Interconnection system for multiprocessor structure

~ 3 2 ~ 3 1 Backqround of the Inv~ntion This invention relates generally to digital data processing systems, and, in particular, to a bus structure suitable for use with multiprocessor computer systems.

Multiprocessor computer systems prov;de multiple independent central processing units (CPUs) which can be coherently interconnected. Recent efforts in the multiprocessor field haYe concentrated on multiprocessor systems wherein each of a plurality of processors is equipped with a dedicated random access or cache memory unit.

These multiple processors typically communicate with one another via a common system bus structure ~iOe. shared bus systems), or by signalling with;n a shared memory address area ~i.e. shared address space systems).

In recent years, a wide range of structures and methods have been proposed or developed to interconnect the processors of a shared bus system multiprocessor~ .

One such shared bus muLtiprocessing computer syst~m is disclosed in United Kingdom Patent Applicati~n ~o. 2,1~8,205 ~publishsd February 4, 19~7).

Th~t sy~tem is understood to comprise a plurality of processors, each havin~ its own dedica~ed cache memory, and wherein the cache memories are connected to one another over a shared bus.

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~320~03 1 Convention~l shared bus systems, how~ver, lack adequate bandwidth to provide multiple processors with short effective access times during periods of high bus contention.

Although a number of caching schemes have been proposed and developed for the purpo~e of reducing bus contention, bus saturation still limits the speed and size of multiprocessor computers.

Additionally, the speed of a conventional bus structure i5 limited by the speed of light and by bus length.

In particular, as morQ processors are linked to a conventional bus, bus length incraases and thus the time required for signal transfer increases.

Another class of interconnection systems, known as crossbar networks, avoid some of the limitations of conventional bus syst*ms. In a crossbar network, however, the path taken by a given ~ignal cannot be uniquely specified.

Moreover, cost increases as the s~uare of the number of interconnected processors.

The!;e characteristics make crossbar networks unsuitab:Le or multiprocessor systems.

There accordingly e~ists a need for an interconne~tion system for multiprocessor computer systems which ~an accommodate the lar~e volume of interconnect access request~ generated by multiple processor~.

In particular~ there e~ists a need for an interconnection system in which transfer speed i~ independent o~ the number of interconnected processors.

, - . , _3_ 1 It is thus an object of the invention to provide an improved multiprocessor digital data processing system.

It is ano~her object of the invention to provide an interconnection system for ~ multiprocessor digital computer structure which can provide multiple processors with short effective access times under all operating conditions.

It is yet another object of the invention to provide an interconnection system having the above characteristics which is applicable to both shared memory an~ non-shared memory multiprocessors.

It is a further object of the invention to provide an interconnection system having high bandwidth and the capability of transferring signals at rates sufficient to 3110w multiprocessors to operate at full speed.

It is another object of. the invention to provide an interconnection system for a multiprocessor wherein bandwidth increases in : 25 proportion to the number of processors.

It is a further object of the invention to provide an interconnection system wherein transfer speed is independent of the number of interconnected processors, and is limited only by the switching speed o$ an individual interconnect~ . ~, -4_ ~ 32~3 1 Other general and specific objects of the invention will in part be obvious and will in part appear hereinaf~er.

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'' : , - ~ -5~ 3 1 Summary of the Invention The aforementioned objects are attained by the invention, which provides a digital data processing apparatus haYing a bus structure for transferring information-representative digital signals, the bus structure inclu~ing a shi~t register element having a set of digital storage and transf~r stages connected in series, for se~uentially storing and transf~rring said information-representative digital signals.

The invention also includes a plurality of processing cells, connected in a ring configuration to the bus structure, wherein at least one of the cells includes a central process;ng unit, an associated memory element for storing information-representative digital signals, coupled with the central processing unit for information transfer therebetween, and associated cell interconnect units, connected in circuit with the shift register element, and with an associated central processing unit, for transferring information-repxesentative si~nals onto the shift register element.

In accordanc~ with one aspect of the invention, the bus structure comprises unidirectional information-representative signal flow paths, and the cell interconnect units include elements ~or driving inormatio~-representative ~ignals alon~ the flow path define~ by the bus structure.

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-6- 132~3 1 In another aspect of the invention, each stage of the shift register element includes latch and register elements for storin~ a digital information-representative signal word of (n) digital bits, where ~n) is a posit;ve integer, ~nd the cell interconnect units include timing control elements respons~ve to applied digital clock cycle signals for sequentially driving information-representative digital words through successive stages of the shift register element at a rate controlled by the digital clock cycle rate.

