Bus arbitration system

Abstract: The invention relates to a device adapted to be connected to a non-pended bus, a circuit of this device for providing communications interface for that device to the non-pended bus, and a process by which this device arbitrates an access to a bus. The interconnecting circuit comprises controller means (300) coupled to a no arbitration and busy lines of the bus for indicating an arbitration cycle, driver means (334) coupled to the controller means for asserting one or two data lines of the bus corresponding to the device during the arbitration, priority resolution means (380) coupled to the data lines to indicate that the device has the highest priority during the arbitration cycle, receive register means (304) coupled to information lines of the bus for comparing the ID number of the device with the ID number of the device, and switching means (308) connected to receive register and to driver means for asserting low or high priority data line to the device during the arbitration cycle.

Description: A. Field of the Invention The invention relates to digital computer architecture and, more particularly, to means for interconnecting for communication with each other such diverse devices as processors, memory (main memory) and I/O devices such as mass storage (e.g. disks and tapes), console terminals, printers, and other such devices in a digital computer system. The particular invention claimed herein relates to an improved means for arbitrating among devices seeking control of a communications path in a digital computer system. B. Prior Art As the cost of digital computer systems and their components continues to decrease, more and more different types of data handling devices are being interconnected into these systems. The devices have widely varying characteristics with respect to speed (i.e., the rate at which they can accept or transmit data), required control information, data format, and other such characteristics, yet they must communicate with each other. For example, processors must often communicate with main memory (very high speed), mass storage devices such as disk memory (high speed), and output devices such as printers (very low speed). An important aspect of an interconnecting means is its ability to support arbitration among the competing demands of devices wishing to communicate with each other. Some form of arbitration must be performed to grant a request for access to the communications path, and thus it is essential that the arbitration process be efficient, since it may otherwise consume an undue portion of the computer system's resources. Further, it is generally desirable that the arbitration process provide some measure of flexibility in allocating the communications path among the requesting devices. In environments which allow a wide variety of devices to be attached to the communications path, particularly in environments which additionally allow the connection of multiple processors to the communications path, the competing demands on the arbitration mechanism often lead to undesirable constraints on system operation and flexibility. Another important aspect of an interconnecting means is its support of interrupts. The manner in which these interrupts are posted often results in significant restrictions on the achievable flexibility of device attachment to the communications path. In addition to providing communications among devices attached to a single central processor, it is frequently desirable to provide access between such devices and one or more additional processors, as well as between the several processors themselves. This requirement of communication among processors adds substantially increased complexity to the interconnection problem because of the need to insure coordinated operation. One aspect of interprocessor communications that requires particular attention is the problem caused by utilization of caches on one or more of the processors. Such caches can cause processing errors if appropriate steps are not taken to insure that access to the cache is allowed only when the cached data is "valid", that is, has not been altered in main memory since it was cached. If cache control is not performed efficiently, the performance of the system as a whole may be significantly degraded. Brief Description of the Invention A. Objects of the Invention Accordingly, it is an object of the invention to provide an improved means for interconnecting diverse devices in a digital computer system. Further, it is an object of the invention to provide an improved means for interconnecting devices in a digital computer system that allows attachment of a wide variety of devices with minimal attachment restrictions. Still a further object of the invention to provide an improved means for interconnecting devices that efficiently allocates interconnection resources among devices requesting communications with each other. Yet another object of the invention is to provide a means for interconnecting devices in a digital computer system that effectively accommodates communications among devices having different access priorities. The invention is mainly directed to an interconnecting circuit in a device for providing a communications interface for that device to a non-pended bus the device having a unique ID number and the device initiating a transaction at times when a request signal is asserted by asserting a busy line to obtain access to the bus, the bus including a plurality of information lines, a plurality of data lines, a no arbitration line, and the busy line, and wherein each of the transactions has a command/address cycle followed by an imbedded arbitration cycle which is then followed by at least one data cycle, the no arbitration line being asserted by any device except during the command/address cycle the circuit comprising: controller means coupled to the no arbitration and busy lines and responsive to a de-assertion of the no arbitration line for indicating when the imbedded arbitration cycle is occurring during a current transaction; driver means coupled to the controller means for asserting one of two data lines corresponding to the device during the imbedded arbitration cycle; priority resolution means coupled to the data lines for asserting a won arbitration signal to indicate that the device had the highest priority during the imbedded arbitration cycle, wherein assertion of the won arbitration signal enables the device to assert the busy line during a subsequent cycle to initiate the next transaction after said busy line is deasserted by a previous device having access to the bus during the current transaction; receive register and comparing means coupled to the information lines for comparing the ID number of the device with an ID number of the previous device which is present on the information lines during the imbedded arbitration cycle, and for setting an indicator in either a first or second state according to whether the ID number of the device seeking to initiate a transaction is greater than that of the previous device; and switching means, responsive to the receive register and comparing means and coupled to the driver means, for asserting a low priority data line corresponding to the device during the imbedded arbitration cycle of the next transaction when the indicator is in the first state, and for asserting a high priority data line corresponding to the device during the imbedded arbitration cycle when the indicator is in the second state, wherein the device has a variable priority and is allocated access to the bus fairly according to a dual-count, dual round robin algorithm. B. Summary Description of the Invention This application is directed to one of several related aspects of the interconnecting means. Specifically, it is directed to the means by which access to a communications path is granted to the devices seeking access at a given time. Because of the interrelation among the separate aspects of the complete system, the structure of the complete system will be described as a whole first, and those aspects specific to the present invention will then be described in somewhat further detail. The claims set forth herein define the specific invention of the present application, namely, the means for determining and granting access to a communications path. 1. General Description Of The Interconnecting Means The interconnecting means described herein is associated with, and preferably forms part of, each device to be interconnected. It controls the transmission and reception of signals on a communications path (e.g., a parallel wired bus) interconnecting each of the devices. The interconnecting means provides uniform control of communications among the devices interconnected by the communications path. These devices are connected in parallel to the communications path, and their operation is independent of physical placement on the path. Each device connected to the communications path is assigned an identification number ("ID") which is used for a number of purposes as described hereafter. In one implementation of the interconnecting means, the assignment is made by a physical plug inserted into the device and wire to specify the identification number. Since this physical plug may be moved from slot to slot, there is no logical dependency between the device and the slot in which it resides. This number is loaded into a control register during system initialization, and is thereafter available for use by the device. The interconnecting means implements a specific set of commands providing efficient communication between devices. These commands are implemented and transmitted in a number of different operations (hereinafter called "transactions"). Each transaction is subdivided into a number of cycles, including a Command/Address cycle in which the operation code for the particular transaction (e.g., Read, Write, Interrupt, etc.) is transmitted over the bus to other devices, together with information identifying the devices to which the command is directed or providing other information pertinent to the command; an Imbedded Arbitration cycle for identifying the device which will next be allowed access to the communications path; and one or more data cycles in which user data (i.e., the ultimate object of the processing) or other information is transmitted. The transaction signals are transmitted over the communications path via separate groups of lines referred to herein as Information Transfer Class lines, Response Class lines, Control Class lines, and Power Class lines. Except for Time and Phase Signals (described later) these signals are detected as being asserted whenever one or more interconnecting means asserts them. The information Transfer Class lines, in turn, comprise Information Data and Parity lines, and transmit command, data status and certain other information used in the transaction. The Response Class Lines provide positive confirmation of error-free reception, as well as additional responses to control or alter the transaction. This error monitoring significantly contributes to system reliability, requires little or no additional bandwidth, and allows the responding device to alter the normal progress of the transaction, thus contributing greatly to system flexibility. For example, a device which requires additional time to respond to a command directed to it beyond that normally provided for by the command may utilize one or more of the response signals to delay completion of the transaction (within predetermined limits) until the device is ready to respond, or may notify the device of its inability to respond at that time and thus free the communication paths for other transactions. A set of control signals is generated and utilized by the interconnecting means in each device to provide efficient and orderly transfer of access to the communications path from one device to another. Additionally, each device generates local timing signals from a common system clock to thereby insure synchronous operation. These signals, as well as test control signals, are also transmitted on separate lines over the bus. Finally, the devices monitor the status of the AC and DC power within the system, and provide signals indicating the status of these power sources so that appropriate action may be taken when necessary. The interconnecting means described herein is unusually powerful and versatile, and readily lends itself to economical manufacture by large scale integration techniques currently available. This results from the relatively limited number of physically separate wires required to carry the command, control, information and data signals among devices, arising from the efficient selection and distribution of functions among these lines. Nontheless, the interconnecting means imposes essentially no restrictions on the physical placement of the devices attached to it. Further, it allows interconnection of a wide variety of devices, and efficiently accomodates both single-processor and multi-processor configurations. European publication EP.A.0.076.196 teaches a system wherein several processors share a common resource (eg., a memory bank), through a common bus. When there are conflicts regarding the access requests (to the bus) made by the processors, a resolution circuit allocates selected priorities to the access requests of the processors. With some types of prior art, there may be a bus dead time resulting in a waiting time to fetch or otherwise transfer the desired data. Also, with known prior art arrangements,there may be a monopoly of one or the other processor in getting access to the services of a bus, even if the bus is nonpended, i.e., has no pended read transactions. Specifically dedicated arbitration lines are needed in some known types of prior art arrangements. 