Direct memory access (DMA) transmitter

[0002] This invention relates generally to data storage systems, and more particularly to data storage systems having redundancy arrangements to protect against total system failure in the event of a failure in a component or subassembly of the storage system.

[0003] As is known in the art, large host computers and servers (collectively referred to herein as “host computer/servers”) require large capacity data storage systems. These large computer/servers generally includes data processors, which perform many operations on data introduced to the host computer/server through peripherals including the data storage system. The results of these operations are output to peripherals, including the storage system.

[0004] One type of data storage system is a magnetic disk storage system. Here a bank of disk drives and the host computer/server are coupled together through an interface. The interface includes “front end” or host computer/server controllers (or directors) and “back-end” or disk controllers (or directors). The interface operates the controllers (or directors) in such a way that they are transparent to the host computer/server. That is, data is stored in, and retrieved from, the bank of disk drives in such a way that the host computer/server merely thinks it is operating with its own local disk drive. One such system is described in U.S. Pat. No. 5,206,939, entitled “System and Method for Disk Mapping and Data Retrieval”, inventors Moshe Yanai, Natan Vishlitzky, Bruno Alterescu and Daniel Castel, issued Apr. 27, 1993, and assigned to the same assignee as the present invention.

[0005] As described in such U.S. Patent, the interface may also include, in addition to the host computer/server controllers (or directors) and disk controllers (or directors), addressable cache memories. The cache memory is a semiconductor memory and is provided to rapidly store data from the host computer/server before storage in the disk drives, and, on the other hand, store data from the disk drives prior to being sent to the host computer/server. The cache memory being a semiconductor memory, as distinguished from a magnetic memory as in the case of the disk drives, is much faster than the disk drives in reading and writing data.

[0006] The host computer/server controllers, disk controllers and cache memory are interconnected through a backplane printed circuit board. More particularly, disk controllers are mounted on disk controller printed circuit boards. The host computer/server controllers are mounted on host computer/server controller printed circuit boards. And, cache memories are mounted on cache memory printed circuit boards. The disk directors, host computer/server directors, and cache memory printed circuit boards plug into the backplane printed circuit board. In order to provide data integrity in case of a failure in a director, the backplane printed circuit board has a pair of buses. One set the disk directors is connected to one bus and another set of the disk directors is connected to the other bus. Likewise, one set the host computer/server directors is connected to one bus and another set of the host computer/server directors is directors connected to the other bus. The cache memories are connected to both buses. Each one of the buses provides data, address and control information.

[0007] The arrangement is shown schematically in FIG. 1. Thus, the use of two buses B1, B2 provides a degree of redundancy to protect against a total system failure in the event that the controllers or disk drives connected to one bus, fail. Further, the use of two buses increases the data transfer bandwidth of the system compared to a system having a single bus. Thus, in operation, when the host computer/server 12 wishes to store data, the host computer 12 issues a write request to one of the front-end directors 14 (i.e., host computer/server directors) to perform a write command. One of the front-end directors 14 replies to the request and asks the host computer 12 for the data. After the request has passed to the requesting one of the front-end directors 14, the director 14 determines the size of the data and reserves space in the cache memory 18 to store the request. The front-end director 14 then produces control signals on one of the address memory busses B1, B2 connected to such front-end director 14 to enable the transfer to the cache memory 18. The host computer/server 12 then transfers the data to the front-end director 14. The front-end director 14 then advises the host computer/server 12 that the transfer is complete. The front-end director 14 looks up in a Table, not shown, stored in the cache memory 18 to determine which one of the back-end directors 20 (i.e., disk directors) is to handle this request. The Table maps the host computer/server 12 addresses into an address in the bank 14 of disk drives. The front-end director 14 then puts a notification in a “mail box” (not shown and stored in the cache memory 18) for the back-end director 20, which is to handle the request, the amount of the data and the disk address for the data. Other back-end directors 20 poll the cache memory 18 when they are idle to check their “mail boxes”. If the polled “mail box” indicates a transfer is to be made, the back-end director 20 processes the request, addresses the disk drive in the bank 22, reads the data from the cache memory 18 and writes it into the addresses of a disk drive in the bank 22.

[0008] When data is to be read from a disk drive in bank 22 to the host computer/server 12 the system operates in a reciprocal manner. More particularly, during a read operation, a read request is instituted by the host computer/server 12 for data at specified memory locations (i.e., a requested data block). One of the front-end directors 14 receives the read request and examines the cache memory 18 to determine whether the requested data block is stored in the cache memory 18. If the requested data block is in the cache memory 18, the requested data block is read from the cache memory 18 and is sent to the host computer/server 12. If the front-end director 14 determines that the requested data block is not in the cache memory 18 (i.e., a so-called “cache miss”) and the director 14 writes a note in the cache memory 18 (i.e., the “mail box”) that it needs to receive the requested data block. The back-end directors 20 poll the cache memory 18 to determine whether there is an action to be taken (i.e., a read operation of the requested block of data). The one of the back-end directors 20 which poll the cache memory 18 mail box and detects a read operation reads the requested data block and initiates storage of such requested data block stored in the cache memory 18. When the storage is completely written into the cache memory 18, a read complete indication is placed in the “mail box” in the cache memory 18. It is to be noted that the front-end directors 14 are polling the cache memory 18 for read complete indications. When one of the polling front-end directors 14 detects a read complete indication, such front-end director 14 completes the transfer of the requested data which is now stored in the cache memory 18 to the host computer/server 12.

[0009] The use of mailboxes and polling requires time to transfer data between the host computer/server 12 and the bank 22 of disk drives thus reducing the operating bandwidth of the interface.

[0010] In accordance with the present invention, a direct memory access (DMA) transmitter is provided adapted to transfer data from a random access memory to an output. The data is generated by a central processing unit coupled to a bus. The generated data is initially stored in a local cache memory connected to the central processing unit. The DMA transmitter is also coupled to the bus. The DMA transmitter includes: (a) a data register; and (b) a transmitter state machine. Requested data at an address provided by a source is read from the random access memory then transferred for storage in the data register. The central processing unit also sends a control signal to the transmit state machine. The control signal indicates to the transmit state machine whether the read data is a most recent copy of the requested data in random access memory or whether the most recent copy of the requested data is still resident in the local cache memory. In response to the control signal, if the most recent data is in the local cache memory, the transmit state machine inhibits the data that was read from random access memory and now stored in data register from passing to the transmitter output. Transmit state machine then performs a second data transfer request at the same address, the second requested data being transferred from the local cache memory to the random access memory. The transmit state machine reads the second requested data from the random access memory. The second requested data is the most recent data available in the random access memory. The transmit state machine then stores such second requested data into the data register, such stored second requested data then being transferred to the transmitter output.

[0011] In accordance with one embodiment, a direct memory access (DMA) transmitter is provided adapted to transfer data from a random access memory to an output, such data having been generated by a central processing unit coupled to a bus. Such generated data is initially stored in a local cache memory connected to the central processing unit. The transmitter is operative in response to a transmit write enable signal produced from an external source requesting the data. The external source also sends to the DMA transmitter a random access memory address. The DMA transmitter is coupled to the bus. The transmitter includes an address register, a data register, and a transmitter state machine. The transmit state machine placing the address in the address register in response to the write enable signal. The requested data at such address is read from the random access memory then transferred for storage in the data register. The central processing unit also sends a control signal to the transmit state machine. The control signal indicates to the transmit state machine whether the read data is a most recent copy of the requested data in random access memory or whether the most recent copy of the requested data is still resident in the local cache memory. In response to the control signal, if the most recent data is in the local cache memory, the transmit state machine inhibits the data that was read from random access memory and now stored in data register from passing to the transmitter output. The transmit state machine then performs a second data transfer request at the same address. The second requested data is transferred from the local cache memory to the random access memory. The transmit state machine reads the second requested data from the random access memory, such second requested data being the most recent data available in the random access memory, the transmit state machine then storing such second requested data into the data register. The stored second requested data is then transferred to the transmitter output.

[0012] In accordance with another embodiment, a method for performing a direct memory access (DMA) transmitter transfers data from a random access memory to an output is provided. The method includes generating the data in a central processing unit coupled to a bus, such generated data being initially stored in a local cache memory connected to the central processing unit. A random access memory address is received from an external source, for storage in an address register. A bus request signal is sent to a bus arbiter connected to the bus. The arbiter performs a bus arbitration and, when appropriate, such arbiter grant access to the bus. When granted such bus, the address currently in the address register is placed on an address portion of the bus for the random access memory along with appropriate read control signals on a control signal portion of the bus. The control signal indicates whether the read data is a most recent copy of the requested data in random access memory or whether the most recent copy of the requested data is still resident in the local cache memory. The data at the address on the bus is read from the random access memory. The read data is and passed, via a data portion of the bus, to the data register. If the control signal indicates that the most recent data is in the local cache memory, the data that was just read from random access memory, and which has been stored in data register, is inhibited from passing to the transmitter output. A second data transfer request at the same address location is made. The second requested data is transferred by the central processing unit from the local cache memory to the random access memory. A re-arbitration is made for the bus; and after the bus is granted, the second requested data is read from the random access memory. The second requested data is the most recent data available in the random access memory. The second requested data is loaded into the data memory in response to the assertion of the signal produced by the transmit state machine. The stored second requested data then being transferred to the transmitter output.