In accordance with this aspect of the invention, the shift register element includes.

elements for storing in a giv~n sta~e of the shift register element a g;ven digital word for one applied digital clock cycle, and transferring a given digital word to a succeeding stage of the shift register element after an applied digital clock cycle.

Moreover, in this aspect of the invention, each cell interconnect unit has associated therewith a subset of ~s~ ~tages of the shift register strueture, where (s) is a positive integer, so that a given digital word is resident in a stage associated with each cell interconnect unit for ~s~ applied digital clock cycles.

In another àsp~ct of the invention, the 6hift regi~ter structure includes eleme~ts for se~uentially transferring digital signal pa~kets comprising ~w) correspond;ng digital words, where ~w~ i~ a positive integer, so that a digital word correspo~din~ to a given digital signal packet is resident in at least one stage a~sociated with a given cel~l interconnect unit or ~s) X (w) digital cl~ ycl~s.

-7- ~ 3 ~ 3 The invention f~rther provides data processing apparatus wherein the shift register structure includes elements for simultaneously transferring to successive shift register stages ~p) digital signal packets, where ~p) ;s a positive integer giYen by (p) = ~c) (s~w) where (c) is the nurnber of cell interconnect units, (s) is the number of shift register stages associated with each cell interconnect unit and (w~ is the number of digital words in each digital signal packet.

In accordance with the invention, as the number of cell interconnect units in a ring is increased, the flux of transfer operat;ons through the ring is constant, and the number of bus operations which can be executed during a completP bus cycle increases linearly.

Another aspect of the invention provide~ data processing apparatus whersiLn at least one of the processing cells described above~ includes elements for generating and transmitting to an associated cell interconnect unit a cell interconnect ~ontrol signal repreæentative of a re~ues~ to stor~ an information-r~presentative signal in a first associated stage of the shift register structure, In accordance with this aspect o~ the inYention, at least one of the processing cells i~cludes elements for ~enerating and transmitting to .

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'. . ,- ' ' , 32~0~3 an associated cell interconnect unit a cell interconnect control signal representative of a request for access to an information representative signal stored in a irst associated stage of the shift register structure.

The associated cell interconnect unit in ~urn includes elementsO responsive to the cell interconnect control signal, for extracting the information-representative signal stored in the first associated sta~e of the shift register structure and for transferring the extracted information-representative signal to the associated cell.

In a digital data processing apparatus of the type described above, the associated cell interconnect unit can also include an element, responsive to the cell interconnect control signal, for replicating the information-representative signal stored in the first associated stage of the shift register structure and for transferrinq the replicated information-representative signal to the cell.

In yet another aspect of the invention, at least one of the processing cells includes an element for ge`nerating and transmitting to an associated cell interconn~Gt unit, a cell interconnect control signal representative of a re~uest to transf~r, unchanged, an information representative signal stored in a first associated stage of the shift register structure to a second, succeeding associated s~age of the shift register structure.

~5 -9- ~32~3 1 In accordance with this aspect of the invention, the associated cell interconnect unit can ;nclude an element, responsive to the cell interconnect control signal, for ~nabling transfer to S the second, succeeding associated stage of the shift register structure the information-representative signal stored in the first associated stage of the shift register structure.

The invention further contemplates data processing apparatus of the type described above, wherein at least one of the processing cells include an element for generating and transmitting to an associated cell interconnect means a cell interconnect control signal representative of a request to identify a given digital word stored in an associatecl stage of the shi f t register structure as the first word of a data packet.

In accordance with this aspect of the invention, the associated cell interconnect unit can include means, responsive to the cell interconnect control s;gnal, for setting a portion of the given digital word to a selected value identifying the given digital word as the first word of the data packetO Yet ano her aspect of the invention provides a di~ital data processing apparatus having a bus structure and plurality of processing cells of the type described above, whaxein at least one o~ the cell~ includes an associated cell interconnect unit, connected in circuit with the shift register structure, and with an associated central processing unit, or transferrin9 information-representative 1320~3 signals onto the shift reyist~r structure, the cell interconnect unit comprising a sub~et of serially connec~ed ~tages of the shift register struc~ure.