2. General Description of the Specific Invention Defined Herein In accordance with the invention specifically defined in this application, each device which seeks control of the path in order to engage in a transaction with other devices monitors the NO ARB control line. Whenever NO ARB is found to be deasserted, the device may arbitrate in the following cycle. This cycle may be a "idle" arbitration cycle, that is, one which occurs at times when no current transaction is taking place on the pathway, or may be an "imbedded" arbitration cycle, that is, one which occurs during the progress of a transaction on the path. On detecting an arbitration cycle, each such device asserts a NO ARB and single line corresponding to its priority. The signals are asserted in decoded form (i.e., as "one of N", where N is the number of different priority levels provided for) on the data lines of the communications path. At the same time, each device monitors the data lines and makes its own determination as to whether it is the device which has a predefined priority characteristic among those devices arbitrating. In the particular implementation specifically described herein, arbitration is performed at two priority levels, namely "high" and "low". Further, within each level there is a sublevel priority ranking from high to low in inverse relation to the device identification number, that is, the sublevel priority decreases as the identification number increases. As particularly described herein, each device determines for itself whether it is the highest priority device arbitrating for access to the communications path at that time. The device determining that is the highest priority device seeking access assumes the status of "Pending Master" and continues to assert NO ARB to prevent other devices from arbitrating for control of the communications path until it assumes control of the path. The Pending Master can assume control of the communications path on the cycle following that in which BSY is deasserted and thereby become Current Master. Further in accordance with the present invention, arbitration is performed not only at different priority levels, but also in different modes. Specifically, a mode control means is provided to establish arbitration by a device at one of a number of fixed priority levels or on a dynamically changing basis (for example, on a "dual round robin" arbitration mode as described hereafter). Further, the device may be set to disable arbitration; this prevents it from gaining control of the communications path. Arbitration mode control is established via a control register which is accessible to, and potentially changeable by, all devices in the system. Thus the arbitration mode of the device may be altered as required in accordance with system needs. Further, in the "dual round robin mode, access to the communications pathway is provided on a basis which gives essentially equal access to the devices over a period of time. The term "dual round robin" is used herein to characterize the behavior of the dynamic arbitration mode of the interconnection means, as opposed to a "pure" round robin scheme. In the latter, if all devices connected to the communications path are in this mode, each obtains control of the path exactly once before any device obtains control of the path a second time. In the "dual round robin" mode, in contrast, two robin "rings" are established, each in itself a "pure" round robin. This results from using the ID of the previous communications path Master in the priority calculation. These rings are effectively overlapped, and the "dual round robin" mode thus provides the same "fairness" as pure round robin to the extent that no device is ever locked out of control of the communications path, and in the worst case latency for any device is the same as in pure round robin. The dual round robin arbitration as such is already described in GB-A-1 158 889. If greater bandwidth is needed by a device at any particular time, the arbitration priority mode of that device may be changed from a dynamic mode to a fixed mode, specifically including a fixed high-priority mode. In this mode, the given device will be provided with access to the communications pathway a larger proportion of time than otherwise, and thus may transfer even greater amounts of data over a period of time. Each device operating in the "dual round robin" mode of arbitration compares its identification number during each Imbedded Arbitration cycle with the identification number of the Current Master. If the identification number of a given device is higher than the identification number of the Current Master, the device updates its priority to the high priority level; otherwise, its priority remains in, or is set to, the low priority level. The decision to arbitrate in a particular Imbedded Arbitration cycle is made prior to updating priority in that cycle so that the arbitration decision is based on the identification number of the previous Master. Accordingly, devices which have low ID numbers and which otherwise might deny access to the communications path by high ID devices are periodically demoted to a low priority level. The interconnecting means of the present invention provides significant benefits. To begin with, it uses only a single additional line in the communications path to provide the facilities for the arbitration process. The remaining lines required for the arbitration are provided by the data lines which are necessarly present to transmit the data which is the end object of the intercommunication. Thus, the system accomodates itself to implementation on a single integrated circuit, an advantage which is significant, and oftentimes critical, in constructing an economical system. Further, the arbitration system provides an extremely flexible manner of allocating resources among competing devices seeking access to a limited facility, the communications path. The allocation of relative priorities among the devices may be changed in a predefined manner among devices competing for access at given times, or may be changed in accordance with varying system demands. Further, because arbitration is distributed among all devices connected to the communications path, the multiple lines, physical placement constraints, and significant overhead usually associated with centralized arbitration is eliminated. Accordingly, it comprises a powerful and unusually flexible mode of operation. Detailed Description The foregoing and other and further objects and features of the invention will more readily be understood from the following detailed description of the invention, when taken in conjunction with the accompanying drawings, in which: Figs. 1A-1C are block and line diagrams of various processor and device configurations which can be implemented with the interconnecting means described herein. Fig. 2 illustrates the signal structure of the interconnecting means; Figs. 3A-3C illustrate various timing signals used in a particular implementation of the interconnecting means, the manner in which local timing signals are generated, and their use in defining a "transaction" among devices connected to the interconnecting means; Fig. 3D illustrates the arbitration function sequence. Fig. 3E illustrates BSY and NO ARB sequences. Figs. 4A-4H are tables setting forth the structure of each of the transaction types utilized by the interconnecting means; Fig. 5A is a table summarizing the command codes of the interconnecting means, while Fig. 5B is a table summarizing the data status codes of the interconnecting means; Fig. 5C is a summary of data length codes of the interconnecting means. Fig. 6 is a Response Code Summary table; Figs. 7A-7I are diagrams of the basic register set utilized by the interconnecting means showing the specific utilization of various bits within each register; and Fig. 8 is a block and line diagram illustrating logic circuitry for implementing the arbitration function of the interconnecting means. 1. Detailed Description of the Interconnecting Means Fig. 1A illustrates the utilization of the interconnecting means described herein in a configuration typical of small and relatively inexpensive computer systems. As there illustrated, a processor 10, memory 12, terminals 14 and mass storage units (disks) 16 are interconnected to each other via interconnecting means 18 and a communications path 20. In the case of processor 10 and memory 12, the interconnecting means 18 are preferably located integrally within the device and thus provide the communications interface to the device. In the case of the terminals 14 and storage units 16, intermediate adapters 22, 24, respectively, may be provided in order to allow the connection of a number of terminal or storage devices to a single interconnecting means 18. The adapters serve to interface the communications path 20 to the remainder of the device. As utilized herein, the term "device" denotes one or more entities connected to the communications path by a common interconnecting means. Thus, in Fig. 1A, the terminals 14 and adapter 22 comprise a single device 26; similarly, processor 10 and main memory 12 are each devices. In Fig. 1B, the processor 32 and memory 34, together with adapter 40 comprise a single device. In Fig. 1A, it will be noted that the processor 10 shares the memory 12 with the other devices connected to communications path 20. This results in lower system cost, but limits system speed because of the need to share the path 20. In Fig. 1B, this problem is resolved by providing a separate memory path 30 between a processor 32 and a memory 34. The processor and memory are then connected to terminal devices 36 and mass storage devices 38 via an adapter 40, a path 42, and adapters 46 and 48. The adapter 40 has an interconnecting means 18 integral with it and connecting adapter to the path 42. Similarly adapters 46 and 48 each have an interconnecting means 18 integral therewith and connecting them to the path 42. A system of this type offers higher performance, but at a higher cost. However, it is still fully compatible with the interconnecting means described herein. Finally, Fig. 1C illustrates the use of the device interconnecting means in a multi-processor system. In this Figure, processors 50 and 52 are connected to primary memories 