Bus control architecture for microprocessors

Abstract: A transportable bus control architecture for single-chip microprocessors is disclosed. The bus control architecture consists of an interface control unit that is logically indepen­ dent of the associated co-resident, common clock-driven microprocessing unit. This independence allows the inter­ face control unit logic to be used with a variety of micro­ processing units. Further, the interface control unit presents an external appearance that is compatible with the peripher­ al devices of a specific microprocessor referred to as the "compatible microprocessor", thereby making available to an associated co-resident microprocessing unit the support devices of the compatible microprocessor. The interface control unit can also access other external devices not related and transparent to the devices of the compatible microprocessor. The interface control unit is logically divided into an execution section and a control section. The ex­ ectuion section is controlled by the control section and comprises various registers, latches, multiplexers, logic, and data and address paths that provide communication be­ tween the co-resident microprocessing unit and off-chip devices. The control section of the interface control unit executes commands from the co-resident microprocessing unit and also performs bus arbitration, interrupt, and exter­ nal reset functions. Bus cycles are of two types: memory-­ access or service, depending on the command from the co-resident microprocessing unit. Service cycles perform the interrupt acknowledge functions and other sense and control functions requested by the co-resident microprocessing unit. These sense and control functions have the special feature of being pin-programmable and pin-readable by the microp­ rocessing unit. All action initiated by commands from the microprocessing unit elicits a comprehensive status re­ sponse from the interface control unit.

Description: The present invention relates to microprocessor bus controllers and, particularly, to a logically distinct interface control unit (ICU) which resides on the same very large scale integrated (VLSI) circuit chip as the microprocessing unit (MPU). The ICU of the invention, in effect, surrounds the MPU and is the means by which the MPU communicates with the outside world. In the past, in the design of a microprocessor circuit chip, the bus controller has been integrated into and functions as part of the microprocessor architecture. This was certainly the expedient design for the early, relatively simple microprocessors for which also a large repertoire of peripheral devices was not yet in existence. But, because the bus controller was integrated into the architecture of the microprocessor, it was impossible to separate and use the bus controller with other MPU architectures. And, consequently the peripheral devices of one microprocessor could not, in general, be used directly with bus of another microprocessor. Mini-Micro Conference Record, Paper 8/1, published May 1984, pages 1 to 7 describe a single chip microprocessor comprising a microprocessing unit and external signal pins to which off-chip devices can be connected. The microprocessor further comprises an interface control unit connected to the microprocessing unit and to the external signal pins to permit the microprocessing unit and any off-chip devices connected to the pins to communicate with one another in response to commands from the microprocessing unit and control signals from off chip devices. With the proliferation and increased sophistication of the many commercially available microprocessors, it can be desirable to separate the architecture of a microprocessor into two logically distinct entities, a bus controller and an MPU. The purpose of such separation is to facilitate (1) the design process and (2) the transferability of peripheral devices among different microprocessors. With the many 8- and 16-bit microprocessors currently in use, there are a large number of 8- and 16-bit support devices which have been developed for these microprocessors. Such support devices include, for example, memory management units, CRT controllers, floppy disk controllers, arithmetic co-processors and the like. It would be a desirable feature of a newly designed microprocessor, and in particular a microprocessor of the new generation of 32-bit microprocessor, to be able to use many of these already existing devices without the immediate need for special design of new support devices. It is therefore an object of the present invention to define a bus controller that is logically independent of MPU architecture, thereby permitting the bus controller to be used with variety of MPUs. A further object of the invention is to define a bus controller that extends the functional capability of the external interface of the compatible microprocessor by means which are completely transparent to devices designed for direct attachment to the compatible microprocessor. Such transparent extensions to the functional capability of the external interface of the compatible microprocessor may include: (a) capability for mixed-width device handling, wherein the operation of the bus controller is goverened dynamically by each device's width-identifying response to the bus cycle; (b) capability for accessing operands at any byte location in memory and, in particular, at locations which do not correspond to the integral boundary for that operand; (c) capability to perform special additional bus cycles having a somewhat different form and function than those of the compatible microprocessor, to thereby extend the functional capability of the associated MPU; (d) capability to perform and identify a chain of logically related bus cycles (called a "macrocycle") whose normal execution cannot be interrupted by external devices and whose purpose is to provide means for serialized access to shared resources; (e) capability for varying the minimum duration of a bus cycle, under external control; (f) capability for detecting, resolving, and reporting to the MPU additional response signals from devices unrelated to the bus architecture of the compatible microprocessor; (g) capability for distinguishing between re-run requests of external devices of the compatible microprocessor and of other external devices, with additional means for the latter type of device to cancel a re-run request and have that cancellation indicated to the MPU; (h) capability for detecting a synchronous "cancellation" signal which a device may use to cancel its earlier response to a bus cycle, thereby providing means for a device to overlap its operation with that of the bus controller; and/or (i) capability for detecting and reporting interrupt signals that are separate and additional to those associated with the compatible microprocessor. Still another object of the invention is to define a bus controller whose control logic is broken down into various distinct, clearly identifiable control sequences which comprise its operation. Associated with this object is a means for integrating any form of bus arbitration architecture with the essentially autonomous bus cycle control logic to effect an orderly, prioritized, dynamic means for sharing the external bus of the bus controller. In summary, the overall object of the present invention, therefore, is to define a bus controller characterised by (1) logical distinction from the MPU, (2) total responsibility for communicating with external devices on behalf of the MPU, (3) compatibility with the support devices of the "compatible" microprocessor, (4) transparent extended capability, and (5) definitive functional specification. These elements and their relationship to one another are illustrated in figure 1. In accordance with the present invention, there is now provided a single chip microprocessor comprising: a microprocessing unit; external signal pins to which off-chip devices are connectable; and an interface control unit connected to the microprocessing unit and to the external signal pins to permit the microprocessing unit and any off-chip devices connected to the pins to communicate with one another in response to commands from the microprocessing unit and control signals from off chip devices; characterised in that the interface control unit comprises: execution logic connected to the microprocessing unit and the external signal pins; and control logic connected to the execution logic and responsive to both commands from the microprocessing unit and control signals from off chip devices for configuring the execution logic to perform a particular data transfer between the microprocessing unit and off chip devices that are designed for use with other and different microprocessing units in response to a command from the microprocessing unit requesting said data transfer, and for sending a status output to the microprocessing unit, the status output describing the outcome of the command. The interface control unit (ICU) according to the invention is characterised by a unique logical and functional capability which, in the disclosed preferred embodiment, presents an external appearance that is fully compatible with the support devices of an industry standard microprocessor, the Motorola MC68000 is, therefore, the "compatible microprocessor" of the preferred embodiment. The reader is referred to the MC68000 literature available from Motorola, Inc., of Austin, Texas, for a description of pin assignments and signals associated with each of the pins. The MC68000 microprocessor is actually part of a family of microcprocessors which include, in addition to the MC68000, the MC68008 and at least three other microprocessors with various differences in bus architecture. The following table briefly summarises the data bus and address bus sizes of the two named members of this family of microprocessors: Microprocessor Data Bus Address Bus MC68⊘⊘⊘ 16 23 MC68⊘⊘8 8 2⊘ In addition to the foregoing, there is a MC68⊘⊘ microprocessor having an 8-bit data bus and a 16-bit address bus. These microprocessors are supported by a wide variety of peripheral devices. By making the ICU fully compatible with the support devices of the MC68⊘⊘⊘ and the MC68⊘⊘, and therefore closely compatible with the support devices of the other members of the MC68⊘⊘⊘ family of Motorola microprocessors, a very large number of support devices are immediately available for use with the associated MPU to build a complete system. According to a preferred embodiment of the invention, the ICU bus has 32 bits of address and up to 32 bits of data. Address and data are separate, not multiplexed. Sixty signal pins of a VLSI chip which incorporates the ICU correspond precisely to those of the MC68⊘⊘⊘ microprocessor, which itself comes in a 64-pin package, four pins of which provide power and ground. Thus, the ICU has the property of exactly emulating the MC68⊘⊘⊘ bus but with an MPU that may be of totally different design. While the invention is disclosed with respect to a preferred embodiment which emulates the MC68⊘⊘⊘ bus, it should be understood by those skilled in the art that the principles of the invention can be applied to other families of microprocessors and their support devices. For example, with modification to allow for the different bus architecture, the invention could be practiced to support the Intel family of storage and peripheral devices associated with the 16-bit 8⊘86/8⊘88 microprocessors and the newer iAPX 186 and iAPX 286 microprocessors. The 32-bit address bus is a uni-directional, three-state bus which carries the address for each bus cycle. For memory accesses, bits A31-A⊘1 identify aligned half-words in "memory space" Bytes within each halfword are separately identified by data strobes. A32 is not used for addressing but is used to distinguish real from virtual addresses. These facilities provide the MPU with means for addressing up to 2³¹ (about 2 billion) halfword locations (4 gigabytes) in memory space. The 32-bit data bus is a bi-directional three-state bus which is used to transfer data to and from external devices. The data may be transferred in widths of a byte (8 bits), halfword (16 bits), or a word (32 bits). For a byte output, the byte is duplicated on all quadrants of the bus. For a halfword output, the halfword is duplicated on the high-order and low-order halves of the bus. For input cycles, the ICU reads Bytes and halfwords from the high-order pins of the data bus; i.e., Bytes from D31-D24 or D23-D16 and halfwords from D31-D16. The ICU is designed to operate normally with any mix of concurrently attached 8-, 16-, and 32-bit peripheral devices. Operation is governed dynamically by each device with its width-identifying response to the bus cycle. If a device cannot fulfill the MPU's access request in a single bus cycle, the ICU will itself initiate additional bus cycles to fulfill that request. This process is transparent to the MPU. Each 8-bit and 16-bit device may occupy any range of halfword address space. Similarly, word devices may occupy any range of aligned word address space. The ICU is designed, moreover, to initiate an access request from the MPU at any byte location. The request may be for a byte, halfword or word operand. If the operand is not aligned with its integral boundary, the ICU will access the operand piecewise using separate bus cycles directed at each aligned halfword field of memory within which any portion of the operand is contained or directed. This capability of the ICU frees the MPU from this responsibility of relating its logical address space to the physical constraints of the storage device (s). The ICU bus cycles are of two distinct types, distinguished by the bus cycle function code. Bus cycles having any one of the seven function codes ⊘⊘⊘-11⊘ are memory access cycles for which bits A31-A⊘1 of the address bus identify the addressed halfword location in "memory space". Bus cycles having a function code of 111 are of the second type and are called service cycles. These special bus cycles perform the MC68⊘⊘⊘ interrupt acknowledge function and other sense and control functions for the MPU. Based on additional control information available at the pins of the ICU, service cycles are further identified as one of three types as follows: Level Code (LLL) Read/ Write R/ W Service Address (AAA) Service Cycle Type ⊘⊘⊘ ⊘ ⊘⊘⊘-111 Control/AAA ⊘⊘⊘ 1 ⊘⊘⊘-111 Sense/AAA ⊘⊘1-111 1 none Inter. Ack. ⊘⊘1-111 ⊘ - (not used) The service cycle repertoire thus comprises eight control cycles, eight sense cycles, and seven (MC68⊘⊘⊘) interrupt acknowledge cycles. The special features of these cycles are summarized as follows: (a) The service cycles do not reference the MPU memory address space. Instead, they reference their own service cycle "address space" which is subdivided into eight separate levels, designated Levels ⊘ to 7. Levels 1 to 7 of service address space are dedicated to the MC68⊘⊘⊘ interrupt-acknowledge function. Level ⊘ address space, on the other hand, is dedicated to the MPU sense and control functions and is itself subdivided into a set of eight service address spaces, SA⊘ to SA7. The complete address space of the specific preferred embodiment is illustrated in Figure 2. In the application of the ICU to a particular MPU, the interrupt acknowledge address space can be accessed exclusively by the MPU microcode; i.e., interrupt acknowledge cycles need not be instruction-driven. Similarly, level-zero service address spaces ⊘-2 can be reserved for the exclusive use of the program through two special instructions called, respectively, SENSE and CONTROL. Level-zero address spaces 3-7, on the other hand, may be accessed by either the MPU or the system program, the latter through the same SENSE and CONTROL instructions just described. Under these circumstances, service-address spaces 3-7 are therefore "shared" address space, tacitly requiring the system program and the MPU microcode to be consistent in their application of these address spaces. Such shared service-cycle address space is shown in cross-hatching in Figure 2. (b) For service cycles, the significance of the content of the address bus is shown in Figure 3. The low-order bits A⊘3 to A⊘1 denote the service cycle level code, LLL. The seven non-zero service cycle level codes identify MC68⊘⊘⊘-compatible interrupt acknowledge cycles, while the level code zero indicates that a SENSE (input) or CONTROL (output) cycle is in progress. The high-order bits A31-A⊘4 comprise the address/command/data (ACD) field. The significance of this field is dependent upon the particular cycle. For all interrupt acknowledge cycles, the ACD field is always all 1's (active high), in accordance with the Motorola specification for these cycles. For SENSE and CONTROL cycles, on the other hand, the significance of the ACD field is dependent upon the 3-bit service address appearing on the data strobe pins. The table shown in Figure 4 designates the ACD field for each of the SENSE and CONTROL cycles of a particular MPU. Note that the specification of "shared" service cycles would be tacitly established by the MPU microcode. The system program should conform to this specification whenever it shares service