APPARATUS AND METHOD FOR SERVICING INTERRUPTS UTILIZING A PENDED BUS

Abstract: Apparatus and method for servicing interrupt requests on a pended bus. An interrupt servicing node provides interrupt acknowledge messages including destination data specifying a particular interrupting node at times when the interrupt servicing node is ready to service an interrupt request message. The interrupt servicing node includes storage means for specifying the identity of a particular interrupting node and for indicating that an interrupt request is pending from a particular interrupting node. An interrupting node provides to the pended bus an interrupt request message including ID data for identifying the interrupting node as the source of an interrupt request. The interrupting node detects whether an interrupt acknowledge message on the bus includes destination data specifying that interrupting node.

Description: APPARATUS AND METHOD FOR SERVICING INTERRUPTS UTILIZING A PENDED BUS FIELD OF THE INVENTION This invention relates to the servicing of interrupts over a pended bus for transferring messages between multiple nodes in a computer system. BACKGROUND OF THE INVENTION Modern computer systems have the capability of utilizing a number of resources that must be able to communicate with each other over a data path. A bus is provided so thac a number of system resources can transfer data among themselves. Typical resources included in a computer system are processors, memory devices, and input/output (I/O) devices. I/O devices are usually the means used in computer systems to interface the ele¬ ments in the system with sources and destinations outside the computer system. An interrupt is the means by which an I/O device draws- the attention of the system so that the system can attend to the needs of the I/O device. In many conventional systems, an I/O device generates a special "interrupt request" signal which is sent by way of dedicated control lines. Alternatively, in a message-based interrupt scheme, an I/O device sends interrupt request messages to the system over the system bus. Thus, in message-based systems, no separate and distinct status and control lines are needed. The rules for creation and interpretation of messages, such as interrupt re¬ quest messages, are referred to as the "protocol." According to some protocols for servicing an interrupt request, the resource servicing the interrupt will solicit addi¬ tional information from the I/O device. One way to obtain such additional information is to provide an interrupt acknowledge message. In response o the interrupt acknowledge, the I/O device returns an interrupt vector containing information par¬ ticular to the requested interrup . The interrupt vector, for example, can provide information about the starting address of a routine to service the interruptinς I/O device. In any system in which several I/O devices transmit interrupt requests, or in which one I/O device transmits several interrupt requests, some method must be provided for arbitrating among the requests. For example, either the interrupting devices or the individual interrupt requests can be assigned in¬ terrupt priorities which the system can use to aid arbitration. The concept and some of the benefits of utilizing a "pended" bus are described below in the System Overview section. Ideally, to gain the advantages of both message-based interrupt schemes and pended buses, it would be desirable to combine these two approaches. That combination, however, is difficult to achieve for several reasons. One source of difficulty is that,, when using a pended bus, there is a need for additional informa¬ tion for routing messages between the interrupting node and the interrupt servicing node. Furthermore, the parallel activity in a pended bus, which is inherent because interrupt requests and the servicing of those requests occur simultaneously and asychronously, makes it difficult to establish coherent or up-to-date status information on pending interrupt requests. Adding to these problems is the complexity involved in maintaining interrupt pending status information for all the in¬ terrupting nodes at all the different priority levels. This is less difficult if dedicated control wires are used since status -could be determined by examination of those wires. However, as the number of processors and other nodes ori a bus increases, the number of dedicated control lines must increase, rendering the use of a dedicated control line system impractical for large multiprocessor systems. The use of a message-based interrupt system, however, while eliminating this problem, creates other problems in main¬ taining pending status information. For example, in a message-based system using a pended bus, it is possible that an interrupting node may specify several interrupt servicing nodes. This introduces an additional level of difficulty in finding a solution to the problem of preventing several interrupt servicing nodes from servicing the same request. Thus, a message-based interrupt scheme on a pended bus requires some mechanism so that interrupt servicing nodes can detect actions taken bv other interrupt servicinσ nodes. A related problem, when messages are used to provide interrupt requests, is how an interrupt servicing node can dif¬ ferentiate between outstanding interrupt requests from different interrupting nodes. Otherwise, there would be contention among several interrupting nodes when an interrupt acknowledge message is sent, and the losing interrupting nodes would be forced to reissue their interrupt requests. Accordingly, it is an object of the present invention to provide a message-based interrupt scheme on a pended bus. It is a further object of the invention to support distributed interrupt servicing in a multiprocessor system on a pended bus, and to provide, simultaneously with distributed in¬ terrupt servicing, direct interrupt servicing between a particu¬ lar interrupting node and a particular interrupt servicing node. It is another object of the present invention to pro¬ vide a message-based interrupt scheme which functions with a pended bus interfacing multiple interrupting devices. It is an additional object of the invention to main¬ tain up-to-date status information on interrupt requests and to send only a single interrupt acknowledge message in response to each request. It is a still further object of the invention to elim¬ inate the need for interrupting devices to reissue interrupt re¬ quests for a single interrupting event. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instru¬ mentalities and combinations particularly pointed out in the appended claims. SUMMARY OF THE INVENTION To achieve the objects and in accordance with the pur¬ poses of the invention, as 'embodied and broadly described here¬ in, a method is described for performing an interrupt function on a pended bus for transferring messages between an inter¬ rupting node and an interrupt servicing node. The method ccm- Drises the steos of: orovidinc to the bus from the interruotinc node an interrupt request message including ID data for identi¬ fying the interrupting node; providing to the bus from the in¬ terrupt servicing node an interrupt acknowledge message including destination data specifying the interrupting node at times when the interrupt servicing node is ready to service the interrupt request message; monitoring interrupt acknowledge mes¬ sages on the bus at the interrupting node to detect the inter¬ rupt acknowledge message including destination data specifying the interrupting node; and providing to the bus from the inter¬ rupting node, in response to the detection of the interrupt acknowledge message including destination data .specifying to the interrupting node, an interrupt vector message. Additional objects in accordance with the purposes of the invention are achieved by providing a system for servicing an interrupt request, comprising: a pended bus for transferring messageJ, including interrupt request, interrupt acknowledge, and interrupt, vector messages; an interrupting node coupled to ' the bus and including: first means for providing to the bus an interrupt request message including interrupting node ID data for identifying the interrupting node; second means for moni¬ toring interrupt acknowledge messages on the bus to detect an interrupt acknowledge message including destination data speci¬ fying the interrupting node; and third means for providing to the bus, in response to the detection of the interrupt acknowl¬ edge message including the destination data specifying the in¬ terrupting " node, an interrupt vector message; and an inter-upt servicing node coupled to the bus for providing the interrupt acknowledge message at times when the interrupt servicing node is ready to service the interrupt request message, including: fourth means for detecting the interrupt request message on the bus provided by the interrupting node; fifth eans for indicating that an interrupt request is pending from the inter¬ rupting node; sixth means for providing to the bus the interrupt acknowledge message including destination data specifying the interrupting node; and seventh means for detecting the interrupt vector message on the bus provided by the interrupting node. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention, and, together with the description, serve to explain the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a data processing systen including a system bus using the present invention; Fig. 2 is a block diagram of a node in the data pro¬ cessing system of Fig. 1; Fig. 3 is a timing diagram showing timing signals used in the data processing system of Fig. 1; Fig. 4 is a block diagram of the data interface in the node of Fig. 2; Fig. 5 is a block diagram of the arbiter in the data processing system of Fig. 1; Fig. 6 is a block diagram of an interrupting node; Fig. 7 is a block diagram of an interrupt servicing node; ' Fig. 8 ' is a block diagram of storage elements included in the interrupt servicing node; and Figs. 9A and 9B are a flowchart of a method for servicing interrupt requests. DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. A. System Overview Fig. 1 shows an example of a data processing system : which embodies the present invention. The heart of system 20 • a system bus 25 which is a synchronous bus that allows communi cation between several processors, memory subsystems, and I/O systems. Communications over system bus 25 occur synchronousl using periodic bus cycles. A typical bus cycle time for syster. bus 25 is 64 nsec. In Fig. i, system bus 25 is coupled to two processors 31 and 35, a memory 39, one I/O interface 41 and one I/O unit 51. I/O unit 53, is coupled to system bus 25 by way of I/ bus 45 and I/O unit interface 41. A central ar iter 28 is also connected to system bus 25 in the preferred embodiment of data processing system 22 . Arbiter 28 provides certain timing and bus arbitration signals directly to the other devices on system bus 25 and shares some signals with those devices. The implementation shown in Fig. 1 is one which is presently preferred and should not necessarily be interpreted as limiting the present invention. For example, I/O unit 53 could be coupled directly to system bus 25, and arbiter 28 need not operate in the manner described for the present invention. In the nomenclature used to describe the present invention, processors 31 and 33, memory 39, and I/O interface 41, and I/O device 51 are all called nodes. A "node" is defined as a hardware device which connects to system bus 25. According to the nomenclature used to describe the present invention, the terms "signals" or "lines" are used in¬ terchangeably to refer to the names of the physical wires. The terms "data" or "levels" are used to refer to the values which the signals or lines can assume. Nodes perform transfers with other nodes over system bus 25. A "transfer" is one or more contiguous cycles that share a common transmitter and common arbitration. For example, a read operation initiated by one node to obtain information from another node on system bus 25 requires a command transfer from the first to the second node followed by one or more return data transfers from the second node to the first node at some later time. A "transaction" is defined as the complete logical task being performed on system bus 25 and can include more than one transfer. For example, a read operation consisting of a command transfer followed later by one or more return data transfers is one transaction. In the preferred embodiment of system bus 25, the permissible transactions support the transfer of different data lengths and include read, write (masked), interlock read, unlock write, nd interrupt operations. The difference between an interlock read and a regular or noninterlock read is that an interlock read to a specific loca¬ tion retrieves information stored at that location and restricts access to the stored information by subsequent interlock read comraands. Access restriction is oerfcr ed bv settin< mechanism. A subsequent unlock write command stores informati in the specified location and restores access to the stored in formation by resetting the lock mechanism at that location. Thus, the Interlock read/unlock write operations are a form of read-modify-write operation. Since system bus 25 is a "pended" bus, it fosters efficient use of bus resources by allowing other nodes to use bus cycles which otherwise would have been wasted waiting for responses. In a pended bus, after one node initiates a transa tion, other nodes can have access to the bus before that trans action is complete. Thus, the node initiating that transactio does not tie up the bus for the entire transaction time. This contrasts with a non-pended bus in which the bus is tied up fo an entire transaction. For example in system bus 25, after a node initiates a read transaction and makes a command transfer, the node to which that command transfer is directed may not be able to return the requested data immediately. Cycles en bus would then be available between the command transfer and the r turn data transfer of the read transaction. System bus 25 allows other nodes to use those cycles. In using system bus 25, each of the nodes can assume different roles in order to effect the transfer of information. One of those roles is a "commander" whic ι is defined as a node which has initiated a transaction currently ' in progress. For example, in a write or read operation, the commander is the no that requested the write or read operation; it is not necessari ly the node that sends or receives the data. In the preferred protocol for system bus 25, a node remains as the commander throughout an entire transaction even though another node may take ownership of the system bus 25 during certain cycles of th transaction. For example, although one node has control of sys tem bus 25 during the transfer of data in response to the command transfer of a read transaction, that one node does not become the commander of the bus. Instead, this node is called "responder. " A responder responds to the commander. For example, if a commander initiated a write operation to '-rite data from node A to node 3, node 3 would be the responder. In addition, in data processing system 20 a node can simultaneously be a com¬ mander and a responder. Transmitters and receivers are roles which the nodes assume in an individual transfer. A "transmitter" is defined as a node which is the source of information placed on system bus 25 during a transfer. A "receiver" is the complement of the transmitter and is defined as the node which receives the infor¬ mation placed on system bus 25 during a transfer. During a read transaction, for example, a commander can first be a transmitter • during the command transfer and then a receiver during the re¬ turn data transfer. When a node connected to system bus 25 desires to become a transmitter on system bus 25, that node asserts one of two request lines, CMD REQ (commander request) and RES REQ (re¬ sponder request) , which are connected between central arbiter 28 and that particular node. In general, a node uses its CMD REQ line to request to become comman er and initiate transactions on, system bus 25, and a node uses its RES REQ line to become a re¬ sponder to return data or message to a commander. Generally, central arbiter 28 detects which nodes desire access to the bus (i.e., which request lines are asserted). The arbiter then re¬ sponds to one of the asserted request lines to grant the corre¬ sponding node access to bus 25 according ;o a priority algorithm. In the preferred embodiment, arbiter 28 maintains two independent, circular queues: one for the commander re¬ quests and one for the responder requests. Preferably, the re¬ sponder requests have a higher priority than the commander re¬ quests and are handled before the commander requests. The commander request lines and responder request lines are considered to be arbitration signals. As illustrated in Fig. 1, arbitration signals also include point-to-point con¬ ditional grant signals from central arbiter 23 to each node r system bus extend signals to implement multi-bus cycle trans- f s, and system bus suppression signals to control the i -tiation of new bus transactions when, for example, a node .e such as memory is momentarily unable to keep up with traf- on the svs em bus. Other types of signals which can constitute system bus 25 include information transfer signals, respond signals, con¬ trol signals, console/front panel signals, and a few miscella¬ neous signals. Information transfer signals include data sig¬ nals, function signals which represent the function being performed on the system bus during a current cycle, identifier signals identifying the commander, and parity signals. The re¬ spond signals generally include acknowledge or confirmation sig¬ nals from a receiver to notify the transmitter of the status of the data transfer. Control signals include clock signals, warning sig¬ nals, such as those ■ identifying low line voltages or low DC voltages, reset signals used during initialization, node failure signals, default signals used during idle bus cycles, and error default signals. The console/front panel signals include sig¬ nals to transmit and receive serial data to a system console, boot signals to control the behavior of a boot processor during power-up, signals to enable modification of the erasable -PROM of processors on system bus 25, a signal to control a -RUN LIGHT on the front panel, and signals providing battery power to clock logic on certain nodes. The miscellaneous signals, in addition to spare signals, include identification signals which allow each node to define its identification code. Fig. 2 shows an example of a node 60 connected to sys¬ tem bus 25. Node 60 could be a processor, a memory, an I/O unit or an I/O interface. In the example shown in Fig. 2, node 60 includes node specific logic 65, a node bus 67, and a system bus interface 64 containing a data interface 61 and a clock decoder 63. Preferably, data inter ace 61, clock decoder 63, and node bus 67 are standard elements for nodes connected to system bus 25. The node specific logic 65, which uses different integrated circuits from system bus interface 64, preferably includes, in addition to the circuitry designed by a user to carry out the specific function of a node, standard circuitry to interface with the node bus 67. In general, data interface 61 is the pri¬ mary logical and electrical interface between node 60 and system bus 25, clock decoder 63 provides timing signals to node όJ based or. centrally generated clock signals, and node bus 67 provides a high speed interface between data interface 61 and node specific logic 65. In the preferred embodiment of node 60 and system bus interface 64 shown in Fig. 2, clock decoder 63 contains control circuitry for forming signals to be placed on system bus 25 and processes clock signals received from central arbiter 28 to obtain timing signals for node specific logic 65 and data inter¬ face 61. Since the timing signals obtained by clock decoder 63 use the centrally generated clock signals, node 60 will operate synchronously with system bus 25. Fig. 3 is a timing diagram showing one bus cycle, the clock signals received by clock decoder 63, and certain of the timing signals generated by clock decoder 63. The clock signals received by clock decoder 63 include a Time H signal, a Time L signal, and a Phase signal as shown in Fig. 3. Time H and Time L are inverses of the fundamental clock signals and the Phase signal is obtained by dividing the fundamental clock signal by three * The timing signals generated by clock decoder 63 include C12, C23, C34, C45, C56 and C61, all of which are shown in Fig. 3. Those timing signals required by data interface 61, which occur once per bus cycle, are provided to data interface 61, and a complete set of timing signals, including equivalent ones of the timing signals provided