54, 56, respectively, by memory paths 58, 60, respectively. The processor-memory pairs, in turn, are connected to the remainder of the system via adapters 62, 64, respectively, having interconnecting means 18 incorporated integrally therewith and interconnected by path 68. A cache memory 52 is associated with one of the processors, e.g., processor 50. The remainder of the system is then essentially that shown in Fig. 1B, namely, one or more terminals 70 connected to the path 68 via an adapter 72 having an interconnecting means 18 therein, and a mass storage device 74 interconnected to the path 68 via an adapter 76 having an interconnecting means 18. In this configuration, not only can each processor communicate with each device in the system, but the processors can communicate directly with each other. Further, cache memory 51 effectively accommodated. Despite the differing nature and level of complexity imposed by this demanding mixture of devices in the same system, the interconnecting means described herein efficiently controls all the communications in essentially the same way. Turning now to Figure 2, the various categories of signals generated and utilized by interconnecting means are summarized in accordance with their principal functional class. Within each class, they are grouped by their separate subfunctions. Additionally, the specific wires of the group of wires (or communications path) 78 which carry these signals from one device to another are also shown in order to facilitate subsequent discussion. A line is considered to be asserted if any device attached to the line asserts it. The line is deasserted only if no device is asserting it. For purposes of illustration, two separate interconnecting means, designated A and B, respectively, and integral with the corresponding physical devices whose communications they control, are illustrated schematically by the signals utilized by them, and are shown as interconnected for signal exchange purposes by path 78. However, it should be understood that path 78 will typically physically link more than two devices at any one time, although only those devices selected by the Current Master will actually participate in a transaction. The remaining devices remain physically connected tothe communications path but do not participate in the transaction. As illustrated in Fig. 2, there are four broad classes of signals utilized by the interconnecting means, namely, Information Transfer class signals; Response class signals; Control class signals; and Power class signals. The "Information Transfer" class signals include an Information field, designated I[3:0], which is transmitted and received over four separate lines 80 of the path 78. The information field carries information such as the command code, code identifying the device initiating the transaction (the "Current Master"), and information specifying the status of data transmitted during the cycle, among other information. A thirty two bit data word transmitted over lines 82, labeled D[31:0] in Fig.2 provides certain information needed in the transaction, such as the length of a data transfer that is to take place (used in Read-type and Write-type transactions); the identity of a device which is selected to participate in the transaction; the address of memory locations which are to be accessed for data transfer; and the data which is to be transferred. This word is transmitted and received over thirty two separate lines 82. Two lines, 84 and 86, designated "PO," used for indicating the parity on the information and data lines, and BAD, signalling an error condition, are also provided. The "Response" class of signals comprises a three-bit field, designated CNF[2:0] and transmitted over lines 88, which provides a response to various information transmitted to a device and which allows the devices to alter the progress of the transactions, as described in more detail subsequently. The "Control" class signals are transmitted over a group of eight lines 90-104. The first of these signals, NO ARB, controls the arbitration process. The second of these, BSY, indicates current control of the communications path by a device. These two signals are used in conjunction with each other to provide an orderly transition of control among device seeking control of the communications path. Of the remaining signals in the control class, the Time (+) and Time (-) signals comprise waveforms generated by a single source connected to the path 98 and transmitted over lines 94, 96, respectively; they are used in conjunction with the Phase (+) and Phase (-) waveforms, also generated by a single source, and transmitted over lines 98 and 100, respectively, to establish the local timing reference for operation of the interconnecting means at each device. Specifically, the interconnecting means of each device connected to the path 78 generates local transmitting and receiving clock signals, TCLK and RCLK, respectively, from the Time and Phase signals. Finally, the STF signal, transmitted over line 102, is used to enable a "Fast Self Test" of the local devices, as described in more detail hereinafter, while the RESET signal, transmitted over line 104, provides a means of initializing (setting to a known status) the devices attached to the communications path. In the "Power" signal class, the AC LO and DC LO signals are transmitted over lines 104, 106, respectively and are monitored by each device to determine the status of the AC and DC power within the system. A Spare line 110 provides for future expansion. The interconnecting means described herein performs its function of establishing communication among selected devices by performing a sequence of operations that are specific to the type of communication to be undertaken. Each operation comprises a sequence of cycles during which various elements of information are placed on, or received from, the communications path in order to effectuate the desired communication with another device or devices also connected to this path. These cycles are defined by the Time and Phase clocks as may be understood more clearly on reference to Fig. 3A which shows Time (+) and Time (-) clock signals 120 and 122, respectively, as well as Phase (+) and Phase (-) signals 124 and 126, respectively. These signals are generated by a single Master clock connected to the communications path. They are received by the interconnecting means ofeach device and used to generate the local TCLK and RCLK signals 128 and 130, respectively, which control the transmission and reception of information by them. Thus, as shown in Fig. 3B, a number of devices 140, 142 etc. are connected in parallel to the communications path so as to transmit and receive information over these lines. These devices, may be input/output (I/O) devices such as printers, display terminals, etc. or may be devices such as processors. The physical placement of the devices on the path is immaterial. A Master Clock 144 also connected to the path generates the Time and Phase signals which are transmitted to each device over lines 94-100. Each interconnecting means includes timing circuitry for generating local transmitting and receiving clocks TCLK and RCLK, respectively. For example, device 140 may include a flip-flop 146 whose Q output produces TCLK. The flip-flop is set from a gate 148 and is clocked by the Time (+) signal from line 94. Gate 148 in turn is enabled by line 98 and the Q bar output. In similar fashion, the local Slave receive clock, RCLK, is generated from the received Time (+) and Phase (-) signals. As shown in Fig. 3C, the time between successive TCLK signals defines a cycle. A sequence of successive cycles which is utilized to perform a desired interchange of information is herein called a "transaction." Although the detailed characteristics of each transaction varies in accordance with the operation performed by it, each transaction consists, generally, of a Command/Address cycle; an Imbedded Arbitration cycle; and one or more additional cycles, most commonly designated as "Data" cycles. For purposes of illustration only two such data cycles are shown in Fig. 3C. In general, information is placed on the communications path 78 at the leading edge of TCLK and is latched into the interconnecting means of a device during RCLK of the same cycle. A state diagram of the arbitration function performed by each interconnecting means is shown in Fig. 3D. The arbitration function remains in the idle state 150 until some element in the device causes it to seek to initiate a transaction as indicated by REQ in Fig. 3D. When this occurs, the interconnecting means determines whether it is free to assert its arbitration signals on the path 78 by examining the NO ARB line. As long as NO ARB is asserted, the arbitration function must remain in the idle state. However, as soon as NO ARB is deasserted, the device may arbitrate during the following cycle, provided that REQ is still asserted. Under these conditions, it enters the arbitration state 152 in which the device arbitrates with the other devices seeking access to the communications path. The manner of arbitration will be described in more detail hereinafter. A device losing the arbitration returns to the idle state 150, from which it may again seek to arbitrate as long as REQ is asserted. Conversely, a device winning the arbitration enters either the Current Master state (if BSY is deasserted) or the Pending Master state (if BSY is asserted.) A Pending Master remains Pending Master as long as BSY is asserted, and becomes Current Master following the deassertion of BSY. Before describing the operation sequence of each of the transactions provided for by the interconnect, it will be helpful to obtain a more detailed understanding of some of the Control, Response, and Information Transfer class signals themselves, as these are common to essentially all the transaction types. Control Signals: NO ARB, BSY The NO ARB signal controls access to the data lines for purposes of arbitration. Devices may arbitrate for use of the communications path only in those cycles for which NO ARB has been deasserted for the previous cycle. The device which has control of the interconnect (the "Current Master") asserts NO ARB throughout the transaction except during the first cycle and the last expected data cycle. (The last expected data cycle of a transaction is usually the last data cycle in fact; however, as described more fully hereafter, devices may delay completion of a transaction under certain conditions. When they do, the cycle that is expected to be the last data cycle no longer is, and subsequent cycles follow before all the data is transferred. NO ARB is also asserted by the Pending Master until it becomes the Current Master. At any one time, there is at most only one Current Master and one Pending Master. No ARB is also asserted during an arbitration cycle by all arbitrating devices. During an Imbedded Arbitration cycle, this assertion is in addition to the assertion of NO ARB by the Current Master. During an Idle Arbitration cycle, assertion of NO ARB by an arbitrating device will preclude subsequent arbitrations until one of the devices currently arbitrating becomes Current Master. NO ARB is additionally asserted by Slave devices (devices selected by the Current Master) for all cycles in which the Slave asserts STALL, as well as for all data cycles except the last. It is also asserted by a device (coincidentally with assertion of BSY) during special modes when the interconnecting means is occupied servicing its own device. In these modes, the device does not use any communications path lines other than BSY and NO ARB. Due to the potential of being selected as Slave, a device prevented from entering a special mode during a command/address cycle. A device may operate in a special mode, for example, in order to access registers in the interconnecting means without requiring use of Information Transfer class lines of the communications path. Further, it may also be desirable to allow the Current Master to continue assertion