[0013] These and other features of the invention will become more readily apparent from the following detailed description when read together with the accompanying drawings, in which:

[0014] FIG. 1 is a block diagram of a data storage system according to the PRIOR ART;

[0015] FIG. 2 is a block diagram of a data storage system according to the invention;

[0016] FIG. 2A shows the fields of a descriptor used in the system interface of the data storage system of FIG. 2;

[0017] FIG. 2B shows the filed used in a MAC packet used in the system interface of the data storage system of FIG. 2;

[0018] FIG. 3 is a sketch of an electrical cabinet storing a system interface used in the data storage system of FIG. 2;

[0019] FIG. 4 is a diagramatical, isometric sketch showing printed circuit boards providing the system interface of the data storage system of FIG. 2;

[0020] FIG. 5 is a block diagram of the system interface used in the data storage system of FIG. 2;

[0021] FIG. 6 is a block diagram showing the connections between front-end and back-end directors to one of a pair of message network boards used in the system interface of the data storage system of FIG. 2;

[0022] FIG. 7 is a block diagram of an exemplary one of the director boards used in the system interface of he data storage system of FIG. 2;

[0023] FIG. 8 is a block diagram of the system interface used in the data storage system of FIG. 2;

[0024] FIG. 8A is a diagram of an exemplary global cache memory board used in the system interface of FIG. 8;

[0025] FIG. 8B is a diagram showing a pair of director boards coupled between a pair of host processors and global cache memory boards used in the system interface of FIG. 8;

[0026] FIG. 8C is a block diagram of an exemplary crossbar switch used in the front-end and rear-end directors of the system interface of FIG. 8;

[0027] FIG. 9 is a block diagram of a transmit Direct Memory Access (DMA) used in the system interface of the FIG. 8;

[0028] FIG. 10 is a block diagram of a receive DMA used in the system interface of FIG. 8;

[0029] FIG. 11 shows the relationship between FIGS. 11A and 11B, such FIGS. 11A and 11B together showing a process flow diagram of the send operation of a message network used in the system interface of FIG. 8;

[0030] FIGS. 11C-11E are examples of digital words used by the message network in the system interface of FIG. 8;

[0031] FIG. 11F shows bits in a mask used in such message network,

[0032] FIG. 11G shows the result of the mask of FIG. 11F applied to the digital word shown in FIG. 11E;

[0033] FIG. 12 shows the relationship between FIGS. 12A and 12B, such FIGS. 12A and 12B Showing a process flow diagram of the receive operation of a message network used in the system interface of FIG. 8;

[0034] FIG. 13 shows the relationship between FIGS. 11A and 11B, such FIGS. 11A and 11B together showing a process flow diagram of the acknowledgement operation of a message network used in the system interface of FIG. 8;

[0035] FIGS. 14A and 14B show process flow diagrams of the transmit DMA operation of the transmit DMA of FIG. 9; and

[0036] FIGS. 15A and 15B show process flow diagrams of the receive DMA operation of the receive DMA of FIG. 10.

[0037] Referring now to FIG. 2, a data storage system 100 is shown for transferring data between a host computer/server 120 and a bank of disk drives 140 through a system interface 160. The system interface 160 includes: a plurality of, here 32 front-end directors 180₁-180₃₂coupled to the host computer/server 120 via ports-123₃₂; a plurality of back-end directors 200₁-200₃₂coupled to the bank of disk drives 140 via ports 123₃₃-123₆₄; a data transfer section 240, having a global cache memory 220, coupled to the plurality of front-end directors 180₁-180₁₆and the back-end directors 200₁-200₁₆; and a messaging network 260, operative independently of the data transfer section 240, coupled to the plurality of front-end directors 180₁-180₃₂and the plurality of back-end directors 200₁-200₃₂, as shown. The front-end and back-end directors 180₁-180₃₂, 200₁-200₃₂are functionally similar and include a microprocessor (μp) 299 (i.e., a central processing unit (CPU) and RAM), a message engine/CPU controller 314 and a data pipe 316 to be described in detail in connection with FIGS. 5, 6 and 7. Suffice it to say here, however, that the front-end and back-end directors 180₁-180₃₂, 200₁-200₃₂control data transfer between the host computer/server 120 and the bank of disk drives 140 in response to messages passing between the directors 180₁-180₃₂, 200₁-200₃₂through the messaging network 260. The messages facilitate the data transfer between host computer/server 120 and the bank of disk drives 140 with such data passing through the global cache memory 220 via the data transfer section 240. More particularly, in the case of the front-end directors 180₁-180₃₂, the data passes between the host computer to the global cache memory 220 through the data pipe 316 in the front-end directors 180₁-180₃₂and the messages pass through the message engine/CPU controller 314 in such front-end directors 180₁-180₃₂. In the case of the back-end directors 200₁-200₃₂the data passes between the back-end directors 200₁-200₃₂and the bank of disk drives 140 and the global cache memory 220 through the data pipe 316 in the back-end directors 200₁-200₃₂and again the messages pass through the message engine/CPU controller 314 in such back-end director 200₁-200₃₂.

[0038] With such an arrangement, the cache memory 220 in the data transfer section 240 is not burdened with the task of transferring the director messaging. Rather the messaging network 260 operates independent of the data transfer section 240 thereby increasing the operating bandwidth of the system interface 160.

[0039] In operation, and considering first a read request by the host computer/server 120 (i.e., the host computer/server 120 requests data from the bank of disk drives 140), the request is passed from one of a plurality of, here 32, host computer processors 121₁-121₃₂in the host computer 120 to one or more of the pair of the front-end directors 180₁-180₃₂connected to such host computer processor 121₁-121₃₂. (It is noted that in the host computer 120, each one of the host computer processors 121₁-121₃₂is coupled to here a pair (but not limited to a pair) of the front-end directors 180₁-180₃₂, to provide redundancy in the event of a failure in one of the front end-directors 181₁-181₃₂coupled thereto. Likewise, the bank of disk drives 140 has a plurality of, here 32, disk drives 141₁-141₃₂, each disk drive 141₁-141₃₂being coupled to here a pair (but not limited to a pair) of the back-end directors 200₁-200₃₂, to provide redundancy in the event of a failure in one of the back-end directors 200₁-200₃₂coupled thereto). Each front-end director 180₁-180₃₂includes a microprocessor (μP) 299 (i.e., a central processing unit (CPU) and RAM) and will be described in detail in connection with FIGS. 5 and 7. Suffice it to say here, however, that the microprocessor 299 makes a request for the data from the global cache memory 220. The global cache memory 220 has a resident cache management table, not shown. Every director 180₁-180₃₂, 200₁-200₃₂has access to the resident cache management table and every time a front-end director 180₁-180₃₂requests a data transfer, the front-end director 180₁-180₃₂must query the global cache memory 220 to determine whether the requested data is in the global cache memory 220. If the requested data is in the global cache memory 220 (i.e., a read “hit”), the front-end director 180₁-180₃₂, more particularly the microprocessor 299 therein, mediates a DMA (Direct Memory Access) operation for the global cache memory 220 and the requested data is transferred to the requesting host computer processor 121₁-121₃₂.

[0040] If, on the other hand, the front-end director 180₁-180₃₂receiving the data request determines that the requested data is not in the global cache memory 220 (i.e., a “miss”) as a result of a query of the cache management table in the global cache memory 220, such front-end director 180₁-180₃₂concludes that the requested data is in the bank of disk drives 140. Thus the front-end director 180₁-180₃₂that received the request for the data must make a request for the data from one of the back-end directors 200₁-200₃₂in order for such back-end director 200₁-200₃₂to request the data from the bank of disk drives 140. The mapping of which back-end directors 200₁-200₃₂control which disk drives 141₁-141₃₂in the bank of disk drives 140 is determined during a power-up initialization phase. The map is stored in the global cache memory 220. Thus, when the front-end director 180₁-180₃₂makes a request for data from the global cache memory 220 and determines that the requested data is not in the global cache memory 220 (i.e., a “miss”), the front-end director 180₁-180₃₂is also advised by the map in the global cache memory 220 of the back-end director 200₁-200₃₂responsible for the requested data in the bank of disk drives 140. The requesting front-end director 180₁-180₃₂then must make a request for the data in the bank of disk drives 140 from the map designated back-end director 200₁-200₃₂. This request between the front-end director 180₁-180₃₂and the appropriate one of the back-end directors 200₁-200₃₂(as determined by the map stored in the global cache memory 200) is by a message which passes from the front-end director 180₁-180₃₂through the message network 260 to the appropriate back-end director 200₁-200₃₂. It is noted then that the message does not pass through the global cache memory 220 (i.e., does not pass through the data transfer section 240) but rather passes through the separate, independent message network 260. Thus, communication between the directors 180₁-180₃₂, 200₁-200₃₂is through the message network 260 and not through the global cache memory 220. Consequently, valuable bandwidth for the global cache memory 220 is not used for messaging among the directors 180₁-180₃₂, 200₁-200₃₂.

[0041] Thus, on a global cache memory 220 “read miss”, the front-end director 180₁-180₃₂sends a message to the appropriate one of the back-end directors 200₁-200₃₂through the message network 260 to instruct such back-end director 200₁-200₃₂to transfer the requested data from the bank of disk drives 140 to the global cache memory 220. When accomplished, the back-end director 200₁-200₃₂advises the requesting front-end director 180₁-180₃₂that the transfer is accomplished by a message, which passes from the back-end director 200₁-200₃₂to the front-end director 180₁-180₃₂through the message network 260. In response to the acknowledgement signal, the front-end director 180₁-180₃₂is thereby advised that such front-end director 180₁-180₃₂can transfer the data from the global cache memory 220 to the requesting host computer processor 121₁-121₃₂as described above when there is a cache “read hit”.

[0042] It should be noted that there might be one or more back-end directors 200₁-200₃₂responsible for the requested data. Thus, if only one back-end director 200₁-200₃₂is responsible for the requested data, the requesting front-end director 180₁-180₃₂sends a uni-cast message via the message network 260 to only that specific one of the back-end directors 200₁-200₃₂. On the other hand, if more than one of the back-end directors 200₁-200₃₂is responsible for the requested data, a multi-cast message (here implemented as a series of uni-cast messages) is sent by the requesting one of the front-end directors 180₁-180₃₂to all of the back-end directors 200₁-200₃₂having responsibility for the requested data. In any event, with both a uni-cast or multi-cast message, such message is passed through the message network 260 and not through the data transfer section 240 (i.e., not through the global cache memory 220).

[0043] Likewise, it should be noted that while one of the host computer processors 121₁-121₃₂might request data, the acknowledgement signal may be sent to the requesting host computer processor 121₁or one or more other host computer processors 121₁-121₃₂via a multi-cast (i.e., sequence of uni-cast) messages through the message network 260 to complete the data read operation.

[0044] Considering a write operation, the host computer 120 wishes to write data into storage (i.e., into the bank of disk drives 140). One of the front-end directors 180₁-180₃₂receives the data from the host computer 120 and writes it into the global cache memory 220. The front-end director 180₁-180₃₂then requests the transfer of such data after some period of time when the back-end director 200₁-200₃₂determines that the data can be removed from such cache memory 220 and stored in the bank of disk drives 140. Before the transfer to the bank of disk drives 140, the data in the cache memory 220 is tagged with a bit as “fresh data” (i.e., data which has not been transferred to the bank of disk drives 140, that is data which is “write pending”). Thus, if there are multiple write requests for the same memory location in the global cache memory 220 (e.g., a particular bank account) before being transferred to the bank of disk drives 140, the data is overwritten in the cache memory 220 with the most recent data. Each time data is transferred to the global cache memory 220, the front-end director 180₁-180₃₂controlling the transfer also informs the host computer 120 that the transfer is complete to thereby free-up the host computer 120 for other data transfers.