to data interfεce 61, is buffered and provided to the node specific logic 65. The purpose of buffering is to insure that node specific logic 65 cannot adversely effect the operation of the system bus interface 64 by improperly loading the timing signals. Clock 63 uses the clock signals to create six subcycles for each bus cycle and then uses the subcycles to create the six timing signals CXY, where X and Y. represent two adjacent " subcycles which are combined to form one timing signal. Each node in the syte bus has its own corresoondinσ set of timing signals generated by its clock decoder 63. While nominally the corresponding signals occur at exactly the same time in every node throughout the system, variations between clock decoder 63 and other circuitry in multiple nodes introduce timing variations between corresponding signals, .nne p s ς eo ~ iπil c variations are commonly known as ' "'clock -:ev." Fig. 4 shows a preferred embodiment of data inter 61. Data interface 61 contains both temporary storage circ and bus driver circuitry to provide a bidirectional and hig speed interface between each of the lines of node bus 67 an each of the lines of system bus 25. As shown in Fig. 4, da interface 61 preferably includes storage elements 70 and 72 system bus driver 74 to provide a communication path from n bus 67 to system bus 25. Data interface 61 also includes s age element 80 and node bus driver 82 to provide communicat path from system bus 25 to node bus 67. As used in the des tion of data interface 61, the term "storage element" refer generally to bistable storage devices such as a transparent latch or a master-slave storage element, and not to a speci implementa ion. Persons of ordinary skill will recognize w types of storage elements are appropriate. As shown in Fig. 4, storage element 70 has an inp connected to receive data from node bus 67 and an output co nected to the input of storage element 72. The output of s age element 72 is connected to an input of system bus drive whose output is connected to system bus 25. Storage elemen and 72 are controlled by node bus control signals 76 and 78, spectively, which are derived from the timing signals gener by clock decoder 63. Storage elements 70 and 72 provide a two-stage temporary storage for pipelining data from node b to system bus 25. Different numbers of storage stages can a be used. System bus driver 74 is controlled by system bus d er enable 79. According to the state of the system bus driv enable 79, the input of system bus driver 74 either is coupl to its output, thereby transferring the data at the output o storage element 72 to system bus 25, or decoupled from that put. When system bus drive enable 79 decouples the input an output of the system bus driver 74, system bus driver 74 pre ents a high impedance to system bus 25. The system bus driv enable 79 is also generated by clock decoder 63 in accordanc with clock signals received from system bus 25 and control s nals received from the node soecific lccic 65. Storage element 80 has an input terminal connected to system bus 25 and an output terminal connected to an input of node bus driver 82. The output o: node bus driver 82 is con¬ nected back to node bus 67. Storage element 80, preferably a transparent latch, is controlled by a system bus latch control signal 85 which is derived from the timing signals generated by clock decoder 63. A node bus drive signal 87 controls node bus driver 82 similar to the manner in which system bus drive signal 79 controls system bus driver 74. Thus, in response to node bus driver signal 87, node bus driver 82 either couples its input to its output or decouples its input from its output and provides a high impedance to node bus 67. In order to explain how data is transferred over sys¬ tem bus 25, it is important to understand the relationship between system bus drive enable 79 and control signal 85. In the present embodiment, this. . relationship is shown in Fig. 3. System bus drive enable 79 is nominally driven from the begin¬ ning to the end of a bus cycle. The new data become available for receipt from system bus 25 at some time later in the bus cycle after driver propagation and bus settling time has oc¬ curred. In the present embodiment, storage element 80 is a transparent latch. Control signal 85 is logically equivalent to clock C45. The bus timing assures that system bus 25 data is available for receipt sometime prior to the deassertion of con¬ trol signal 85. Storage element 80 stores bus data that is sta¬ ble at least, a set-up time ' prior to the deassertion of control signal 85 and remains stable a hold time after the deassertion of control signal 85. Node bus 67 is preferably a very high speed data bus which allows bidirectional data transfer between the node spe¬ cific logic 65 and system bus 25 by way of data interface 61. In the preferred embodiment of node 60 shown in Fig. 2, node bus 67 is an interconnect system consisting of point-to-point con¬ nections between the system bus interface 64 and the node spe¬ cific logic 65. In accordance with the present invention, how¬ ever, there is no requirement for such a point-to-point interconnection. Fig. 5 shows a preferred embodiment of the central arbiter 28 which is also connected to system bus 25. Central arbiter 28 provides the clock signals for system bus 25 and grants ownership of the bus to the nodes on system bus 25. Cen¬ tral arbiter 28 preferably includes an arbitration circuit 90, a clock circuit 95, and a oscillator 97. Oscillator 97 generates the fundamental clock signals. Clock 95 provides timing signals for arbitration ' circuit 71 and the basic Time H, Time L, and Phase clock signals for timing on system bus 25. Arbitration circuit 71 receives the commander and responder request signals, arbitrates conflicts between nodes desiring access to system bus 25, and maintains the-queues referred to above for the commander and responder requests. Arbitration circuit 71 also provides certain control signals to clock 95. B. Interrupt Servicing An interrupting node which embodies the present inven¬ tion is shown in Fig. 6 and is represented generally by the nu¬ meral 110. Node 110, which has the characteristics of node 60 shown in Fig. 2, is adapted to be coupled to system bus 25, which is a pended bus. As explained above, system bus 25 trans¬ fers messages, including interrupt request, interrupt acknowl¬ edge, and interrupt vector messages, between nodes, such as a processor and an I/O device, that are adapted to be coupled to the bus. As here embodied, node 110 is adapted to be coupled to bus 25 by means of system bus interface 64 and a set of genera¬ tors and decoders for creating and interpreting messages in accordance with the system protocol. Node bus 67 is provided for enabling messages to be transferred between the components in node 110 and system bus interface 64. Interrupting node 110 provides interrupt requests to system bus 25, and the requests are subsequently serviced by an interrupt servicing node. In accordance with tne present invention, the inter¬ rupting node includes first means for providing to the bus an interrupt request message including ID data for identifying -he interrupting node as the source of the interrupt request. As embodied herein, such means includes interrupt request command generator 118. Command generator 113 creates a message, in accordance with the protocol used on bus 25, which includes function and command codes that are recognizable as an interrupt request message by any other nodes using that protocol. In the preferred embodiment, slot ID signals are input to command generator 118 in order, to provide an interrupt re¬ quest message with ID data identifying interrupting node 110. When a -node is included in the computer system and has access to system bus 25, a physical connection is made between the node and a system backplane, which includes a number of physical slots. The connection to the backplane includes a number of wires that must be mated between the backplane and each of the nodes. Some of these wires, however, are unique to each slot on the backplane. Included in these slot unique wires are wires that are hard-coded to generate a particular ID for that slot that is distinguishable from each of the other slots. There¬ fore, it is preferred that the node generate the slot ID input into generator 118 using the unique set of wires from the backplane that are hard-coded with the particular ID for the slot in which the node resides. Of course, various modifica¬ tions and variations can be made in the way in which ID data identifying interrupting node 110 is generated and input to command generator 118. In the preferred embodiment, there are additional in¬ puts to the interrupt request command generator 118. An inter¬ rupt request enable signal is input from interrupting node 110 to generator 118 in order to indicate that some event has oc¬ curred that requires node 110 to generate an interrupt that must be serviced by one of the no-- s in data processing system 20. In accordance with the present invention, an inter¬ rupting node can also include means for providing the interrupt request message with destination data for specifying particular interrupt servicing nodes. In node 110 shown in Fig. 6, command generator 118 receives destination data from an interruot re¬ quest destination mask register 120. The data in register 12C can be set in any of a number of ways, such as hard-coding the register, writing data into the register using a message trans¬ ferred over bus 25, or internally writing data into register 121 from one of the other comDonent≤ in ' node 110. The data stored in register 120 is a destination mask that determines which of the nodes in data processing system 20 will be specified as the destination for the interrupt request message provided to bus 25 from generator 118. If a number of nodes are included in data processing system 20 and it is desired to have several of those nodes available as resources to service the interrupt request being made by node 110, then the data in the destination mask can specify a plurality of those nodes. For example, if eight nodes in the system are processors and it is desirable to permit interrupt requests from node 110 to be serviced by four of those processors, the mask stored in register 120 can be chosen so that four specific bits will be set, each corresponding to one selected processor in ' data pro¬ cessing system 20. In accordance with the present invention, the means