of NO ARB beyond its usual termination cycle to thereby perform a sequence of transactions without relinquishing control of the communications path. This would be particularly useful for high speed devices to allow extended information transfer cycles, and thus effectively increase the available bandwidth for that device. BSY indicates that a transaction is in progress. It is asserted by the Current Master during the entire transaction, except during the last expected cycle. It is also asserted by Slave devices which need to delay progress of the transaction (e.g., a memory device which needs additional time to access a particular memory location); the delay is accomplished by asserting BSY and NO ARB together with a STALL response code (to be described later). In addition, BSY is also asserted for all data cycles except the last. A device may also extend the assertion of BSY in order to delay the start of the next transaction, or when operating in the special modes discussed above. BSY is examined by devices at the end of each cycle; when deasserted, a Pending Master may assert it and assume control as Current Master. Figure 3E is a state diagram of possibly sequences of the BSY and NO ARB control lines in the present implementation. It will be used to illustrate the manner in which the joint observation these signals efficiently control the exchange of information from device to device on the communications path. On the power up all devices assert NO ARB (State "A") effectively preventing access by any device until all devices deassert the line (State "B"), whenever the communications path enters the IDLE state. This allows time for all devices to complete any power up initialization sequence if required. Once NO ARB is desasserted and State "B" is thereby entered, devices may freely seek to contend for control of the communications path. Once a device arbitrates, State "A" is again entered whereupon the "winning" device enters Command/Address State "C". It is important to not that this Command/Address cycle is recognized by all devices not only by the transition assertions of BSY from the deasserted to the asserted state but in conjunction with the assertion of NO ARB in the previous cycle. The observation of NO ARB is necessitated for devices to ignore the special mode state as a Command/Address. The first entry of State "D" from the Command/Address state is indicative of the Imbedded Arbitration cycle of a transaction. It is this cycle that devices update their dynamic priority (if in "dual round robin" mode) by observation of the encoded Master ID. Depending on the data length of the transaction, control may remain in this state for subsequent cycles. If no arbitration occurs, the Master and Slave eventually relinquish control of the communications path and flow proceeds again back to State "B", the deassertion of both control signals. If, however, a Pending Master exists, state F will be subsequently entered, whereupon the device asserting NO ARB will notice the deassertion of BSY in this cycle and proceed either to Command/Address "C" or "G" depending on whether the decision to preclude further arbitration by other devices (referred to as "BURST MODE" in the diagram) is determined by the Master. Note that in State "G" the Command/Address control signals show that NO ARB and BSY are both asserted which differentiates this from Command/Address State "C". If the previous transaction was extended by the assertion of BSY, and no Pending Master had existed, control would have sequenced from State "D" to "E", and remain in State "E" for one or more cycles as required. The witnessed assertion of BSY would cause control to remain in this state for one or more cycles, whereupon the sequence may continue back to IDLE State "B" and relinquish the communications path for future transfers. As described above, a special mode of operation may alternatively caused control to return to State "D" for one or more cycles in one particular device wished preclusion of selection as a Slave by any other device. The simultaneous deassertion of BSY and NO ARB would then again return control to State "B", the IDLE condition. The figure therefore shows that the joint operation of NO ARB and BSY regulates the orderly flow of control exchange as well as information transfer on the communications path. Response Signals: ACK, NO ACK, STALL, RETRY System reliability is greatly increased by requiring a response to transmissions over the Information and Data lines. Generally, response is expected exactly two cycles after the particular transmission. The response code for these devices is shown in Fig. 6, where a "O" bit indicates assertion (low level) and a "1" bit indicates deassertion (high level). The ACK response indicates successful completion of a transmission reception by the intended recipient of the transmission, For all transaction types, the assertion of ACK during the first data cycle of the transaction confirms correct receipt (i.e., no parity error) of the Command/Address information transmitted two cycles earlier. Additionally, in the first data cycle as well as in subsequent data cycles in Read-type and Ident transactions, ACK also indicates that read or vector data is being assertion by the Slave, while in Write-type transactions ACK also indicates that the Slave is prepared to accept Write data. NO ACK indicates either a failure in the transmission/reception or that no Slave has been selected. Both ACK and NO ACK are permissible responses to command transmissions, as well as to data transmissions; in the latter case, the responses occur through the two cycles following the last data cycle, even though these cycles may coincide with a subsequent transaction. NO ACK is the default state of the response lines. It is defined in such a way that any other code may override it. STALL may be asserted by a Slave device during data cycles. For example, it is used by memories to extend the time allowed for a read access or to provide time for a refresh or error correction cycle during a transaction. It is also used by memories to delay further data transmission from the Master when the memory write buffer is full. It is used by devices to synchronize to another communication path. One or more STALLS may be used to delay an ACK or NO ACK command confirmation if the device recognizes that it is the Slave. RETRY is asserted by a Slave device which cannot immediately respond to a transaction. For example, it is used by devices requiring a long internal initialization sequence; by devices waiting for access to another communications path; and by memories which have been locked by an Interlock Read command as described below. The Current Master responds to the Slave RETRY response by terminating the transaction. In the present implementation, RETRY is not used after the first data cycle of a transaction. This simplifies the interconnection logic. One or more STALLS may precede the assertion of Retry. In order to prevent a device from monopolizing the communications path, a limit is placed on the extensions or successive assertions of STALL, RETRY, BSY and NO ARB. System Architecture: Specific Transaction Sequences Figs. 4A-H set forth in detail the specific characteristics of the transactions provided for by the interconnecting means. In particular, transactions for reading and writing data (READ, READ WITH CACHE INTENT, INTERLOCK READ WITH CACHE INTENT, WRITE, WRITE WITH CACHE INTENT, WRITE MASK WITH CACHE INTENT, and UNLOCK WRITE MASK WITH CACHE INTENT); for invalidating obsolete cached data (INVALIDATE); for handling interrupts (INTERRUPT, INTERPROCESSOR INTERRUPT, IDENTIFY): for halting transaction generation by devices (STOP); and for transmitting information to a number of devices simultaneously (BROADCAST) are illustrated in detail. In each of the Figures, the range of permissible CNF responses is set forth, and the particular response illustrated is marked by a dot (.). Further, for purposes of illustration only, the transactions are shown as including only two cycles of data transfer although a larger or smaller number of cycles may be used. The commands described herein are of two general types, namely, single responder commands (Read-type, Write-type commands, and IDENTIFY) and multi-responder commands (STOP, INVALIDATE, INTERRUPT, INTERPROCESSOR INTERRUPT, and BROADCAST). In order to insure the unique recognition of responses when multiple responses are being asserted on the same lines, the permissible responses to multi-responder commands are limited to ACK and NO ACK. Read-Type Transactions Referring now to Fig. 4A, the characteristics of a Read-type transaction are set forth in detail. This type of transaction includes not only the READ command, but also the READ WITH CACHE INTENT and the INTERLOCK READ WITH CACHE INTENT commands as well. The four-bit codes for these commands are shown in Fig. 5A, together with the codes for the other commands utilized by the device interconnecting means. Note that additional codes may subsequently be added, as indicated by the dash (-) in this Figure. The transaction comprises a number of successive cycles, namely, a command/address cycle 180, an Imbedded Arbitration cycle 182, and a number of data cycles. For purposes of illustration only, the transaction is shown as including two data cycles 184, 186, respectively. The principal lines on which information is transmitted (cf. Fig. 2) are indicated by their functional names, namely, the Information lines I[3:0], the Data lines D[31:0], the Confirmation lines CNF[3:0], and the NO ARB, BSY and P (parity) lines. For clarity of illustration, the remaining lines (i.e., Time, Phase, STF, RESET, AC, LO, DC, LO, BAD and SPARE) are omitted in Fig. 4 since they are not essential to understanding the operation of the transactions. As indicated in Fig. 4A, during the command/address cycle of a Read-type transaction, the four-bit command code is placed on the information lines I[3:0]. Additional data required in connection with the command is placed on the data lines D[31:0]. Specifically, a two-bit data length code specifying the length of the transfer which is to take place is applied by the interconnecting means to data lines D[31:30], while the "address" of the device with which the transfer is to take place is applied to data lines D[29:0]. The fact that these signals are asserted on the appropriate lines by the device which currently has control of the interconnect (the "Current Master") is indicated by the letter "M" in the appropriate block in Fig. 4A. The assertion of information on a given line of set of lines by a Slave device is indicated by the letter "S" in Fig. 4A. In similar fashion, the letters "AD", "ADD", "APS" and "PM" (i.e. "All Devices", "All Arbitrating Devices, "All Potential Slaves", and "Pending Master", respectively) indicate various other devices which may assert signals on selected lines of the communications path during particular cycles. The address comprises a single thirty-bit word designating the specific storage location with which a Read-type or Write-type transaction is to take place. A separate block of addresses is assigned to each device. The location of the block is based on the identification number of the associated device. During the Command/Address cycle, the Current Master deasserts NO ARB as shown at 158 in Fig. 4A. (For purposes of discussion herein, a signal is considered "asserted" when at a low level, and "deasserted" when at a high level). Deassertion of NO ARB allows other devices desiring control of the communications path to arbitrate for such access during the following cycle. At the same time, the device asserts BSY to prevent other devices from gaining control of the communications path while the current transaction is in process. No signals are applied to the CNF lines at this time by the Current Master, although it should be understood that, in the course of a sequence of transactions, one or more Response signals may be applied to the CNF lines by other devices during a transaction by a Current Master. The