[0045] When it is time to transfer the data in the global cache memory 220 to the bank of disk drives 140, as determined by the back-end director 200₁-200₃₂, the back-end director 200₁-200₃₂transfers the data from the global cache memory 220 to the bank of disk drives 140 and resets the tag associated with data in the global cache memory 220 (i.e., un-tags the data) to indicate that the data in the global cache memory 220 has been transferred to the bank of disk drives 140. It is noted that the un-tagged data in the global cache memory 220 remains there until overwritten with new data.

[0046] Referring now to FIGS. 3 and 4, the system interface 160 is shown to include an electrical cabinet 300 having stored therein: a plurality of, here eight front-end director boards 190₁-190₈, each one having here four of the front-end directors 180₁-180₃₂; a plurality of, here eight back-end director boards 210₁-210₈, each one having here four of the back-end directors 200₁-200₃₂; and a plurality of, here eight, memory boards 220′ which together make up the global cache memory 220. These boards plug into the front side of a backplane 302. (It is noted that the backplane 302 is a mid-plane printed circuit board). Plugged into the backside of the backplane 302 are message network boards 304₁, 304₂. The backside of the backplane 302 has plugged into it adapter boards, not shown in FIGS. 2-4, which couple the boards plugged into the back-side of the backplane 302 with the computer 120 and the bank of disk drives 140 as shown in FIG. 2. That is, referring again briefly to FIG. 2, an I/O adapter, not shown, is coupled between each one of the front-end directors 180₁-180₃₂and the host computer 120 and an I/O adapter, not shown, is coupled between each one of the back-end directors 200₁-200₃₂and the bank of disk drives 140.

[0047] Referring now to FIG. 5, the system interface 160 is shown to include the director boards 190₁-190₈, 210₁-210₈and the global cache memory 220, plugged into the backplane 302 and the disk drives 141₁-141₃₂in the bank of disk drives along with the host computer 120 also plugged into the backplane 302 via I/O adapter boards, not shown. The message network 260 (FIG. 2) includes the message network boards 304₁and 304₂. Each one of the message network boards 304₁and 304₂is identical in construction. A pair of message network boards 304₁and 304₂is used for redundancy and for message load balancing. Thus, each message network board 304₁, 304₂, includes a controller 306, (i.e., an initialization and diagnostic processor comprising a CPU, system controller interface and memory, as shown in FIG. 6 for one of the message network boards 304₁, 304₂, here board 304₁) and a crossbar switch section 308 (e.g., a switching fabric made up of here four switches 308₁-308₄).

[0048] Referring again to FIG. 5, each one of the director boards 190₁-210₈includes, as noted above four of the directors 180₁-180₃₂, 200₁-200₃₂(FIG. 2). It is noted that the director boards 190₁-190₈having four front-end directors per board, 180₁-180₃₂are referred to as front-end directors and the director boards 210₁-210₈having four back-end directors per board, 200₁-200₃₂are referred to as back-end directors. Each one of the directors 180₁-180₃₂, 200₁-200₃₂includes a CPU 310, a RAM 312 (which make up the microprocessor 299 referred to above), the message engine/CPU controller 314, and the data pipe 316.

[0049] Each one of the director boards 190₁-210₈includes a crossbar switch 318. The crossbar switch 318 has four input/output ports 319, each one being coupled to the data pipe 316 of a corresponding one of the four directors 180₁-180₃₂, 200₁-200₃₂on the director board 190₁-210₈. The crossbar switch 318 has eight output/input ports collectively identified in FIG. 5 by numerical designation 321 (which plug into the backplane 302. The crossbar switch 318 on the front-end director boards 191₁-191₈is used for coupling the data pipe 316 of a selected one of the four front-end directors 180₁-180₃₂on the front-end director board 190₁-190₈to the global cache memory 220 via the backplane 302 and I/O adapter, not shown. The crossbar switch 318 on the back-end director boards 210₁-210₈is used for coupling the data pipe 316 of a selected one of the four back-end directors 200₁-200₃₂on the back-end director board 210₁-210₈to the global cache memory 220 via the backplane 302 and I/O adapter, not shown. Thus, referring to FIG. 2, the data pipe 316 in the front-end directors 180₁-180₃₂couples data between the host computer 120 and the global cache memory 220 while the data pipe 316 in the back-end directors 200₁-200₃₂couples data between the bank of disk drives 140 and the global cache memory 220. It is noted that there are separate point-to-point data paths P₁-P₆₄(FIG. 2) between each one of the directors 180₁-180₃₂, 200₁-200₃₂and the global cache memory 220. It is also noted that the backplane 302 is a passive backplane because it is made up of only etched conductors on one or more layers of a printed circuit board. That is, the backplane 302 does not have any active components.

[0050] Referring again to FIG. 5, each one of the director boards 190₁-210₈includes a crossbar switch 320. Each crossbar switch 320 has four input/output ports 323, each one of the four input/output ports 323 being coupled to the message engine/CPU controller 314 of a corresponding one of the four directors 180₁-180₃₂, 200₁-200₃₂on the director board 190₁-210₈. Each crossbar switch 320 has a pair of output/input ports 325₁, 325₂, which plug into the backplane 302. Each port 325₁-325₂is coupled to a corresponding one of the message network boards 304₁, 304₂, respectively, through the backplane 302. The crossbar switch 320 on the front-end director boards 190₁-190₈is used to couple the messages between the message engine/CPUcontroller 314 of a selected one of the four front-end directors 180₁-180₃₂on the front-end director boards 190₁-190₈and the message network 260, FIG. 2. Likewise, the back-end director boards 210₁-210₈are used to couple the messages produced by a selected one of the four back-end directors 200₁-200₃₂on the back-end director board 210₁-210₈between the message engine/CPU controller 314 of a selected one of such four back-end directors and the message network 260 (FIG. 2). Thus, referring also to FIG. 2, instead of having a separate dedicated message path between each one of the directors 180₁-180₃₂, 200₁-200₃₂and the message network 260 (which would require M individual connections to the backplane 302 for each of the directors, where M is an integer), here only M/4 individual connections are required). Thus, the total number of connections between the directors 180₁-180₃₂, 200₁-200₃₂and the backplane 302 is reduced to ¼th. Thus, it should be noted from FIGS. 2 and 5 that the message network 260 (FIG. 2) includes the crossbar switch 320 and the message network boards 304₁, 304₂.

[0051] Each message is a 64-byte descriptor, shown in FIG. 2A) which is created by the CPU 310 (FIG. 5) under software control and is stored in a send queue in RAM 312. When the message is to be read from the send queue in RAM 312 and transmitted through the message network 260 (FIG. 2) to one or more other directors via a DMA operation to be described, it is packetized in the packetizer portion of packetizer/de-packetizer 428 (FIG. 7) into a MAC type packet, shown in FIG. 2B, here using the NGIO protocol specification. There are three types of packets: a message packet section; an acknowledgement packet; and a message network fabric management packet, the latter being used to establish the message network routing during initialization (i.e., during power-up). Each one of the MAC packets has: an 8-byte header which includes source (i.e., transmitting director) and destination (i.e., receiving director) address; a payload; and terminates with a 4-byte Cyclic Redundancy Check (CRC), as shown in FIG. 2B. The acknowledgement packet (i.e., signal) has a 4-byte acknowledgment payload section. The message packet has a 32-byte payload section. The Fabric Management Packet (FMP) has a 256-byte payload section. The MAC packet is sent to the crossbar switch 320. The destination portion of the packet is used to indicate the destination for the message and is decoded by the switch 320 to determine which port the message is to be routed. The decoding process uses a decoder table 327 in the switch 318, such table being initialized by controller during power-up by the initialization and diagnostic processor (controller) 306 (FIG. 5). The table 327 (FIG. 7) provides the relationship between the destination address portion of the MAC packet, which identifies the routing for the message and the one of the four directors 180₁-180₃₂, 200₁-200₃₂on the director board 190₁-190₈, 210₁-210₈or to one of the message network boards 304₁, 304₂to which the message is to be directed.

[0052] More particularly, and referring to FIG. 5, a pair of output/input ports 325₁, 325₂is provided for each one of the crossbar switches 320, each one being coupled to a corresponding one of the pair of message network boards 304₁, 304₂. Thus, each one of the message network boards 304₁, 304₂has sixteen input/output ports 322₁-322₁₆, each one being coupled to a corresponding one of the output/input ports 325₁, 325₂, respectively, of a corresponding one of the director boards 190₁-190₈, 210₁-210₈through the backplane 302, as shown. Thus, considering exemplary message network board 304₁, FIG. 6, each switch 308₁-308₄also includes three coupling ports 324₁-324₃. The coupling ports 324₁-324₃are used to interconnect the switches 322₁-322₄, as shown in FIG. 6. Thus, considering message network board 304₁, input/output ports 322₁-322₈are coupled to output/input ports 325₁of front-end director boards 190₁-190₈and input/output ports 322₉-322₁₆are coupled to output/input ports 325₁of back-end director boards 210₁-210₈, as shown. Likewise, considering message network board 304₂, input/output ports 322₁-322₈thereof are coupled, via the backplane 302, to output/input ports 325₂of front-end director boards 190₁-190₈and input/output ports 322₉-322₁₆are coupled, via the backplane 302, to output/input ports 325₂of back-end director boards 210₁-210₈.

[0053] As noted above, each one of the message network boards 304₁, 304₂includes a processor 306 (FIG. 5) and a crossbar switch section 308 having four switches 308₁-308₄, as shown in FIGS. 5 and 6. The switches 308₁-308₄are interconnected as shown so that messages can pass between any pair of the input/output ports 322₁-322₁₆. Thus, it follow that a message from any one of the front-end directors 180₁-180₃₂can be coupled to another one of the front-end directors 180₁-180₃₂and/or to any one of the back-end directors 200₁-200₃₂. Likewise, a message from any one of the back-end directors 180₁-180₃₂can be coupled to another one of the back-end directors 180₁-180₃₂and/or to any one of the front-end directors 200₁-200₃₂.