for providing the interrupt request' ;..essage can also include means for providing the interrupt request message with interrupt priority level (IPL) data for specifying a priority level of the interrupt request. For example, interrupts associated with events such as power failures will typically be at a higher pri¬ ority level and will have to be serviced more quickly than in¬ terrupts associated with events such as the availability of data from a disk drive. In the preferred embodiment, interrupt prior¬ ity data is input to generator 118 by node 110, and this Inter¬ rupt Request (I.R.) IPL data specifies the priority level of the interrupt request currently being generated by node 110. In accordance with the invention, the interrupting node also includes second means for monitoring interrupt acknowledge messages on the bus and for detecting an interrupt ■ acknowledge message on the bus including destination data speci¬ fying the interrupting node and corresponding t' the ID data in the interrupt request message. Preferably, such means includes a command decoder 122 and destination comparator logic 124. At times when a command is transferred by a node onto system bus 25, it is received by node 110 and stored in a responder input queue 126, which typically is a first-in first-out register used as temporary storage during command decode. The messages in σueue 126 are then supplied to command decc er 122. Eacn interrupt acknowledge command sent on system bus 25 will include function and command codes identifying that message as an inter¬ rupt acknowledge command. Command decoder 122 detects whether the message stored in queue 126 is an interrupt acknowledge mes¬ sage, and if so, outputs an interrupt acknowledge present signal to an AND gate 128 at times when an interrupt acknowledge mes¬ sage has been received. If the message in queue 126 is an interrupt acknowl¬ edge message, it will also contain destination data. Command decoder 122 outputs this destination data to destination comparator logic 124. Destination comparator logic 124 compares the destination data with the slot ID signals, which have been previously described and which include ID data identifying in¬ terrupting node 110. At times when an interrupt acknowledge message on system bus 25 includes destination data specifying the particular interr pting node 110, destination comparator logic 124 will output to AND gate 128 a signal indicating there is a match. AND gate 128 asserts as its output an interrupt vector enable signal when command decoder 122 detects that an interrupt acknowledge message is present and destination comparator logic 124 detects that the message includes destina¬ tion data specifying node 110. The interrupt vector enable sig¬ nal is sent to an interrupt vector response generator 130. In accordance with an interrupting node of the present invention, there is third means for providing to the bus, in re¬ sponse to the detection of an interrupt acknowledge message includirg the destination data specifying the interrupting node, an interrupt vector message. As embodied herein, such means in¬ cludes interrupt vector response generator 130. Response gener¬ ator 130 is similar to command generator 118 in that both gener¬ ators transform input data into a message in the format required by the system protocol. Response generator 130 is enabled by the int rrupt vector enable signal level from AND gate- 128. The information transmitted by i;terrupting node 110 to system bus 25 in response to an interrupt acknowledge message is stored in a vector register file 132. As shown in Fig. 6, vector reg¬ ister file 132 incl les vector registers 134, 136, 138, and 140. In the preferred embodiment of the invention shown in Fig. 6, several vector registers may be used because node 110 may be capable of generating interrupt request messages in re¬ sponse to separate events that require service by execution of a different set of program instructions. Therefore, an inter¬ rupting node of the present invention can include means for se¬ lecting one of a plurality of interrupt vector messages according to priority level data from the interrupt acknowledge message. As embodied herein, this means can comprise a vector register file 132 and a multiplexer 142 to select the interrupt vector data contained in one of the vector registers in vector register file 132. An interrupt acknowledge message on bus 25 will pref¬ erably be provided with interrupt acknowledge interrupt priority level (I.A. IPL) data specifying the priority level of the in¬ terrupt request message that an interrupt servicing node is ready to service. The I.A. IPL data is supplied by command decoder 122 to multiplexer 142 which selects the interrupt vector data in accordance with the received I.A. IPL data. As a result, the selected interrupt vector data will be supplied to interrupt vector response generator 130 and will be included in the interrupt vector message provided by generator 130. Re¬ sponse generator 130 then outputs the interrupt vector message to system bus 25 so that the message can h-i received by the in¬ terrupt servicing node. The interrupting node of this invention can also in¬ clude means for providing the interrupt vector message with source data identifying a partici .ar interrupt servicing node. In the preferred embodiment, the interrupt acknowledge message provided by an interrupt servicing node and detected by node 110 includes source data for identifying a particular interrupt servicing node as the source of the interrupt acknowledge mes¬ sage. This source data is supplied from command decoder 122 to interrupt vector response generator 130. When generator 130 creates an interrupt vector message, it utilizes the source data received from command decoder 122 in order to provide to the bus an interrupt vector message with source data. The inclusion of source data in the interrupt vector messaqe facilitates the reception of the interrupt vector data requested by a particular interrupt servicing node. An interrupt servicing node which embodies the princi¬ ples of the present invention is shown in Figs. 7 and 8 and is represented generally by the numeral 144. The slot ID signals, system bus interface 64, node bus 67, input queues, command gen¬ erator, decoders, and destination comparator logic shown in Fig. 7 are similar in structure and operation to corresponding ele¬ ments shown in Fig. 6 for node 110 and described in the discus¬ sion of interrupting node 110. Using these elements, in con¬ junction with the elements shown in Fig. 6, it is possible for a node to function both as an interrupting node and an interrupt servicing node. Interrupt servicing node 144 services interrupt request messages on bus 25 which were provided by an inter¬ rupting node. Node 144 is adapted to be coupled to system bus 25 through system bus interface 64 and a set of generators and decoders. The interrupt servicing node of this invention in¬ cludes fourth means for detecting interrupt request messages on the bus including ID data identifying the interrupting node. Preferably, messages transferred on pended bus 25 are stored by interrupt servicing node 144 in a responder input queue 146, which is a first-in first-out register used as temporary storage during command decode. Each stored message is then supplied to a command decoder 148, which is included in the detection means. An interrupt request message transferred on system bus 25 in¬ cludes function and command codes identifying it as an interrupt request message, and decoder 148 will supply an interrupt re¬ quest present signal to an AND gate 150 in response to the detection of an interrupt request message on bus 25. Destination comparator logic 152 in interrupt servicing node 144 :s used to examine destination data in inter¬ rupt reqiest messages. As described previously, the interrupt request message provided by interrupting node 110 can include destination data for specifying particular interrupt servicing nodes. This destination data, in the form of a destina ion mask, is output by decoder 143 to destination comparator logic 152. Comparator logic 152 compares the destination data with slot ID signals that uniquely identify interrupt servicing node 144. If interrupt servicing node 144 is one of the nodes specified or selected in the destination mask, comparator logic 152 will assert to AND gate 150 a signal indicating that there is a match. In the preferred embodiment, AND gate 150 will assert a set storage element signal, indicating that an inter¬ rupt request message has be -n detected which includes destina¬ tion data specifying interrupt servicing node 144. In accordance with the invention, an interrupt servicing node also includes fifth means for indicating that an interrupt request is pending from an interrupting node. As embodied in interrupt servicing node 144, such indicating means are provided by a storage array 154. The particular interrupting node making an interrupt request is determined by node 144 utilizing the interrupting node ID data included in the inter¬ rupt request message provided to the bus by interrupting node 110. This information is then stored using storage array 154 to indicate that an interrupt request is pending from the corre¬ sponding interrupting node. In accordance with the present invention, the inter¬ rupt servicing node can have storage means including node stor¬ age elements, each corresponding to a different one of the in¬ terrupting nodes, for indicating whether an incerrupt request is pending from the corresponding node. As embodied in node 144, such storage means may comprise storage array 154. The interrupt servicing node of the present invention may also include means for setting one of the node storage ele¬ ments in response to the detecting of the interrupt request mes¬ sage including the ID data which identifies a corresponding in¬ terrupting node as the source of the interrupt request. This indicates that the corrresponding interrupting node has a pend¬ ing interrupt request. The preferred embodiment of storage array 154 is shown in Fig. 8. Storage elements ' are included in array 154 and cor¬ respond to particular interrupting nodes. Preferably, set-reset flip-flops are utilized as the storage elements, with each stor¬ age element 15τ corresponding to a particular interrupting node. As shown in Fics. 7 and 3, .nterruotinc node ID data is ou ut from command decoder 148 to a demultiplexer 158. The inter¬ rupting node ID data is included in the interrupt