Accordingly, in one of i~s emb~diments, the present invention relates to a digital data proce3sing apparatus comprising A. bus means for transferring packets of information-representative digital signals, said bus means including shift register means comprising a plurality of digital storage and transfer stages connected in series in a ring configuration for sequentially storing and transferring said information-representative digital signals, wherein each said stage within said shit register means includes means for storing an informationrepresentative signal of (M) bits, where (M) is greater than one, B. a plurality of processing cells, connected in a ring configuration through said bus means, each processing cell being in communication with an associated subset of (N) said stages, where ~N) is greater than one, at least one of said cells having associated memory means coupled thereto for storing a plurality of information-representatiYe digital signals, C. said at least one said processing cell further including cell interconnect means, conne~ted to said associated subset of stages and said associated memory means, for selectively transferring information-representative signals between said associated subset of stages and said associated memory means, D. said cell interconnect means including means or performing.at least one of modifying, e~tracting, replicating and transferring a packet of ~, -lOa ~32~0~3 digital information-representative signals, wherein at least a portion of said packet is stored within said associated subset of stages, based on an association, if any, between an informationrepresentative signal identified in that packet andone or more information-representative signals of said plurality of information-representative signals stored in said associated memory means, and E. said cell interconnect means including means responsive to applied digital clock cycle signals for simultaneously transferring at least a selected digital signal packet through successive sta~es of said associated subset of stages, at a rate responsive to said digital clock cycle rate, while performing said at least one modifying, e~tracting, replicating and transferring operation on that same digital signal packet.

In a further embodiment, the present invention relates to a digital data processing apparatus comprising A. bus means for trans~Eerring packets of information-representative digital signals, said bus means including shift register rneans comprising a plurality of digital storage and transfer stages connected in series in a ring configuration for se~uentially storing and transferring said in~ormation-representative digital sign~ls, wherein each said stage within said shift register means includes means for storing an informationrepresentative signal of ~M) bits, where (M) is greater than one, B. a plurality of processing cells, connected in a ring configuration through said bus means, each processing cell being in co~nunication with an associated subset o (N) said stages, where (N) is greater than one, at least one of said cells having associated memory means coupled thereto for storing information-representative digital signals, `B ~32~3 -lOh 1 C. said at least one said proces~sin~ cell further including cell interconnect means, connected to said associated subset of stages and said associated memory means, for selectively transferring inEormationrepresentative signals between said associated subset of stages and said associated memory means, and D. said cell interconnect means including means responsive to applied digital clock cycle signals for simultaneously transferring at least a selected digital signal packet thro~gh successive stages of said associated subset of stages, at a rate responsive to said digital clock cycle rate, while performing at subset o~ stages, at a rate responsive to said digital clock signals, while performing said at least one modifying, extracting, replicating and transferring operation on said digital signal packet.

In another aspect ~he invention resides in a method of operating a digital data processing apparatus comprising A. providing a plurality of processing cells, at least a selected one of which includes an associated memory element for storing a plurality of informationrepresentative digital signals, BD transferring packets of informationrepresentative digital signals between said processing cells on a bus comprising a plurality of digital storage and transfer stages connected in series in a ring configuration, wherein each said storage and transfer stage stores an information representative signal o~ (M~ bits, where (M) is greater than one, and wherein each processing cell is in communication with an associated subset oE tN) said stages, where (N) is greater than one, ~i`' -lOc- ~2~3 l C. selectively transferring information representative signals between said memory element and the subset of stages associated with said selected processing cell, D. responding to an association~ if any, between i) an information-representative signal identified in a selected packet of digital information-representative signals at least a portion of which packet is stored within the IO subset of sta~es associated with said selected processing cell, and ii) one or more information-representative signals stored in the memory element associated with that processing cell, to selectively perform at least one of a mod.i~ying/ extracting, replicating and transferring operation on that packet of digital information~representative signals, E. performing said at least one modifying, extracting, replicating and transferring operation on said selected packet of digital information-representative signals while simultaneously transerring, at a rate responsive to applied digital clock cycle signals, that packet through the successive stages associated with said selected processing cell.

The invention accordingly comprises an apparatus and method embodying eatures of constructionr combinations of elements and arrangements of parts as exemplified in the following detailed disclosure, and the l~ scope o the invention is indicated in the claims.

~"~ 32Q~Q3 1 ~rie Description of the Drawi~~ For a fuller understanding of the nature and objects of the invention, reference should be mad~ to the following detailed description and the accompanying drawings~ in which:

FIGURE 1 depicts the structure of a multiprocessor computer system constructed in accordance with the invention FIGURE 2 depicts detail of the structure of a processing cell illustrated in FIGURE 19 FIGURE 3 depicts a plurality of the processing cells of FIGURE 2 interconnected by a bus system constructed in accordance with the invention;

FIGURE 4 depicts detail of the structure of a cell interconnect of FIGURE 3;

FIGURE 5 depicts detail of the ~tructure of a cell interconnect unit in the ~ell interconnect of FIGUR~ 4;

, FIGURE 6 depîcts the shift registe~ stages associated with the cell interconnect~ of FIGURE 3;

FI~URE 7 depict~ clnck ~ignal distribution in the embodiment o~ FI~URE 3; and FIGVRE 8 depicts the contents of an exemplary data packet processed by the embodiment of FIGURE 3.