second cycle of the transaction comprises an arbitration cycle. It is referred to as an "imbedded" arbitration cycle since it is contained within a transaction. Arbitration which occurs outside of a transaction is referred to as an "Idle" arbitration cycle. During the Imbedded Arbitration cycle of Fig. 4A, the Current Master places its identification number (ID) on the information lines I[3:0]. This code is used by all devices to update their arbitration priority, as previously described. At this time also, those devices seeking use of the communications path assert a single-bit signal corresponding to their identification number on either the low priority level lines, D[31:16], or the high priority level lines, D[15:0] e.g., device 11 asserts line D[11] if arbitrating at high priority and asserts line D[27] if arbitrating at low priority. The level at which the device arbitrates is determined by its arbitration mode as well as by the ID of the previous Master. In the present implementation, the arbitration mode is defined by bits 4 and 5 of the particular device's control and status register, i.e., CSR[5:4] (see Fig., 7C). As presently implemented, four modes are provided for, namely, fixed high priority, fixed low priority, "dual round robin", and arbitration disabled. The interconnecting means supports mixing these modes at will be appropriately setting the arbitration mode bits CSR[5:4]. In the case of arbitration in a fixed-priority mode, whether fixed high or fixed low, the priority does not vary from transaction to transaction. In contrast, in the case of "dual round robin" arbitration the priority of a device may change from one transaction to another as described previously. In particular, in the "dual round robin arbitration" mode, during a given transaction, a device will arbitrate at a low priority level (i.e., on lines D[31:16]) if its ID number is equal to or less than the ID number of Master in the immediately preceding transaction, and will arbitrate at a high priority level (i.e., lines D[15:0]) otherwise. Continuing on with the transaction of Fig. 4A, at the conclusion of the Imbedded Arbitration cycle, a device which has arbitrated during this cycle and won the arbitration becomes Pending Master, and asserts NO ARB until it becomes Current Master, as shown in dotted lines in Fig. 4A. This prevents other devices from subsequently arbitrating for, and possibly gaining control of, the communications path before the Pending Master can assume such control. The arbitration cycle is followed by one or more data cycles. For purposes of illustration, Fig. 4A shows such data cycles only. As noted previously, the actual amount of data to be transferred in each transaction, and thus the number of data cycles utilized by the transaction, is specified in the command/address cycle by bits D[31:30]. In the particular implementation described in Fig. 4A, from one to four cycles of data (here, 32 bits per cycle) may be transmitted in a transaction. Of course, by providing fewer or more bits for the data length specification, a lesser or greater number of data cycles, and thus transaction cycles, may be provided for. In the case of Read-type transaction as shown in Fig. 4A, the data called for by the transaction is supplied by the Slave to which the transaction is addressed. This device may be a memory device or it may be some other device such as an input/output terminal. In either event, the device so selected asserts its data on the data lines D[31:0] during the data cycle. At this time, also, it asserts a code on lines I[3:0] which indicates the status of the data. For example, for memory references, the code may indicate whether the data is data that has been retrieved without utilization of any correction algorithms (referred to simply as "read data"), data that has been corrected before being asserted on the data lines (referred to simply as "read data"); or data that, for one reason or another, cannot be relied on ("read data substitute"). Further, the status code indicates whether or not, for each of these data categories, the data may be cached. The use of the "don't cache" facility will greatly enhance performance in some systems. These codes are illustrated in Fig. 5B. During the first data cycle, the Slave returns to the Master a confirmation code on lines CNF[2:0] which confirms receipt of the Command/Address information from the Master and which may provide further information to the Master with respect to the Slave's response. Thus, the first assertion of the confirmation signals, for the current transaction, is made during the first data cycle, two cycles after the Command/Address cycle which began the transaction. For the Read transaction described in Fig. 4A, the permissible responses in the first data cycle are the ACK ("Acknowledge"), NO ACK ("Not Acknowledge"), STALL and RETRY. These are largely common to all transactions, with certain exceptions which will be described in connection with the particular transactions. In general, the assertion of ACK during the first data cycle indicates correct receipt of Command/Address information, together with the ability of the Slave to take the requested action i.e., return read data. Conversely, the assertion of NO ACK indicates either an error in transmission of the command or some other inability of a Slave to respond. The assertion of STALL allows the Slave to extend the transaction in order to prepare itself to provide the read data requested by the Master, while the assertion of RETRY indicates current inability to respond to the command, accompanied by a request that the Master try again at a subsequent time. RETRY is appropriately used when the expected response time of the Slave would be so long that it would be undesirable to extend the transaction an excessive number of cycles by asserting general STALL responses. In Fig. 4A, the ACK response (designated by a dot (.) before the response) is illustrated. If the response were NO ACK, the action taken by the Master would differ from that taken in response to ACK, e.g., the Master may seek to repeat the transaction a limited number of times, may call for an interrupt, etc. A STALL response is similar to an ACK response but the transaction will be extended by one or more "blank" cycles (cycles in which no valid data is present on the data lines) before the requested data is returned. The second, and last, data cycle in Fig. 4A is similar to the preceeding data cycle, that is, the Slave asserts the requested data on lines D[31:0] together with a code indicating the status of the data on lines I[3:0]. At the same time, it asserts a confirmation signal on CNF[2:0]. Unlike the Slave's response to the first data cycle, however, the Slave may respond only with ACK, NO ACK, or STALL; it may not assert RETRY. Further, since the second data cycle is the last data cycle of the transaction in Fig. 4A, the Slave deasserts both NO ARB and BSY. If the Slave were to extend the transaction by asserting STALL so that the return of read data would be deferred to a subsequent cycle, the Slave would continue its assertion of NO ARB and BSY until the last data cycle in fact occurred. It would then deassert NO ARB and BSY during that last data cycle. As noted previously, deassertion of BSY allows a Pending Master to assume control of the communications path on the following cycle, while the Slave's deassertion of NO ARB is preparatory to allowing subsequent arbitration to occur for access to the communications path. With the completion of the second and last data cycle, the principal information transfer functions of the transactions of Fig. 4A are completed. However, it is still necessary to confirm the correct receipt of the data. This is accomplished during the two cycles following the last data cycle during which the Master asserts the appropriate confirmation signal on CNF[2:0] with respect to receipt of the data. As shown, the appropriate confirmation is either ACK or NO ACK. Note that the confirmation extends beyond the last data cycle and may thus overlap with the Command/Address and Imbedded Arbitration cycles of a following transaction. However, no error will arise from this since the confirmation lines are not used by the following transaction during its first two cycles. During the Command/Address cycle parity is generated by the Current Master on the I[3:0] and D[31:0] lines, and is checked by all devices. During the Imbedded Arbitration cycle, it is generated by the Master on the I[3:0] lines only and checked by all devices. During the data cycles, parity is generated by the Slave on the I[3:0] and D[31:0] lines and is checked by the Current Master. The specific consequence of a parity error will depend on the nature of the information being transmitted during the given cycle when the error occurs. At a minimum, devices detecting a parity error during the Command/Address cycles should not respond to selection; additionally, they may indicate the parity error by setting an error flag, initiating an interrupt, or other such action. As noted previously, the Read With Cache Intent command has the same format as the Read transaction. It is generated by devices with cache to indicate to the Slave that the requested read data may be placed in the Master's cache. When this command is used in conjunction with the INVALIDATE command described below, it can provide a significant performance enhancement in certain systems with cached devices. The Interlock Read transaction also has the same format as the Read transaction. It is used with shared data structures to provide exclusive access to data by processors and other intelligent devices. Slaves supporting the Interlock Read command have one or more interlock bits corresponding to designated storage locations. When accessed by an Interlock Read Command, a Slave sets the appropriate bit corresponding to the addressed location. This prevents subsequent Interlock Read accesses to the location until the bit is reset to thereby unlock the given location. This bit is typically reset by the UNLOCK WRITE MASK WITH CACHE INTENT Command described below. The INTERLOCK READ command is especially useful in systems having processors which provide read-modify-write operations to insure that intervening devices using the Interlock Read Command are precluded from access to data after the initiation, but before the completion, of such an operation. Slaves addressed by INTERLOCK READS while the interlock is set issue a RETRY. Note that the interlock bit is set only if the Interlock Read transaction is successful, i.e., the Master confirms correct receipt of the Slave's read date. Write-Type Transaction Turning now to Fig. 4B, the Write-type transactions (as implemented, WRITE, WRITE WITH CACHE INTENT, WRITE MASK WITH CACHE INTENT, and UNLOCK WRITE MASK WITH CACHE INTENT) are shown in detail. Starting with the Command/Address cycle, the current Master places the appropriate four bit code for the command on information lines I[3:0]; a two-bit code identifying the length of the data transmission on data lines D[31:30]; and an address on data lines D[29:0]. At the same time, it asserts BSY to indicate the occupied status of the communications path, and deasserts NO ARB to signal the availability of the data lines for arbitration during the immediately following cycle. During the second cycle, the Current Master places its ID on information lines I[3:0]. Devices seeking control of the communications path for a subsequent transaction assert a singly bit corresponding to their ID on the data lines at this time. As was previously the case, the assertion is made of one of the low priority data lines D[31:16] for arbitration at the low priority level, and is made on the high priority data lines D[15:0] for arbitration at the high priority level. The Master continues to assert BSY at this time, and the Master, as well as devices participating in the arbitration, assert NO ARB at this time also. In the example shown in Fig. 4B, the third and fifth cycles are data cycles. Although two data cycles are shown, a