request mes¬ sage provided by an interrupting node to system bus 25. Demultiplexer 158, in response to the ID data, activates a sin¬ gle node select line and asserts a signal enabling one of a num¬ ber of AND gates 160, each of which is coupled to the set termi¬ nal of the storage element 156 correspond! .g to a particular interrupting node. The set storage element signal from AND gate 150 is also received by each of the AND gates 160. AND gate 160 thus acts as a setting means. As a result, a single storage element 156 corresponding to the particular interrupting node will be set to indicate that there is a pending interrupt re¬ quest from that node. The present invention can also include means for clearing the node storage element corresponding to a particular interrupting node in response to the providing to the bus of an interrupt acknowledge message including the destination data specifying the interrupting node. As embodied herein, inte rupt acknowledge messages provided on system bus 25 include destina¬ tion data specifying node 110 as having an interrupt request -that an interrupt servicing node is ready to service. This des¬ tination data is output by command decoder 148 to storage array 154-, and is shown in Fig. 8 by a set of destination data lines, each line providing an enable input to one of a number of AND gates 162. As a result, in response to an interrupt acknowledge " message on ~ system bus 25, a selected AND gate 162 will be enabled. The particular AND gate 162 selected will depend upon the destination data included in the interrupt acknowledge mes¬ sage. The output of the selected AND gate 162 will be coupled with the reset input of storage element 156 corresponding to the particular interrupting node specif .ed as the destination of the interrupt acknowledge rαessaς-. In response to an interrupt acknowledge message, command decoder 148 will output an inter¬ rupt acknowledge present signal to each of the AND gates 162, so that clearing of a storage element occurs. AND gates 162 thus act as a clearing means As a result, a single storage element 156 corresponding to node 1 J is cleared whenever an interrupt ac ' tnowledce rr.essaαe indicates that the interrupt reσuest fro . artιc _ar noαe is oe Interrupt servicing nodes may include means for speci¬ fying the identity of the interrupting node corresponding to each storage element. As embodied herein, such specifying means includes an encoder 164. The outputs from each of the storage elements 156 are coupled to encoder 164. Encoder 164 scans each of the inputs in some predetermined order and selects a particu¬ lar one of the storages elements 156 that are set. Encoder 164 outputs selected node data that identifies the particular inter¬ rupting node that will be serviced by node 144. As shown in Fig. 7, this selected node data is output from the storage array 154 to an interrupt acknowledge command generator 166, and specifies the identity of the particular interrupting node being serviced by the interrupt servicing node. In accordance with the present invention, the inter¬ rupt servicing node includes sixth means for providing to the bus an interrupt acknowledge message including destination data specifying a particular interrupting node at times when the in¬ terrupt servicing node is ready to service an interrupt request message from that node. As embodied herein, the means for pro¬ viding the interrupt acknowledge message includes an interrupt acknowledge command generator 166. Preferably, an interrupt re¬ quest message rece .ved from system bus 25 includes ID data iden¬ tifying the particular interrupting node 110. As described pre¬ viously, this ID data is output from decoder 148 to demultiplexer 158 and is used to set a storage element corre¬ sponding to the particular interrupting node 110 in storage array 154. The outputs from each of the storage elements 156 are coupled to one of the OR gates 168. Each OR gate 168 has an interrupt request pending output provided to other logic ele¬ ments in the interrupt servicing node. The interrupt request pending output indicates that a pending interrupt request is present. When node 144 is ready to service an inter: pt re¬ quest, logic in node "' 144, such as a CPU, asserts an interrupt acknowledge enable signal to interrupt acknowledge command gen¬ erator 166. Generator 156 can create an interrupt acknowledge message including destination data specifying a particular in¬ terrupting node .aving a pending interrupt request because encoder 164 outputs selected node data to Generator 156. The selected node data specifies the identity of the particular in¬ terrupting node 110 that will be the destination of the inter¬ rupt acknowledge message being generated. The generator 166 then provides the interrupt acknowledge message to system bus 25. In accordance with the invention, an interrupt servicing node also includes seventh means for detecting an in¬ terrupt vector message on the bus which can include source data identifying the interrupt servicing node. As embodied herein and shown in Fig. 7, node 144 includes a coinmander input queue 170, which is a first-in first-out register, for storing mes¬ sages transferred on system bus 25. The stored messages are transferred to response decoder 172, which provides the detec¬ tion means. At times when the decoder 172 detects the r resence of an interrupt vector message, it enables AND gate 174. In the referred embodiment of the invention, interrupt vector messages on syste bus 25 will include source data specifying a particu- ■ lar interrupt servicing node 144. This source data is output by decoder 172 to destination comparator logic 176, which compares it with slot ID signals that uniquely identify interrupt servicing node 144. If node 144 is the intended destination of the interupt vector data contained in the interrupt vector mes¬ sage, the inputs to comparator logic 176 will match, and comparator logic 176 will enable AND gate 174. When both of the above inputs to AND gate 174 are enabled, AND gate 174 will assert an interrupt vector valid signal to logic in node 144. Response decoder 172 will also output the interrupt vector data contained in the interrupt vector message to logic in node 144. This information will allow interrupt servicing node 144 to com¬ pute the starting address of the program corresponding to the interrupt request generated by interrupting node 110. Prefer¬ ably, interrupt servicing node 144 will include a CPU that enters a wait state after node 144 provides an interrupt acknowledge message to system bus 25, and awaits the receipt of an interrupt vector message from system bus 25. In accordance with one aspect of the invention, an in¬ terrupt servicing node includes means for obtaining control of the bus at times when the node is read" to service a pendia interrupt request and for performing null operations for a pre¬ determined time period before providing the interrupt acknowl¬ edge message to the pended bus. As embodied herein and shown in Fig. 7, such means includes interrupt acknowledge command gener¬ ator 166. At times when interrupt acknowledge generator 166 is ready to provide an interrupt acknowledge message to system bus 25, it outputs a commander request signal to demand access to system bus 25. When node 144 is granted access to bus 25, interrupt acknowledge generator 166 begins by providing bus cy¬ cles interpreted as null operations to system bus 25. These cy¬ cles provided ~o the bus will contain a "no-operation" function code which, according to the computer system protocol, will be interpreted by every node in the computer system as not requiring the execution of any operations. The null operations are provided by generator 166 before an actual interrupt acknowledge message is provided to system bus 25. By waiting a predetermined time period, node 144 ensures that the particular • interrupting node 110 still has a pending interrupt request that has not been previously serviced by another interrupt servicing node. The predetermined time period accounts for the "pipeline" delay associated with the operation of clearing the storage ele¬ ment, which may require additional cycles to complete subsequent to the transmission of an interrupt acknowledge message across bus 25. By waiting this predetermined time, node 144 thus ensures that the storage elements in storage array 154 provide coherent informatio . that accurately indicates whether a partic¬ ular interrupting node 110 still has a pending interrupt re¬ quest. In the preferred embodiment of data processing system 20 shown in Figs. 1-8, it takes the interrupt servicing nodes approximately four to eight bus cycles to clear the correspond¬ ing storage elements after an interrupt acknowledge message is provided on bus 25. Therefore, by waiting for the clearing operation to complete before ' providing an interrupt acknowle r 'e message, the storage arrays will all contain up-to-date inter¬ rupt request status information. At times when node 144 has control of the system bus 25 and is performing null operations, no further interrupt ackr rwieαge or interrupt request messages can be transmitted on svs era O' .s 25. As embodied herein, the output of OR gate 168 is re¬ ceived by interrupt acknowledge command generator 166 and indi¬ cates whether any interrupt requests are still pending prior to the end of the predetermined time period. If no signal is pres¬ ent, this indicates that there are no pending interrupt re¬ quests, and interrupt acknowledge generator 166 may abort before providing an interrupt acknowledge message to the pended bus. If the interrupt request from node 110 is no longer pending, but different interrupt requests are still pending in other storage elements in storage array 154, the output of OR gate 168 will indicate this pendency, and the selected node data output to interrupt acknowledge generator 166 will reflect the identity of one of the nodes having a pending interrupt request. Cn the preferred embodiment shown in Figs. 7 and 8, the encoder 164 scans through a set of inputs from storage elements 156, re- lects one of the storage elements that is set, and presents the identity of that storage element as selected node data to generator 166. As has been discussed previously, an interrupt servicing node of the present invention can include means for clearing the storage element corresponding to a particular in¬ terrupting node in response to the providing to the bus by any node of an interrupt acknowledge message including