[0054] As noted above, each MAC packet (FIG. 2B) includes in an address destination portion and a data payload portion. The MAC header is used to indicate the destination for the MAC packet and such MAC header is decoded by the switch to determine which port the MAC packet is to be routed. The decoding process uses a table in the switch 308₁-308₄, such table being initialized by processor 306 during power-up. The table provides the relationship between the MAC header, which identifies the destination for the MAC packet and the route to be taken through the message network. Thus, after initialization, the switches 320 and the switches 308₁-308₄in switch section 308 provides packet routing which enables each one of the directors 180₁-180₃₂, 200₁-200₃₂to transmit a message between itself and any other one of the directors, regardless of whether such other director is on the same director board 190₁-190₈, 210₁-210₈or on a different director board. Further, the MAC packet has an additional bit B in the header thereof, as shown in FIG. 2B, which enables the message to pass through message network board 304₁or through message network board 304₂. During normal operation, this additional bit B is toggled between a logic 1 and a logic 0 so that one message passes through one of the redundant message network boards 304₁, 304₂and the next message to pass through the other one of the message network boards 304₁, 304₂to balance the load requirement on the system. However, in the event of a failure in one of the message network boards 304₁, 304₂, the non-failed one of the boards 304₁, 304₂is used exclusively until the failed message network board is replaced.

[0055] Referring now to FIG. 7, an exemplary one of the director boards 190₁-190₈, 210₁-210₈, here director board 190₁is shown to include directors 180₁, 180₃, 180₅and 180₇. An exemplary one of the directors 180₁-180₄, here director 180₁is shown in detail to include the data pipe 316, the message engine/CPU controller 314, the RAM 312, and the CPU 310 all coupled to the CPU interface bus 317, as shown. The exemplary director 180₁also includes: a local cache memory 319 (which is coupled to the CPU 310); the crossbar switch 318; and, the crossbar switch 320, described briefly above in connection with FIGS. 5 and 6. The data pipe 316 includes a protocol translator 400, a quad port RAM 402 and a quad port RAM controller 404 arranged as shown. Briefly, the protocol translator 400 converts between the protocol of the host computer 120, in the case of a front-end director 180₁-180₃₂, (and between the protocol used by the disk drives in bank 140 in the case of a back-end director 200₁-200₃₂) and the protocol between the directors 180₁-180₃, 200₁-200₃₂and the global memory 220 (FIG. 2). More particularly, the protocol used the host computer 120 may, for example, be fibre channel, SCSI, ESCON or FICON, for example, as determined by the manufacture of the host computer 120 while the protocol used internal to the system interface 160 (FIG. 2) may be selected by the manufacturer of the interface 160. The quad port RAM 402 is a FIFO controlled by controller 404 because the rate data coming into the RAM 402 may be different from the rate data leaving the RAM 402. The RAM 402 has four ports, each adapted to handle an 18 bit digital word. Here, the protocol translator 400 produces 36 bit digital words for the system interface 160 (FIG. 2) protocol, one 18 bit portion of the word is coupled to one of a pair of the ports of the quad port RAM 402 and the other 18 bit portion of the word is coupled to the other one of the pair of the ports of the quad port RAM 402. The quad port RAM has a pair of ports 402A, 402B, each one of to ports 402A, 402B being adapted to handle an 18 bit digital word. Each one of the ports 402A, 402B is independently controllable and has independent, but arbitrated, access to the memory array within the RAM 402. Data is transferred between the ports 402A, 402B and the cache memory 220 (FIG. 2) through the crossbar switch 318, as shown.

[0056] The crossbar switch 318 includes a pair of switches 406A, 406B. Each one of the switches 406A, 406B includes four input/output director-side ports D₁-D₄(collectively referred to above in connection with FIG. 5 as port 319) and four input/output memory-side ports M₁-M₄, M₅-M₈, respectively, as indicated. The input/output memory-side ports M₁-M₄, M₅-M₈were collectively referred to above in connection with FIG. 5 as port 317). The director-side ports D₁-D₄of switch 406A are connected to the 402A ports of the quad port RAMs 402 in each one the directors 180₁, 180₃, 180₅and 180₇, as indicated. Likewise, director-side ports of switch 406B are connected to the 402B ports of the quad port RAMs 402 in each one the directors 180₁, 180₃, 180₅, and 180₇, as indicated. The ports D₁-D₄are selectively coupled to the ports M₁-M₄in accordance with control words provided to the switch 406A by the controllers in directors 180₁, 180₃, 180₅, 180₇on busses R_A1-R_A4, respectively, and the ports D₁-D₄are coupled to ports M₅-M₈in accordance with the control words provided to switch 406B by the controllers in directors 180₁, 180₃, 180₅, 180₇on busses R_B1-R_B4, as indicated. The signals on buses R_A1-R_A4are request signals. Thus, port 402A of any one of the directors 180₁, 180₃, 180₅, 180₇may be coupled to any one of the ports M₁-M₄of switch 406A, selectively in accordance with the request signals on buses R_A1-R_A4. Likewise, port 402B of any one of the directors 180₁-180₄may be coupled to any one of the ports M₅-M₈of switch 406B, selectively in accordance with the request signals on buses R_B1-R_B4. The coupling between the director boards 190₁-190₈, 210₁-210₈and the global cache memory 220 is shown in FIG. 8.

[0057] More particularly, and referring also to FIG. 2, as noted above, each one of the host computer processors 121₁-121₃₂in the host computer 120 is coupled to a pair of the front-end directors 180₁-180₃₂, to provide redundancy in the event of a failure in one of the front end-directors 181₁-181₃₂coupled thereto. Likewise, the bank of disk drives 140 has a plurality of, here 32, disk drives 141₁-141₃₂, each disk drive 141₁-141₃₂being coupled to a pair of the back-end directors 200₁-200₃₂, to provide redundancy in the event of a failure in one of the back-end directors 200₁-200₃₂coupled thereto). Thus, considering exemplary host computer processor 121₁, such processor 121₁is coupled to a pair of front-end directors 180₁, 180₂. Thus, if director 180₁fails, the host computer processor 121₁can still access the system interface 160, albeit by the other front-end director 180₂. Thus, directors 180₁and 180₂are considered redundancy pairs of directors. Likewise, other redundancy pairs of front-end directors are: front-end directors 180₃, 180₄; 180₅, 180₆; 180₇, 180₈; 180₉, 180₁₀; 180₁₁, 180₁₂; 180₁₃, 180₁₄; 180₁₅, 180₁₆; 180₁₇, 180₁₈; 180₁₉, 180₂₀; 180₂₁, 180₂₂; 180₂₃, 180₂₄; 180₂₅, 180₂₆; 180₂₇, 180₂₈; 180₂₉, 180₃₀; and 180₃₁, 180₃₂(only directors 180₃₁and 180₃₂being shown in FIG. 2).

[0058] Likewise, disk drive 141₁is coupled to a pair of back-end directors 200₁, 200₂. Thus, if director 200₁fails, the disk drive 141₁can still access the system interface 160, albeit by the other back-end director 180₂. Thus, directors 200₁and 200₂are considered redundancy pairs of directors. Likewise, other redundancy pairs of back-end directors are: back-end directors 200₃, 200₄; 200₅, 200₆; 200₇, 200₈; 200₉, 200₁₀; 200₁₁, 200₁₂; 200₁₃, 200₁₄; 200₁₅, 200₁₆; 200₁₇, 200₁₈; 200₁₉, 200₂₀; 200₂₁, 200₂₂; 200₂₃, 200₂₄; 200₂₅, 200₂₆; 200₂₇, 200₂₈; 200₂₉, 200₃₀; and 200₃₁, 200₃₂(only directors 200₃₁and 200₃₂being shown in FIG. 2). Further, referring also to FIG. 8, the global cache memory 220 includes a plurality of, here eight, cache memory boards 220₁-220₈, as shown. Still further, referring to FIG. 8A, an exemplary one of the cache memory boards, here board 220₁is shown in detail and is described in detail in U.S. Pat. No. 5,943,287 entitled “Fault Tolerant Memory System”, John K. Walton, inventor, issued Aug. 24, 1999 and assigned to the same assignee as the present invention, the entire subject matter therein being incorporated herein by reference. Thus, as shown in FIG. 8A, the board 220₁includes a plurality of, here four RAM memory arrays, each one of the arrays has a pair of redundant ports, i.e., an A port and a B port. The board itself has sixteen ports; a set of eight A ports M_A1-M_A8and a set of eight B ports M_B1-M_B8. Four of the eight A port, here A ports M_A1-M_A4are coupled to the M₁port of each of the front-end director boards 190₁, 190₃, 190₅, and 190₇, respectively, as indicated in FIG. 8. Four of the eight B port, here B ports M_B1-M_B4are coupled to the M₁port of each of the front-end director boards 190₂, 190₄, 190₆, and 190₈, respectively, as indicated in FIG. 8. The other four of the eight A port, here A ports M_A5-M_A8are coupled to the M₁port of each of the back-end director boards 210₁, 210₃, 210₅, and 210₇, respectively, as indicated in FIG. 8. The other four of the eight B port, here B ports M_B5-M₄₈are coupled to the M₁port of each of the back-end director boards 210₂, 210₄, 210₆, and 210₈, respectively, as indicated in FIG. 8.

[0059] Considering the exemplary four A ports M_A1-M_A4, each one of the four A ports M_A1-M_A4can be coupled to the A port of any one of the memory arrays through the logic network 221_1A. Thus, considering port M_A1, such port can be coupled to the A port of the four memory arrays. Likewise, considering the four A ports M_A5-M_A8, each one of the four A ports M_A5-M_A8can be coupled to the A port of any one of the memory arrays through the logic network 221_1B. Likewise, considering the four B ports M_B1-M_B4, each one of the four B ports M_B1-M_B4can be coupled to the B port of any one of the memory arrays through logic network 221_1B. Likewise, considering the four B ports M_B5-M_B8, each one of the four B ports M_B5-M_B8can be coupled to the B port of any one of the memory arrays through the logic network 221_2B. Thus, considering port M_B1, such port can be coupled to the B port of the four memory arrays. Thus, there are two paths data and control from either a front-end director 180₁-180₃₂or a back-end director 200₁-200₃₂can reach each one of the four memory arrays on the memory board. Thus, there are eight sets of redundant ports on a memory board, i.e., ports M_A1, M_B1; M_A2, M_B2; M_A3, M_B3; M_A4, M_B4; M_A5, M_B5; M_A6, M_B6; M_A7, M_B7; and M_A8, M_B8. Further, as noted above each one of the directors has a pair of redundant ports, i.e. a 402A port and a 402B port (FIG. 7). Thus, for each pair of redundant directors, the A port (i.e., port 402A) of one of the directors in the pair is connected to one of the pair of redundant memory ports and the B port (i.e., 402B) of the other one of the directors in such pair is connected to the other one of the pair of redundant memory ports.