lesser or greater number may be utilized, in accordance with the transfer length specified on lines D[31:30] in the Command/Address cycle. The data being written by the Master is applied to data lines D[29:0] during these cycles. The Information lines I[3:0] carry either a write mask (in the case of a Write Mask transaction) during the data cycles to indicate the selected byte or bytes which are to be written during the transaction, or are "undefined" (in the case of Write and Write With Cache Intent transactions). The "undefined" status of the I[3:0] lines indicates that any information on these lines is to be ignored by the devices for purposes of the transaction. During the first data cycle, the Current Master continues to assert BSY and NO ARB. During the fourth data cycle, which the Current Master expects to be the last data cycle, the Current Master deasserts both BSY and NO ARB to prepare for an orderly transition of communications path control. In order to illustrate the capability of a Slave to extend a transaction, the fourth cycle (Data 2) is shown as stalled by way of the Slave's assertion of STALL. For example, this may be done when the Slave is unable to accept the second data word at this time. The Slave asserts BSY and NO ARB during this cycle. The last data cycle of this transaction is cycle 5. During this cycle the Master responds to the assertion of STALL by retransmitting Data 2. The Slave asserts ACK on the CNF lines; and the Slave deasserts both BSY and NO ARB. In the two cycles following the last data cycle, the Slave continues to assert ACK to confirm the correct receipt of Write data. When a Write-type transaction occurs on the communications path, devices connected to the path and having resident cache memory invalidate any cached data within the address range of the write command. As was the case with the READ WITH CACHE INTENT command, the WRITE WITH CACHE INTENT command, when used with the Invalidate command offers significant performance advantages in certain systems. The write mask is a four-bit code indicating, by the presence of asserted bits in one or more of the four-bit positions, the selection of the corresponding eight-bit bytes to be written. Thus, the code 1001 indicates that only the first and fourth bytes (corresponding to D[7:0] and D[31:24], respectively) of a four byte (32 bit) word are to be written. The UNLOCK WRITE MASK WITH CACHE INTENT command is used in conjunction with the Interlock Read command to implement indivisible operations such as a read-modify-write operation. As may be seen from Fig. 4B, during a WRITE-type transaction, parity is generated by the Master during all cycles of the transaction. It is checked by all devices during the Command/Address and Imbedded Arbitration cycle; and by the Slave during the data cycles. Invalidate Transaction The Invalidate transaction is used by systems having cache memories associated therewith. It is issued by devices under certain conditions to guarantee that obsolete data that may be present in the caches of other devices is not used. In the Command/Address cycle of this transaction, as shown in Fig. 4C, the Current Master asserts the Invalidate command on information lines I[3:0] and the starting address of the data to be invalidated on data lines D[29:0]. The number of consecutive locations of cached memory to be invalidated is indicated by the data length code on lines D[31:30]. The Command/Address cycle is followed by the usual Imbedded Arbitration cycle, and a data cycle during which no information is transmitted. As with other multi-responder commands, the specified permissible responses are ACK and NO ACK. Interrupt and Identify Transactions An interrupt transaction is illustrated in Fig. 4D. The purpose of the transaction is to notify other devices (typically, processors) of the need to interrupt current activities in order to take other action. The interrupted device responds with an IDENT command to solicit the Interrupt Vector. This vector serves as a pointer to the address of an interrupt routine stored in memory which will establish the required action. The Interrupt transaction comprises a Command/Address cycle, an Imbedded Arbitration cycle, and a data cycle, in which no information is transmitted. During the Command/ Address cycle, the Interrupt command code is asserted on the Information lines I[3:0] by the device seeking to interrupt. During this cycle, the interrupting device also asserts one or more interrupt priority levels on data lines D[19:16] to identify the immediacy of requested services. The interrupting device also places an interrupt destination mask on data lines D[15:0]. This mask specifies the devices to which the interrupt is directed. All devices on the communications path receive this mask. If any asserted bit in the mask corresponds to the device's decoded ID, then the device is selected. This device will later respond with an Identify transaction. Devices which have been selected by the interrupt respond by transmitting an ACK signal two cycles after the Command/Address cycle, As with all other multi-responder commands, ACK and NO ACK are the only permissible responses. Devices selected during an interrupt may be expected to engage in a subsequent transaction with the interrupt-requesting device in order to complete the interrupt process. Accordingly, each responding device maintains a record for each interrupt level to indicate whether an interrupt was received at the corresponding level. Typically, the "record" comprises a flag bit in a flip flop (hereinafter referred to as an Interrupt Pending Flip-Flop). Each bit remains set until the corresponding interrupt has been serviced. The second and third cycles comprise the usual Imbedded Arbitration cycle as previously described, as well as a data cycle in which no further information is transmitted. Confirmation is made by one of the confirmation codes permissible for multi-responder commands, ACK or NO ACK. Fig. 4E illustrates an Identify transaction. This transaction takes place in response to an Interrupt transaction. During the Command/Address cycle, the Current Master asserts the Identify command code on information lines I[3:0] and asserts on data lines D[19:16] a code corresponding to one or more interrupt levels to be serviced. It also asserts BSY and deasserts NO ARB. The following cycle is the usual Imbedded Arbitration cycle. In the next cycle, the Current Master reasserts its ID number, this time in decoded form on data lines D[31:16]. Each device that requires service at an interrupt level specified in the Command/Address cycle compares the decoded Master ID with the interrupt destination mask that it had earlier transmitted in order to determine whether it is one of the devices to which the Identify command is directed. If it determines that it is, it manifests its status as a Potential Slave participating in the Interrupt Arbitration cycle. During both the Decoded Master and the Interrupt Arbitration cycles, the interrupting Slaves also assert BSY and NO ARB. During the Interrupt Arbitration cycle, the devices arbitrating to transmit their interrupt vector assert their decoded ID number on the appropriate one of the data lines D[31:16]. Arbitration takes place in the manner previously described, that is, the device having the highest priority (lowest ID number) "wins" the arbitration, thereby becoming the Slave. The Slave then asserts its interrupt vector on the data lines. This vector points to a location in memory which contains a further vector identifying the start of the interrupt service routine. At the same time, the Slave transmits a vector status code on information lines I[3:0] indicating the status of the vector in much the same manner as the data status indicated the status of the read data on these lines during a Read transaction. As was the case with previously described transactions, the BSY signal is asserted by the Master during the transaction from the first cycle to the last expected cycle, while NO ARB is asserted from the Imbedded Arbitration cycle to the last expected cycle. ACK, NO ACK, STALL and RETRY may be asserted by the Slave in response to the Identify command. This response occurs in cycle five, which is two cycles later than for all other transaction types. During the two cycles following the vector cycle, the Master asserts the ACK confirmation code to indicate successful completion of the transaction. On receipt of the Slave's acknowledgement of the Identify command, the Master resets the Interrupt Pending flip flop corresponding to the interrupt level for which the interrupt vector was transmitted. If the Slave does not receive the Master's acknowledgement to its transmission of the Interrupt Vector, it retransmits the Interrupt transaction. A device may not participate in the interrupt arbitration cycle if it has detected a parity error in either the Command/Address or the Decoded Master ID cycles. Devices which have been arbitrated during the Interrupt Arbitration cycle but which have lost the arbitration are required to reissue the Interrupt Command. This prevents loss of previously posted interrupts. Interprocessor Interrupt Transaction A simplified form of interrupt is provided for multiprocessor systems when one processor seeks to interrupt one or more other processors. The Interprocessor Interrupt transaction, illustrated in Fig. 4F, comprises a Command/Address cycle, an Imbedded Arbitration cycle, and a data cycle in which no information is transmitted. In the particular implementation used to illustrate the intercommunicating means herein, this transaction makes use of three registers, namely, Interprocessor Interrupt Mask, Destination, and Source Registers 212, 214 and 216 respectively (Fig. 7A). The Mask Register contains a field that identifies the processors from which Interprocessor interrupt commands will be accepted. The Destination register contains a field that identifies the processors to which an Interprocessor Interrupt Command is to be directed; the Source Register contains a field that identifies the source of Interprocessor Interrupt transaction received by a processor. During the Command/Address cycle, the interrupting processor asserts the interprocessor interrupt command code on the information lines I[3:0]. At the same time, it asserts its decoded Master ID on the data lines D[31:16] and asserts a destination code (e.g., from its Interprocessor Interrupt Destination Register) on data lines D[15:0]. During the following Imbedded Arbitration cycle, the interrupting processor asserts its ID on the Information lines I[3:0], and arbitration proceeds in the usual manner. During the third cycle, the devices addressed by the Destination Code asserted in the Command/Address cycle compare the decoded Master ID with the mask in the Mask Register to determine whether the Master is a device to which they may respond. If so, in addition, the Decoded Master ID is preferably stored in the Interprocessor Interrupt Source register in order to maintain the identity of interrupting devices. This saves the processor the overhead of later seeking an Interrupt Vector as is done in the Interrupt transaction. The permissible Slave confirmation signals are ACK and NO ACK as for any other multi-responder command. Stop Transaction The Stop transaction is illustrated in Fig. 4G. It facilitates diagnosis of failed systems by stopping further generation of transactions by selected devices while allowing them to continue responding as Slaves. Devices selected by a Stop Transaction must abort any Pending Master state and deassert NO ARB. In order to facilitate error diagnosis, it is preferred that such devices maintain at least certain minimum information concerning error conditions existing at the time of the Stop Transaction. For example, it is desirable that the information contained in Communications Path Error Register 204 (Fig. 7D) be maintained for subsequent analysis. During the Command/Address cycle, the Current Master performing a Stop transaction asserts the appropriate command on information