destination data specifying the particular interrupting node. Therefore, whenever an interrupt request from a particular interrupting node 110 is being serviced by any of the interrupt servicing nodes in data processing system 20, the storage element 156 cor¬ responding to the particular interrupting node 110 will be cleared to indicate that the interrupt request of node 110 i_ being serviced. This pre"ents interrupt servicing node 144 from providing an additional interrupt acknowledge message and attempting to service a particular node 110 that no longer has a pending interrupt request. In accordance with the present invention, an interrupt se vicing node _ * an also include means for providing the inter¬ rupt acknowledge message with source data for identifying the inte r rupt servicing node. Provision of this source data facili¬ tates the tercetinc of an interrupt vector messαce to the particular interrupt servicing node 144 that previously provided an interrupt acknowledge message. As shown in Fig. 7, the in¬ terrupt acknowledge generator 166 is supplied with the slot ID signals uniquely identifying interrupt servicing node 144. As a result, generator 166 can create an interrupt acknowledge mes¬ sage including source data that identifies interrupt servicing node 144. In accordance with another aspect of the invention, an interrupt servicing node can include service priority level means for specifying a service priority level for the interrupt servicing node and for indicating that the node is ready to ser¬ vice an interrupt request at times when the priority level of the interrupt request message is equal to the service priority level. As embodied herein, interrupt servicing node 144 can assert an interrupt acknowledge enable signal to the command generator 166 indicating that the node is ready to service a pending interrupt request. Service IPL (interrupt priority level) data specifies a selected set of pending interrupt re¬ quests that can be serviced, and together with the interrupt acknowledge enable signal provides the function of a service priority level means. In the preferred embodiment, interrupt request messages include IPL data specifying a priority level of the interrupt request, and this IPL data is output by command decoder 148 to storage array 154. The present invention can also include means for setting a storage element corresponding to the particular inter¬ rupting node at the specified priority level of the interrupt request message, and means for indicating there is a pending in¬ terrupt request from a particular interrupting node at that pri¬ ority level. Similarly, the invention may include means for clearing a storage element corresponding to a particular inter¬ rupting node at a specified priori v level, thereby indicating that the interrupt request of an interrupting node at a specified IPL is being serviced. As embodied herein, such means include AND gates 160 and 162. In accordance with the system protocol of the preferred embodiment, interrupt acknowledce mes¬ sages provided by node 144 also include data specifying the pri¬ ority level of the interrupt request message that the interru; servi ing node s r acv to s=rv.; . In the preferred embodiment, when either an interrupt request message or an interrupt acknowledge message is received by interrupt servicing node 144 from system bus 25, -it contains IPL data that is output by command decoder 148 to storage array 154. Node 144 can employ several banks of storage ele¬ ments, where each bank corresponds to a different IPL, and the storage elements in each bank correspond to each possible inter¬ rupting node. Demultiplexer 177 is included in storage array 154 of a node 144 and selects one of the several banks of storage elements in .accordance with the priority level data of the interrupt being requested or acknowledged. There is a -sepa¬ rate storage element 156 for each particular interrupting node at each specified priority level. At times when an interrupt request message is provided on system bus 25, demultiplexer 177 ensures that the only AND gates 160 which will be enabled are those that are coupled to storage elements 156 corresponding to the specified priority level. Therefore, onl -the storage element corresponding to the particular node at the priority level specified by the IPL data in the interrupt request message will be set. Similarly, at times when an interrupt acknowledge message is provided on sys¬ tem bun 25, demultiplexer 177 also ensures that the only AND gates 162 that will be enabled are those that are coupled to the storage elements 156 corresponding to the IPL data contained in the interrupt acknowledge message. As a result, only the stor¬ age element corresponding to the particular interrupting node at the priority level specified by the IPL data will be cleared to indicate which interrupt request is being serviced. Preferably, multiplexer 178 is included in node 144 as part of a means for selecting a storage element corresponding to a particular interrupting node at a specified IPL. There can be a separate OR gate 168 associated with each specified priority level, and thus, each bank, and the outputs from th OR gates 168 are sent to otner logic Elements in node 144 and to a multi¬ plexer 180. Service IPL data is sent by node 144 to multiplexer 178 so that the selected node data input to inter¬ rupt acknowledce Generator 156 will correspond to a oarticular interrupting node having a pending interrupt request at an IPL equal to that specified by the service IPL data generated by node 144. The interrupt servicing node typically will include a CPU for servicing interrupt requests. The service IPL data determines the priority level of the interrupt requests that a CPU, for example, in interrupt servicing node 144 is ready to service. During its operations, the CPU will operate at a par¬ ticular IPL which changes as the CPU executes instructions. The CPU will periodically examine the outputs of the OR gates 168 to determine whether there are any pending inter¬ rupt requests at an IPL greater than the interrupt priority level at which the CPU is currently operating. If there is a pending interrupt request at an IPL greater than the interrupt priority level of the CPU, node 144 asserts an interrupt acknowledge enable signal and supplies service IPL data to the interrupt acknowledge command generator to indicate that node 144 is ready to service a pending interrupt request at the specified IPL. Node 144 will select a particular node ID from among the storage elements at which an interrupt request is pending corresponding to the specified service interrupt priori¬ ty level (service IPL) data. Multiplexer 1 " 8 selects an output from one of the encoders 164, and outputs selected node dat- to generator 166. This data identifies a particular interrupting node having a pending request at the priority level specified by the service IPL data. In the preferred embodiment of the invention, the in¬ terrupt request pending signal to be output to interrupt acknowledge command generator 166 from multiplexer 180 is se¬ lected from the outputs of OR gates 168 in accordance with the service IPL data. If after null operations have been provided for a predetermined t me period, there are no pending interrupt requests at the priority level specified by the service IPL data, the interrupt acknowledge generator 166 will abort and will not provide an interrupt acknowledge message. In the preferred embodiment, the interrupt acknowledge message created by interrupt acknowledge generator 166 also in¬ cludes IPL data specifying the interrupt priority level of the i erruot recuest messace bei c serviced bv node 144. Generator L66 receives the service IPL data as an input and includes this IPL data in the interrupt acknowledge message provided to the bus. To understand the operation of the entire interrupt servicing system and method of the present invention, a flow¬ chart is provided in Figs. 9A-B. In the method demonstrated by the flowchart, the system bus receives from an interrupting node an interrupt request message including ID data for identifying that node. The interrupt servicing node provides to the system bus an interrupt acknowledge message including destination data specifying the particular interrupting node at times when the interrupt servicing node is ready to service the interrupt re¬ quest message. The interrupting node detects whether an inter¬ rupt acknowledge message on the bus includes destination data specifying itself. In response to tne detection of an interrupt acknowledge message including destination data specifying that node, the interrupting node provides an interrupt vector message to the bus. Preferably, the interrupt servicing node also pro¬ vides IPL data specifying the priority level of the interrupt request message, so that the interrupting node can select one of a plurality of interrupt vector messages in accordance with the IP-, data. In step 182 of the flowchart in Figs. 9A and 9B, in¬ terrupting node 110 arbitrates for access to the bus. In order to contend for the bus, a node must output a bus request signal. In step 184, node 110 determines whether access has been granted. If not, node 110 returns to step 182. If bus access has been granted, in step 186 the interrupting node transmits an interrupt request message on the system bus. The interrupt re¬ quest message includes function and command codes identifying the message as an interrupt request command, IPL data specifying a priority level for the interrupt request, a destination mask for specifying the interrupt servicing nodes that can service the request, and ID data fc specifying node 110 as the source of the interrupt request. In step 188, the interrupt servicing node 144 designated by the destination data will set a storr :e element corresponding to the ID data and IPL data of the inter¬ rupt request message. The setting cf the storage element indicates that there is a pending interrupt request that can be serviced by node 144. In step 190, signa. levels from storage array 154 in node 144 indicate to the logic of node 144 that there is pending interrupt request. In step 192, the interrupt servicing node 144 is operating at a particular interrupt prior¬ ity level (IPL). In step 194, a determination is made whether there is a pending interrupt request having an IPL greater than the current operating IPL of node 144. If rot, node 144 con¬ tinues with step 192. If the IPL of a pending interrupt request is greater than