D ~L32~3 - 1 1 a 1 FIGURES 9A and 9B depict a cell interconnect schematic.

FIGURES 10A to 10G depict a cell interconnect unit directory schematic.

FIGURES 11A to 11C depict a cell interconnect unit cache group.

FIGURES 12A and 12B depict a cell interconnect unlt datapath schematic.

FIGURES 13A to 13D depict a cell interconnect datapath cell schematic.

FIGURES 14A and 14B depict datapath cache group low schematic.

FIGURES 15A and 15B depict a datapath cache group high schematic.

FIUGRES 16A to 16C depict a cell interconnect unit datapath extract FIFO schematic.

FIGURES 17A to 17E depict a datapath insert FIFO schematic.

FIGURES 18A to 18E depict ideal cell interconnect unit cloc k d iagrams.

-12- ~32~3 1 DescriptiQn of th~ Illustrated Emhodiment Str~cture FIGURE 1 depicts a multiprocessor computer utilizing a bus system constructe~ in accordance with the invention.

The multiprocessor system is hierarchically constructed from processors, cells and domains.

Each of the processing cells 0, 1, 2 and 3 contains a pr~cessor and cache memory, as discussed below in connection with Figure 2.

The cells 0-3 are interconnected by cell interconnects (CIs~ 10-13 and bus 8, thereby collectively forming Domain 0.

Domains, in turn, are interconnected by domain interconnects ~not shown), to form a complete system.

The structure of cell interconnects is described hereinafter in connection with FIGURES 4 and 5, and the structure and operation of the illustrated multiprocessor system is more fully discus~ed in Can~dian Patent Application No. 582,560, filed on even date herewith.

FIGURE 2 depicts the components of processing cell 0, including processor (PROC) 50, cache 40 and cell interconnect (CI) 10.

Data, parity, and control signals passed between processor 50, cache 40 a~d cell interconnect 10 are indicated in F~URE 2.

The datapath width associated with each respective signal is in~icated by numera~s in brackets~ For esample, cach~ data ~ignals ~cache_data t64]) passed between cell int rconnect 10 and cache 40 hav~ a 64 bit datapath width, as do processor ~ata signals (p~data~64J~ passed between cache 40 and processor 50.

~3 -13- ~3~ 1 As FIGURE 2 illustrates, cell interconnect 10 receives and transmits DOMAIN DATA ~ignals (dmn_data), DOMAIN PARITY signals dmn_parity), DOMAIN EMPTY signals (dmn_empty), DOMAIN HEADER signals (dmn_hdr), DOMAIN CELL ADDRESS signals (dmn_cell_addr), and DOM~IN CLOCK signals (dmn_clkSOj discussed in greater detail hereinafter.

In addition, cell interconnect 10 processes cache arbitration, routing, operation, and parity signals as indicated in FIGURE 2.

The structure of cell interconnect 10 is discussed in greater detail below ;n connection with FIGURE 4.

Moreover, urther understanding of the logic components and structure of cache 40 and processor 50 may be had by reference 1~ to Figures 9 to 18, and by reference to Canadian Patent Application No. 582,560~ Cell intercon~ect 10 provides interconnection of c~ll O into a multiple-c~ll domain lik~ tha~ depicted in FIGUR~ 3. .

FIGURE 3 illustrates the configuration of a ten cell domain, containing cells 0-9 interconnected in accordance with the inventi.on in a dual ring bus structure organize~ as ring A and ring B.

Utilizing plural rings is an important ieature of the invention, which enables the system to continue operating in the event of ~ingle point component failures, and increases the bandwidth o~ the interconnection system.

In a preferred practice of the invention, utilizing two rings, A and ~, rlng A is configured ~or transfers involving even page addresses in memor~, and ring B for odd page address~s in memory.

This interleaving mode is -14- ~32~Q~3 1 discussed in greater detail hereinafter.

Those skilled in the art will understand that the invention may be practiced in an embodiment having more than two rings.

Rings A and B are preferably 50 megahertz synchronous shift registers having plural data storage stages with a 1~8 bit datapath width, as indicated in FIGURE 3. Each of the cells 0-9 communicates with rings A and B through two associated Cell Interconnects ~CIs~.