[0060] More particularly, referring to FIG. 8B, an exemplary pair of redundant directors is shown, here, for example, front-end director 180₁and front end-director 180₂. It is first noted that the directors 180₁, 180₂in each redundant pair of directors must be on different director boards, here boards 190₁, 190₂, respectively. Thus, here front-end director boards 190₁-190₈have thereon: front-end directors 180₁, 180₃, 180₅and 180₇; front-end directors 180₂, 180₄, 180₆and 180₈; front end directors 180₉, 180₁₁, 180₁₃and 180₁₅; front end directors 180₁₀, 180₁₂, 180₁₄and 180₁₆; front-end directors 180₁₇, 180₁₉, 180₂₁and 180₂₃; front-end directors 180₁₈, 180₂₀, 180₂₂and 180₂₄; front-end directors 180₂₅, 180₂₇, 180₂₉and 180₃₁; front-end directors 180₁₈, 180₂₀, 180₂₂and 180₂₄. Thus, here back-end director boards 210₁-210₈have thereon: back-end directors 200₁, 200₃, 200₅and 200₇; back-end directors 200₂, 200₄, 200₆and 200₈; back-end directors 200₉, 200₁₁, 200₁₃and 200₁₅; back-end directors 200₁₀, 200₁₂, 200₁₄and 200₁₆; back-end directors 200₁₇, 200₁₉, 200₂₁and 200₂₃; back-end directors 200₁₈, 200₂₀, 200₂₂and 200₂₄; back-end directors 200₂₅, 200₂₇, 200₂₉and 200₃₁; back-end directors 200₁₈, 200₂₀, 200₂₂and 200₂₄;

[0061] Thus, here front-end director 180₁, shown in FIG. 8A, is on front-end director board 190₁and its redundant front-end director 180₂, shown in FIG. 8B, is on anther front-end director board, here for example, front-end director board 190₂. As described above, the port 402A of the quad port RAM 402 (i.e., the A port referred to above) is connected to switch 406A of crossbar switch 318 and the port 402B of the quad port RAM 402 (i.e., the B port referred to above) is connected to switch 406B of crossbar switch 318. Likewise, for redundant director 180₂, However, the ports M₁-M₄of switch 406A of director 180₁are connected to the M_A1ports of global cache memory boards 220₁-200₄, as shown, while for its redundancy director 180₂, the ports M₁-M₄of switch 406A are connected to the redundant M_B1ports of global cache memory boards 220₁-200₄, as shown.

[0062] Referring in more detail to the crossbar switch 318 (FIG. 7), as noted above, each one of the director boards 190₁-210₈has such a switch 318 and such switch 318 includes a pair of switches 406A, 406B. Each one of the switches 406A, 406B is identical in construction, an exemplary one thereof, here switch 406A being shown in detail in FIG. 8C. Thus switch 406A includes four input/output director-side ports D₁-D₄as described in connection with exemplary director board 190₁. Thus, for the director board 190₁shown in FIG. 7, the four input/output director-side ports D₁-D₄of switch 406A are each coupled to the port 402A of a corresponding one of the directors 180₁, 180₃, 180₅, and 180₇on the director board 190₁.

[0063] Referring again to FIG. 8C, the exemplary switch 406A includes a plurality of, here four, switch sections 430₁-430₄. Each one of the switch sections 430₁-430₄is identical in construction and is coupled between a corresponding one of the input/output director-side ports D₁-D₄and a corresponding one of the output/input memory-side ports M₁-M₄, respectively, as shown. (It should be understood that the output/input memory-side ports of switch 406B (FIG. 7) are designated as ports M₅-M₈, as shown. It should also be understood that while switch 406A is responsive to request signals on busses R_A1-R_A4from quad port controller 404 in directors 180₁, 180₃, 180₅, 180₇(FIG. 7), switch 406B is responsive in like manner to request signals on busses R_B1-R_B4from controller 404 in directors 180₁, 180₃, 180₅and 180₇). More particularly, controller 404 of director 180₁produces request signals on busses R_A1or R_B1. In like manner, controller 404 of director 180₃produces request signals on busses R_A2or R_B2, controller 404 of director 180₅produces request signals on busses R_A3or R_B3, and controller 404 of director 180₇produces request signals on busses R_A4or R_B4.

[0064] Considering exemplary switch section 430₁, such switch section 403₁is shown in FIG. 8C to include a FIFO 432 fed by the request signal on bus R_1A. (It should be understood that the FIFOs, not shown, in switch sections 430₂-430₄are fed by request signals R_A2-R_A4, respectively). The switch section 406₁also includes a request generation 434, and arbiter 436, and selectors 442 and 446, all arranged as shown. The data at the memory-side ports M₁-M₄are on busses DM1-DM4 are fed as inputs to selector 446. Also fed to selector 446 is a control signal produced by the request generator on bus 449 in response to the request signal R_A1stored in FIFO 432. The control signal on bus 449 indicates to the selector 446 the one of the memory-side ports M₁-M₄which is to be coupled to director-side port D₁. The other switch sections 430₂-430₄operate in like manner with regard to director-side ports D₁-D₄, respectively and the memory-side ports M₁-M₄.

[0065] It is to be noted that the data portion of the word at port D₁(i.e., the word on bus DD1) is also coupled to the other switch sections 4302-430₄. It is further noted that the data portion of the words at ports D₂-D₄(i.e., the words on busses DD2-DD4, respectively), are fed to the switch sections 430₁-430₄, as indicated. That is, each one of the switch sections 430₁-430₄has the data portion of the words on ports D₁-D₄(i.e., busses DD1-DD4), as indicated. It is also noted that the data portion of the word at port M₁(i.e., the word on bus DM1) is also coupled to the other switch sections 430₂-430₄. It if further noted that the data portion of the words at ports M₂-M₄(i.e., the words on busses DM2-DM4, respectively), are fed to the switch sections 430₂-430₄, as indicated. That is, each one of the switch sections 430₁-430₄has the data portion of the words on ports M₁-M₄(i.e., busses DM1-DM4), as indicated.

[0066] As will be described in more detail below, a request on bus R_A1to switch section 430₁is a request from the director 180₁which identifies the one of the four ports M₁-M₄in switch 430₁is to be coupled to port 402A of director 180₁(director side port D₁). Thus, port 402A of director 180₁may be coupled to one of the memory side ports M₁-M₄selectively in accordance with the data on bus R_A1. Likewise, a request on buses R_A2, R_A3, R_A4to switch section 430₂-430₄, respectively, are requests from the directors 180₃, 180₅, and 180₇, respectively, which identifies the one of the four ports M₁-M₄in switch 430₁-430₄is to be coupled to port 402A of directors 180₃, 180₅and 180₇, respectively.

[0067] More particularly, the requests R_A1are stored as they are produced by the quad port RAM controller 440 (FIG. 7) in receive FIFO 432. The request generator 434 receives from FIFO 432 the requests and determines which one of the four memory-side ports M₁-M₄is to be coupled to port 402A of director 180₁. These requests for memory-side ports M₁-M₄are produced on lines RA1,1-RA1,4, respectively. Thus, line RA1,1 (i.e., the request for memory side port M₁) is fed to arbiter 436 and the requests from switch sections 430₂-430₄(which are coupled to port 402A of directors 180₃, 180₅, and 180₇) on line RA2,1, RA3,1 and RA4,1, respectively are also fed to the arbiter 436, as indicated. The arbiter 436 resolves multiple requests for memory-side port M₁on a first come-first serve basis. The arbiter 436 then produces a control signal on bus 435 indicating the one of the directors 180₁, 180₃, 180₅or 180₇which is to be coupled to memory-side port M₁.

[0068] The control signal on bus 435 is fed to selector 442. Also fed to selector 442 are the data portion of the data at port D₁, i.e., the data on data bus DD1) along with the data portion of the data at ports D₂-D₄, i.e., the data on data busses DD2-DD4, respectively, as indicated. Thus, the control signal on bus 435 causes the selector 442 to couple to the output thereof the data busses DD1-DD4 from the one of the directors 180₁, 180₃, 180₅, 180₇being granted access to memory-side port M₁by the arbiter 436. The selected outputs of selector 442 is coupled to memory-side port M₁. It should be noted that when the arbiter 436 receives a request via the signals on lines RA1,1, RA2,1, RA3,1 and RA4,1, acknowledgements are returned by the arbiter 436 via acknowledgement signals on line AK1,1, AK1,2, AK1,3, AK1,4, respectively such signals being fed to the request generators 434 in switch section 430₁, 430₂, 430₃, 430₄, respectively.

[0069] Thus, the data on any port D₁-D₄can be coupled to and one of the ports M₁-M₄to effectuate the point-to-point data paths P₁-P₆₄described above in connection with FIG. 2.

[0070] Referring again to FIG. 7, data from host computer 120 (FIG. 2) is presented to the system interface 160 (FIG. 2) in batches from many host computer processors 121₁-121₃₂. Thus, the data from the host computer processors 121₁-121₃₂are interleaved with each other as they are presented to a director 180₁-180₃₂. The batch from each host computer processor 180₁-180₃₂(i.e., source) is tagged by the protocol translator 400. More particularly by a Tacheon ASIC in the case of a fibre channel connection. The controller 404 has a look-up table formed during initialization. As the data comes into the protocol translator 400 and is put into the quad port RAM 420 under the control of controller 404, the protocol translator 400 informs the controller that the data is in the quad port RAM 420. The controller 404 looks at the configuration of its look-up table to determine the global cache memory 220 location (e.g., cache memory board 220₁-220₈) the data is to be stored into. The controller 404 thus produces the request signals on the appropriate bus R_A1, R_B1, and then tells the quad port RAM 402 that there is a block of data at a particular location in the quad port RAM 402, move it to the particular location in the global cache memory 220. The crossbar switch 318 also takes a look at what other controllers 404 in the directors 180₃, 180₅, and 180₇on that particular director board 190₁are asking by making request signal on busses R_A2, R_B2, R_A3, R_B3, R_A4, R_B4, respectively. The arbitration of multiple requests is handled by the arbiter 436 as described above in connection with FIG. 8C.