cycles; otherwise, operation is unpredictable. Compatibility with the MC68⊘⊘⊘ "memory-mapped I/O'' is thus achieved through the use of the level zero address space which is transparent to the MC68⊘⊘⊘ peripheral devices. (c) For SENSE and CONTROL cycles, the data strobe pins WS , UDS and LDS provide the service address bits SA3, SA2 and SA1, respectively. By this means, eight distinct SENSE cycles (SENSE/AAA) and eight distinct CONTROL cycles (CONTROL/AAA) are established, where AAA denotes the encoded service address. The service address signals have approximately the same timing as the address strobe ( AS ). Because of the unavailability of the data strobe pins for data strobe control under these circumstances, other means are provided for the actual data strobe control function. For SENSE cycles, the address strobe ( AS ) performs the dual function of an address/data strobe. For CONTROL cycles, an output service strobe signal ( OSS ) is generated on the VMA pin. The timing of OSS coincides with that of the data strobes of memory-access write cycles. (d) All 32-bits of the data bus are used during each service cycle. The ICU does not inspect or modify data, nor does it initiate additional cycles to complete a service cycle request from the MPU. For output service cycles, 32-bits of "data" are taken from the MPU and transferred over D31-D⊘⊘ in a single bus cycle. For input service cycles, the content of D31-D⊘⊘ is transferred to the MPU. For SENSE and CONTROL cycles, the format of the data bus for the specific MPU specified in the table of Figure 4 is as shown in the table of Figure 5. For this MPU, program-initiated SENSE cycles can recognize any one-of-four data bus formats, depending upon the device response ( WTACK , DTACK , BTACK or VPA ). MPU-initiated SENSE cycles do not have this capability; the MPU interprets the data bus identically for each of these normal responses. For the "shared" SENSE cycles (3-7), therefore, only the single data format shown in Figure 5 should be used. To do otherwise would lead to unpredictable results inasmuch as the source of the "shared" service cycle is not known to the device. For the MC68⊘⊘⊘ interrupt acknowledge cycle, external logic is expected to place the 8-bit interrupt vector number on the data bus as follows: Device Response Location of Interrupt Vector Number BTACK D31-D24 DTACK D23-D26 (MC68⊘⊘ compatible) WTACK D⊘7-D⊘⊘ VPA No Vector (Auto-Vectoring) (e) For service cycles, the ICU unconditionally reports all device responses to the MPU. For input service cycles, one-of-four normal responses, WTACK , DTACK , BTACK or VPA , can be reported, while for output service cycles, one-of-three responses, WTACK , DTACK or BTACK , is available. This behavior of the ICU provides means to distinguish between any one-of-four/one-of-three normal outcomes to a SENSE/CONTROL bus cycle and is one of the features of the programmable bus architecture of the ICU according to the invention. By contrast, for memory access cycles, the ICU works with the accessed device(s) to satisfy the MPU's access request. If more than one bus cycle is required, the ICU will automatically initiate such additional cycles and continue to do so until the MPU's access request is completely satisfied. All of this bus action is transparent to the MPU. (f) The service cycle architecture may be seen to be synonymous with the important capability of "pin-programmability". This is the capability to specify, by means external to the ICU, the precise state of various output signal pins during a bus cycle. Service cycles have this property, and the content of the fields of the address bus, the service address, and the entire content of the data bus for CONTROL cycles are, in effect, all controlled by the MPU. To appreciate the uniqueness of this property, consider the way ordinary memory-access cycles are performed. For these cycles, the ICU acts to satisfy the MPU'S access request by performing one or more bus cycles and adjusting the access address, the choice of data strobes, and the content of the data bus (for CONTROL cycles) to satisfy the requirements of each particular bus cycle. This behavior is in sharp contrast to the operation of the service cycles according to the present invention. The ICU according to the invention does not intervene but, instead, allows the MPU control information to "pass-through" directly to the output pins. This property has the important consequence that the access request and all of its attributes may originate not only at the MPU but also at the user program itself. While the ICU does not care about from there the request for a SENSE or CONTROL cycle ultimately comes, its architecture is nevertheless the basis for its user-programmable interface feature. This feature is further enhanced by the high-resolution status response of the ICU to each request for a service cycle. The ICU architecture also sets forth the concept of a "macrocycle". As used herein, a macrocycle is defined as a logically unbroken sequence of bus cycles identified by the macrocycle-in-progress ( MIP ) signal and having the following charactersistics: 1) During a macrocycle, the ICU will not respond to a normal halt request. A normal halt can be taken only after the last bus cycle of the macrocycle. 2) Upon initiating a macrocycle, the ICU will not respond to any request for bus mastership until the macrocycle has been completed. 3) Any normal attempt by a peripheral device to rerun any bus cycle of a macrocycle will cause the ICU to terminate the macrocycle and present "rerun request rejected" status to the MPU. A macrocycle is initiated and normally terminated in response to "start" and "stop" command signals from the MPU. The purpose of a macrocycle is to provide a means for serializing access to shared memory. Macrocycles are used by the MPU in the execution of test-and-set and compare-and-swap instructions which are extended forms of MC68⊘⊘⊘ capability. With MC68⊘⊘⊘ microprocessors, a bus cycle can be synchronized externally with the common clock through use of the address strobe. This signal begins to appear just after the start of the second clock period of the bus cycle and is guaranteed to be valid before the start of the next (third) clock period. The ICU according to the present invention Provides means to establish this synchronization a full clock period earlier than the MC68⊘⊘⊘. This is accomplished with the bus cycle in-progress ( BCIP ) signal which is activated at the beginning of each bus cycle and is held active until the start of the last clock period of the bus cycle. Using the BCIP signal, external logic can be "started" a full clock period earlier than would otherwise be the case. This capability provides a basis for simpler external logic and/or better performance. The minimum duration of an ICU bus cycle is three clock periods for input and four clock periods for output. This is the latent capability of the ICU. The actual duration of a bus cycle, however, depends upon the responsiveness of the peripheral device. Devices which utilize a "strapped" acknowledge signal, or which otherwise activate their acknowledge signal within one and a half clock periods of the start of a bus input cycle, can achieve the three clock period performance. In order to make the ICU compatible with already existing external device logic which utilizes strapped acknowledge but which cannot accomodate the three clock period read cycle, the ICU features a pin-selectable minimum duration of the bus input cycle. Through the use of the 3T/ 4T pin, a "normal" four-clock period (4T) or a "fast" three-clock period (3T) minimum duration of the bus input cycle can be selected. In the implementation of the preferred embodiment, a 16-bit data path is used to move data in and out of the MPU. This means that the ICU must perform two internal (MPU/ICU) cycles in order to transfer a full word (32 bits) over the external bus in a single bus cycle. As a result, the minimum effective cycle time for storing an aligned word is seven clock periods, comprising three clock periods for buffering the first halfword and four clock periods for the bus cycle itself. This is a saving of one clock period over the two back-to-back cycles which would otherwise be required, but this saving is not without a price. Since an MPU may not check far operand alignment, the ICU must check all incipient word output for alignment during the three clock period "buffering" time. Inasmuch as alignment checking is completed in the first two clock periods, a bus cycle can be started with only a two clock period delay if the operand is found to be non-aligned. This bus cycle will transfer the first halfword or byte of the word, and one, two, or three more cycles will ensue to complete the transfer, depending upon the target address. Thus, if the word is aligned, the procedure saves one clock period; on the other hand, if the word is not aligned, two clock periods are lost. Application studies show that aligned words occur far more frequently in typical instruction mixes than do non-aligned words, indicating that the procedure will, in practice, yield better performance than if the buffering/checking period and single 32-bit output cycle were completely eliminated. This is particularly true in the presence of high bus contention for which the bufffering/checking will often be overlapped with "off-line" use of the bus. Applications for which there are no 32-bit peripheral devices cannot benefit from this capability. The ICU architecture therefore provides a Suppress 32-bit Output pin by which the user can force the ICU to store full words a byte and/or a halfword at a time without the buffering/checking delays. The ICU provides means for external devices to respond to a bus cycle in any one of fifteen distinct ways, plus several variations thereto. This feature of the ICU, associated with ten pins of its external interface, enables off-chip logic to terminate a bus cycle with a signal that concurrently describes the outcome of that cycle. This feature is a basis for improving performance and for reducing the amount of response logic at external devices, and it also contributes to the programmability of the ICU. The fifteen responses may be classified into four distinct response classes as shown in Figure 6. The four normal responses are the basis for the mixed-width device handling and device acknowledge (ACK) reporting features of the ICU, briefly described above. Specifically, the normal responses are the dynamic means for identifying (a) device bit-width during memory access cycles and (b) one-of-four "normal" outcomes of a service cycle. For service cycles, the ICU always reports the particular normal response to the MPU. The MPU, in turn, can use that information itself for non-program-initiated service cycles or it can set a condition code for program-initiated service cycles. In the latter instance, the program may branch at once on the particular outcome, thereby eliminating the need for additional instructions (and bus cycles) to sense the same information. Any of the eight device exceptions listed in Figure 6 is always reported to the MPU. This applies to all bus cycles, whether memory access or service. The action of the MPU in response to a device exception is arbitrary. For purposes of illustration, the action of a particular MPU in response to a device exception could comprise one of the following three conditional actions: Case A. If, and only if, the associated bus cycle was a program-initiated SENSE or CONTROL cycle, the MPU can encode the device response and store it in a general-purpose register designated by the SENSE/CONTROL instruction. It will then resume normal operations without any further action peculiar to the exception. Thus, under these circumstances, the exception is essentially transparent to the MPU. Case B. If the associated bus cycle was not a program-initiated SENSE or CONTROL cycle and the device response is either BERR , DTEXN/⊘ , DTEXN/1 , or DTEXN/2 (a single signal) , the MPU can force a machine check and create and store a 65-bit interruption code containing the device response in encoded from. Case C. If the associated bus cycle was not a program-initiated SENSE or CONTROL cycle and the device response comprises more than one DTEXN signal, the MPU can take an action peculiar to the signal combination and also move a device-generated interruption code (corresponding to a program check or machine check) from the device to the appropriate pre-assigned location in main storage. Thus, as a consequence of the immediate reporting of the particular exception to the MPU, the MPU may begin its specific response to that exception at once without any further bus cycles to identify the exception. This contributes to performance, and moreover, in all instances, the device response is effectively buffered, either in a general purpose register (Case A) or in main storage (Cases B and C). This eliminates the need to retain at the device the outcome status of the bus cycle, thereby providing a means for simplifying external device logic. In addition, Case A contributes to the programmability of the ICU by providing the program with the precise details of the exceptional response to SENSE or CONTROL cycles. The single interface check (see Figure 6) is handled by the ICU exactly as it handles a device exception; i.e., it is individually reported to the MPU. The MPU action, in turn, can be the same as for device exception Cases A and B. Thus, the consequences would also be the same, providing a basis for better performance, simplified device design, and enhanced programmability. It should be noted that the classification distinction between "interface check" responses and "exception" responses is essentially arbitrary. A device could, for example, intentionally signal a particular exception using multiple ACK signals, or a device could assign a particular meaning to the VPA response to a control cycle. The interface check classification thus identifies responses that would ordinarily be unintentional and, for simplicity, they are lumped together, indistinguishable from one another, in a common response class. The ICU will re-run any bus cycle in response to an appropriate, external device-initiated re-run request sequence. Every re-run request sequence comprises three basic phases, as follows: (a) Prepare Phase. The external device presents BERR and HALT in response to the bus cycle to be rerun. The ICU responds by terminating that bus cycle and unconditionally enabling bus arbitration. If a macrocycle is also in progress, it too is terminated along with the bus cycle. (b) Wait Phase. The external device holds the BERR and HALT signals active while it performs whatever logical functions it desires before initiating the proceed phase of the request sequence. The duration of the wait phase is arbitrary, and bus arbitration proceeds at the ICU. (c) Proceed Phase. When the external device is ready to proceed, it activates one of the following three "proceed" signals to the ICU appropriate to the circumstances and consistent with its capability: (c1) Rerun. The external device performs a MC68⊘⊘⊘ compatible "preferred release" sequence, defined as the removal of BERR followed at least one clock period later by the removal of the HALT signal. this requests the ICU to rerun the prior bus cycle. The ICU will rerun the cycle in response to this signal only if that cycle had not been within a macrocycle. If a macrocycle had been in progress, the ICU will not rerun the prior bus cycle and will, instead, present rerun request rejected status to the MPU. (c2) Rerun Unconditionally. Here, the external device performs the preferred release sequence augmented by a momentary activation of the auxiliary reset ( XRESET ) signal synchronized with the deactivation of the HALT signal. The ICU will unconditionally rerun the prior bus cycle and, if a macrocycle has also been in progress, it too would be resumed, and the MIP signal would be re-activated along with the address strobe. All of this action is transparent to the MPU. (c3) Cancel. The external device performs a "reverse release" sequence, defined as either the concurrent removal of BERR and HALT or the removal of HALT prior to the removal of BERR . This tells the ICU to proceed without rerunning the previous bus cycle and to transmit rerun request cancelled status to the MPU. The ICU rerun architecture thus provides compatible means for the associated MPU to operate with MC68⊘⊘⊘-type devices without, however, exposing its macrocycles to invisible penetration during the rerun process. This follows from the fact that the MC68⊘⊘⊘ devices, which cannot recognize ICU macrocycles, also cannot perform the unconditional rerun request sequence. The only "proceed" alternatives available to MC68⊘⊘⊘-type devices, therefore, are rerun and cancel. An unqualified rerun request, in the presence of a macrocycle, will always be rejected by the ICU, thereby protecting the macrocycle from possible penetration. In this case, recovery must be achieved by indirect, software-supported means. It should be noted, however, that use of the unconditional rerun request (c2) by external devices designed specifically for use with the ICU tacitly