As FIGURE 3 illustrates, cell interconnects 10-19 connect cells 0-9, respectively, to r;ng B, while cell interconnects 20-29 connect cells 0-9, respectively, to rinq A.

A preferred cell interconnect structure is illustrated in FIGURE 4. Two cell interconnect units (CIUs~ 72 and 73 and two 64K X 4 static RAMs (SRAMs) 70 and 71 are configured in pairs to form a single cell interconnect 20.

Similarly, cell interconnect units 62 ~nd 63, and SR~Ms 60 and 61 are utilized to form cell interconnect 10.

Each cell interconnect presents two 6~ bit data connections from its cell to a respective ring (dmn_data) and one S4 bit connection to its cell cache bus ~cache_data)~ The structure and operation of such ~ cell cache bus are described in Canadian Patent Application No. 582,560.

Through these connect;ons, the cell interconnect moves req~ests and responses between th~ cell and a respect;ve ring.

The ring connection~ of each cell interconnect collectively form ~n input port and an output port.

In operation, each cell interconnect ~B .

-15- ~320~3 1 moves the data on its input port through two stages (comprising ~our latches), modîfies the data as required by a given cell interconnect unit operation and presents the data on its output port.

Accordingly, when a number of cell interconnects are linked in a loop, the delay stages form a shift register such as Ring A or Ring B.

Each cell interconnect receiv~s data from the previous cell interconnect in its ring and forwards data to the ne~t.

An insertion and extraction protocol described in greater detail hereinafter allows the cell interconnects to pass data between cells.

As FI~URE 4 illustrates, each cell interconnect unit (CI) is formed by two cell interconnect units (CIUsa, and associated SRAMs for storing state bits.

Each c~ll interconn2ct unit ~CIU), in turn, is constructed from a plurality of integrated circuits.

The integrated circuits which form cell interconnect unit (CIU) 72 are depicted in FIGURE 5.

The cache bus connection of the cel~ interconnect is a bî-directional interface.

The cell interconn~ct recsives data from the cache bus to send to the ring and places data it receives from the ring onto the cache bus to be serviced by the cache control unit or domain routin~ unit.

The structure and operation of pref2rred cache control and domain routing units are described in Canadian Patent No.

5~2,560.

~L 3 ~ 3 -16 1 As illustrated in FIGURE 6, each cell interconnect contributes two shift register stages to the ~hift register structures of rings A and ~.

For example, a ring with ten cell interconnects, such as rings A and B ~hown in FIGURE 6, consists of twenty pipeline stages.

Each pipeline ætage is capable of selectively storing and transferring an information-repxesentative signal representing one data word.

All data words circulate throu~h the ring by progressing, in parallel, at the rate of one stage per applied clock cycle.

It is this feature o the invention which allows each cell to uni~uely identify the source and destination of each data word on the bus, an~ determine appropriate processing steps for each data word.

One example of a preferred clock signal distribution ~onfiguration is depicted in FIGURE 7.

In accordance wlth the invention, cell interconnect unit (CIU) 72 is constructed from periphery unit 30, CIU tag unit 81, SRAM control unit 8~, cache ~us control unit 83, CIU data path unit 84, CIU master control unit 85, and CIU directory unit 86.

The integrated circuits illustrated in FIGURE S ~ontain latches, FIFO b~ffers, multiplexors (MUXs~ and other conventional logic ~lements.

In particular, the CIU datapath associated wi~h CIU datapath circuit 84 is a 36 bit wide datapath including low ana high cache group units, an extract FIFO and an insert FIFO.

These four units collectively provide paths or (i) moving addre~ses -17- ~37J~3 1 from the domain interconnected by rings A and B, and from the cache bus, to the directory for address lookup, ~ii) moving packets through the two pipeline stages of each CIU, (iii) moving packets from the domain to the cache bus, and (iv) moving packets from the cache bus to the domain.

~he low and high cache group units direct appropriate addresses to the CIU directory circuit 86 or lookup, and provide mo~ification of directory entries.

In particular, the cache group units can pass domain~ cache or recirculated addresses for lookup operations, modify directory entries, and move data from directory 86 to the associated cache bus.

The extract FIFO unit mo-es data from the CIU domain inputs into a holding register file, and subsequently passes the data to the associated cache bus.

Th~ insert FIFO unit moves data from the cache bus ;nputs into a holding reg;ster file, and subsequently passes this data to the CIU domain outputs.

Addit;onally, the insert FIFO uni~ provides for modifying packets on the domain formed by rings A and B.

The datapath control section associa~ed with CIU datapath unit 84 receives commands from the master control unit 85 and converts them into comman~ signals for use by the elements of the CIU datapath.