[0071] Referring again to FIG. 7, the exemplary director 180₁is shown to include in the message engine/CPU controller 314. The message engine/CPU controller 314 is contained in a field programmable gate array (FPGA). The message engine (ME) 315 is coupled to the CPU bus 317 and the DMA section 408 as shown. The message engine (ME) 315 includes a Direct Memory Access (DMA) section 408, a message engine (ME) state machine 410, a transmit buffer 424 and receive buffer 424, a MAC packetizer/depacketizer 428, send and receive pointer registers 420, and a parity generator 321. The DMA section 408 includes a DMA transmitter 418, shown and to be described below in detail in connection with FIG. 9, and a DMA receiver 424, shown and to be described below in detail in connection with FIG. 10, each of which is coupled to the CPU bus interface 317, as shown in FIG. 7. The message engine (ME) 315 includes a transmit data buffer 422 coupled to the DMA transmitter 418, a receive data buffer 424 coupled to the DMA receiver 421, registers 420 coupled to the CPU bus 317 through an address decoder 401, the packetizer/de-packetizer 428, described above, coupled to the transmit data buffer 422, the receive data buffer 424 and the crossbar switch 320, as shown, and a parity generator 321 coupled between the transmit data buffer 422 and the crossbar switch 320. More particularly, the packetizer portion 428P is used to packetize the message payload into a MAC packet (FIG. 2B) passing from the transmit data buffer 422 to the crossbar switch 320 and the de-packetizer portion 428D is used to de-packetize the MAC packet into message payload data passing from the crossbar switch 320 to the receive data buffer 424. The packetization is here performed by a MAC core which builds a MAC packet and appends to each message such things as a source and destination address designation indicating the director sending and receiving the message and a cyclic redundancy check (CRC), as described above. The message engine (ME) 315 also includes: a receive write pointer 450, a receive read pointer 452; a send write pointer 454, and a send read pointer 456.

[0072] Referring now to FIGS. 11 and 12, the transmission of a message from a director 180₁-180₃₂, 200₁-200₃₂and the reception of a message by a director 210₁-210₃₂, here exemplary director 180₁shown in FIG. 7) will be described. Considering first transmission of a message, reference is made to FIGS. 7 and 11. First, as noted above, at power-up the controller 306 (FIG. 5) of both message network boards 304₁, 304₂initialize the message routing mapping described above for the switches 308₁-308₄in switch section 308 and for the crossbar switches 320. As noted above, a request is made by the host computer 120. The request is sent to the protocol translator 400. The protocol translator 400 sends the request to the microprocessor 299 via CPU bus 317 and buffer 301. When the CPU 310 (FIG. 7) in the microprocessor 299 of exemplary director 180₁determines that a message is to be sent to another one of the directors 180₂-180₃₂, 200₁-200₃₂, (e.g., the CPU 310 determines that there has been a “miss” in the global cache memory 220 (FIG. 2) and wants to send a message to the appropriate one of the back-end directors 200₁-200₃₂, as described above in connection with FIG. 2), the CPU 310 builds a 64 byte descriptor (FIG. 2A) which includes a 32 byte message payload indicating the addresses of the batch of data to be read from the bank of disk drives 140 (FIG. 2) (Step 500) and a 32 byte command field (Step 510) which indicates the message destination via an 8-byte bit vector, i.e., the director, or directors, which are to receive the message. An 8-byte portion of the command field indicates the director or directors, which are to receive the message. That is, each one of the 64 bits in the 8-byte portion corresponds to one of the 64 directors. Here, a logic 1 in a bit indicates that the corresponding director is to receive a message and a logic 0 indicates that such corresponding director is not to receive the message. Thus, if the 8-byte word has more than one logic 1 bit more than one director will receive the same message. As will be described, the same message will not be sent in parallel to all such directors but rather the same message will be sent sequentially to all such directors. In any event, the resulting 64-byte descriptor is generated by the CPU 310 (FIG. 7) (Step 512) is written into the RAM 312 (Step 514), as shown in FIG. 11.

[0073] More particularly, the RAM 512 includes a pair of queues; a send queue and a receive queue, as shown in FIG. 7. The RAM 312 is coupled to the CPU bus 317 through an Error Detection and Correction (EDAC)/Memory control section 303, as shown. The CPU 310 then indicates to the message engine (ME) 315 state machine 410 (FIG. 7) that a descriptor has been written into the RAM 312. It should be noted that the message engine (ME) 315 also includes: a receive write pointer or counter 450, the receive read pointer or counter 452, the send write pointer or counter 454, and the send read pointer or counter 454, shown in FIG. 7. All four pointers 450, 452, 454 and 456 are reset to zero on power-up. As is also noted above, the message engine/CPU controller 314 also includes: the de-packetizer portion 428D of packetizer/de-packetizer 428, coupled to the receive data buffer 424 (FIG. 7) and a packetizer portion 428P of the packetizer/de-packetizer 428, coupled to the transmit data buffer 422 (FIG. 7). Thus, referring again to FIG. 11, when the CPU 310 indicates that a descriptor has been written into the RAM 312 and is now ready to be sent, the CPU 310 increments the send write pointer and sends it to the send write pointer register 454 via the register decoder 401. Thus, the contents of the send write pointer register 454 indicates the number of messages in the send queue 312S of RAM 312, which have not been sent. The state machine 410 checks the send write pointer register 454 and the send read pointer register 456, Step 518. As noted above, both the send write pointer register 454 and the send read pointer register 456 are initially reset to zero during power-up. Thus, if the send read pointer register 456 and the send write pointer register 454 are different, the state machine knows that there is a message is in RAM 312 and that such message is ready for transmission. If a message is to be sent, the state machine 410 initiates a transfer of the stored 64-byte descriptor to the message engine (ME) 315 via the DMA transmitter 418, FIG. 7 (Steps 520, 522). The descriptor is sent from the send queues 312S in RAM 312 until the send read pointer 456 is equal to the send write pointer 454.

[0074] As described above in connection with Step 510, the CPU 310 generates a destination vector indicating the director, or directors, which are to receive the message. As also indicated above the command field is 32-bytes, eight bytes thereof having a bit representing a corresponding one of the 64 directors to receive the message. For example, referring to FIG. 11C, each of the bit positions 1-64 represents directors 180₁-180₃₂, 200₁-200₃₁, respectively. Here, in this example, because a logic 1 is only in bit position 1, the eight-byte vector indicates that the destination director is only front-end director 108₁. In the example in FIG. 11D, because a logic 1 is only in bit position 2, the eight-byte vector indicates that the destination director is only front-end director 108₂. In the example in FIG. 11E, because a logic 1 is more than one bit position, the destination for the message is to more than one director, i.e., a multi-cast message. In the example in FIG. 11E, a logic 1 is only in bit positions 2, 3, 63 and 64. Thus, the eight-byte vector indicates that the destination directors are only front-end director 108₂and 180₃and back-end directors 200₃₁and 200₃₂. There is a mask vector stored in a register of register section 420 (FIG. 7) in the message engine (ME) 315 which identifies director or directors which may be not available to use (e.g. a defective director or a director not in the system at that time), Step 524, 525, for a uni-cast transmission). If the message engine (ME) 315 state machine 410 indicates that the director is available by examining the transmit vector mask (FIG. 11F) stored in register 420, the message engine (ME) 315 encapsulates the message payload with a MAC header and CRC inside the packetizer portion 428P, discussed above (Step 526). An example of the mask is shown in FIG. 11F. The mask has 64 bit positions, one for each one of the directors. Thus, as with the destination vectors described above in connection with FIGS. 11C-11E, bit positions 1-64 represents directors 180₁-180₃₂, 200₁-200₃₂, respectively. Here in this example, a logic 1 in a bit position in the mask indicates that the representative director is available and a logic 0 in such bit position indicates that the representative director is not available. Here, in the example shown in FIG. 11F, only director 200₃₂is unavailable. Thus, if the message has a destination vector as indicated in FIG. 11E, the destination vector, after passing through the mask of FIG. 11F modifies the destination vector to that shown in FIG. 11G. Thus, director 200₃₂will not receive the message. Such mask modification to the destination vector is important because, as will be described, the messages on a multi-cast are sent sequentially and not in parallel. Thus, elimination of message transmission to an unavailable director or directors increases the message transmission efficiency of the system.

[0075] Having packetized the message into a MAC packet via the packetizer portion of the packetizer/de-packetizer 428 (FIG. 7), the message engine (ME) 315 transfers the MAC packet to the crossbar switch 320 (Step 528) and the MAC packet is routed to the destination by the message network 260 (Step 530) via message network boards 304₁, 304₂or on the same director board via the crossbar switch 320 on such director board.

[0076] Referring to FIG. 12, the message read operation is described. Thus, in Step 600 the director waits for a message. When a message is received, the message engine (ME) 315 state machine 410 receives the packet (Step 602). The state machine 410 checks the receive bit vector mask (FIG. 11 stored in register 426) against the source address of the packet (Step 604). If the state machine 410 determines that the message is from an improper source (i.e., a faulty director as indicated in the mask, FIG. 11F, for example), the packet is discarded (Step 606). On the other hand, if the state machine 410 determines that the packet is from a proper or valid director (i.e., source), the message engine (ME) 315 de-encapsulates the message from the packet (Step 608) in de-packetizer 428D. The state machine 410 in the message engine (ME) 315 initiates a 32-byte payload transfer via the DMA receive operation (Step 610). The DMA writes the 32 byte message to the memory receive queue 312R in the RAM 312 (Step 612). The message engine (ME) 315 state machine 410 then increments the receive write pointer register 450 (Step 614). The CPU 310 then checks whether the receive write pointer 450 is equal to the receive read pointer 452 (Step 616). If they are equal, such condition indicates to the CPU 310 that a message has not been received (Step 618). On the other hand, if the receive write pointer 450 and the receive read pointer 452 are not equal, such condition indicates to the CPU 310 that a message has been received and the CPU 310 processes the message in the receive queue 314R of RAM 312 and then the CPU 310 increments the receive read pointer and writes it into the receive read pointer register 452. Thus, messages are stored in the receive queue 312R of RAM 312 until the contents of the receive read pointer 452 and the contents of the receive write pointer 450, which are initialized to zero during power-up, are equal.