implies that these devices will also be designed to make appropriate use of the MIP -Out signal, thereby guaranteeing that shared memory space will not be violated during the rerun process. The ICU will "halt" any bus cycle in response to an appropriate request from the peripheral device. The term "halt" refers to completion of the current bus cycle and enablement of bus arbitration, followed by a wait state during which the ICU is effectively idle, except for its bus arbitration and interrupt detection functions. The duration of the wait state is arbitrary and ends whenever the peripheral device "releases" the halt signal. The ICU thus halts a bus cycle during every rerun request sequence. Two distinct types of halt are defined: normal and pseudo. A normal halt is always transparent to the MPU, and the associated bus cycle is always completed normally. A normal halt, moreover, always occurs in response to signals which include the HALT signal. In the absence of a macrocycle, a normal halt may be achieved merely by activating the HALT signal along with the normal termination signal. Normal halt can also be achieved with all bus cycles through the appropriate rerun process, but the bus cycle will be rerun upon halt release. A pseudo halt occurs whenever the device response falls into one of the following three classes, under the conditions noted: (a) device exception, always; (b) re-run, if the bus cycle is not rerun; and (c) interface check, if HALT is concurrently active. Upon removal of the corresponding termination signal (s), the ICU will resume operation and transmit the exception status to the MPU. Data transfer may or may not have been completed correctly. Thus, the conditions which cause a pseudo halt are never transparent to the MPU. For pseudo halts, moreover, there is a distinction between the halt action and the HALT signal. Some pseudo halts may occur independently of the HALT signal as, for example, in the case of the device exceptions listed in the table of Figure 6, for which concurrent use of the HALT signal is either unnecessary or ineffective. The basis for the ICU halt architecture is the same as for its rerun architecture. The ICU is designed to be compatible with standard MC68⊘⊘⊘-type peripherals without exposing its macrocycles to invisible penetration during the halt process, for which bus arbitration is enabled. Normal halt requests from MC68⊘⊘⊘-type devices are simply ignored during a macrocycle. Bus cycles of a macrocycle can be halted normally, however, by special procedures (i.e., unconditional rerun) which are intended for use only when the integrity of shared resources can be guaranteed. The ICU provides means for a peripheral device to, in effect, "hedge" its synchronous response to a bus cycle and thereby achieve a full clock period reduction in bus cycle duration over what would otherwise be required. This feature of the ICU may be explained as follows with reference to the timing diagram of the "4T" bus cycle shown in Figure 7. The duration of this bus cycle is always (4+N)T, where T is the clock period and (N+1)T is the duration of the detection sequence (phase) of the bus cycle. For normal terminations, N≧⊘, and for exceptional terminations, N≧1. The value of N is determined entirely by the peripheral device; specifically, by how quickly it can respond to the output controls of the bus cycle. The ICU samples the device response signals at the S4 i S5 i negative transitions of the clock, and the ICU detection logic resolves that response during S5 i . It is the output of the detection logic that determines the subsequent action of the ICU. Thus, if during S5 i . the detection logic detects a termination signal, i=N and states S6 and S7 unconditionally follow, thereby terminating the bus cycle. Now, consider the situation of a peripheral device whose response logic requires just under (3+N)T to complete, measured from the very start of the bus cycle. In order to keep this explanation simple, the specific case of N=⊘ will be described from which the general case can be readily inferred. For this device, a response to the bus cycle is not available until the end of S5 ⊘ . This means that another full clock period (S4 i S5 i ) would be required in order for the ICU to latch and resolve the device response, loading to a bus cycle duration of 5T. If the peripheral device were allowed to penetrate the ICU logic and apply a special, device-generated CANCEL signal to the output of the ICU detection logic using an AND gate, the device could present a tentative normal response to the bus cycle during S4 ⊘ and complete its operation during S5 ⊘ ; i.e., it could overlap its operation with that of the ICU as the latter resolves the initial (tentative) response If, during S5 ⊘ , the device finds it cannot complete the bus cycle at this time, it merely asserts its CANCEL signal, thereby preventing the output of the ICU detection sequence from terminating the bus cycle. This operation is illustrated by the timing diagram of the CANCEL signal shown in Figure 8. On the other hand, if the device finds that it can complete the bus cycle as it had indicated a half-clock period earlier, it does not interfere (by asserting CANCEL) with the output of the detection sequence. By these means, therefore, the device would be able to perform its normal functions with a 4T rather than a 5T bus cycle. In simple terms, such means allow the device to anticipate the successful completion of operations and rely on a cancellation scheme in the event it does not succeed. This capability is included in the ICU architecture. It is achieved through the use of the Auxiliary Reset ( XRESET ) pin, through which the CANCEL signal may be applied. The CANCEL signal is effective for the initial signals of all classes of device response; it is not, however, effective for the "release" phases of the non-normal responses. The CANCEL signal, moreover, may be applied without danger of external reset as long as its duration is under 1⊘T, the latter duration being well beyond the needs of the cancellation process. The ICU implements three distinct interrupt request groups providing a total capability for detecting up to seven, concurrent interrupt requests. These groups are set forth in the table of Figure 9. Each interrupt request cannot be guaranteed if that request is removed prematurely. Requests may be presented at any time. Synchronization and deskewing are performed by the ICU. The ICU samples the ten interrupt request pins at every negative transition of the input clock. If any request of (or within) a group is identical with that detected one clock period earlier, that request or its removal is latched in the ICU's interrupt request register (IRR) where it can be sampled by the MPU. Propagation time through the ICU, from first detection to latching in its IRR, is 2.5T, where T is the input clock period. This detection scheme is designed to shield the MPU/ICU VLSI chip from any spurious activity on the interrupt request lines. Interrupt processing is handled entirely by the MPU. Each interrupt request will, in due course, elicit a response from the MPU and, in order to transmit that response to the requestor, the MPU must again draw upon the facilities of the ICU. The facilities used for this purpose by a specific MPU associated with the preferred embodiment of the ICU are summarized in the table shown in Figure 1⊘. For any of the five system interruption requests, the MPU response in a SENSE/4 and/or SENSE/5 service cycle called, respectively, Sense System Interruption Code Low/High. These service cycles can be microcode or program-initiated; i.e., they are among the "shared" service cycles of the system. The IBM System/37⊘ interruption class (one-of- five) is itself identical in the ACD field of the address bus as indicated in the table of Figure 4. The requestor should remove the corresponding interrupt request in response to either of these service cycles. The response to interrupt requests of the priority request group appears solely on the System/37⊘ CPU State Indicator pins and, in the case of the E-unit Dump, in the action itself. The dump request should be removed before completion of the dump, or the dump will be repeated. For the vectored interrupts of the MC68⊘⊘⊘ group, the MPU response is an "interrupt acknowledge" cycle which corresponds in all respects to that of the MC68⊘⊘⊘. The requestor may, however, place the 8-bit interrupt vector number in any of the three previously described positions on the data bus and define that location with the appropriate acknowledgement. Auto-vectoring is indicated exactly as with the MC68⊘⊘⊘, with a VPA response. The MC68⊘⊘⊘ device response should, of course, be removed in response to the interrupt acknowledge cycle. The bus arbitration architecture of the ICU is similar to that of the MC68⊘⊘⊘. It utilizes the same three signals, Bus Request ( BR -In), Bus Grant ( BG -Out), and Bus Grant Acknowledge ( BGACK -In), and insofar as access to its external bus is concerned, encompasses the same three classes of device: supreme master (the ICU itself), master, and slave. The ICU can initiate a bus cycle with any device of the other two classes, whereas an ordinary master device can initiate a bus cycle only with slave devices. Slave devices do not have the capability to initiate bus cycles. At any given time, no more than one master device can be in control of the bus. There are, however, some specific differences between the two architectures that may affect compatibility with some master-type devices designed for the MC68⊘⊘⊘. For the ICU, a bus grant response to bus request is a necessary condition for the assertion by a requesting master of BGACK . Violations of this requirement will, in general, lead to conflict with the ICU and unpredictable results. The complete picture is set forth below where, for simplicity, the signal mnemonics denote logical states and not actual levels: 1) BR can be activated at any time and is ordinarily negated by the requestor just after it asserts BGACK. 2) BG is the ICU's response to BR. It is never activated for any other reason. The maximum response time is not specified and cannot be predicted by the requestor. Whenever BG is activated during a bus cycle, that activation occurs at the beginning of S6 of the bus cycle, one clock period later than would be the case for the MC68⊘⊘⊘. BG is negated in response to the assertion of BGACK or to the prior removal (cancellation) of BR. 3) BGACK is a response to BG· AS ; i.e., BGACK must not be asserted unless BG is active and AS is concurrently inactive. Once activated, BGACK must be held active for the entire duration of bus mastership, for which there is no specified limit. 4) The bus "busy" condition is synonymous with BGACK+AS ICU , where AS ICU denotes the ICU-driven address strobe. There is no specified limit on the duration of a given busy condition. The system designer must, therefore, take into account the needs of overrunable devices. The underlying reason for the "tight control" of bus arbitration relates to the ICU's capacity for performing sequences of related bus cycles for which, unlike the MC68⊘⊘⊘, the target address may change from cycle to cycle. Specifically, bus availability to others is affected by macrocycles, unaligned operands, and word transfers that require more than one bus cycle. For the ICU, the BG signal is definitive and must be obeyed. This is quite different from its role in the MC68⊘⊘⊘. For that microprocessor, the BG signal is essentially only a convenience providing (1) early notification of incipient bus release, if activated during a bus cycle, and (2) means for implementing external, serial priority allocation logic. With the MC68⊘⊘⊘, a requestor can, for example, infer bus release merely by presenting BR or BGACK and watching the action of AS . If, 4.5T after presentation of either of these signals, AS is inactive, the bus is immediately available. On the other hand, if at that time AS is active, it may be concluded that the bus will nevertheless become available as soon as AS is subsequently negated. For the MC68⊘⊘⊘, this need not be long, as a read/modify/write cycle is the longest duration MC68⊘⊘⊘ cycle. With the exception of only two cases, the ICU, like the MC68⊘⊘⊘, will never take more than two back-to-back bus cycles without enabling bus arbitration. The two exceptions are (1) when running macrocycles, and (2) then storing a full word that straddles a non-word aligned physical boundary of a halfword memory followed by a byte memory. Finally, it is helpful here to note the specific circumstances under which the ICU will tri-state its controls or maintain them in that high-impedance state. These circumstances are the concurrence of the following three conditions: (1) The bus is not needed by the MPU, (2) the ICU is not driving the address strobe, and (3) any one of the conditions of (a) HALT, driven by the ICU in response to a command for the MPU, (b) the bus arbitration "state machine" has detected a logically valid request for bus mastership, or (c) the bus is "busy" under control of an external master. The ICU will also tri-state its controls during any of its three external reset sequences. Brief Description of the Drawings The foregoing and other objects, aspects and advantages of the invention will be better understood from the following detailed description of the preferred embodiment. This description references the accompanying drawings, in which: Figure 1 is a block diagram illustrating the nomenclature of the invention; Figure 2 is a functional diagram illustrating the structure of the address space of a specific example of an MPU used with the preferred embodiment of the ICU according to the invention; Figure 3 is a functional diagram illustrating the format of the address bus for service cycles; Figure 4 is a table of the SENSE and CONTROL cycles designating the ACD fields for a specific example of an MPU used with the preferred embodiment of the ICU according to the invention; Figure 5 is a table of the SENSE and CONTROL cycles designating the data bus formats for a specific example of an MPU used with the preferred embodiment of the ICU according to the invention; Figure 6 is a table of distinguishable device responses to an ICU bus cycle; Figure 7 is a timing diagram of the ICU 4T bus cycle; Figure 8 is a timing diagram illustrating the CANCEL signal timing; Figure 9 is a table of the ICU interrupt request groups; Figure 1⊘ is a table of the application of the ICU facilities to the interrupt response process of a specific example of an MPU used with the preferred embodiment of the ICU according to the invention; Figure 11 is a block diagram showing a typical attachment of external devices to the ICU; Figure 12 is a table summarizing the ICU pin designations according to signal direction and function; Figure 13 is a functional block diagram illustrating the relationship of the ICU to its external interface and to the MPU; Figure 14 is a functional diagram and table detailing the required content of the MPU's Processor Command Register (PCR); Figures 15 and 16 are timing diagrams of the MPU/ICU interface cycles; Figure 17 is a detailed logic diagram of the ICU bus-cycle-in-progress control logic; Figure 18 is a timing diagram of the operation of the bus-cycle-in-progress signal; Figure 19 is a detailed logic diagram of the ICU function code logic; Figure 20 is a detailed logic diagram of the ICU read/write control logic; Figure 21 is a detailed logic diagram of the ICU addressing facilities; Figure 22 is a truth table which defines the address selection logic shown in Figure 21; Figure 23 is a detailed block and logic diagram of the ICU data flow facilities; Figures 24 and 25 are truth tables which define the asynchronous control logic for the output and input multiplexers, respectively, shown in Figure 23; Figure 26 is a detailed logic diagram of the control facilities for the address and data strobes of the ICU; Figure 27 is a truth table which defines the data strobe selection logic shown in Figure 26; Figure 28 is a block diagram of the logical structure and operation of the ICU control section; Figures 29, 3⊘ and 31 are detailed logic diagrams of the control inputs to the address selection logic shown in Figure 21; Figure 32 is a timing diagram describing the basic features of the bus arbitration architecture of the ICU according to the invention; Figure 33 is a detailed logic diagram of the bus arbitration logic; Figure 34 is a truth table defining the G logic of the bus arbitration logic; Figures 35 and 36 are timing diagrams illustrating the operation of the bus arbitration logic; Figure 37 is a table which summarizes the set/reset conditions of the BGE flip-flop; Figure 38 is the specification timing diagram for the command sequence; Figure 39 is the specification timing diagram for the command stop; Figure 4⊘ is the command sequence specification timing diagram for clearing the HOLD flip-flop; Figure 41 is a detailed logic diagram of the ICU command sequence and its in bound and out bound control signals; Figure 42 is a detailed logic diagram of the ICU macrocycle control logic; Figures 43 to 51, inclusive, are flow diagrams describing the required