the operating IPL of node 144, node 144 will arbitrate for access to the system bus. As shown in Fig. 7, generator 166 outputs a commander request signal to demand access. If bus access is not granted in step 193, node 144 continues with step 196. When bus access is granted in the preferred embodiment, node 144 in step 200 will issue a series of null cycles after obtaining access to the bus in order to allow its storage array 154 to be updated to reflect previous interrupt acknowledge mes¬ sages. In step 202, node 144 determines whether storage array 154 indicates that there is still a pending interrupt request at the specified IPL. If not, the interrupt servicing node 144 goes to step 218 for a determination of the resence of any pending interrupt requests at other levels. If a pending inter¬ rupt request still exists, in step 204 the interrupt servicing node selects one of the storage elements that is set to indicate an interrupt request is pending that corresponds to a particular interrupting node at the specified IPL. In step 206, node 144 provides an interrupt acknowl¬ edge message to the bus. The interrupt acknowledge message in¬ cludes function and command codes identifying the message as an acknowledge command, IPL data, source data identifying node 144 as the source of the interrupt acknowledge message, and destina¬ tion data specifying node 110. This will cause interrupt servicing nodes in the system to eventually clear the storage element corresponding to the interrupt request message being serviced. In step 208, the interrupt acknowledge message is detected by the interrupting node 110 designated by the destina¬ tion data in the interrupt acknowledge message. In step 210, interrupting node 110 arbitrates for the bus. If access to the bus is not granted in step 212, node 110 continues w th step 210. If node 110 obtains control of the bus, the interrupting node provides an interrupt vector message to the bus in step 214 corresponding to priority data contained in the interrupt acknowledge message. The interrupt vector message includes the source data identifying the interrupt servicing node- 144, a function code indicating that the message is an interrupt vector message, and the interrupt vector data needed by node 144 in order to compute the starting address of the program to service the pending interrupt request. In step 216, the interrupt servicing node 144 desig¬ nated by the source data is in a wait state and detects the in¬ terrupt vector message. It uses this interrupt vector informa¬ tion in order to perform an interrupt service routine. Finally, in step 218, node 144 determines whether any pending interrupt requests are indicated by storage array 154. If not, the method returns to step 182. If there are other pending interrupt re¬ quests, the method continues at step 19_. In th-? foregoing description, the interrupting nodes can include input/output devices, such as disk drives and disk drive controllers, and can include nodes which are used to cou¬ pler a number or a variety of I/O devices to a system bus 25. Similarly, the interrupt servicing nodes can include a pro¬ cessing means for servicing interrupt requests. Typically, the interrupt servicing nodes will contain a CPU in order to service interrupt requests. Additionally, it is possible for a single node to generate and service interrupt requests. It will be apparent to those skilled in the art that -various modi ications and vεriations can be made in the present invention without departing from the scope or spirit of the invention. Thus, it is intended tnat the present invention cover the modifications and variations of this invention provid¬ ed they come within the scope of the appended claims and their equivalents.

Claims: WHAT IS CLAIMED IS: 1. A method for performing an interrupt function on a pended bus for transferring messages between an ' interrupting node and an interrupt servicing node, the method comprising the steps of: providing to the bus from the interrupting node an in¬ terrupt request message including ID data for identifying the interrupting node; providing to the bus from the interrupt servicing node an interrupt acknowledge message including destination data specifying the interrupting node at times when the interrupt servicing node is ready to service the interrupt request mes¬ sage; monitoring interrupt acknowledge messages on the bus at the interrupting node to detect the interrupt acknowledge message including destination data specifying the interrupting node; and providing to the bus from the interrupting node, in response to the detection of the interrupt acknowledge message including destination data specifying to the interrupting node, an interrupt vector message. 2. A method in accordance with claim 1 in which the step of providing an interrupt request message further comprises the step of supplying the interrupt request message with desti¬ nation data specifying the interrupt servicing node. 3. A method in accordance with claim 2 in which the step of providing an interrupt request message further comprises the step of supplying the interrupt request message with desti¬ nation data specifying a plurality of interrupt servicing nodes. 4. A method for performing an interrupt function on a pended bus for transferring messages between an interrupting node and an interrupt servicing node, the method comprising the steps of: providing to the bus from the interrupting node an in¬ terrupt request message including ID data for identifying ths. interrupting node and priority level data for specifying a pri¬ ority level of the interrupt request message; providing to the bus from the interrupt servicing node an interrupt acknowledge message including destination data specifying the interrupting node and priority level data speci¬ fying the priority level of the interrupt request message at times when the interrupt servicing node is ready to service the interrupt request message at the specified priority level; monitoring interrupt acknowledge messages on the bus at the interrupting node to detect the interrupt acknowledge message including destination data specifying the interrupting node and priority level data specifying the priority level of the interrupt request message; and providing to the bus from the interrupting node, in response to the detection of tre interrupt acknowledge message including destination data specifying the interrupting node and priority level data specifying the priority level of the inter¬ rupt request message, an interrupt vector message. 5. A method in accordance with claim 4 in which the step of providing an interrupt vector message includes the step of selecting one ' of a ' plurality of interrupt vector messages in accordance with the priority level data of the interrupt acknowledge message. 6. A method in accordance with claim 4 in which the step of providing an interrupt request message furthe * comprises the step of supplying the interrupt request message with desti¬ nation data specifying the interrupt servicing node. 7. A method in accordance with cl-_άm 6 in which the step of providing an interrupt request message further comprises the step of supplying the interrupt request message with desti¬ nation data specifying a plurality of interrupt servicing nodes. 8» A method in accordance with claim 1, in which the step of providing an interrupt acknowledge message further com¬ prises the step of supplying the interrupt acknowledge message with source data for identifying the interrupt servicing node; and in which the step of " providing an interrupt vector message further comprises the step of supplying the interrupt vector message with the source data identifying the interrupt servicing node. 9. A method in accordance with claim 4, in which the step of providing an interrupt acknowledge message further comprises the step of supplying the interrupt acknowledge mes¬ sage with source data for identifying the interrupt servicing node; and in which the step of providing an interrupt vector message further comprises the step of supplying the interrupt vector message with the source data identifying the interrupt servicing node. 10. A system for servicing an interrupt request, com¬ prising: a pended bus for transferring messages, including in¬ terrupt request, interrupt acknowledge, and interrupt vector messages; an interrupting node coupled to the bus and including: first means for providing to the bus an interrupt request message including interrupting node ID data for identifying the interrupting node; second means for monitoring interrupt acknowledge messages on the bus and for detecting an interrupt acknowledge message including destination data speci¬ fying the interrupting node; and third m-ians for providing to the bus, in response to the detection of the interrupt acknowledge message including the destination data specifying the inter¬ rupting node, an interrupt vector message; and an interrupt servicing node coupled to the bus for providing the int irrupt acknowledge message at times when the interrupt servicing node is ready to service the interrupt re¬ quest message, including: fourth means for detecting the interrupt request message on the bus provided by the interrupting node; fifth means for indicating t ιat the interrupt re¬ quest is pending from th interrupting node; sixth means for providing to the bus the inter¬ rupt acknowledge message including destination data specifying the interrupting node; and seventh means for detecting the interrupt vector message on the bus pr o vided bv the interruptinσ node. 11. A system in accordance with claim 10 further com¬ prising: a plurality of interrupting nodes each including the first, second, and third means. 12. A system in accordance with claim 10 further com¬ prising: a plurality of interrupt servicing nodes each including the fourth, fifth, sixth, and seventh means. 13. A system in accordance with claim 11, in which: at least one of the interrupting nodes comprises an input/output device. 14. A system in accordance with claim 12, in which: at least one of the interrupt servicing nodes com¬ prises a processor. 15. A system in accordance with claim 10, in which the interrupt ,servicing node includes: means for obtaining control of the bus at times when the interrupt servicing node is ready to service ' a pending in¬ terrupt request from the interrupting node and for performing null operations for a predetermined time period before providing the interrupt acknowledge message. 