Detailed schematics and timing diagrams ~or the elements o~ these integrated circuits are set forth in ~lGVRES 9 to 18.

Top level control of the CIU 72 is managed by the CIU master control circuit 85, the ~RAM .

-18- ~3~ 3 1 control circuik 82, and the ca~he bus control CiL'CUit 83.

The master control circuit 85 receives PACKET HEADER and EMPTY STATUS bits, and provides sequencing to the directory block to perform address lookups, The master control circuit 85 utilizes the results of these lookup operation~ to determine which of the PASS, EXTRACT and INSERT operations, discussed in greater detail hereinafter, is appropriate.

The master control circuit 85 performs these operations based upon signals from the CIU data path circuit 84 and cache bus control circuit 83.

The SRAM control circuit 82 generates control signals for addressing the external SRAMS 70 and 71 used by the CIU 72 and illustrated in FIGURE 4.

The cache bus control circuit 83 manages arbitration and flow control on the cache ~us, as described in Canadian Patent Application No. 58~,560.

The cache bus control circuit ~3 receives command signals from the master control circuit 85, and in turn, tran~mi~s status report signal~ to the master control circuit ~5.

Siqna1s and As FIGURE 7 illustrates, a single domain clock signal ~h,l), ~enerated by clock generator 30, is distributed to the entire domain interconnect formed by rings A and B.

Domain clock (h,l) provides 50mh~ æynchronous timing information to the cell interconnects within the domain interconnect ~ormed by rings ~ and B.

By properly distributing domain clock (h,l~, the effective clock ~kew for a cell interconnect such ' ~3~0~3 --19- 1 as, for e~ample, the cell interconnect 14 corresponding to cell 4, is the clock skew between that cell interconnect's input stage 14.0 and prior adjacent cell inter~onnect (cell 2) and its output stage 14.1 and nest adjacent cell (cell 6).

An important advantage of the invention is that clock ~kew is not accumulative, and propagation time between cell interconnects is independent of the number of cell interconnects or stages.

The fundamental result is that the clock cycle time --i.e. the inverse of the clock fr~quency -- of the domain interconnect is simply the cycle time between two adjacent cell interconnects.

The lS clock cycle time does not increase and requency does not decrease as the number of cell interconnects is increased.

Thus, as the number of cell interconnects in a ring is increased, the flux of operations through the ring is constant, while the number of bus operations which can be e~ecuted during a complete bus cycle increases linearly. 'rhis is an important feature of the invention, which is ideally suited for multiprocessor structures.

The high ~peed nature o~ a domain interconnect constructed in accordance with the invent;on is further enhanced by two topological factor~.

First, the output (i.e. second~ stage of each cell interconnect ~ives a single load, ~he input ~tage of the adjacent cell interconnect.

Second, each cell interconnect requires connection to only its two neighboring cell interconnect~, allowing close pro~imity of all directly connected cell interconnects.

Thi~ combination of absolute minimal ~32~3 l loading and very short physical distance between adjacent cells minimizes propagation time between cell interconnect~.

Those skilled in the art will understand that while the ambodiment described above in connection with FIGURE 7 utilizes a synchronous clock, the invention can be practiced in connection with an asynchronous or self-timed clock confi~uration.

In accordance with the invention, data circulating through a given ring is divided into data packets of ten data words, corresponding to ten shift register ring stages.

The nurnber of shift register stages must be an exact multiple of the number of data worcls in a data packet.

Given, for example, a ring with twenty cells and two register stages per cell, the ring consists of forty staqes.

Thus, four ten-word packets can be transferred simultaneously in this ring~ This property is generalized below in Table I.

Number of CI Number of Stages Number of Packets 2~ 2 2~ 40 4 : : .

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-21- ~ 3 ~ 3 1 The invention is preferably practiced ~n connection with the packet configuration shown in FIGURE 8.

The first data word in each half is an address, the second data word is a command and the remainin~ data words are data, as indicated in FIGURE B.

Those skille~ in the art will understand that alternative packet configurations are possible and within the ~cope o~ the invention.

In addition to the operations described abov~, a cell interconnect can modify the com~and field of a packet.

For example, a cell interconnect can extract a packet by copy;ng the packet from the ring and changing the command field to EMPTY.

Alt~rnatively, if a cell interconnect merely copies a packet, the command field would remain unchanged, allowing the packet to continue to circulate through the ring.

All packets circulate through the domain interconnect only once.

This property results from an operational protocol in which each operation is created and retir~d by the same cell interconnect.

Cells which extract a packet to add response data must later re-insert the packet.