action of the command sequence control logic; Figures 52 and 53 are flow chart descriptions of the operation of the buffer read/write logic; Figure 54 is a timing diagram illustrating the operations of the initiation sequence; Figure 55 is a detailed logic diagram of the initiation sequence; Figure 56 is a flow diagram describing the operations of the initiation sequence; Figure 57 is a timing diagram illustrating the operation of the detection sequence; Figure 58 is a detailed logic diagram of the ICU status response logic which communicates with the MPU; Figure 59 is a table summarizing the status response signals and how they are generated; Figure 6⊘ is is a detailed logic diagram of the implementation of the detection sequence; Figures 61 to 74, inclusive, are truth tables defining the detection and resolution logic of the detection sequence; Figure 75 is a timing diagram illustrating the operation of the post termination sequence; Figure 76 is the post termination sequence 3 specification timing diagram; and Figures 77 and 78 are, respectively, the post termination sequence 3 (PTS3) logical implementation and the truth table defining the resolution logic. Detailed Description of the Preferred Embodiment of the Invention The following description is divided into several parts by topic in order to facilitate an understanding of the invention. The first main topic is the external architecture of the ICU. This architecture includes both the pin-out specification and the MPU/ICU interface. The MPU/ICU interface is properly considered a part of the external architecture because, although the MPU and ICU physically share the same VLSI chip, they are logically independent. This is an important and novel departure from the prior art wherein the distinction is not made The second main topic is the actual implementation of this architecture in the form of the preferred embodiment. This implementation is described in two subtopics dealing separately with execution logic and control logic. Like an MPU, the ICU may be characterized as comprising an execution unit and a control unit. The execution unit is controlled by the control unit and comprises various registers, latches, multiplexers, logic, and data and address buses which provide the physical interface between the MPU and off-chip devices. The brain of the ICU is the control unit. In the discussion of the execution logic, no attempt is made to describe in detail the control signals generated by the control logic. These signals are, instead, discussed in the latter subtopic. The execution logic forms the mechanism by which the various functions of the ICU are performed. The control logic determines when those functions are performed. External Architecture, Pin-Out Specification Referring now to Figure 11, the ICU 1⊘⊘ is shown as being connected by an address bus 1⊘1 and a four byte-wide data bus 1⊘2a, 1⊘2b, 1⊘2c and 1⊘2d to a plurality of external devices having different address and data widths. For example, a typical 16-bit MC68⊘⊘⊘ device 1⊘3 is shown connected to 23 lines of the address bus 1⊘1 and to the high-order two bytes of the data bus 1⊘2c and 1⊘2d. Similarly, a typical 8-bit MC68⊘⊘8 device 1⊘4 is shown connected to 19 lines of the address bus 1⊘1 and to the high-order byte of the data bus 1⊘2d. Both of these devices, despite differences in data widths, belong to the same family of devices. There is also shown a typical MC68⊘⊘ device 1⊘5 connected to 15 lines of the address bus 1⊘1 and to the high-order byte of the data bus 1⊘2d. This is another 8-bit device, but it belongs to a different family of devices. All of these devices are also connected to the various control signal pins of the ICU by which these devices and the ICU communicate. In the case of the MC68⊘⊘8 and MC68⊘⊘ devices 1⊘4 and 135, additional small-scale integration (SSI) logic 1⊘6 is required to generate some of the required control signals. In addition to the MC68⊘⊘⊘ family of devices which arc already commercially available, the ICU according to the invention also contemplates the design and attachment of its own 32-bit devices. A generic device of this type is illustrated at 1⊘7. The combination of devices shown in Figure 11 is merely illustrative, and in a given system there may be a different combination of these types of devices. The pin designations and functions of the ICU are summarized in the table shown in Figure 12. From this table, it will be understood that the SSI logic 1⊘6 in Figure 11 combines the upper data strobe UDS and the lower data strobe LDS , which are used by MC68⊘⊘⊘ devices, to produce the data strobe DS , which is used by MC68⊘⊘ devices. In addition, the low-order address bit A⊘, which is required to address MC68⊘⊘8 and MC68⊘⊘ devices, is also derived from UDS and LDS in the logic 1⊘6. The logic 1⊘6 is represented by a truth table and an AND gate. The word strobe WS is activated for non-service cycles only during output operations, and in particular, only during those for which the ICU has prepared the data bus for a full, aligned 32-bit word transfer. For these transfers, UDS and LDS are also activated concurrently to maintain MC68⊘⊘⊘ compatibility, and A1 is set to zero. Byte and halfword transfers are indicated by the ICU using only UDS and/or LDS . For read/write cycles, the address bus 1⊘1 has exactly the same significance as the address bus of the MC68⊘⊘⊘ microprocessor except that the MPU can address up to 2³¹ halfword (16-bit) locations using address bits A31-A⊘1. A32 of the bus is not used for addressing and is, instead, the real address tag. When this tag is active (high), the associated address on bus lines A31-A⊘1 is a real address. When A32 is inactive, the associated address is interpreted as a virtual address. The level of A32 is determined solely by the MPU. The word transfer acknowledge WTACK signal is used by 32-bit devices to indicate a normal response to the output controls of a bus cycle. When used with an input operation, WTACK signifies that the addressed device has placed, or will place within the time period T31 (see MC68⊘⊘⊘ electrical specifications), on the data bus the full 32-bit aligned word corresponding to the address bits A31-A2. This word will be maintained until the device detects the deactivation of the strobe signals from the ICU. When used in response to an output operation, WTACK signifies that the device has accepted the byte, halfword, or word on the data bus and that the ICU may now proceed to terminate the bus cycle. The format of the data bus is indicated to the device at the outset of the bus-cycle by the data strobes: All input service cycles are terminated normally in response to any one of the three data transfer acknowledge signals, WTACK , DTAC K, BTACK , and VPA , and each of these signals is identified to the MPU. Output service cycles (CONTROL) are treated similarly except that a VPA response will cause an interface check. For interrupt acknowledge service cycles, each acknowledge signal has the following significance: DEVICE RESPONSE LOCATION OF INTERRUPT VECTOR WTACK D⊘7-D⊘⊘ DTACK D23-D16 BTACK D31-D24 A VPA response to an interrupt-acknowledge cycle signifies that no vector was transmitted and that the processor should auto-vector the interrupt request. This is a normal response. Each of thirteen bus cycle response signals feeds the D-input of its own dedicated, negative-edge triggered D-type flip-flop. The outputs of these synchronizing flip-flops, in turn, drive the ICU detection logic which produces a single status response to the MPU. Detection of any one of the four acknowledge signals ( WTACK , DTACK , BTACK , or VPA ) generates the normal status response to the MPU. Detection of more than one acknowledge signal always generates an "interface check". Whenever the ICU latches up an acknowledge signal, and no exception signal has also been latched, the ICU will next enter state 6 of the bus cycle, as shown in Figure 7. The byte transfer acknowledge BTACK signal is used by non-MC68⊘⊘ 8-bit devices to indicate their normal response to the output controls of a bus cycle. When used during an input (read) operation, BTACK signifies that the addressed device has placed, or will place within time period T31, on D31-D24 of the data bus 1⊘2d the addressed byte, if a byte fetch, or the high-order byte of a 16-bit fetch. If the fetch request had been for a 16-bit halfword, the ICU 1⊘⊘ will, in response to BTACK , immediately act to fetch the low-order byte of the halfword. For output (write) operations, the activation of BTACK signifies that the addressed device has taken only the information on D32-D24 of the data bus 1⊘2d and that the ICU may now proceed to terminate the associated bus cycle. The device will store the byte at the location corresponding to A31-A⊘, where A⊘=⊘, if UDS =1, and A⊘=1, if UDS =⊘ and LDS =1. If BTACK is received in response to a 16-bit store ( UDS = LDS =1) , the ICU1⊘⊘ will automatically perform another bus cycle to store the low-order byte. For all byte output transfers, the byte is duplicated on all quadrants of the data bus. The valid memory address VMA and enable E signals are supplied in response to VPA , in conformity with the MC68⊘⊘ specification of these signals. The complete signal repertoire of the ICU is defined in the table shown in Figure 12. Not all of the signals defined in Figure 12 are represented in Figure 11, this latter figure having been simplified to provide a clear overview of the attachment capability and basic communication between the ICU 1⊘⊘ and externally connected devices. External Architecture, MPU/ICU Interface As shown in Figure 13, the ICU 1⊘⊘ provides communication between external devices and the on-chip MPU. Commands to the ICU originate at the processor command register (PCR) 1⊘8 located in the MPU. The content of this register is detailed in Figure 14. Status responses to commands originate in the detection logic 1⊘9 within the ICU 1⊘⊘ and are transmitted to the MPU via eighteen status response signal lines SR⊘-SR17 as described in more detail hereinafter. As indicated in the timing diagrams of Figures 15 and 16, commands are taken by the ICU only on falling edges of the input block. If service to a prior command is not in progress, or if service to the MPU is not temporarily inhibited because of a halt condition, the ICU will strobe and buffer the content of PCR 1⊘8 on every falling edge until a non-zero command is detected. The PCR hierarchy comprises three levels, denoted PCR, PCR', and PCR''. PCR 1⊘8 is the highest level and represents the full command at the MPU. It will be understood by those skilled in the art that PCR 1⊘8 need not be a physical register in the MPU. PCR' 1⊘8' is the replica of the full PCR 1⊘8 as buffered in the ICU during the command sequence. Since the command sequence may be overlapped with states 6 and 7 of the bus cycle, the content of the PCR' 1⊘8' may change during the bus cycle. The PCR' 1⊘8' cannot, therefore, be used to control address selection or to establish the function code, for these signals must be maintained for the entire bus cycle. Thus, a third level of buffering is required. This is provided by PCR'' 1⊘8'' which receives selected bits of PCR' 1⊘8' that must be fixed for the entire bus cycle , namely PCR1(S), PCR2(P), and PCR3(B₃). Also, PCR4(B₄), PCR1⊘(W₁₀), PCR11(W₁₁), and PCR12(A) are buffered in PCR'' 1⊘8''. PCR'' 1⊘8'' is implemented as indicated in Figure 13 and is loaded from PCR' 1⊘8' at the start of each bus cycle and buffer enabled by the command sequence. Execution of the command, e.g., a bus cycle, begins as early as one-half clock period after PCR' 1⊘8' is loaded. Upon completion of service to the current command, and in the absence of a bus exception, the ICU 1⊘⊘ will begin seeking the next command immediately after the rising edge at which it presents proceed status for the current command. As shown in Figure 16 for the case of exceptional status (e.g.., bus error), the ICU will not begin seeking the next command until 2T after the rising edge at which it presents the exceptional status signal. Loading of PCR' 1⊘8' nevertheless occurs only on negative transitions of the clock. In other words, command sensing is delayed by 2T beyond the normal case. This allows the MPU time to replace or cancel the current content of the PCR. The MPU should, therefore, be designed to post commands to the ICU on rising edges of the input clock so that they will be stable for loading into the PCR' at negative transitions of the clock. The PCR 1⊘8 and the 31-bit address out (shown coming from address out register AOR 11⊘) together provide all the information needed by the ICU 1⊘⊘ to conduct its operations. The processor inter face also contains two, 32-bit data paths, D1 (shown directed to an MPU data-in register DIR 111) and DO (data out) 112, as shown in Figure 13. The high-order byte of each path always corresponds to the lower storage address associated with any set of bytes contained thereon. The most significant bit is the high-order bit of the high-order byte. Each 16-bit half of either data path DO of DI to be used may be specified by the MPU with a 1-bit alignment code. All halfword transfers take place through either of these high-order or low-order halves of DO or DI. All byte transfers take place through the low-order byte position of the designated halfword data path. For a specific MPU used with the preferred embodiment of the ICU according to the invention, all word transfers are accomplished a halfword at a time by two microinstructions. Each of these two microinstructions references a halfword. The first of these microinstructions, moreover, may reference a halfword at any byte address A in storage. This microinstruction is identified in the PCR 1⊘8 by the MPU as a fetch or store halfword high (HWH) and implies that a second halfword operation will follow. This second microinstruction will always reference the halfword at a byte address (A+2) in storage. The referenced halfword operands may use either the high or low-order half of either DI 111 or DO 112, as specified by the alignment code for each register. Each of these two commands to the ICU 1⊘⊘ elicits its own status response, as is the case for all commands. The specific MPU used with the preferred embodiment of the invention never checks alignment. The ICU 1⊘⊘ must, therefore, check the low-order address bits before it can perform output cycles on words (herein denoting four contiguous bytes). Moreover, in the case of instruction fetches, if A⊘=1 the ICU will not execute the command and will, instead, generate odd-instruction address exceptional status in response to the MPU command. In addition to the MPU/ICU interface facilities already mentioned, an interrupt request register (IRR) 113 and an external reset register (XRR) 114 are provided in the ICU 1⊘⊘ as indicated in Figure 13. The IRR 113 buffers the interrupt control signals derived from the ten interrupt pins of the ICU. Note that the state indicators are represented by direct connections between the MPU and external pins of the ICU. These, as well as the other pins represented at the bottom of Figure 13, are defined in the table of Figure 12. These state indicators are not germane to the ICU 1⊘⊘ as disclosed herein but are instead a feature of the specific MPU used with the preferred embodiment of the invention. More specifically, the MPU conforms to IBM System/37⊘ processor architecture. Thus, the preferred embodiment of the invention provides an interface between an IBM System/37⊘ processor and Motorola MC68⊘⊘⊘-compatible devices. But it must be emphasized once again that this is but one example, and other combinations can be envisioned based on the teachings of the invention. In view of the foregoing and in the interest of simplicity, the specific MPU used with the preferred embodiment of the ICU according to the invention will be referred to simply as the System/37⊘ MPU. Implementation, Execution Logic Figure 21 shows in detail the addressing facilities of the ICU 1⊘⊘. This includes asynchronous address selection logic 115 which receives address bits A⊘1 and AO⊘ from the address output AO 11⊘ of the MPU. The address selection logic 115 also receives signals Z', Y' and WIP' from the control logic described hereinafter and generates the output gating signals AG1 and AG2 in accordance with the following truth table, where the X denotes a "don't care" condition: The AG1 output of logic 115 is used to select either AO1 or AO1 as the low-order address bit A1 on the address bus. The AG2 output of logic 115 is used to select either the AO31-AO2 bits from AO 11⊘ or the AO31-AO2 address incremented by one. The incremented address is generated by the ALU 116 which adds one to the AO31-AO2 bits of AO 11⊘. The address bits selected by AG1 and AG2 and address bit AO32 from AO 11⊘ feed the tri-state address bus represented by the tri-state drive 117. The tri-state address bus is enabled by the address enable flip-flop 118. The data flow logic is shown in Figure 23 and comprises an input multiplexer (IMUX) 121, an output multiplexer (OMUX) 122, aid an input/output register (IOR) 123, interconnected as shown. The OMUX 122 is also used during the first bus cycle of double-cycle inputs to steer the