16. A system i.. accordance with claim 12, in which the interrupt servicing node includes: means for obtaining control of the bus at times when the interrupt servicing node is ready to service a pending in¬ terrupt request from the interrupting node and for performing null operations for a predetermined time period before providing the interrupt acknowledge message. 17. A system in accordance with claim 12 in which the interrupting node further includes means for providing the in¬ terrupt request message with destination data specifying a se¬ lected plurality of the plurality of interrupt servicing nodes. 18. A sy-tem in accordance with claim 10 in which the interrupting node further includes means for -roviding the in¬ terrupt request message with destination data specifying the in¬ terrupt servicing node. 19. A system for servicing an interrupt request, com¬ prising: a pended bus for transferring messages, including in¬ terrupt request, interrupt acknowledge, and interrupt vector messages; an interrupting node coupled to the bus and including: first means for providing to the bus an interrupt request message including interrupting node ID data for identifying the interrupting node and priority level data for specifying an interrupt request priori¬ ty level of the interrupt request message; second means for monitoring interrupt acknowledge messages on the bus and for detecting an interrupt acknowledge message including destination data speci¬ fying the interrupting node and priority level data specifying the priority level of the -interrupt request message; and third means for providing to the bus, in response to the detection of the interrupt acknowledge message including the destination data specifying the inter¬ rupting node and the priority level data specifying the priority level of the interrupt request message, an interrupt vector message; and an interrupt servicing node coupled to the bus for providing the interrupt acknowledge message at times when the interrupt servicing node is ready to service the interrupt re¬ quest message at the specified interrupt request priority level, including: fourth means for detecting the interrupt reciest message on t e bus provided by the interrupting node; fifth means for indicating that the interrupt re¬ quest is pending from the interrupting node and for indicating the interrupt request is at the interrupt request priority level specified by the priority level data of the interrupt request message; sixth means for providing to the bus the inter¬ rupt ackπo- ledge message including de ti.tation data specifying the interrupting ncde and priority level data specifying the priority level of the interrupt request message; seventh means for detecting the interrupt vector message on the bus provided by interrupting node; and service priority level means for specifying a service priority level for the interrupt servicing node and for indicating that the interrupt servicing node is ready to service the interrupt request message at times when the priority level of the interrupt re¬ quest message is equal to the service priority level. 20. A system in accordance with claim 19 further com¬ prising: a plurality of interrupting nodes each including the first, second, and third means. 21. A system in accordance with claim 19 further com¬ prising: a plurality of interrupt servicing nodes each including the fourth, fifth, sixth, seventh, and service priori¬ ty level means. 22. A system in accordance with claim 20, in which: at least one of the interrupting nodes comprises an input/output device. 23. A system in accordance with claim 21, in which: at least one of the interrupt servicing nodes com¬ prises a processor. 24. A system in accordance with claim 19 in which t '~ e interrupt servicing node includes: means for obtaining control of the bus at times when the interrupt servicing node is rea iy to service a pending in¬ terrupt request from the interrupting node at the specified in¬ terrupt request priority level and for performing null opera¬ tions for a predetermined time period before providing the interrupt acknowledge message. 25. A system in accordance with claim 21 in which the interrupt servicing node includes: means for obtaining control of the bus at times when the interrupt servicing node is ready to service a pending in- • ter "UPt reσuest from the interruotinc node at the specified interrupt request priority level and for performing null opera¬ tions for a predetermined time period before providing the in¬ terrupt acknowledge message. 26. A system in accordance with claim 21 in which the interrupting node further includes means for providing the in¬ terrupt request message with destination data specifying a se¬ lected plurality of the plurality of interrupt servicing nodes. 27. A system in accordance with claim 19, in which the interrupting node further includes means for providing the interrupt request message with destination data specifying the interrupt servicing node. 28. A system in accordance with claim 10 in which the sixth means in the interrupt servicing node includes means for providing the interrupt acknowledge message with source data for identifying the interrupt servicing node. 29. A system in accordance with claim 19 in which the sixth means in the interrupt servicing node includes means for providing the i-nterrupt acknowledge message with source data for identifying the interrupt servicing node. 30. A system in accordance with claim 28 in which the third means in the interrupting node includes means for provid¬ ing the interrupt vector message with the source data identi¬ fying the interrupt servicing node. 31. A system in accordance with, claim 29 in which the third means in the interrupting node includes means for provid¬ ing the interrupt vector message with the source data identi¬ fying the interrupt servicing node. 32. A system in accordance with claim 19 in which the third means in the interrupting node includes means for se¬ lecting one of a plurality of interrupt vector messages in accordance with the priority level data from the interrupt acknowledge message. 33. A system for servicing an interrupt request in accordance with claim 11, wherein the fifth means includes: a plurality of node storage elements, each correspond¬ ing to a different one of the interrupting nodes, for indicating whether an interrupt reσuest is pending from the corresponding interrup ing node; eans for setting one of the node storage elements in response to the detecting of an interrupt request message including ID data identifying the corresponding interrupting node; and means for clearing one of the node storage elements in response to the providing to the bus of an interrupt acknowledge message including destination data specifying the corresponding interrupting node. 34. A system for servicing an interrupt request in accordance with claim 20, wherein the fifth means includes: a plurality of node storage elements, each correspond¬ ing to a different one of the interrupting nodes at a different one of the interrupt request priority levels, for indicating whether an interrupt request is pending from the corresponding interrupting node at the corresponding priority level; means for setting one of the node storage elements in response to the detecting of an interrupt request message including ID data identifying the corresponding interrupting node and priority level data specifying the corresponding prior¬ ity level; and means for clearing one of the node storage elements in response to the providing to the bus of an interrupt acknow l edge message including destination data specifying the corresponding interrupting node and priority level data specifying the corre¬ sponding priorityg level. 35. A system for servicing an interrupt request, com¬ prising: a pended bus for transferring messages, including in¬ terrupt request, interrupt acknowledge, and interrupt vector messages; an interrupting node coupled to the bus and including: first means for providing to the bus an interrupt request message including interrupting node ID data for identifying the ' interrupting node, destination data for specifying an interrupt servicing node, and priority level data for specifying an interrupt re¬ quest priority level of the interrupt reσuest message; second means for monitoring interrupt acknowledge messages on the bus and for detecting an interrupt acknowledge message including destination data speci¬ fying the interruptir g node and priority level data specifying the priority level of the interrupt request message; and third means for providing to the bus, in response to the detection of the interrupt acknowledge message including the destination data specifying the inter¬ rupting node and the priority level data specifying the priority level of the interrupt request message, an interrupt vector message including source data identifying an interrupt servicing node; and an interrupt servicing node coupled to the bus for providing the interrupt acknowledge message at times when the interrupt servicing node is ready to service the interrupt re¬ quest message at the specified interrupt request priority level, including: fourth means for detecting the interrupt request message on the bus provided by the interrupting node; fifth means including a plurality of node storage elements, each element corresponding to an inter¬ rupting node at a specified priority level, for indicating that the interrupt request is pending from the interrupting node and for indicating the interrupt request is at the interrupt -equest priority level specified by the priority level data of the interrupt request message; sixth means for providing to the bus the inter¬ rupt acknowledge message including destination data specifying the interrupting node, source data for identifying the interrupt servicing node, and priority level data specifying the priority level of the inter¬ rupt request message; seventh means tor detecting the interrupt vector message on the bus provided by interrupting node; and service priority level means for specifying a service priority level for the interrupt servici .g node and for indicating that the interrupt servicing node is ready to service the interrupt request message at times when the priority level of the interrupt re¬ quest message is equal to the service priority level.

Inventors: WILLIAMS DOUGLAS D, WILLIAMS, DOUGLAS, D.
Applicants: DIGITAL EQUIPMENT CORP, DIGITAL EQUIPMENT CORPORATION
Application year: 1988
Main classification: G06F13
IPC codes: G06F13, G06F15