[0077] Referring now to FIG. 13, the acknowledgement of a message operation is described. In Step 700 the receive DMA engine 420 successfully completes a message transfer to the receive queue in RAM 312 (FIG. 7). The state machine 410 in the message engine (ME) 315 generates an acknowledgement MAC packet and transmits the MAC packet to the sending director via the message network 260 (FIG. 2) (Steps 702, 704). The message engine (ME) 315 at the sending director de-encapsulates a 16 byte status payload in the acknowledgement MAC packet and transfers such status payload via a receive DMA operation (Step 706). The DMA of the sending (i.e., source) director writes to a status field of the descriptor within the RAM memory send queue 314S (Step 708). The state machine 410 of the message engine (ME) 315 of the sending director (which received the acknowledgement message) increments its send read pointer 454 (Step 712). The CPU 310 of the sending director (which received the acknowledgement message) processes the descriptor status and removes the descriptor from the send queue 312S of RAM 312 (Step 714). It should be noted that the send and receive queues 312S and 312R are each circular queues.

[0078] As noted above, the MAC packets are, during normal operation, transmitted alternatively to one of the pair of message network boards 304₁, 304₂by hardware a selector S in the crossbar switch 320. The selector S is responsive to the bit B in the header of the MAC packet (FIG. 2B) and, when such bit B is one logic state the data is coupled to one of the message networks boards 402A and in response to the opposite logic state the data is coupled to the other one of the message networks boards 402B. That is, when one message is transmitted to board 304₁the next message is transmitted to board 304₂.

[0079] Referring again to FIG. 9, the details of an exemplary transmit DMA 418 is shown. As noted above, after a descriptor has been created by the CPU 310 (FIG. 7) and is then stored in the RAM 312. If the send write pointer 450 (FIG. 7) and send read pointer 452, described above, have different counts an indication is provided by the state machine 410 in the message engine (ME) 315 (FIG. 7) that the created descriptor is available for DMA transmission to the message engine (ME) 315, the payload off the descriptor is packetized into a MAC packet and sent through the message network 360 (FIG. 2) to one or more directors 180₁-180₃₂, 200₁-200₃₂. More particularly, the descriptor created by the CPU 310 is first stored in the local cache memory 319 and is later transferred to the send queue 312S in RAM 312. When the send write pointer 450 and send read pointer 452 have different counts, the message engine (ME) 315 state machine 410 initiates a DMA transmission as discussed above in connection with Step 520 (FIG. 11). Further, as noted above, the descriptor resides in send queues 312R within the RAM 312. Further, as noted above, each descriptor which contains the message is a fixed size, here 64-bytes. As each new, non-transmitted descriptor is created by the CPU 310, it is sequentially stored in a sequential location, or address in the send queue 312S. Here, the address is a 32-bit address.

[0080] When the transmit DMA is initiated, the state machine 410 in the message engine (ME) 315 (FIG. 7), sends the queue address on bus 411 to an address register 413 in the DMA transmitter 418 (FIG. 9) along with a transmit write enable signal Tx_WE signal. The DMA transmitter 418 requests the CPU bus 317 by asserting a signal on Xmit_Br. The CPU bus arbiter 414 (FIG. 7) performs a bus arbitration and when appropriate the arbiter 414 grants the DMA transmitter 418 access to the CPU bus 317. The Xmit Cpu state machine 419 then places the address currently available in the address register 413 on the Address bus portion 317A of CPU bus 317 by loading the output address register 403. Odd parity is generated by a Parity generator 405 before loading the output address register 403. The address in register 403 is placed on the CPU bus 317 (FIG. 7) for RAM 312 send queue 312S, along with appropriate read control signals via CPU bus 317 portion 317C. The data at the address from the RAM 312 passes, via the data bus portion 317D of CPU bus 317, through a parity checker 415 to a data input register 417. The control signals from the CPU 310 are fed to a Xmit CPU state machine 419 via CPU bus 317 bus portion 317C. One of the control signals indicates whether the most recent copy of the requested descriptor is in the send queue 312S of the RAM 312 or still resident in the local cache memory 319. That is, the most recent descriptor at any given address is first formed by the CPU 310 in the local cache memory 319 and is later transferred by the CPU 310 to the queue in the RAM 312. Thus, there may be two descriptors with the same address; one in the RAM 312 and one in the local cache memory 319 (FIG. 7), the most recent one being in the local cache memory 319. In either event, the transmit DMA 418 must obtain the descriptor for DMA transmission from the RAM 312 and this descriptor is stored in the transmit buffer register 421 using signal 402 produced by the state machine 419 to load these registers 421. The control signal from the CPU 310 to the Xmit CPU state machine 419 indicates whether the most recent descriptor is in the local cache memory 319. If the most recent descriptor is in the local cache memory 319, the Xmit CPU state machine 419 inhibits the data that was just read from send queue 312S in the RAM 312 and which has been stored in register 421 from passing to selector 423. In such case, state machine 419 must perform another data transfer at the same address location. The most recent message is then transferred by the CPU 310 from the local cache memory 319 to the send queue 312S in the RAM 312. The transmit message state machine 419 then re-arbitrates for the CPU bus 317 and after it is granted such CPU bus 317, the Xmit CPU state machine 419 then reads the descriptor from the RAM 312. This time, however, there the most recent descriptor is available in the send queue 312s in the RAM 312. The descriptor in the RAM 312 is now loaded into the transmit buffer register 421 in response to the assertion of the signal 402 by the Xmit CPU state machine 419. The descriptor in the register 421 is then transferred through selector 423 to message bus interface 409 under the control of a Xmit message (msg) state machine 427. That is, the descriptor in the transmit buffer register 421 is transferred to the transmit data buffer 422 (FIG. 7) over the 32 bit transmit message bus interface 409 by the Xmit message (msg) state machine 427. The data in the transmit data buffer 422 (FIG. 7) is packetized by the packetizer section of the packetizer/de-packetizer 428 as described in Step 530 in FIG. 11.

[0081] More particularly, and referring also to FIG. 14A, the method of operating the transmit DMA 418 (FIG. 9) is shown. As noted above, each descriptor is 64-byte. Here, the transfer of the descriptor takes place over two interfaces namely, the CPU bus 317 and the transmit message interface bus 409 (FIG. 7). The CPU bus 317 is 64 bits wide and eight, 64-bit double-words constitute a 64-byte descriptor. The Xmit CPU state machine 419 generates the control signals which result in the transfer of the descriptor from the RAM 312 into the transmit buffer register 421 (FIG. 7). The 64-byte descriptor is transferred in two 32-byte burst accesses on the CPU bus 317. Each one of the eight double words is stored sequentially in the transmit buffer register 421 (FIG. 9). Thus, in Step 800, the message engine 315 state machine 410 loads the transmit DMA address register 413 with the address of the descriptor to be transmitted in the send queue 312S in RAM 312. This is done by the asserting the Tx_WE signal and this puts Xmit CPU state machine 419 in step 800, loads the address register 413 and proceeds to step 802. In step 802, The Xmit Cpu state machine 419 loads the CPU transfer counter 431 (FIG. 9) with a 32-byte count, which is 2. This is the number of 32 byte transfers that would be required to transfer the 64-byte descriptor, Step 802. The Xmit Cpu state machine 419 now proceeds to Step 804. In step 804, the transmit DMA state machine 419 checks the validity of the address that is loaded into its address register 413. The address loaded into the address register 413 is checked against the values loaded into the memory address registers 435. The memory address registers 435 contain the base address and the offset of the send queue 312s in the RAM 312. The sum of the base address and the offset is the range of addresses for the send queue 312S in RAM 312. The address check circuitry 437 constantly checks whether the address in the address register 413 is with in the range of the send queue 312S in the RAM 312. If the address is found to be outside the range of the send queue 312S the transfer is aborted, this status is stored in the status register 404 and then passed back to the message engine 315 state machine 410 in Step 416. The check for valid addresses is done in Step 805. If the address is within the range, i.e., valid, the transmit DMA state machine 419 proceeds with the transfer and proceeds to Step 806. In the step 806, the transmit DMA state machine 419 requests the CPU bus 317 by asserting the Xmit_BR signal to the arbiter 414 and then proceeds to Step 807. In Step 807, the Xmit Cpu state machine 419 constantly checks if it has been granted the bus by the arbiter. When the CPU bus 317 is granted, the Xmit CPU state machine proceeds to Step 808. In Step 808, the Xmit Cpu state machine 419 generates an address and a data cycle which essentially reads 32-bytes of the descriptor from the send queue 312S in the RAM 312 into its transmit buffer register 421. The Xmit Cpu state machine 419 now proceeds to step 810. In Step 810, the Xmit Cpu state machine 419 loads the descriptor that was read into its buffer registers 421 and proceeds to Step 811. In Step 811, a check is made for any local cache memory 319 coherency errors (i.e., checks whether the most recent data is in the cache memory 319 and not in the RAM 312) on these 32-bytes of data. If this data is detected to be resident in the local CPU cache memory 319, then the Xmit Cpu state machine 419 discards this data and proceeds to Step 806. The Xmit Cpu state machine 419 now requests for the CPU bus 317 again and when granted, transfers another 32-bytes of data into the transmit buffer register 421, by which time the CPU has already transferred the latest copy of the descriptor into the RAM 312. In cases when the 32-bytes of the descriptor initially fetched from the RAM 312 was not resident in the local CPU cache memory 319 (i.e., if no cache coherency errors were detected), the Xmit Cpu state machine 419 proceeds to Step 812. In Step 812, the Xmit CPU state machine 419 decrements counters 431 and increments the address register 413 so that such address register 413 points to the next address. The Xmit Cpu state machine then proceeds to step 814. When in Step 814, the Transmit CPU state machine 419 checks to see if the transfer counter 431 has expired, i.e., counted to zero, if the count was found to be non-zero, it then, proceeds to Step 804 to start the transfer of the next 32-bytes of the descriptor. In case the counter 431 is zero, the process goes to Step 816 to complete the transfer. The successful transfer of the second 32-bytes of descriptor from the RAM 312 into the transmit DMA buffer register 421 completes the transfer over the CPU bus 317.