high-order byte from the data bus to IOR⊘ where the byte is buffered for use in the final cycle of the double cycle. The use of a pre-established common buffer (IOR⊘) in this way simplifies the process of assembling the halfword during the second cycle. Note, however, that the device response for the second cycle need not be the same as that for the first cycle; i.e., the halfword may span two, physically distinct and different storages. As implemented in the preferred embodiment of the invention, the OMUX 122 connects D⊘, D1, or D3 to IRO⊘ for all input cycles. Actual loading of the IOR 123, however, is determined by the detection sequence. Certain fetch cycles, for example, will end up with the low-order byte of a word in both IOR3 and IRO⊘. The same situation will occur during input service cycles. IOR⊘ is not used in either of these latter two circumstances. The OMUX 122 asynchronous control logic is defined by the table shown in Figure 24. For the case of buffer outbound halfword high action (Q=⊘ in the table shown in Figure 24), the only significant control is the alignment code A''. Loading of the PCR'' 1⊘8'' (Figure 13) is enabled by the command sequence at D23 (during execution of STORE HWH) and at H26 (during the first macrocycle of an output service cycle, described in more detail hereinafter. This means that the PCR'' 1⊘8'' is loaded at the end of the first and second executions of the command sequence; i.e., corresponding to the end of the "wind-up" and "start-up" phases, respectively, of the MPU/ICU cycle (see Figure 15). The IOR 123, on the other hand, is not loaded until the second negative edge of the "command and seek" phase of the MPU/ICU bus cycle (indicated in Figure 53); i.e., 3 1/2 T after the start of the command sequence. The OMUX 122 must, therefore, "settle" within a period of 1 1/2 T after the second loading of PCR'' 1⊘8'' in order to satisfy the timing requirements for this case. For all output bus cycles, the OMUX 122 setting depends, as shown in Figure 24, upon the output values of WIP', Y', Z', and A⊘', as well as on PCR'' 1⊘8'' content. The values of WIP', Y', Z', and A⊘' begin to appear during S1 of the bus cycle and must be stable by the beginning of S3 in order to satisfy the address bus specification. The OMUX 122, in turn, must stabilize by the end of S4 in order to satisfy the output data specifications; i.e., about one clock (T) after the input controls (WIP', Y', Z', and A⊘') settle. In the case of input cycles, the setting of OMUX depends upon the decoded device response (WTACK', etc.), which is not available until into S5 of the detection sequence. The OMUX 122 must then settle in time for the data to be strobed at the end of S6 of the termination sequence. Settling time for the OMUX 122 is therefore T/2, the most stringent of all cases. An IMUX control logic table, similar to the OMUX control logic table of Figure 24 is shown in Figure 25. The IMUX 121, along with the OMUX 122, must set up during the detection and termination sequences and satisfy set-up time at the negative edge between S6 and S7. Four of the byte inputs to IMUX 121 are directly connected to the 32-bit bidirectional data bus. The other four byte inputs to the IMUX are taken from the IOR 123, as shown in Figure 23. The function control logic is shown in Figure 19 which produces signals FC⊘, FC1 and FC2 which are valid only during a bus cycle. The inputs are from PCR'' 1⊘8''. This logic consists of two-input OR gates 13⊘, 131 and 132. PCR''3(B''₃ ), the service cycle indicator bit, is supplied to one input of each of these OR gates. PCR''1(S'') is supplied to the other input of OR gate 13⊘, and PCR''2(P'') is supplied to the other input of OR gate 131. PCR''2(P'') is also supplied to the second input of OR gate 132, but this input is an inverting input. The outputs of the OR gates 13⊘, 131 and 132 are connected to output pins of the ICU via tri-state drivers 133, 134 and 135. The bus cycle controls (see Figures 12 and 13) include various output strobes. The address strobe flip-flop 137 is shown in Figure 26. The output of the address strobe flip-flop 137 is connected to an output pin of the ICU via an inverting tri-state driver 138. The data strobe logic is shown in Figure 26. The data selection logic 14⊘ is defined by the truth table shown in Figure 27. The selection logic 14⊘ receives as inputs the WIP', Y' and Z' signals and A⊘'. Selection logic 14⊘ also receives outputs B'₃ and W'≡W'₉ W'₁₀ W'₁₁ from PCR' 1⊘8'. The outputs generated by the selection logic 14⊘ are LDS', UDS' and WS' which are supplied to respective AND gates 141, 142 and 143. These two-input AND gates are enabled by a data strobe activation flip-flop 144 via OR gate 145. The AND gates 141, 142 and 143 are also enabled by the output of AND gate 146 via OR gate 145. The inputs to AND gate 146 are B'₃ and B'₄ from the PCR' 1⊘8', and this AND gate is enabled by the output of the address strobe flip-flop 137. The outputs of the AND gates 141, 142 and 143 are connected to output pins of the ICU via respective inverting tri-state drivers 147, 148 and 149. Also shown in Figure 26 is the valid memory address (VMA) logic 151 which generates the VMA signal of the MC68⊘⊘ peripheral controls. the output of this logic is connected to an output pin of the ICU via an inverting tri-state driver 152 through an OR gate 153. The other input of OR gate 153 is supplied from AND gate 154 which, in turn, receives as inputs B'₃ and B'₄ from the PCR' 1⊘8'. AND gate 154 is enabled by the data strobe activation flip-flop 144. As will be noted, B'₃=B'₄=1 only for output service cycles. With the logic of Figure 26, level-zero service cycles (SENSE/CONTROL) are completely defined (1) by the content of the address bus and (2) the encoded state of the signals appearing on the three data strobe service-address pins 147, 148 and 149. The service address identifies the particular SENSE/CONTROL cycle. Service address timing is similar to that of the address strobe ( AS ) for all service cycles for which the service address is defined, by virture of AND gate 146. For level-zero input service cycles (SENSE), the address strobe ( AS ) is used both as an address strobe and a data strobe, i.e., it serves the external device in the dual capacity of an address/data strobe. For level-zero output service cycles (CONTROL), the output service strobe ( OSS ), driven by the tri-state driver 152, is activated as a data strobe. Its timing corresponds to that of the data strobes during ordinary store cycles, by virtue of AND gate 154. VPA is a normal response to all input service cycles irrespective of level, thereby maintaining the Motorola-compatible auto-vectoring request. For output service cycles, on the other hand, VPA is disallowed and, if used, will generate an interface check. It will be noted that the timing of the service address for SENSE cycles is therefore different from that of CONTROL cycles. For CONTROL cycles, the service address appears one logic delay later than for SENSE cycles. This is all right since SENSE/CONTROL cycles are not associated with the MC68⊘⊘⊘; the timing of the service address signals need not meet any Motorola specification. The last of the five bus cycle control-out signals is the read/write (R/ W ) signal. The read/write flip-flop 151 is shown in Figure 2⊘. The output of flip-flop 151 is connected to an output pin of the ICU via the tri-state driver 153. The bus-cycle-in-progress (BCIP) signal is one of the timing signals listed in the table shown in Figure 12. The bus-cycle-in-progress flip-flop 154 is shown in Figure 17. The output of the flip-flop 154 is connected to an output pin of the ICU via an inverting tri-state driver 156. Figure 18 is a timing diagram showing the operation of flip-flop 154. External devices respond to the control-out signals of a bus cycle with one or more asynchronous control-in signals. There are nine control-in signals: BTACK BERR DTEXN/⊘ DTACK VPA DTEXN/1 WTACK HALT DTEXN/2 Each control-in signal feeds the D-input of a respective negative-edge triggered D-type flip-flop 161 to 169, as shown at the bottom of Figure 58. In this way, these signals are sampled on every negative edge of the clock. The outputs of the synchronizing flip-flops, in turn, feed the detection logic 17⊘. This asynchronous control logic is discussed under the heading of ICU control, which follows. Implementation, Control Logic A comprehensive overview of the control logic of the ICU is provided by the block diagram of Figure 28. This diagram resolves the complex control function of the ICU into four, distinct, concurrently operating "sequences", as follows: 1) MPU Service Sequences a) Command b) Execution Reset-Out Buffer Read Buffer Write Bus Cycle Halt-Out c) Post-Termination 2) Bus Arbitration Sequence 3) Interrupt Sequence 4) External Reset Sequence The term "sequence" refers to the sequence of logical steps performed by the associated logic, i.e., to what the logic does, as distinguished from how it is implemented. These steps will be described hereinafter. The bus arbitration, interrupt, and external reset sequences are completely independent of one another and of the MPU service sequences. Each of these three independent sequences are enabled at all times and operate continuously to detect and respond to their particular input signals. The MPU service sequences, on the other hand, comprise an endless chain of sequences, each enabling at least one other sequence of the set before ending its own operation. No more than two of the MPU service sequences are ever in operation concurrently, i.e., overlapped, and in most instances the sequences are performed one at a time. Some of the MPU service sequences, however, are mutually exclusive, namely the command sequence and each of the post-termination sequences as indicated in Figure 28 by the available switch positions. The specific concatenation of enablement/disablement of each of these sequences will be made clear in what follows. The MPU service sequences, moreover, are interlocked in three places (command sequence, PTS4 and PTS6) with three signals (BGE, BR' and BGACK) from the bus arbitration logic. This interlock, indicated in Figure 28, acts to prevent the associated sequence from initiating a bus cycle while the bus is temporarily unavailable because of bus arbitration. Because of the overall complexity of the ICU control function, the resolution of function as depicted in Figure 28 is essential to its understanding and to its successful implementation. This resolution of function is believed to be unique and is, therefore, one of the elements of this invention. The implementation of the preferred embodiment is based directly on Figure 28, i.e., each sequence is implemented separately. In practice, such an implementation scheme need not be followed. The MPU service sequences, for example, can be implemented by the logic of a single "state machine". But the description of Figure 28 nevertheless remains as the underlying basis, or specification, of what the state machine, or any other form of implementation, must do. In what follows, the implementation of each of the control sequences according to the invention is described in detail. In general, the description will comprise three elements: (1) a timing diagram, specifying what the sequence is to do; (2) a logic diagram and/or truth table(s) describing a logical implementation of that sequence; and (3) a flow chart, describing the operation of that implementation. Item (3) will be seen to satisfy the requirements set forth by item (1). Before proceeding, however, it is helpful to describe first some of the implementation elements used for inter- and intra-sequence communication. Inter- and Intra-Sequence Communication Logic This logic comprises control flip-flops that are set/reset by a sequence, or by more than one sequence, and whose states are interrogated later, either by the setting/resetting sequence or by another sequence. Among these flip-flops are the WIP, Y and Z flip-flops and their buffers, the WIP', Y' and Z' flip-flops. These flip-flops are described in the present topic and are illustrated in Figure 41. Another such flip-flop is the Bus Grant Enable (BGE) flip-flop. The description of this strategic flip-flop, whose function is more specialized, is deferred to the topic of bus arbitration. The word-in-progress (WIP) flip-flop 2⊘4 (see Figure 29) is used in conjunction with store or fetch halfword commands to indicate that a full word (32 bits) is to be transferred. For input, the WIP flip-flop 2⊘4 is set by the bus cycle detection sequence whenever the ICU is executing a fetch halfword high (HWH) command, the address is word-aligned, and the device has responded with WTACK . The ICU will then transmit the high-order half of the word to the MPU and concurrently buffer the low-order half in its input/output register (IOR) 123 (shown in Figure 23) in anticipation of the next halfword fetch command from the MPU. This next command, detected in the presence of WIP=1, causes the ICU to fetch the low-order half-word form the IOR 123 instead of from the external device, thereby avoiding an additional bus cycle. The WIP flip-flop is then reset. The WIP flip-flop 2⊘4 is also set by the ICU in response to a store halfword high (HWH) command for which the target address is found to be word-aligned. Under these circumstances, the ICU buffers the high-order halfword in its IOR 123. The subsequent store halfword bus cycle, noting that WIP=1, is then modified to deliver the full, 32-bit word using all data strobes including, in particular, WS . The WIP flip-flop 2⊘4 is then reset at the end of the detection sequence of this bus cycle, provided that the device responds with WTACK . If additional cycles are required because of BTACK , DTACK , or VMA responses, the WIP flip-flop 2⊘4 is not reset until all bytes of the word have been transferred. Stated differently, the WIP flip-flop 2⊘4 is normally reset during the bus cycle that completes the storing of the full word. The Y flip-flop 2⊘6 (see Figure 3⊘) is used by the bus cycle controls to identify (1) the second bus cycle (Y=1) of a double cycle and (2) the bus cycles of the word-write operation. The Y flip-flop 2⊘6 is always valid for these purposes at the first negative edge of the initiation sequence of each bus cycle, at which time it is buffered by the bus cycle detection sequence (1) upon recognizing the need for a double cycle, or (2) in response to BTACK or VPA during the Z'Y'=⊘⊘- and Z'Y'=1⊘- cycles of the word-write operation. During the next bus cycle, the Y flip-flop 2⊘6 sets the Y' flip-flop 2⊘7, and the latter remains in the set state for the duration of that bus cycle. The Y flip-flop 2⊘6 is reset at the start of each bus cycle enabled by the command sequence CDS, or by the detection sequence of the Z'Y'=⊘1-cycle of the word-write operation, the latter in response to a normal response from the external device. The Z flip-flop 2⊘8 (see Figure 31) is used by the bus cycle controls in conjunction with the Y flip-flop 2⊘6 to identify the cycles of a word-write operation. The Z flip-flop 2⊘8 is always valid for this purpose at the first negative edge of the initiation sequence of every bus cycle, at which time it is buffered in the Z' flip-flop 2⊘9. The output of the Z' flip-flop 2⊘9 is of significance only when the WIP flip-flop 2⊘4 is in the set state. The Z flip-flop 2⊘8 is set only by the bus cycle detection sequence during a word-write operation in response to DTACK during the Z'Y'=⊘⊘- cycle or in response to BTACK or VPA during the Z'Y'=⊘1-cycle. The Z flip-flop 2⊘8 is always reset at the start of every bus cycle enabled by the command sequence. The WIP' flip-flop 2⊘5 (Figure 29) and Y' flip-flop 2⊘7 (Figure 3⊘) capture the states of the WIP flip-flop 2⊘4 and the Y flip-flop 2⊘6, respectively, at the first negative edge of the bus cycle and retain these states for the entire duration of the bus cycle, independent of possible changes in the WIP and Y flip-flops 2⊘4 and 2⊘6 that may occur during the bus cycle. Inasmuch as the address bus 1⊘1 is also enabled at the first negative edge of the bus cycle, propogation time for the WIP' and Y' flip- flops 2⊘5 and 2⊘7 must be sufficiently short to allow the address to become valid within the corresponding time specified for the MC68⊘⊘⊘. Bus Arbitration The bus arbitration architecture of the ICU is essentially identical with that of the MC68⊘⊘⊘, but includes specific variations that will affect compatibility with some master-type devices designed for MC68⊘⊘⊘, namely those which do not conform to the variations contained herein. The essential elements of the bus arbitration architecture are set forth in what follows. Bus arbitration is available at the completion of every bus cycle, excluding only the following bus cycles: 1) Bus cycles with Hold (bus cycles performed in response to CCC=111) that end normally, with or without HALT. These comprise all bus cycles of a macrocycle. 2) "Qualifying" cycles, that is, bus cycles, not of a double-cycle or of a macrocycle, that qualify for the exclusion of bus arbitration at the end of that cycle, irrespective of the state of Bus Request. Such cycles are associated with storing or fetching the high-order half of full, 32-bit words under specified circumstances. 