lines I[3:0] and asserts a destination mask on data lines D[31:0]. The mask comprises a number of bits which, when set, identify the devices which are to be stopped. The Command/Address cycle is followed by the usual Imbedded Arbitration cycle and a data cycle during which no information is transmitted. The information transmitted during the Command/Address cycle is confirmed two cycles later by all devices selected by the Stop transaction. Broadcast Transaction The Broadcast transaction, illustrated in Fig. 4H, offers a convenient means of broadly notifying devices on the communications path of significant events while avoiding the overhead costs of Interrupt transactions. During the Command/Address cycle of the transaction, the Current Master initiating the Broadcast transaction asserts the appropriate command code on Information lines I[3:0] and places a two-bit data length code on data lines D[31:30]. At the same time, it places a destination mask on data lines D[15:0]. This mask specifies the devices which are selected by the broadcast transaction. For example, a "one" bit asserted on data lines 2, 3, 5, 9, 12, 13, and 14 will select devices 2, 3, 5, 9, 12, 13, and 14 for receipt of the Broadcast. The Command/Address cycle is followed by the usual Imbedded Arbitration cycle which in turn is followed by one or more data cycles. For purposes of illustration only, two data cycles are shown. The data itself is asserted on data lines D[31:0] by the Master. As with Write-type transactions, the Slaves issue either ACK or NO ACK two cycles later. Register Complement Fig. 7A shows the register file contained in the present implementation of the interconnecting means. These comprise a Device-Type Register 200, a Control and Status Register 202, a Bus Error Register 204, an Error Interrupt Control Register 206, an Error Vector Register 208, an Interrupt Destination Register 210, an Interprocessor Interrupt Mask Register 212, an Interprocessor Interrupt Destination Register 214, and an Interprocessor Interrupt Source Register 216. These registers comprise both 32 bit registers (e.g., registers 200, 204) and 16 bit registers (e.g., registers 202, 206, 208, 210, 212, 214 and 216. In the Device-Type Register 200, (Fig. 7B), the code for the device-type is stored in the lower half (DTR[15:0]) of the register. The device-type is loaded into this register on system power-up or on subsequent initialization of the system. This register may be interrogated by other elements in the system, usually a processor, to determine what devices are connected to the system for purposes of optimizing and dynamically rearranging, the system configuration. A Revision Code field (DTR[31:16]) is provided for in the upper half of the Device-Type register. The Control and Status Register 202 contains a number of bits indicating the status of various conditions within the device, as well as within the interconnecting means to which it is attached. Additionally, it stores information utilized in arbitrating for control of the communications path. Thus, bits CSR[3:0] store the encoded form of the device ID which also is loaded into this register on power up or on subsequent initialization. Bits CSR[5:4] specify the arbitration mode in which the device will arbitrate. As described earlier, these modes comprise "Dual Round Robin", Fixed High, Fixed Low, and Arbitration Disabled modes. On power up or on subsequent Initialization, the arbitration mode is set to "dual round robin." However, this mode may be changed by writing to these bits during system operation. CSR[7] and CSR[6] are Hard Error Interrupt Enable and Soft Error Interrupt Enable bits, respectively. When set, they enable the device to generate an Interrupt transaction (referred to hereafter as an Error Interrupt transaction) whenever the Hard Error Summary Bit CSR[15] or Soft Error Summary bit CSR[14], respectively, are set. These latter bits are set when a hard or a soft error, respectively, is detected. A "hard" error is one which affects the integrity of data on this system; for example, a parity error detected on the data lines during transmission of data is a hard error. Conversely, a "soft" error is one which does not affect the integrity of the data in the system; for example, a parity error detected on the Identification I[3:0] lines during the Imbedded Arbitration cycle may lead to an incorrect calculation by a device but will not affect the integrity of data on the communications path. Accordingly, it is a soft error. The Unlock Write Pending bit CSR[8] indicates that an Interlock Read transaction has been successfully transmitted by the device but that a subsequent Unlock Write Mask with Cache Intent command has not yet been transmitted. Start Self Test bit CSR[10], when set, initiates a self test which checks out the operation of the interconnect logic. The Self Test status CSR[11] remains reset until the self test has been successfully completed, at which time the STS bit is set to indicate successful completion of the test. The Broke bit CSR[12] is also set if the device has failed its self test. The Initialization bit CSR[13] is used in conjunction with system initialization. For example, it may be used as a status indicator while the device is undergoing Initialization. CSR[23:16] specifies the particular design of the interconnecting means. Bits CSR[31:24] are presently not used. The Bus Error Register 204 records various error conditions during system operation. The Null parity Error bit BER[0], the Corrected Read Data Bit BER[1] and the ID Parity Error Bit BER[2] records soft errors, while the remaining bits record hard errors. The Null Parity Error Bit is set if incorrect parity was detected during the second cycle of a two-cycle sequence during which NO ARB and BSY were deasserted. The Corrected Read Data bit is set if a Corrected Read Data Status Code is received in response to a Read-type transaction. The ID parity error bit is set if a parity error is detected on the I[3:0] lines carrying the encoded Master ID during an Imbedded Arbitration cycle. Illegal Confirmation Error bit BER[16] indicates receipt of an illegal confirmation code during a transaction. Nonexistent Address bit BER[17] is set on receipt of a NO ACK response to a read-type or write-type command. Bus timeout bit BER[18] is set if a Pending Master ever waits more than a predetermined number of cycles to assume control of the interconnect. In the specific implementation described herein, a timeout of 4096 cycles is implemented. STALL timeout bit BER[19] is set if a responding (Slave) device asserts STALL on the response lines CNF[2:0] for more than a predetermined number of cycles. In the present implementation, the stall timeout occurs after 128 cycles. The RETRY timeout bit BER[20] is set if a Current Master receives a predetermined number of consecutive RETRY responses from a Slave with which it is communicating. In the present implementation, this timeout is set for 128 consecutive RETRY responses. The Read Data Substitute Bit BER[21] is set if a data status comprising a Read Data Substitute or a Reserved Status Code is received during a Read-type or Identify transaction and there has been no parity error during this cycle. The Slave Parity Error bit BER[22] is set when a Slave detects a parity error on the communication path during a data cycle of a Write-type or Broadcast transaction. The Command Parity Error bit BER[23] is set when a parity error is detected during a Command/Address cycle. The Identify Vector error bit BER[24] is set by a Slave on receipt of any confirmation code other than ACK from the Master Identify transaction. The Transmitter During Fault bit BER[25] is set if a device was asserting information on the data and information lines (or, during Imbedded Arbitration, just on the information lines) during a cycle resulting in the setting of the SPE, MPE, CPE, or IPE bit. The Interlock Sequence Error Bit BER[26] is set if a Master successfully transmitted a Write Unlock transaction without having previously transmitted the corresponding Interlock Read transaction. The Master Parity Error bit BER[27] is set if the Master detects a parity error during a data cycle having an ACK confirmation on the CNF[2:0] lines. The Control Transmit Error bit BER[28] is set when a device detects a deasserted state on the NO ARB, BSY, or CNF lines at a time when the device is attempting to assert these lines. Finally, the Master Transmit Check Error bit BER[29] is set when the data that the Master is attempting to assert on the Data, Information or Parity lines fails to match the data actually present on these lines. However, the assertion of the Master ID during an Imbedded Arbitration is not checked. Turning now to Fig. 7E, the structure of the Error Interrupt Control Register 206 is shown in detail. When a bit is set in the Bus Error Register, and the appropriate Error Interrupt Enable bit is set in the Control and Status Register, or when the force bit is set in the Error Interrupt Control Register, an Error Interrupt will occur. Bits EICR[13:2] contain the Error Interrupt vector. If the Force bit EICR[20] is set, the interconnecting means will generate an Error Interrupt transaction at the levels specified by bits EICR[19:16]. The Sent bit EICR[21] is set after an Error Interrupt has been transmitted. When set, it prevents the generation of further interrupts by this register. This bit is reset on losing an Interrupt Arbitration for the Error Interrupt. The Interrupt Complete Bit EICR123] is set on successful transmission of the Error Interrupt Vector. The Interrupt Abort bit EICR[24] is set if an Error Interrupt transaction is not successful. Turning now to Fig. 7F, the Interrupt Destination Register 210 contains an interrupt destination field IDR[15:0] which identifies which devices are to be selected by interrupt commands originated by this device, as previously described. The Interprocessor Interrupt Mask Register 212 is shown in Fig. 7G. This register contains a Mask Field IIMR[31:16] which identifies devices from which interprocessor interrupts will be accepted. Similarly, the interprocessor interrupt destination register 214 contains a destination field IIDR[15:0] which identifies devices to which interprocessor interrupt commands are to be directed. Finally, the Interprocessor Interrupt Source Register 216 contains a source identification field IISR[31:16], which stores the decoded ID of a device sending an interprocessor interrupt command to this device provided the ID of the sending device matches a bit in the Interprocessor Interrupt Mask Register of this device. 