The operations that the cell interconnect units can perform on packets thus include the following:

PA~S PACKET: The cell interconnect unit passes a packet from its ring inputs to its ring outputs w~thout any modification if the packet 5 -~2- ~320~03 1 specifies an address of which the cell interconnect has no knowledge.

EXTRACT PACKET: The cell interconnect unit extracts a packet from the ring if it represents a request the cell interconnect unit made to the ring or contains an address the cell interconnect unit must act upon.

When a cell interconnect unit extracts a packet from the ring it modiies the command of the packet to indicate the extractionO SEND PACKET to the Cache Bus: The cell interconnect unit sends each packet that it extracts from thë ring to the cache bus for service by the cache controller.

RECEIVE PACKET from the C~che Bus: The cache controller can send a packet to the cell interconnect unit for insertion into the ring.

The cell interconnect unit receives these packets and retains them until they can be inserted into the ring.

INSERT PACKET: Th~ cell interconnect unit inserts a packet into the ring whenever it has a packet awaiting insertion and t:he current ring packet is marked as EMPTY.

In a preferred embodiment o~ the invention, the domain interconnect formed by rin~s ~ and B supports two sets o~ fields, referred to as the domain ields and the cell interconnect uni~ f;~lds.

The dsmain fields are established by serial connections from one cell interconnect to the next , ~ . ~ -23- ~3~ 3 1 cell interconnect, which form a ring.

Each cell interconnect has a separate r~ceive and transrnit port or the domain fields, as indicated in FIGURE 4.

The cell interconnect unit fields provide communication among the cell interconnect units of the cell.

The domain fields are summarized below in Table ~I:

TABLE II: DOMAIN FIELDS DOMAIN DATA DOMAIN ECC DOMAIN ~EADER DOMAIN CLOCK CIU ID CIU CELL ADDRESS CIU EXTRACT The DOMAIN DATA and DOMAIN ECC fields are responsible for movin~ the dat~ of ring operations.

Each operation is a packet of ten domain bus transfers.

The DOMAIN HEADER field marks the beginning of an operationD The! DOMAIN CLOCK field provides ~iming for the shift register structure in cooperation with the clock generator ;llustrated in Figure 7.

The CIU ID field identifies the type of cell interconnect unit involved in a given transfer.

The CIU CE~L ADDRESS field identifi~s the domain local address of the cell.

The CIU EXTRACT field communicates information between cell interconnect uni ~.

I~ operation, the DOMAIN DATA field transmits the address, command, and data information 5 -24 L3200~3 corresponding to an operation~ The~ conf iguratiorl o~ the DOMAIN DATA field consists first of an address, then a cornrnand, and inally eight data v~luesO one or more of which may be empty.

The DOMAIN ECC field transmits a Hamming base~ error correction code (ECC) word for eac~ domain data transfer. DOMAIN ECC is not generated or checked by the cell interconnect unit~, but passed unchanged to the target units.

The DOMAIN HEADER field designates a given word in the current domain data transfer as the first word of a packet.

The assertion of the DO~AIN RESET signal by the domaln structure or domain power controller causes each cell interconnect and corresponding cell to enter a reset state.

The reset state is described in Canadian Patent Application No. 582, 560.

The assertion of CELI, RESET by a ell causes the corresponding cell and ~ell interconnect to reset. When reset, cell interconnects perform only P~SS PACKET operations.

The c~ll interco~nect unit control fi~lds provide for communic~ti~n specific to the cell interconnect units. These cell interconnect unit co~trol ~ields are summarized below in Table III.

~32~3 -25 TABLE IIT: CIU Control Field~ CIU ID CIU EXTRACT CIU CELL ADDRESS THE CIU ID ield for each CIU is established by the configuration of electrical connections rom the CIU to power and ground terminals.

Thi~ configuration establishes a unique CIU identification number for each CIU.

The interpretation of the identification number depends upon whether the plural ring structure is configured in a two-way or a four-way memory interleaving mode.

Normally, riny A is configured for even page addresses in memory and ring B for odd page addresses in memory.

However, those skilled in the art will appreciate that the shift register structure can be! c~nfigured to pass all addresses on either riny. ~Page address ; interleaving is normally configur~d at s~stem configuration time by control locations in the cell - interconnec~s.

Table IV below summarizes interpretation of id numbers in the two-way interleaved mode.