3) The first bus cycle ("inner cycle") of a double-cycle, if HALT is inactive. 4) Specific cycles of the word-write operation, as follows: a) The WIP/⊘⊘ cycle, if not a WTACK response and no halt request; and b) The WIP/1⊘ cycle, if a WTACK or DTACK response and no halt request. The processor is effectively halted by a normal halt ( ACK signal and HALT ) or by any exceptional termination arising from the BERR and DTEXN signals (pseudo halt). Bus arbitration is always available while the processor is halted by the active state of any of these signals. Bus-master devices control two, asynchronous signals: BR and BGACK, where the notation BR, BGACK, etc. here denote logic levels, for simplicity. The ICU drives AS and BG. The basic rules governing these signals are embodied in the timing diagram of Figure 32 and comprise the following: 1) BR can be activated at any time, and it is ordinarily negated by the originator just after it activates BGACK. 2) BG is the response to BR, and if it is activated during a bus cycle, this will only occur just after the start of S6. 3) BGACK is a response to BG· AS ; BGACK must never be activated unless BG is active and AS is concurrently inactive. 4) The bus "busy" condition is effectively BGACK+AS. 5) BG is negated in response to the activation of BGACK (or to a prior negation of BR). If the bus arbitration logic of a MC68⊘⊘⊘ device is not consistent with the above, it cannot be used with this implementation of the ICU. In particular, devices which use HALT-In alone to "capture" the bus are excluded from use, as this would violate condition 3 above. The logical implementation of this bus arbitration architecture is described completely in Figure 33 and the truth table shown in Figure 34. The related timing diagrams are shown in Figures 35 and 36. The logic of Figure 33 comprises five basic components: 1) Device-controlled logic, namely the BR flip-flops 226 and 227 and the BGACK (A) flip-flops 229, 229 and 23⊘. 2) State-controlled logic, namely the G logic 221, the G flip-flop 225, and the VA flip-flop 231. 3) The BGE flip-flop, controlled by other sequences of the ICU. 4) The bus grant logic, consisting of the AND gate 223. 5) Tri-state enable (TSE) logic, i.e., OR gate 232 and AND gate 233. The BR logic comprises the negative edge-triggered BR flip-flop 227 and the positive edge-triggered BR' flip-flop 226. The BR' flip-flop is, in effect a conditional slave to the BR flip-flop 227. The BR' flip-flop 226 will copy the state of the BR flip-flop 227 whenever that state is valid, as defined by the SVD signal VR, which gates the clock to the BR' flip-flop 226. If the state of the BR flip-flop is invalid because of untimely transition of BR , VR will be zero. An additional clock period will then elapse before the content of BR can be copied into the BR' flip-flop 226, provided that VR has become active. This scheme is, therefore, a two-stage detection scheme. The BGACK (A) logic is free-running logic that is essentially identical with the BR logic with the sole difference that a third flip-flop (A'') 23⊘ is slaved to the A' flip-flop 229 with a phase relationship of one full clock period (T). A'' thus represents the prior state of A' and may be used along with A' to identify transitions in the BGACK signal. The Unauthorized Acknowledge (UA) flip-flop 231 is set and reset solely by the G logic 221 and flags activation of the A' flip-flop 229 in the absence of a G signal. This is the "unauthorized acknowledge" condition. The UA flip-flop 231 is reset whenever the condition is removed (UA=1, A'=⊘). This logic reflects the requirement that BGACK never be activated by a device in the absence of BG . The G logic 221 is driven by five flip-flops, as shown in Figure 33, and determines the states of the UA and G flip-flops 231 and 225, respectively. These two flip-flops and the G logic 221 thus comprise a "state machine" whose current state is determined by the indicated external events, its own prior state (through feedback of the UA and G signals), and the "history" of the A' flip-flop 229 (provided by A'' flip-flop 23⊘). State changes occur only at positive transitions of the clock, and the G logic must, therefore, have a propagation time of <T in order to satisfy set-up time for the next positive transition. The BGE flip-flop 222 is set and reset as indicated in the table of Figure 37. Unconditional reset occurs at the start of every bus cycle, i.e., at the positive transition of the clock which begins S⊘ of the bus cycle. The conditions for starting a bus cycle are assessed by the command sequence or a post-termination sequence during the clock period just preceding the tentative start of the bus cycle. If BGE is active and the output of the BR' flip-flop 226 is also found to be active, meaning that G and BG are either active or about to become active, a bus cycle is not started. Thus, a BG signal is never aborted by the ICU. The BGE flip-flop 222 is set at the positive edge of the clock that ends the detection sequence of every bus cycle, excluding the following specific cycles: 1) All cycles of a macrocycle that end normally, either with or without HALT. 2) Qualifying cycles as defined above. 3) Inner cycles, not of a macrocycle, for which HALT-in is not detected. 4) Specific cycles of the word-write operation. The BGE flip-flop 222 is also set during the command sequence (whenever a bus cycle is either delayed or the processor halted), during the ICU reset sequence, and whenever an interface check prevents initiation of a bus cycle (sequencing exception). The BGE signal thus synchronizes the enablement of BG with the bus cycle, allowing BG to begin to appear (1) no earlier than at 5 6 of a bus cycle, (2) at the end of each macrocycle, or (3) whenever the ICU is delayed in initiating a bus cycle on behalf of the MPU. Once activated, it is never reset unless BR' and A' are both inactive at the prior positive transition of the clock, i.e., degating of BG is never performed by the BGE flip-flop 222. Instead, deactivation of BG is determined solely by the G signal, in response to BR and/or BGACK . Thus, whenever the ICU sets the BGE flip-flop 222, it is effectively releasing the bus for arbitration. Whenever it next wants the bus for MPU service, it resets the BGE flip-flop 222 as soon as the bus becomes available, indicated by concurrent reset states of BR' and A'. In other words, once the device is given the bus, the device may hold the bus for as long as it wants. The ICU cannot "take it away". But, whenever the ICU has the bus, it too may retain the bus as long as it desires. The bus grant logic comprises only AND gate 223. Whenever BGE is active, BG is activated whenever the device wants the bus (G active). BG is always deactivated by deactivation of G, i.e., when the device releases the bus. The TSE signal enables all tri-state controls of the bus. This explicitly excludes the bi-directional data bus and the address bus which are tri-stated by other controls. The TSE signal is generated as shown in Figure 33, using OR gate 232 and AND gate 233. The ICU controls are removed from the bus (TSE inactive) whenever the following conditions are met: 1) BGE active (bus has been released by the MPU); 2) G or A' active (bus being requested by a device), or HALT-out active; and 3) AS inactive (bus cycle ending or absent, on behalf of the MPU). Thus, during the "last" bus cycle on behalf of the MPU, TSE begins to fall during state 7 of the bus cycle. The device can, in principle, begin its cycle immediately if it activates BGACK at the same time. Finally, with regard to the implementation of the bus arbitration architecture of the ICU, and by way of summary, it is important to note that the logic embraced by the BGE flip-flop 222 and the AND gate 223, the BR' flip-flop 226, and the A' flip-flop 229, is the means by which the bus cycle control logic and the bus arbitration G logic, each with different responsibilities, dynamically share the singe, external bus of the ICU. The bus cycle control logic expresses its needs for the bus by resetting and setting the BGE flip-flop at times consistent with the states of the BR' and A' flip-flops. Similarly, the G logic 221 expresses its needs for the bus through the G flip-flop 225. These two, independent and mutually exclusive actions are then combined by the AND gate 223 to produce the BG signal. The important point is that as a result of the logical structure depicted in Figure 33, the designer of the bus cycle control logic need not know the specific action of the G logic 221 in order to carry out that design. And, the converse is true: any set of arbitration rules can be implemented by the G logic 221 in complete harmony with any form of bus cycle protocol. For the preferred embodiment herein, the G logic defined by the table of Figure 38 conforms closely, but not exactly, to the bus arbitration protocol of the MC68⊘⊘⊘. That behavior did not, however, impact the specification and implementation of the bus cycle control architecture. Thus, the logical structure depicted in Figure 34 provides the designer of a microprocessor with the important benefit of even further separation of function. This, in turn, leads to flexibility in the choice of architectural elements and to simplicity of implementation. Command Sequence The purpose of the command sequence (CDS) 2⊘⊘ (see Figure 28) is to detect, buffer, and initiate execution of commands form the MPU. These functions are unique to this control sequence. The command sequence 2⊘⊘ also performs the normal halt-release function. A logic diagram of the command sequence 2⊘⊘ and its inbound and outbound control signals is shown in Figure 41. The actual control logic is not detailed here and is described separately in what follows. The command sequence 2⊘1 is specified as comprising two states, C⊘ and C1, and a single phase (α) corresponding to the output of the command sequence enable flip-flop. C⊘ corresponds to clock high, and C1 to clock low. See Figure 38. Once enabled, the sequence runs continuously until a detected command can be executed. A busy bus, for example, can delay command execution. the timing is critical: a STOP command becomes valid after a negative clock edge, and the outcome must be resolved by the logic in less than one-half clock period (T/2) in order to satisfy set-up time for the next positive clock edge. Apart from START/STOP commands, commands otherwise become valid after positive edges of the clock. The command sequence may itself generate a status response. This occurs only in the case of command reject or during execution of the buffer write sequence. In either case, status is valid only across the single positive clock edge at which it is intended to appear, i.e., the status signal satisfies both set-up and hold time for that one edge. If the command sequence ends with the presentation of exceptional status, as is the case with command reject, the deactivation of status occurs as a result of disablement of the command sequence. For this case, the command sequence is then resumed after a 2T delay. If, however, the command sequence is repeated while PROCEED status is active, the command sequence logic, operating on the new command will generate a new status response in time for the next positive edge. This new status response will be zero in the case of a NOOP or when no status is forthcoming from the command sequence itself. The command sequence is enabled or enablable by the following sequences: 1) Interface check, upon attempt to initiate a bus cycle; 2) External Reset; 3) Reset-Out; 4) Bus Cycle Detection Sequence (conditional, but normal); 5) Buffer Read Sequence (BRS); (Buffer Write Sequence runs concurrently with Command Sequence); 6) Post-termination (all sequences other than PTS4 and, if rerun occurs, PTS3); and 7) MC68⊘⊘⊘ sequence, if not HALT. The macrocycle controls 21⊘ identify the start of a mcarocycle, its continuous presence (independent of the activation/deactivation of the address strobe), and its termination. These controls are shown in more detail in Figure 42 and include the following basic elements: the HOLD flip-flop 211, the macrocycle-in-progress (MIP) flip-flop 212, the START flip-flop 213, and the STOP flip-flop 214. Specification timing diagrams for Command Stop and for clearing the HOLD flip-flop are shown in Figures 39 and 4⊘, respectively. The HOLD flip-flop 211 denotes bit 6 of the PCR buffer, i.e., PCR'6. This bit is the high-order bit of the Operation field of the PCR; it expresses the MPU's desire to start or maintain a macrocycle. The HOLD flip-flop 211 is therefore set or reset each time PCR' 1⊘8' is loaded, i.e., at negative transitions of the clock during the command sequence. A start macrocycle request from the MPU is detected when this bit toggles from ⊘ to 1. This may occur just as a bus cycle is completing or while the bus is idle relative to the processor. Similarly, a stop macrocycle request from the MPU is detected when this bit toggles the other way, i.e., from 1 to ⊘. This will generally occur during S7 of the last bus cycle of a macrocycle, but it can also occur later, while the bus is idle. The MIP flip-flop 212 identifies an ongoing macrocycle. It also drives the MIP pin on the bus to identify the macrocycle to multi-ported storage devices. This is needed because the address strobe ( AS ) may rise and fall during a macrocycle. The MIP flip-flop 212 is set during the first bus cycle of the macrocycle, at the same time AS is activated. It is always reset on a positive transition of the clock in one of the following ways: 1) In response to a stop request from the MPU. For this case, the MIP flip-flop 212 will be reset one-half clock period after the STOP command is loaded into PCR' 1⊘8' by the command sequence. This will ordinarily occur at the end of S7 of the last bus cycle of the macrocycle. 2) In response to an illegal command from the MPU. Reset will occur at the positive edge of the clock that ends the command sequence. 3) In response to an odd-instruction address. For this case, the MIP flip-flop 212 will be reset at the end of S7 of the bus cycle in progress at the time the address exception is detected. This cycle then becomes an ICU-forced last cycle of the macrocycle. 4) In response to any exceptional device termination. This case is similar to case 3 except for the origin of termination and the moment of detecting the exception. Detection occurs during the detection sequence. The transition time of the MIP flip-flop 212 must be less than one-clock period to assure that MIP is inactive before the earliest time at which AS can next become active. The minimum off time for the MIP flip-flop (inactive state) is two clock periods. The minimum time can occur if a NOOP command ends a macrocycle and is followed one clock period later with a Bus Cycle with Hold command. The START flip-flop 213 requests that a macrocycle begin with the next bus cycle. As a consequence, the MIP flip-flop 212 is set at the S1 S2 transition of that next bus cycle. The START flip-flop 213 is set whenever PCR' 1⊘8' is loaded and PCR'6 changes from ⊘ to 1. It remains set until reset by one of the following three events: 1) Activation of the MIP flip-flop 212 during the first bus cycle of the macrocycle. This bus cycle may not occur at once, however, if the START flip-flop 213 is set by a NOOP with HOLD or by a command that causes a buffer cycle. Additional NOOPs may also follow. This reset of the START flip-flop 213 occurs after the command sequence has terminated. 2) By the command sequence upon any command reject. 3) Setting of the STOP flip-flop 214. This implies that the START flip-flop 213 had been set by a NOOP with HOLD and no subsequent bus cycle was either requested or performed for the macrocycle. There is no net effect from the setting of the START flip-flop under these circumstances. The STOP flip-flop 214 is set whenever the PCR' 1⊘8' is loaded and PCR'6 toggles to ⊘. This denotes a Command Stop and causes the following to occur: 1) The MIP flip-flop 212 is reset at the next positive transition of the clock. 