2. Further Specific Description of the Arbitration Logic Fig. 8 is a block and line diagram illustrating the priority arbitration logic in accordance with the invention particularly described herein. For convenience in discussion, the logic is shown as positive logic in which inputs are present, and outputs are generated, when the respective signals are "high" and are absent otherwise. Since the signals on the communications path are in fact negative logic, that is, are present when "low," they should be regarded as having passed through inverters prior to application to the logic of Fig. 8. A cycle controller 300 receives the NO ARB and BSY signals from the control lines 90 and 92, respectively, of lines 78, and also receives the local clock signals TCLK and RCLK. These signals are used to define the start of a transaction. In particular, in response to the detection of a Command/Address cycle by detecting the assertion of BSY following a cycle in which NO ARB was asserted, the controller 300 generates a delayed signal IARB, to define the Imbedded Arbitration cycle. The controller also generates a signal, ARB, (designated "Arb" in Fig. 7A) that is used to demarcate any arbitration cycle which is asserted (i.e., low) for the duration of the imbedded arbitration cycle. Devices seeking access to the bus lines 78 in order to conduct a transaction assert a single line corresponding to their ID number in one of two groups of lines comprising the data lines 82 of bus lines 78. In particular, a device will assert a single line in the high priority group of lines D[15:0] if it is to arbitrate at a high priority level and will assert a single line in the low priority group of lines D[31:16] if it is to arbitrate at a low priority level. The ID number of the device is applied to these lines from an ID register 302. This register may be the same as bits CSR[3:0] of the control and status register (Fig. 7C), or may be loaded from that register, or may be a separate register loaded from the device ID plug on power up. Regardless of the manner in which the register 302 is implemented, it contains a four-bit code specifying the ID number of the device in question. The output of register 302 is applied to a four-to-sixteen decoder 304 which converts the four bit code (the "encoded" ID) to a one-of-sixteen code (the "decoded" ID) applied to lines 306. These lines in turn are applied to a multiplexer 308 which in turn connects them to the high-priority or low-priority data line groups depending on the status of enabling signals at enable input terminals 310. The enable inputs 310 in turn are driven from a mode select register 312 and a comparator 314 via AND gates 316, 318 and OR gates 320, 322. The mode select register 312 specifies the mode in which arbitration is to take place. As presently provided for, this mode is one of our modes, namely, arbitration at fixed high priority, according to a "dual round robin" basis, or "Arbitration Disabled." A two-bit code is adequate to select these modes. These bits are stored at bits CSR[5:4] (Fig. 7C) of the control and status register. Accordingly, the register 312 may itself comprise these bits of that register, or may be loaded from these bits of that register. Two-to-four decode 324 converts the (encoded) two bit binary form to a (decoded) one-of-four form which are applied to one or more of gates 316, 318, 320 and 322. Lines 326, 328, 330 and 332 are energized, respectively, when the output of decoder 316 indicates that arbitration does not take place ("Arbitration Disabled"); that arbitration is to take place at fixed low priority ("FL"); that arbitration is to take place at a fixed high priority ("FH"); or that arbitration is to take place on a "dual round robin" basis ("RR"). Line 326, when energized, deactivates drivers 334 to prevent transmission of the device's ID onto the data lines 82. When this occurs, the device cannot participate in any arbitration. Line 328, when energized, enables the low priority input 310b of multiplexer 308 via OR gate 322 to thereby switch the priority assertion signal applied to one of the lines 306 at the input of the multiplexer to one of the low priority data lines D[31:16]. Similarly, line 330, when energized, enables input 310a of multiplexer 308 to thereby switch the selected one of the lines 306 to the high priority data lines D[15:0] of data lines 82. Arbitration in the "dual round-robin" mode requires comparison of the device ID number with the ID number of the Current Master of the immediately preceding transaction. This is accomplished by means of Receive Register 340, clocked by IARB ANDed by RCLOCK with input gate 348. The comparator 314 compares the ID of its own device (i.e., the device in which the comparator is located) with the ID of the prior Master. If the ID of its own device is equal to or less than the ID of the prior Master, comparator 314 energizes a line 315 to thereby enable input 310b of multiplexer 308 and thus establish arbitration of the particular device on the low priority data lines. Alternatively, if the ID of the present device is greater than the ID of the prior Master, comparator 314 energizes line 317 and thus enables input 310a of multiplexer 308 via gates 318 and 322 to thereby establish arbitration for this device on the high priority lines of data lines 82. FIG. 8 also illustrates the manner in which a particular device is determined to have "won" an arbitration. In particular, a priority resolution unit 380 within the interconnecting means determines the win/lose result of the arbitration. Receive drivers 341 receive the arbitration cycle bit pattern from data lines 78 when enabled by the ARB signal assertion sourced by Cycle control 300. The outputs of drivers 341 are divided into 16-line groupings D[15:0], the high priority set and D[31:16], the low priority set. The set of lines D[15:0] and D[31:16] input priority encoders 370 and 372 respectively. Each priority encoder contains circuitry for the determination of the encoded ID of the highest priority level asserting device in the 16 bit input pattern, and outputs the result on lines 362 during the arbitration cycle. Multiplexer 367 is enabled as follows. Signal 364 is a "0 DETECT" which asserts if no bits are sensed by 370 to be asserted on D[15:0] during the arbitration cycle. Signal 364 assertion causes input 362 a bit pattern to be channelled to 368. Signal 364 deassertion causes 362b pattern to output to 368. This function enables comparator-366 to compare the device's own ID only with the ID of the highest priority. Comparator 366 outputs WON ARB asserts therefore as follows: ID368 = THIS DEVICES ID WON ARB is deasserted for all other conditions of 366 inputs. Thus, only that device (or devices) determined as having the highest priority will generate a "won arbitration" signal within its interconnect controller. When this signal is generated, the device may assume the status of Pending Master on the next cycle, and will retail that status until it assumes control of the subsequent transaction as Current Master. Conclusion From the foregoing, it will be seen that we have provided a system for efficiently assigning priority among a group of devices, each seeking control of a limited facility such as a communications path. Devices in the system may operate in differing priority modes, including fixed priority and "dual round robin" modes. These modes may be changed by the operating system or by the device itself in order to optimize overall system performance. Accordingly, an usually versatile system is provided. Further, each device determines for itself whether and when it is permitted to control the communications path, independently of the total number of devices either connected to the path or seeking control of it, and its physical placement on the path is unrestricted. This permits the addition or removal of devices from the communications path without the need to modify devices already connected to it or to modify the path itself. Additionally, the system requires only a single additional line in the communications path, over and above that provided for other purposes, for performing the requisite arbitration, and thus facilitates implementation of the interconnecting means on a single integrated circuit with present technology. This results in a usually economical control mechanism for controlling communications among computer devices interconnected to form a digital computer system.

Claims: An interconnecting circuit in a device for providing a communications interface for that device to a non-pended bus (fig. 8: 78), the device having a unique ID number (302), and the device initiating a transaction at times when a request signal is asserted by asserting a busy line (92) to obtain access to the bus, the bus including a plurality of information lines (80), a plurality of data lines (82), a no arbitration line (90), and the busy line, and wherein each of the transactions has a command/address cycle (for example fig. 4A: 180) followed by an imbedded arbitration cycle (182) which is then followed by at least one data cycle (184, 186), the no arbitration line being asserted by any device except during the command/address cycle, the circuit comprising: controller means (300) coupled to the no arbitration (90) and busy lines (92) and responsive to a de-assertion of the no arbitration line for indicating when the imbedded arbitration cycle is occurring during a current transaction; driver means (334) coupled to the controller means for asserting one of two data lines corresponding to the device during the imbedded arbitration cycle; priority resolution means (380) coupled to the data lines (82) for asserting a won arbitration signal to indicate that the device had the highest priority during the imbedded arbitration cycle, wherein assertion of the won arbitration signal enables the device to assert the busy line during a subsequent cycle to initiate the next transaction after said busy line is de-asserted by a previous device having access to the bus during the current transaction; receive register and comparing means (340, 314) coupled to the information lines (80) for comparing the ID number (302) of the device with an ID number of the previous device which is present on the information lines (80) during the imbedded arbitration cycle, and for setting an indicator (315, 317) in either a first or second state according to whether the ID number of the device seeking to initiate a transaction is greater than that of the previous device; and switching means (308), responsive to the receive register and comparing means (340, 314) and coupled to the driver means (334), for asserting a low priority data line corresponding to the device during the imbedded arbitration cycle when the indicator is in the first state, and for asserting a high priority data line corresponding to the device during the imbedded arbitration cycle when the indicator is in the second state, wherein the device has a variable priority and is allocated access to the bus fairly according to a dual-count, dual round robin algorithm. A circuit in accordance with claim 1, wherein the device initiates an error interrupt transaction when an error condition occurs during system operation, and the circuit is further characterized by: bus error register means (204) for recording the error condition occurring during system operation. A circuit in accordance with claim 2 , in which the bus error register means indicates to the device that a parity error was received from the bus during a data cycle of a write-type transaction. A circuit in accordance with claim 3, in which the device is a memory (12). A circuit in accordance with claim 3, in which the device is a mass storage unit (16). A circuit in accordance with claim 2, in which the bus error register means indicates to the device that an illegal confirmation code was received from the bus during a transaction. A circuit in accordance with claim 2, in which the bus error register means indicates to the device that an acknowledgement was not received from the bus during an identify transaction. A circuit in accordance with claim 1, in which the device is a memory (12). A circuit according to claim 1 wherein said transaction is an error interrupt transaction and said controller means further includes: means (18) for asserting a request signal at times when the device seeks to initiate the error interrupt transaction; A circuit including a memory in accordance with claim 9, wherein the device initiates the error interrupt transaction when an error condition occurs during system operation, and further characterized by: bus error register means (204) for recording the error condition occurring during system operation. A circuit in accordance with claim 10, in which the bus error register means indicates that a parity error was received from the bus during a data cycle of a write-type transaction. A circuit in accordance with claim 10, in which the bus error register means indicates that an illegal confirmation code was received from the bus during a transaction. A circuit in accordance with claim 10, in which the bus error register means indicates that an acknowledgement was not received from the bus during an identify transaction.

Inventors: BOMBA FRANCK C, JENKINS STEPHEN R, STRECKER WILLIAM D, BOMBA, FRANCK C., JENKINS, STEPHEN R., STRECKER, WILLIAM D.
Applicants: DIGITAL EQUIPMENT CORP, DIGITAL EQUIPMENT CORPORATION
Application year: 1988
Main classification: G06F13
IPC codes: G06F13