T~E D: TW0 WA~ INTERLE~V~ ID VALUE DEFIN~TI0 o M~ster CIU 1 1 Master CIU 1 a Slave CIU 0 ~ Slave CIU 1 '' ' ~ 26 ~32~ 1 Tabl2 V below summarizes int~rpretation of id numbers in the four-way interleaved mode:

T_E ~T . FOUR WAY I NTERLEAVE12 ID VALUE DE:FINIT.ION o M~STER 0 1 Master CIU 1 2 Master CIU 2 3 Master CIU 3 When two CIUs are partnered as master and slave, the slave cell interconnect unit drives a one-bit CIU EXTRACT signal which is read by its `!

partner master cell interconnect unit.

The CIU EXTRACT ~ignal is asserted or de-asserted accordinq to whether the slave CIU identifies the current operation as requiring removal from th~ shift register structure.

Th~ CIU CEL~ ADDRESS sign~l represents khe address within the domain of tlle cell asser~ing the : signal.

In accordance with the invention, ~11 domain interconnect tran~fers form a single packet.

The domain interconnect formad by the plural rings iæ initialized to co~tain a figed number of~ packets base~ on the number o~ cell interconnect For -e~ampIe, 3 twenty cell doma;n interconnect contains eight packets per ring.

Thus, in this esample, eigh~ : : .

-27- ~20~03 1 packets per ring, or s;xteen packets per domai~ interconnect, can be transferred in parallel.

In a preferred practice of the invention in conjunction with a multiprocessor structure like that described in Canadian Patent Application No. 5B2,560, th~ c~ll interconnect performs two levels of interpretation in order to determine how to operate on a packet~ First, the cell interconnect examines the address specified in the packet.

The cell interconnect may be configured to operate as a positive or a negative fil~er.

As positive filter, it operates on any System Virtual Address (SVA) that has an entry in its cache descriptors.

The interaction of SVA addresses and cache descriptors is further described in Canadian Patent Application NoO 582,560.

The positive filter configuration is used when a cell interconnect i~ connected to a cache either directly or by a remote interface.

The ne~ative filter configuration is used when the cell interconnect is connected to a rout~r.

In ei~her configuration, the cell interconnect operates on SVA addresses directed to it.

.

Secon~ly, having recognized an address, the cell interconnect esamines the command portion of the packet to determine if it ~an modify the response field of the command and allow the packet to proceed, or whether it must extract the packet from the domain - in~erconnect.

A cell interconnect can insert a packet i~to the ring when an empty packet arrives.

An empty packet is indicated by an IDLE operation type in the .

-28_ :L32~3 cornrnand word of the packet.

Evenly distributed usage of the packets of the ring is provided by the invention because no cell interconnect can use a packet that it has just emptied by an extraction operation.

In order to perform an insertion, th2 cell interconnect must place its operation into the ten consecutive stages of the empty packet.

It will be understood that given the plural ring structure of the invention, the cell intarconnect which initially injects a particular operation into the ring will eventually receive that operatiQn back.

At that time, the cell interconnect destroys the operation by changing the command word to IDLE.

Any cell interconnect that removes an operation it did not create must return that operation to the ring.

A cell interconnect e:~tracts an operation from the ring by copying ;ts contents from the ten consecutive stages of its packet and writing thP IDLE operation type into the commant3 word of the packet.

Any cell interconnect that e~tracts an operation i~ did not create mus~ return tha1; operation to the rin~.

The bus structures are initialized in two stepæ.

First, th~ domain stages are formatted into ten word packet~ by initiallzing the domain header signal.

Secondly, each packet is initialize~ ~o an ID~ state. ~ the number of stages in either ring is not an even multiple of ten stages, or if the ci2cular path is logically broken, the ring will not initialize.

-29- ~320~a3 In a preferred embodim~nt of the invention, bus initialization is performed by software~ with cell interconnect as~ist.

Within each cell interconnect is a DOMAlN HEADER STATUS bit which indicates whether the domain is properly formatted by verification of the DOMAIN HEADER ~ignal.

If th~ DOMAIN HEADER STATUS bit indicateæ that a given ring is improperly formatted, then a SETUP DOMAIN command issued by a given cell to a cell interconnect performs domain initializat;on.

It will thus be seen that the invention efficiently attains the objects set forth above, among those made apparent from the preceding description.

It will be understood that changes may be made in the above construct;on and in the foregoing sequences of operation without departing from the scope o~ the invention.

It is accordingly intended tha~ all matter c~ntained in the above description or shown in the accompanying drawings be interpreted as illustrative rather than in a limiting sense.

It is also understood that the ollowing claims are intended to cover all of the generic and specific features of the inve~tion as described herein, and all statements of the scope of the invention which, as a matter of language, might be æaid to fall therebetween.