2) The BGE flip-flop 222 is set at the next positive transition of the clock and the command sequence is repeated if any of the following conditions prevail: a) the HALT₂ flip-flop is active, in which case the halt-release function of the command sequence is enabled; b) the BR' flip-flop 226 is active; or c) the STOP command is a NOOP. A Command Stop cannot occur as the result of the next command following an exceptional termination of a macrocycle because the ICU has itself already cleared PCR'6. This prevents toggling to ⊘ until after another macrocycle has been initiated by the MPU. The STOP flip-flop 214 is unconditionally reset during the positive clock following its setting; i.e., the duration of its active state is approximately one-half clock period (T/2). The required action of the command sequence control logic is specified in detail in the flowcharts of Figures 43 to 51. The form of implementation is arbitrary, but whatever the form (PLA, combinational logic, etc.), it must produce the action specified by these flowcharts. In the paragarphs which follow, these flowcharts will be "read" in some detail to explain the control logic action and, at the same time, how to read the flowcharts of this document. With this introduction to the flowchart form of describing a complex sequence, all other flowcharts herein will be treated in context as essentially stand-alone, self-explanatory description. With reference first to Figure 43, a command sequence begins at a positive edge of the clock. The CMUX select flip-flop 2⊘3 is checked to determine if it is at 1 or ⊘, the former selecting PCR' 1⊘8' and the latter selecting the PCR 1⊘8. If the CMUX select flip-flop 2⊘3 is set at 1, this means that a command has already been buffered but could not yet be executed. The command sequence is therefore repeating. If, on the other hand, the CMUX select flip-flop 2⊘3 is set at ⊘, the contents of PCR 1⊘8 are read into PCR' 1⊘8' on the negative transition between clock periods ⊘ and 1 and, at the same time, the START and STOP flip-flops 213 and 214, respectively, are conditionally set or reset. After the CMUX-select action, the next step in the logic is to check the HALT FLAG. Note here that the command sequence steps A8 and A1⊘ are mutually exclusive because (1) the HALT FLAG cannot be set by the detection sequence (DTS) during a macrocycle and (2) the HALT FLAG is set as the STOP flip-flop 214 is being reset during a normal STOP with HALT. Assuming first that the HALT FLAG is ⊘, the STOP flip-flop 214 is checked to determine its state. If the STOP flip-flop 214 is zero, the operation code C₇C₈ is checked. If the code is ⊘⊘ representing a NOOP, the logic returns to the beginning and the command sequence is repeated upon the start of the next positive clock. If the code is ⊘1, the HALT-out sequence is enabled and the BG flip-flop is enabled. Here, "enabled" means that the J-input of the corresponding flip-flop is activated. From here, the logic goes to the flow diagram shown in Figure 48 for ending the command sequence. If the code is 1⊘, the RESET-Out sequence is enabled and the process also goes to the flow diagram shown in Figure 48. Turning briefly to Figure 48, the end of command sequence involves the activation of the K-input of the CMUX select flip-flop 2⊘3 to select the PCR 1⊘8, the activation of the K-input of the command sequence enable flip-flop 2⊘2, and the activation of the reset input to the delay ring 21⊘. When the K-input of the CMUX select flip-flop 2⊘3 is activated, preparation is made to seek the next command from the MPU when the command sequence is next enabled. Activation of the K-input of the command sequence enable flip-flop 2⊘2 prepares for disabling the sequence at the next positive transition of the clock, and activation of the reset input to the delay ring prepares the delay ring 21⊘ logic for synchronous reset (see Figure 41). Returning to Figure 43, what has been described so far are the initial steps and the final steps (Figure 48) performed by the command sequence logic 2⊘1. Assume now that when the HALT FLAG is checked at step 8 it is found to be a 1. In this case, the logic goes to Figure 44 which shows the normal HALT release function. Note that the BGE flip-flop 222 is always active during execution of this logic. First, the HALT₂ flip-flop is checked and, if set to ⊘, the K-input to the HALT FLAG flip-flop is activated. The logic then returns to Figure 43 to continue with the command sequence at the command decode (step 12). On the other hand, if the HALT₂ flip-flop is set to 1, then a decision is made as to whether this is a NOOP, i.e., is C₇C₈=⊘⊘? If so, the logic simply returns to Figure 43 where the command sequence is repeated at the next positive transition of the clock. If not, the J-input to the CMUX select flip-flop 2⊘3 is activated in preparation to select PCR' 1⊘8' before returning to Figure 43. Again with reference to Figure 43, assume now that the HALT FLAG is ⊘ and the STOP flip-flop 214 is a 1. In this case, the logic next goes to Figure 45 which describes the Command Stop logic. First, preparation is made to reset the STOP flip-flop by activating the J-input to the XS flip-flop 215. In addition, the K-input to the MIP flip-flop 212 is activated. These steps are performed directly by the output of the STOP flip-flop 214 (see Figure 42). The HALT₂ flip-flop is checked and, if set to 1, the J-inputs to the BGE flip-flop 222 and the HALT FLAG flip-flop are both activated, the latter in preparation to perform a normal HALT release function. In this case, HALT was latched in the HALT flip-flop at the negative clock transition between periods 4 and 5 of the bus cycle. After the J-inputs of the BGE and HALT FLAG flip-flops have been activated, a determination is made as to whether this is a NOOP, i.e., is C₇C₈=⊘⊘? If so, the logic returns to Figure 43 to repeat the command sequence at the next positive clock. If not, the J-input to the CMUX select flip-flop 2⊘3 is activated to prepare to select PCR' 1⊘8' before returning to Figure 43 to repeat the command sequence. Returning to the HALT₂ flip-flop, if it is set to ⊘, the BR' flip-flop 226 is checked and, if it is set to 1, the logic proceeds as if the HALT₂ flip-flop had been set to 1. On the other hand, if the BR' flip-flop 226 is set to ⊘, a determination is made as to whether this is a NOOP. If it is, the J-input to the BGE flip-flop 222 is activated before the logic returns to Figure 43 to repeat the command sequence. If not, the logic simply returns to Figure 43 to perform the command decode operation. If the command decode operation of Figure 43 produces a code of 11, a bus cycle is specified. The logic then proceeds to Figure 46 where (1) the K-inputs of the Y and Z flip-flops 2⊘6 and 2⊘8, respectively, are activated in preparation to reset both of these flip-flops, and (2) the bus cycle ID field (BBB) is decoded. If the BBB field is X⊘⊘ or X11, the command reject logic is initiated since these codes are undefined. If the bus cycle code is ⊘⊘1, this denotes a read cycle, and the first determination that must be made is the access width, whether a byte, halfword or the first halfword of a word. In either case, the WIP flip-flop 2⊘4 is checked to determine if the previous bus cycle had extracted an aligned word from memory and, if not, the logic goes to Figure 47 where the set-up of the bus cycle is concluded. Similar logic exists for a decoded write bus cycle. Here again the access width is determined and for byte or halfword high (word) output, the WIP flip-flop 2⊘4 is checked for consistency with the access width. For halfword high (word) output, the Supress pin is checked to determine if full-word output is to be to be suppressed and, if so, a bus cycle is initiated (see Figure 47) to store the high-order halfword. If not, the delay ring 21⊘ is tested, and after two passes through the command sequence, a determination is made as to whether A1'=A⊘'=⊘. This is indicated in the drawing figure by the NOR operator A1'+A⊘' . If A1'≠A⊘'=⊘, a nonaligned word address is detected, and, as before, a bus cycle is initiated (see Figure 47) to store the high-order halfword. Turning now to Figure 47, the set-up for the bus cycle continues by checking the BGE flip-flop 222. This step prevents lock-up as well as provides "express" service. If the output of the BGE flip-flop is 1, the BR' flip-flop 226 is checked, and if ⊘, it is as if the BGE flip-flop bad been set to ⊘. In both cases, the BGACK' flip-flop is checked to determine its level. (Each of these flip-flops is set and reset on the positive edges of the clock. Propagation time is assumed to be adequate to provide hold time for the logic fed by the command sequence.) If the BGACK' flip-flop is ⊘, this indicates finally that the bus is available. The next thing that is done in to activate the K-input of the BGE flip-flop 222 in preparation for disabling bus grant ( BG ). Next, the load input to PCR'' 1⊘8'' and the J-input to the BCIP flip-flop 154 are activated and the initiation sequence is enabled. When this is accomplished, the logic goes to Figure 48 to end the command sequence as previously described. On the other hand, if upon testing the BR' flip-flop 226 or the BGACK' flip-flop, either is found to be set, the bus is not available and the J-input of the CMUX selector flip-flop 2⊘3 is activated to select the PCR'. The logic then returns to Figure 43 to repeat the command sequence at the next positive clock. Returning now to Figure 46, assume that a read cycle has been detected, the access width has been decoded for a halfword, and the WIP flip-flop 2⊘4 is 1, indicating that the MPU is attempting to fetch the low-order halfword or a full word. In this case, the K-input of the WIP flip-flop 2⊘4 is activated in preparation to reset that flip-flop (if it has done its job), and then the buffer read sequence is enabled and the load input to the PCR'' is activated. At this point, the logic again goes to Figure 48 to end the command sequence as previously described. On the other hand, assume either that the bus width was detected to be a byte or a halfword high (HWH) and the WIP flip-flop 2⊘4 is 1, indicating that the MPU has issued an inconsistent command. In this case, the logic proceeds to Figure 49 for the command reject sequence. A similar result is obtained for a write cycle if the WIP flip-flop 2⊘4 is 1. As shown in Figure 49, the first step in the command-reject process is to test the delay ring. If it is ⊘, indicating that an illegal command has just been detected, the J-input to the BGE flip-flop 222 is activated and preparations are made to increment the delay ring. Also, the J-input of the CMUX select flip-flop 2⊘3 is activated to select PCR' 1⊘8'. At this point the logic returns to Figure 43 to repeat the command sequence. The purpose of this procedure is to achieve a one clock-period delay. This delay has been achieved when the delay ring is found to be at 1. Preparation is then made to reset the WIP flip-flop 2⊘4, and the adjuct delay sequence is enabled. Also, the command-reject status signal to the MPU is activated, the J-input to the XH flip-flop 215 is activated in preparation to clear PCR'6, and the START and STOP flip-flops 213 and 214, respectively, are reset. And, finally, preparation is made to reset the MIP flip-flop 212. This done, the logic proceeds to Figure 48 to end the command sequence, as previously described. Returning again to Figure 46, assume that a write cycle has been detected to store the high-order half of a word, the WIP flip-flop 2⊘4 is ⊘, and word output is not suppressed. Again, the delay ring 21⊘ is tested, and if ⊘ or 1, the load input to PCR'' 1⊘8'' is activated, the increment output is activated to prepare for incrementing the delay ring 21⊘, and the J-input of the CMUX select flip-flop 2⊘3 is activated to select PCR' 1⊘8'. At this point, the logic returns to Figure 43 to repeat the command sequence. If, when the delay ring is tested and found to be at 2, the two low-order address bits are both zero, then preparation is made for a buffer write sequence and to seek the next command. The logic then returns to Figure 43 to repeat the command sequence. Preparation for the buffer write sequence involves activating the J-input to the WIP flip-flop 2⊘4 and enabling the buffer write sequence. Preparation for the next command involves activating proceed status to the MPU, the K-input of the CMUX select flip-flop 2⊘3 to select PCR 1⊘8, and the reset input to the delay ring 21⊘. The description thus far has been for the set up for a bus or buffer cycle involving ordinary read or write operations. If, in Figure 46, service cycle codes of 1⊘1 or 11⊘ are detected, the logic proceeds to Figure 5⊘. The code 1⊘1 is the input service cycle, and the code 11⊘ is the output service cycle. When either are detected, the first thing that is done is to check the WIP flip-flop 2⊘4. It should never be 1. If it is, then the MPU is in error and the logic goes to Figure 49 for the command-reject sequence previously described. If the WIP flip-flop 2⊘4 is ⊘, the next thing that is checked is the alignment code. In the case of an input service cycle, if the alignment code is ⊘, or in the case of an output service cycle, if the alignment code is 1, the logic goes to Figure 47 to perform a bus cycle. Assume, however, that the alignment code for the input service cycle is 1, representing the second microcycle. The buffer read sequence is then enabled and the load input to PCR'' 1⊘8'' is activated. At this point the logic goes to Figure 48 to end the command sequence. Assuming on the other hand that the alignment code for an output service cycle is ⊘ (denoting the first microcycle), it is necessary to introduce a 2T delay to give the MPU time to deliver valid data. If the delay ring 21⊘ is at ⊘ or 1, the load input to PCR'' 1⊘8'' is activated, the increment input is activated to prepare for incrementing the delay ring, and the J-input of the CMUX flip-flop 2⊘3 is activated to select PCR' 1⊘8'. At this point, the process returns to Figure 43 to repeat the command sequence. When the delay ring 21⊘ is found to be at 2, the buffer trite sequence is enabled and preparation is made to seek the next command. The logic then returns to Figure 43 to repeat the command sequence. The preparation for the next command involves activating (1) proceed status to the MPU, (2) the K-input of the CMUX select flip-flop 2⊘3 in preparation to reset that flip-flop for selection of PCR 1⊘8, and (3) the reset input to the delay ring 21⊘. Execution sequence, Bus Cycle Control Returning to Figure 28, the five "execution" sequences of the control logic of the ICU are seen to be Reset-Out, Buffer Read, Buffer Write, Bus Cycle, and Halt-Out. The bus cycle sequence is chosen as exemplary of this set and is described in detail in what follows. It is also the sequence of most action. Detailed descriptions of the other execution sequences are not included herein since these sequences can be inferred by those skilled in the art. Flowchart descriptions of the operation of the buffer read and buffer write sequences are nevertheless included in Figures 52 and 53, respectively. Bus cycle control is itself subdivided into four, functionally distinct sequences: initiation, detection, synchronization, and termination. These subsequences are described in this order. The Initiation Sequence 24⊘ "initiates" every bus cycle. Its primary function is to enable the buses and activate the output controls at the appropriate moment peculiar to each signal and to the type of bus cycle in progress. In addition, the initiation sequence enables the next sequence of the bus cycle, namely the detection sequence, and means are provided for early enablement in the event that a "fast read" is requested. The initiation sequence also implements the detection of a sequence exception (Interface Check/B). If a sequence exception is detected, a bus cycle is not initiated. Instead, SR9 is activated, the BGE flip-flop is set, and, after a 2T delay, the command sequence is re-entered. The specification timing diagram for the initiation sequence is given in Figure 54, and one form of implementation is illustrated in Figure 55. With reference to Figure 55, the initiation sequence comprises four states (⊘, 1, 2, and 3) and four phases (ϑ⊘, ϑ1, ϑ2, and ϑ3). It is executed only once for each bus cycle, and its duration is always 2T, as specified in Figure 54. The output of the INS ENABLE flip-flop 241 is designated ϑ. The functions performed depend on the state of the sequencing exception (SEQX) flip-flop 242 which is set at the same time the INS ENABLE flip-flop 241 is set, if an exception is detected. This is shown in more detail in the timing diagram of Figure 54. The enabling sequences which set the INS ENABLE flip-flop 241 are the Command Sequence (see CDS/E12 or E14), Post-Termination Sequence 3 (for rerun), and Post-Termination Sequence 4 (for double-cycle and word-write operation). The four phases are seen to be generated by a ring counter 243. These phase signals perform the gating functions shown in Figure 55, and, in addition, the following: ϑ1 functions enable the clock input at the Y' and WIP' flip-flops 2⊘1 and 2⊘5, respectively, and activate the J-input at the address (bus) enable flip-flop. ϑ2 functions enable the K-input at the R/W flip-flop and the J-input at the MIP flip-flop 212. ϑ3 functions activate the P-input of the address strobe activation flip-flop 144, and enable the J-input of the data bus enable flip-flop 125. The operation of the initiation sequence logic is summarized in the flowchart of Figure 56 which may be read in the same manner as demonstrated previously with respect to Figure 43 to 51 of the Command Sequence logic. The second sequence of the bus cycle control sequences is the detection sequence 25⊘. This critical sequence detects the device response to the output controls and then decides what next steps are to be performed. The specification timing diagram for the detection sequence is given in Figure 57, and the logical implementation of a preferred embodiment is detailed in Figure 58 and in the related truth tables of Figures 61 to 74. Figure 58 shows the relationship of the detection sequence logic to the first-stage external signal synchronization flip-flops, discussed earlier, and to the status reporting elements of the ICU, which carry the termination response for the command just completed. The detection sequence can generate any one of fourteen of the possible eighteen termination status responses to the MPU The other four status responses (SR9, SR1⊘, SR11, and SR13) are generated by other control sequences, as noted in Figure 58. The significance of each status signal is summarized in the table shown in Figure 59. The detection sequence is enabled solely by the initiation sequence 24⊘ and comprises states 4 and 5 of the bus cycle and a single phase, denoted by δ. Once enabled, the sequence runs continuously, i.e., states 4/5 repeat indefinitely, as shown in Figure 57, until a valid response is detected. It then generates its output signals and passes control on to the next sequence of its choice, namely, either the synchronization sequence 26⊘ or the termination sequence 27⊘ and, if the latter, also the command sequence or a post-termination sequence. Exceptional responses, containing the BERR signal and any of the three DTEXN signals, are handled by a two-edge detection scheme that permits completely asynchronous presentation of multiple signals without a tight skew tolerance. First detection of any of these exception signals causes the detection sequence to perform at least one additional cycle (denoted 4'/5') during which it can again sample the device response. Any exceptional or normal response detected during states 4'/5' is then unconditionally recognized. If, however, during states 4'/5' no response is detected (the prior signal was either spurious or removed), the detection sequence returns to state 4 and continues its normal operation. Specifically, the detection sequence performs the following functions: A) Detection of any one of 51⊘ device responses, plus a status cancellation signal ( HALT , occurring alone, is not detected); B) Logical resolution of the device response, in conjunction with the command from the MPU, and other controls; C) Generation and buffering of all status signals, other than SR⊘9, SR1⊘, SR11, and SR13 (see Figure 58); D) Depending upon the outcome of step B, either (a) repeating S4, (b) starting S4', or (c) terminating, upon enabling one sequence from Group 1 below and, conditionally, one sequence from Group 2: 1. (a) The Termination Sequence, comprising states S6/S7 of the bus cycle; or (b) the MC68⊘⊘⊘ Synchronization Sequence. 