[0082] The message interface 409 is 32 bits wide and sixteen, 32 bit words constitute a 64-byte descriptor. The 64-byte descriptor is transferred in batches of 32 bytes each. The Xmit msg state machine 427 controls and manages the interface 409. The Xmit Cpu state machine asserts the signal 433 to indicate that the first 32 bytes have been successfully transferred over the CPU bus 317 (Step 818, FIG. 14B), this puts the Xmit msg state machine into Step 818 and starts the transfer on the message interface. In step 820, the Xmit msg machine 427 resets burst/transfer counters 439 and initiates the transfer over the message interface 409. In Step 820, the transfer is initiated over the message interface 409 by asserting the “transfer valid” (TX_DATA_Vaild) signal indicating to the message engine 315 state machine 410 that valid data is available on the bus 409. The transmit msg machine 427 transfers 32 bits of data on every subsequent clock until its burst counter in burst/transfer counter 439 reaches a value equal to eight, Step 822. The burst counter in burst/transfer counter 439 is incremented with each 32-bit word put on the message bus 409 by a signal on line 433. When the burst count is eight, a check is made by the state machine 427 as to whether the transmit counter 431 has expired, i.e., is zero, Step 824. The expiry of the transfer counter in burst/transfer counter 439 indicates the 64 byte descriptor has been transferred to the transmit buffer 422 in message engine 315. If it has expired, the transmit message state machine 427 proceeds to Step 826. In step 826, the Xmit msg state machine asserts the output End of Transfer (Tx_EOT) indicating the end of transfer over the message bus 409 In this state, after the assertion of the Tx_EOT signal the status of the transfer captured in the status register 404 is sent to the message engine 315 state machine 410. The DMA operation is complete with the descriptor being stored in the transmit buffer 422 (FIG. 7).

[0083] On the other hand, if the transfer counter in burst/transfer counter 439 has not expired, the process goes to Step 800 and repeats the above described procedure to transfer the 2^nd32 bytes of descriptor data, at which time the transfer will be complete.

[0084] Referring now to FIG. 10, the receive DMA 420 is shown. Here, a message received from another director is to be written into the RAM 312 (FIG. 7). The receive DMA 420 is adapted to handle three types of information: error information which is 8 bytes in size; acknowledgement information which is 16 bytes in size; and receive message payload and/or fabric management information which is 32 byes in size. Referring also to FIG. 7, the message engine 315 state machine 410 asserts the Rx_WE signal, indicating to the Receive DMA 420 that it is ready transfer the Data in its Rec buffer 426 FIG. 7. The data in the Receive buffer could be the 8-byte error information, the 16-byte Acknowledgment information or the 32-byte Fabric management/Receive message payload information. It places a 2 bit encoded receive transfer count, on the Rx_transfer count signal indicating the type of information and an address which is the address where this information is to be stored in the receive queue of RAM 312. In response to the receive write enable signal Rx_WE, the Receive message machine 450 (FIG. 10) loads the address into the address register 452 and the transfer count indicating the type of information, into the receive transfer counter 454. The address loaded into the address register 452 is checked by the address check circuitry 456 to see if it is with in the range of the Receive queue addresses, in the RAM 312. This is done by checking the address against the values loaded into the memory registers 457 (i.e., a base address register and an offset register therein). The base address register contains the start address of the receive queue 312R residing in the RAM 312 and the offset register contains the size of this receive queue 312R in RAM 312. Therefore the additive sum of, the values stored in the base address register and the offset register specifies the range of addresses of the receive queue in the RAM 312R. The memory registers 457 are loaded during initialization. On the subsequent clock after the assertion of the Rx_WE signal, the message engine 315 state machine 410 the proceeds to place the data on a 32-bit message engine 315 data bus 407, FIG. 10. A Rx_data_valid signal accompanies each 32 bits of data, indicating that the data on the message engine data bus 407 is valid. In response to this Rx_data_valid signal the receive message state machine 450 loads the data on the data bus into the receive buffer register 460. The end of the transfer over the message engine data bus. 407dis indicated by the assertion of the Rx_EOT signal at which time the Receive message state machine 450 loads the last 32 bits of data on the message engine data bus 407D of bus 407, into the receive buffer registers 460. This signals the end of the transfer over the message engine data bus 407D portion of bus 407. At the end of such transfer is conveyed to the Rx_Cpu state machine 462 by the assertion of the signal 464. The Receive CPU machine 462 now, requests for the CPU bus 317 by asserting the signal REC_Br. After an arbitration by CPU bus arbiter 414 (FIG. 7) the receive DMA 420 (FIG. 10) is given access to the CPU bus 317. The Receive CPU state machine 462 proceeds to transfer the data in its buffer registers 424 over the CPU bus 317 into the Receive queue 312R in the RAM 312. Simultaneously, this data is also transferred into a duplicate buffer register 466. The data at the output of the receive buffer register 460 passes to one input of a selector 470 and also passes to a duplicate data receive-buffer register 460. The output of the duplicate receive buffer register 466 is fed to a second input of the selector 470. As the data is being transferred by the Receive CPU state machine 462, it is also checked for cache coherency errors. If the data corresponding to the address being written into the RAM 312, is located in the CPU's local cache memory 319 (FIG. 7), the receive DMA machine 420 waits for the CPU 310 to copy the old data in its local cache memory 319 back to the receive queue 312R in the RAM 312 and then overwrites this old data with a copy of the new data from the duplicate buffer register 466.

[0085] More particularly, if central processing unit 310 indicates to the DMA receiver 420 that the data the receive buffer register 460 is available in the local cache memory 319, the receive CPU state machine 462 produces a select signal on line 463 which couples the data in the duplicate buffer register 466 to the output of selector 470 and then to the bus 317 for store in the random access memory 312.

[0086] The successful write into the RAM 312 completes the DMA transfer. The receive DMA 420 then signals the message engine 315 state machine 410 on the status of the transfer. The status of the transfer is captured in the status register 459.

[0087] Thus, with both the receive DMA and the transmit DMA, there is a checking of the local cache memory 319 to determine whether it has “old” data, in the case of the receive DMA or whether it has “new data” in the case of the transmit DMA.

[0088] Referring now to FIG. 15A, the operation of the receive DMA 420 is shown. Thus, in Step 830 the Receive message machine 450 checks if the write enable signal Rx_WE is asserted. If found asserted, the receive DMA 420 proceeds to load the address register 452 and the transfer counter 454. The value loaded into the transfer counter 454 determines the type of DMA transfer requested by the Message engine state machine 310 in FIG. 7. The assertion of the Rx_WE signal starts the DMA receive transfer operation. This puts the Rx msg state machine 450 in Step 832. In Step 832 the Rec msg state machine 450 loads the address register 452, the transfer counter 454 and then proceeds to Step 834. In Step 834, it checks to see if the Rx_DATA_VALID signal is asserted. If asserted it proceeds to step 836. The Rx msg state machine loads the buffer register 460 (FIG. 10) in Step 836 with the data on the message engine data bus 407D of bus 407 FIG. 10. The Rx_DATA_VALID signal accompanies each piece of data put on the bus 407. The data is sequentially loaded into the buffer registers 460 (FIG. 10). The End of the transfer on the message engine data bus 407D of bus 407 is indicated by the assertion of the Rx_EOT signal. When the Receive message state machine 450 is in the End of transfer state Step 840 it signals the Receive CPU state machine 462 and this starts the transfer on the CPU bus 317 side.

[0089] The flow for the Receive CPU state machine is explained below. Thus, referring to FIG. 15B, the End of the transfer on the Message engine data bus 407D portion of bus 407 starts the Receive CPU state machine 462 and puts it in Step 842. The Receive CPU state machine 462 checks for validity of the address in this state (Step 844). This is done by the address check circuitry 456. If the address loaded in the address register 452 is outside the range of the receive queue 312R in the RAM 312, the transfer is aborted and the status is captured in the Receive status register 459 and the Rec Cpu state machine 462 proceeds to Step 845. On a valid address the Receive CPU state machine 462 goes to Step 846. In Step 846 the Receive Cpu state machine 462 requests for access of the CPU bus 317. It then proceeds to Step 848. In step 848 it checks for a grant on the bus 317. On a qualified grant it proceeds to step 850. In Step 850, The Rec Cpu state machine 462 performs an address and a data cycle, which essentially writes the data in the buffer registers 460 into the receive queue 312R in RAM 312. Simultaneously with the write to the RAM 312, the data put on the CPU bus 317 is also loaded into the duplicate buffer register 466. At same time, the CPU 310 also indicates on one of the control lines, if the data corresponding to the address written to in the RAM 312 is available in its local cache memory 319. At the end of the address and data cycle the Rec Cpu state machine 462 proceeds to Step 850. In this step it checks for cache coherency errors of the type described above in connection with the transmit DMA 418 (FIG. 9). If cache coherency error is detected and the receive CPU state machine 462 proceeds to Step 846 and retries the transaction more particularly, the Receive CPU state machine 462 now generates another address and data cycle to the previous address and this time the data from the duplicate buffer 466 is put on to the CPU data bus 317. If there were no cache coherency errors the Receive CPU state machine 462 proceeds to Step 852 where it decrements the transfer counter 454 and increment the address in the address register 452. The Receive Cpu state machine 462 then proceeds to Step 854. In Step 854, the state machine 462 checks if the transfer counter has expired, i.e., is zero. On a non zero transfer count the receive Cpu state machine 462 proceeds to Step 844 and repeats the above described procedure until the transfer becomes zero. A zero transfer count when in step 854 completes the write into the receive queue 312R in RAM 312 and the Rec Cpu state machine proceeds to 845. In step 845, it conveys status stored in the status register back to status is conveyed to the message engine 315 state machine 410.

[0090] Referring again to FIG. 7, the interrupt control status register 412 will be described in more detail. As described above, a packet is sent by the pocketsize portion of the packetizer/de-packetizer 428 to the crossbar switch 320 for transmission to one or more of the directors. It is to be noted that the packet sent by the packetizer portion of the packetizer/de-packetizer 428 passes through a parity generator PG in the message engine 315 prior to passing to the crossbar switch 320. When such packet is sent by the message engine 315 in exemplary director 180₁, to the crossbar switch 320, a parity bit is added to the packet by parity bit generator PG prior to passing to the crossbar switch 320. The parity of the packet is checked in the parity checker portion of a parity checker/generator (PG/C) in the crossbar switch 320. The result of the check is sent by the PG/C in the crossbar switch 320 to the interrupt control status register 412 in the director 180₁.

[0091] Likewise, when a packet is transmitted from the crossbar switch 320 to the message engine 315 of exemplary director 180₁, the packet passes through a parity generator portion of the parity checker/generator (PG/C) in the crossbar switch 320 prior to being transmitted to the message engine 315 in director 180₁. The parity of the packet is then checked in the parity checker portion of the parity checker (PC) in director 180₁and is the result (i.e., status) is transmitted to the status register 412.

[0092] A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.