CPU B Denied Bridge discard writes No access to D Slot Access to CPU A OK 1101 Read/Write bridge, Read/Write bridge Access to CPU B Denied No access to D slot Bridge discards writes 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 processmg set, and provides indeterminate data to satisfy any device dnvers, 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 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 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 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 in the bridge memory 126, along with the posted wnte 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 venfy the conecmess 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 conespondmg 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 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 bemg allocated to identify which of two bndges 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 Accordmgly, by comparing 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 ananged 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 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 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 appropnate for the slot concerned Either way, the address decode logic 142 is ananged to permit DMA to proceed if the DMA addresses fall withm 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 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 SRRfl] 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 SRRfl] 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 SRRf l] 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 SRRf2] 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 SRRfl] 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 processmg sets causes the necessary memory operations (read or wnte as appropπate) to be effected on the processmg set memory There now follows a descnption of an example of a mechanism for enabling automatic recovery from an EState (see Figure 11) The automatic recovery process includes remtegration of the state of the processmg sets to a common status m 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 copymg of the content of the memory from one processmg set to the other is that durmg this copymg 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 m 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 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 occuned durmg the penod of the copy However, it is likely that further DMA operations would be performed durmg the copy retry, and accordingly 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 du y RAM 124 is configured as part of the bndge SRAM memory 126 The dirty RAM 124 comprises a bit map havmg a dirty mdicator, for example a dirty 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 instructions at the beginning of a copy pass, so that it can start to record pages which are dirtied smce a given time The dirty RAM 124 can be read word by word If a large word size is chosen for readmg the dirty RAM 124, this will optimise the readmg and resetting of the dirty RAM 124 Accordmgly, at the end of the copy pass the bits in the dirty RAM 124 will mdicate 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 dutied 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 dirty RAM 124 is set and cleared in both the combmed and split modes This means that in split mode the dirty RAM 124 may be cleared by either processmg set The dirty 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 combinations of the address bits 29 to 31 are mapped into the dirty RAM 124 and a bit is dutied on a write even though the bndge will not pass these transactions to the processmg sets When reading 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 in this range on a read As an alternative to providing a single dirty RAM 124 which is cleared on being read, another alternative would be to provide two dirty RAMs which are used in a toggle mode, with one being written to while another is read Figure 28 is a flow diagram summansmg the operation of the dirty RAM 124 In stage S41, the primary processmg set reads the duty 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 dirty RAM 124 which has the effect of resetting the duty RAM 124 In stage S44, the primary processor determines whether less than a predetermined number of bits have been written m the dirty RAM 124 If more than the predetermined number of bits have been set, then the processor m stage S45 copies those pages of its memory 56 which have been dutied, as mdicated by the duty bits read from the duty 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 predetermined number of bits have been written 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 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 Durmg the penod m 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 conespondmg to the bits set m the dirty 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 withm the spmt and scope of the present mvention as defined m the appended claims For example, although m 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 comprising a first processor bus interface for connection to an I/O bus of a first processmg set, a second processor bus mterface for connection to an I/O bus of a second processmg set, a device bus interface for connection to a device bus and a bridge control mechanism configured to be operable to provide geographic addressing for devices on the device bus and to be responsive to a request from a device on the device bus for direct access to a resource of a processing set to venfy that an address supplied by the device falls withm a conect geographic range 2 The bridge of claim 1, wherein a different geographic address range is allocated to each of a plurality of device slots on the device bus 3 The bridge of claim 1 , wherein a different geographic address range is allocated to the processor set resources 4 The bridge of claim 2, wherem the bridge control mechanism is configured to be responsive to the request for direct memory access from the device on the device bus to verify that an address supplied by a device falls withm the conect geographic range for the slot m which the device is located 5 The bndge of claim 4, wherein the bndge control mechanism includes an address decodmg mechanism configured to be operable to maintain geographic address mappings and to v erify geographic addresses for direct memory access 6 The bridge of claim 5, wherein geographic address mappings are configured in random access memory in the bridge 7 The bndge of claim 5, wherein geographic address mappings are configured in a register in the bndge 8 The bridge of claim 2, compnsmg a slot response register associated with each slot on the device bus, wherem the slot response register records ownership of a device by the first processing set, the second processmg set or neither processmg set 9 The bridge of claim 8, wherein the bndge control mechanism is configured to be responsive to a direct memory access request from a device on the device bus to access the slot response register for the slot of the requestmg device for identifying the owning processor set and for enabling access to the memory of the owning processor set 10 The bridge of claim 9, wherein the slot response registers are configured in random access memory m the bridge 11 The bridge of claim 9 wherem the slot response registers are configured m random access memory m the bndge 12 The bndge of claim 1, comprising at least one further processor bus interface for connection to an I/O bus of a further processing set 13 A bndge for a multi-processor system, the bridge compnsmg means for interfacing with 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 providmg geographic addressmg for devices on the device bus and means responsive to a request from a device on the device bus for direct access to a resource of a processing set to venfy that an address supplied by the device falls withm a conect geographic range 14 A computer system compnsmg a first processmg set havmg a first I/O bus, a second processmg set havmg a second I/O bus, a device bus with at least one device on the device bus and a bndge, the bndge bemg connected to the first I/O bus, the second I/O bus and the device bus, compnsmg a bridge control mechanism configured to be operable to provide geographic addressmg for the devιce(s) on the device bus and to be responsive to a request from a device on the device bus for direct access to a resource of a processing set to verify that an address supplied by the device falls withm a conect geographic range 15 A computer system accordmg to claim 14, wherem each processmg set comprises at least one processor, memory and a processmg set I/O bus controller 16 The computer system of claim 15, wherem the resource is the processor set memory 17 The computer system of claim 14, further compnsmg at least one further processmg set 18 The computer system of claim 14, compnsmg a plurality of switchable device slots, each device slot havmg a respective geographic address range associated therewith 19 A method of operating a multi-processor system compnsmg a first processmg set having a first I/O bus, a second processmg set having a second I/O bus, a device bus with at least one device on the device bus and a bndge, the bridge bemg connected to the first I/O bus, the second I/O bus and device bus, the method compnsmg maintaining respective geographic address ranges for the devιce(s) on the device bus, and respondmg to a request from a device on the device bus for direct memory access to a memory of a processmg set to verify that an address supplied by a device falls withm a conect geographic range 20 The method of claim 19, comprising permitting direct memory access to proceed where the address falls withm the conect geographic range and discarding a direct memory access request otherwise

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