2. One of the following, in conjunction only with the Termination Sequence (1(a) above): (a) Command Sequence; or (b) a Post-Termination Sequence (one of six), to deal with unusual conditions. E) If D2(a) is an outcome, the detection sequence also gates status to the MPU (see G⊘ in Figure 58), so that valid status will be available to the MPU at the positive edge of S6. F) Enables loading of the IOR 123 at 6 7 of input bus cycles, whenever buffering is required. G) Conditional enablement of Bus Grant ( BG ). H) Detection of the fetch of an aligned full word in response to a corresponding command from the MPU, in which case the WIP flip-flop 2⊘4 iset to flag the subsequent command sequence. I) Reset of the WIP flip-flop 2⊘4, upon detecting a bus cycle that stores a full word (WIP'=1). J) Detection of the need for a double cycle, in which case the Y flip-flop 2⊘6 is set and PTS4 is enabled; and, similarly, control fo the word-write operation, through set and reset action on the Y and Z flip-flops 2⊘6 and 2⊘8, respectively. K) Permits the use of multiple, asynchronous, exceptional device-response signals, without the need for tight tolerances to skew, using a two-edge detection scheme implemented with the XRV flip-flop 215. L) "Proceed" cancellation, through means for detecting the synchronous presentation of a CANCEL signal. Proceed cancellation allows the device to anticipate (and signal) a normal response T/2 before it determines the validity of that response. If not valid, timely activation of the CANCEL signal allows the bus cycle to continue without interruption. All of these functions A through L may be seen to be carried out by the preferred embodiment described in Figure 6⊘ and its associated truth tables in Figures 61 to 74. The synchronization sequence 26⊘ in Figure 28 is provided to handle Motorola MC68⊘⊘ peripheral devices. These devices respond to the bus cycle with a VPA signal, and upon detection of this signal, the detection sequence enables the synchronization sequence (symbolic path 3 in Figure 28) rather than proceeding directly to the termination sequence 27⊘ (paths 1 or 2). It is during the synchronization sequence that the ICU responds to VPA with VMA ; VMA is the effective "proceed" signal to the MC68⊘⊘ device. The device then performs its input/output (I/O) function, and the ICU completes its synchronization sequence in concurrence with a common synchronization signal (ENABLE). The duration of the synchronization sequence is dependent entirely upon when the VPA is asserted relative to the ENABLE signal. The ENABLE signal (E) is a free-running, "slow clock" derived exclusively from the input clock (CLK) It is completely independent of the bus cycle and of any action on the part of MC68⊘⊘ devices. The purpose of the E signal is to provide synchronization for MC68⊘⊘ devices. Again, in the interest of brevity, a description of the preferred embodiment of the synchronization sequence is not provided herein. This sequence must satisfy the known Motorola specification for MC68⊘⊘ devices and can, therefore, be implemented by those skilled in the art without further description. Moreover, the sequence is not essential to the invention. The termination sequence 27⊘ (see Figure 28) is the final sequence of each bus cycle. It performs the following termination functions: 1) Negation of all strobes at the 6 7 transition of the clock, specifically, negation of AS, WS , UDS , and LDS . 2) Activation or deactivation of other controls at the end of S7 of the bus cycle, as follows: R/ W (activated), MIP (activation, conditionally), and VMA (deactivated). 3) Tri-stating of the address bus and data bus, at the end of S7. The termination sequence comprises two states (6 and 7) and one phase, denoted by τ. It is executed only once for each bus cycle, and its duration, therefore, is one clock period. It does not itself enable any subsequent sequence. The sequences that enable the termination sequence are the detection sequence 25⊘, indirectly, each time the command sequence 2⊘⊘ or any post-termination sequence (PTS) is enabled, or by the MC68⊘⊘ synchronization sequence 26⊘, as shown in Figure 28. Implementation of this sequence is straight-forward and is not, therefore, further described. Post Termination Sequences At the end of the detection sequence 25⊘ or at the end of the synchronization sequence 26⊘, i.e., at the end of S5 or S5', one of the following two sequences (in addition to the termination sequence) may be enabled: (1) the command sequence, or (2) a post-termination sequence. A post-termination sequence is always enabled under any of the following circumstances: 1) All exceptional terminations; 2) Normal termination with HALT, for non-macrocycles; and 3) Normal termination of a detected inner cycle of a double-cycle or a word-write operation. Post-termination sequences perform the following functions, in the order given: 1) Release detection, where the term "release" refers to removal of the associated device response signal(s); 2) Status generation, if the order of release of multiple signals is significant; 3) Presentation of status to the MPU, via the status buffer register or directly; 4) Generation of post-status delay (2T), if required; and 5) Enablement of either (1) the command sequence, (2) the second bus cycle of a double-cycle or the ⊘1, 1⊘, and 11 cycles of the word-write operation, or (3) a rerun of the same cycle. The typical features of a post-termination sequence are illustrated by the timing diagram shown in Figure 75. Six separate post-termination sequences are specified. They are the following wherein names denote the device response to the associated bus cycle: PTS1-Interface Check: This sequence merely supplies post-status delay before enabling the command sequence. There is no release constraint on the ACK signals, and status, formed earlier, is gated from the status buffer. The HALT signal is checked by this sequence. The presence of HALT will delay presentation of status and enablement of the command sequence. Neither BERR nor any of the DTEXN signals is significant to this sequence. PTS2-DTEXN (Data Transfer Exception) : This sequence presents exceptional status to the MPU upon negation of the exception signal(s), then inserts the post-status delay before enabling the command sequence. Status is gated from the status buffer register (SBR). For simplicity, all DTEXN must be inactive to satisfy the release condition, independent of the particular DTEXN previously latched in the SBR. HALT release is performed. BERR is not checked. PTS3-BERR with HALT (Rerun Request) : This sequence provides preferred/reverse release detection, and, upon: a) Preferred release, the bus cycle is rerun, provided that the rerun request is unconditional, or that the cycle is not part of a macrocycle; for cycles of a macrocycle in the absence of an unconditional rerun request, rerun reject status is generated followed by post-status delay and enablement of the command sequence; and b) Reverse release, an exceptional status (rerun cancelled) is presented to the MPU, followed by post-status delay, and enabling of the command sequence. Independent of the type of release, the DTEXN signals are not checked. Moreover, the interface is checked for availability before enabling INS (rerun) and resetting the BGE flip-flop 222. Whenever rerun rejected or rerun cancelled status is set, the WIP flip-flop 2⊘4 is also reset. PTS4-Normal Termination/Inner Cycles : This sequence enables the second bus cycle of a double-cycle and the ⊘1, 1⊘, and 11 cycles of the word-write operation, following normal termination of the prior cycle. That prior cycle determines the state of the BGE flip-flop 222. The initiation sequence, not the command sequence, is the successor sequence to PTS4. Status is not gated to the MPU. If BGE=1, the interface is checked for HALT release and availability before enabling INS and resetting the BGE flip-flop 222. PTS5-BERR and Not HALT (Time-Out) : This sequence presents buffered time-out status to the MPU upon negation of BERR, then inserts the post-status delay (2T) before enabling the command sequence. Note that, as applied here, Time-Out implies the absence of any other initial response from the device, i.e., the response which terminates the detection sequence. The HALT signal is ignored by PTS5. PTS6-General Release : This sequence is used when exceptional status is generated independently of the device response, e.g., odd-instruction address. PTS6 does the following: (1) waits for release of HALT, BERR, and all DTEXN signals, if any of these is present, and (2) upon total release, gates status from the status buffer and, after a 2T delay, enables the command sequence. The post-termination architecture of this invention is believed to be unusual in the design of microprocessors as it precisely specifies the action that will be taken for all possible responses to a bus cycle. No longer is the resolution of unusual responses left to whatever eventually falls out of the circuit design process. The term "post-termination" may be seen to refer to the fact that these sequences always operate beyond the moment that the associated bus cycle actually ends, namely S7. But, in all instances, the termination sequence is enabled concurrently with any post-termination sequence. In retrospect, therefore, these sequences could be accurately called "co-termination" sequences. The actual implementation of the post-termination sequences is straightforward and is illustrated by the specification and logical implementation of PTS3 illustrated in Figures 76, 77 and 78.

Claims: Single chip microprocessor comprising: a microprocessing unit; external signal pins to which off-chip devices are connectable; and an interface control unit connected to the microprocessing unit and to the external signal pins to permit the microprocessing unit and any off-chip devices connected to the pins to communicate with one another in response to commands from the microprocessing unit and control signals from off chip devices; characterised in that the interface control unit comprises: execution logic (Figs 21, 23, and 26) connected to the microprocessing unit and the external signal pins; and control logic (Fig 28) connected to the execution logic and responsive to both commands from the microprocessing unit and control signals from off chip devices for configuring the execution logic to perform a particular data transfer between the microprocessing unit and off chip devices that are designed for use with other and different microprocessing units in response to a command from the microprocessing unit requesting said data transfer, and for sending a status output to the microprocessing unit, the status output describing the outcome of the command. Microprocessor according to claim 1, wherein the control logic (Fig 28) is adapted to perform signal sequencing at said external signal pins, said signal sequencing being called a bus cycle; said control logic including command logic responsive to said commands from said co-resident microprocessing unit and status logic for presenting status to said co-resident microprocessing unit describing the outcome of each command, wherein said command logic causes said execution logic to perform a particular data transfer in response to a particular command from said co-resident microprocessing unit, and said status logic presents the outcome status of said particular command upon logical completion of said data transfer. Microprocessor according to claims 1 and 2 wherein said execution logic and said control logic are designed to provide an external appearance at said external signal pins that is compatible with the peripheral devices of a specific micro-processor called the compatible microprocessor, thereby enabling at least one compatible microprocessor, thereby enabling at least one peripheral device of said compatible microprocessor to be attached to said single-chip microprocessor and to communicate with said co-resident microprocessing unit. Microprocessor according to claim 3 wherein said execution logic and control logic further include extended logic for attaching and operating other peripheral devices not associated with said compatible microprocessor, said extended logic aid its operation being transparent to any co-attached device of said compatible microprocessor. Microprocessor according to claim 4 wherein said extended logic comprises: logic for generating non-compatible signals not physically detectable by any attached device of said compatible microprocessor; and logic for generating compatible signals physically detectable by, but logically transparent to, all attached devices of said compatible microprocessor, said compatible signals conforming to the natural communication protocol of the devices of said compatible microprocessor, said other peripheral devices not associated with said compatible microprocessor using said non-compatible signals and said compatible signals in the process of communicating with said interface control unit. Microprocessor according to claim 4 wherein said control logic includes mixed-width device handling logic for dynamically controlling said execution logic in response to an addressed device's width-identifying response to a bus cycle. Microprocessor according to claim 4 wherein said control logic includes accessing logic for accessing operands from said off-chip devices at any byte location including those locations which do not correspond to an integral boundary of the accessed operand. Microprocessor according to claim 4 wherein said control logic is responsive to an external control signal for varying the minimum duration of a bus cycle. Microprocessor according to claim 2 wherein said control logic further comprises: initiation sequence logic for generating initiation signals identifying said bus cycle to at least one off-chip device; detection sequence logic for seeking and responding to the external signals produced by said off-chip device in response to said initiation signals; and termination sequence logic for concluding said bus cycle. Microprocessor according to claim 2 wherein said control logic further comprises: bus arbitration logic for granting temporary and exclusive use to an off-chip master device of any bus derived from said external pins of said single-chip microprocessor, said arbitration logic being logically independent of said control logic responsive to said commands from said co-resident microprocessing unit, and said granting being subject to enablement by said control logic as a condition for being exhibited to said off-chip master device; and bus grant enable logic for enabling and disabling said granting of said bus.

Inventors: GAVRIL BRUCE DAVID, GAVRIL, BRUCE DAVID
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: G06F13
IPC codes: G06F13, G06F15