Preference circuit for a computer system

Abstract: A logic controlled gate is inserted in the arbitration logic of a computer system that supports multiple masters on a data bus. In such a system with arbitration for data bus ownership, the gate is so controlled that competitors for the data bus cannot force the main processor (CPU) from the data bus until certain system conditions are met. In particular, a pattern of CPU "hits" to memory cache is recognized as an opportunity for the CPU to relinquish the data bus.

Description: The present invention relates to a preference circuit for computer systems which include a data bus which may be controlled by plural masters including a main processor and which allocate bus time according to a predefined arbitration scheme. It is known in computer systems, such as those conforming to IBM Corporation's Micro Channel architecture, to have multiple masters which each can manage data transfers over the system's main data bus. The use of such masters serves to relieve the main processing unit (CPU) from detailed involvement in data transfers between input /output (I/O) devices and main memory and also from I/O device to I/O device. By offloading these duties the CPU is permitted to more fully concentrate its efforts on processing data and setting up transfers that other devices will execute. In such systems, priorities are established for the respective devices on the data bus to allocate bus time. The CPU is assigned the default or residual priority recognizing that the main work on the bus is desirably allocated to other devices. If a queue develops the devices including the CPU vie for the bus each arbitration cycle and the highest in priority wins. As burst devices may continually assert their high priority, a "fairness" limitation is often used with the burst devices to force them to wait until the existing queue disappears before they may reenter the queue. This process works well to allocate efficiently bus time unless the duty on the data bus becomes high. In that case, the CPU gets its chance on the bus infrequently, as the priority scheme permits, and then gets "bumped off" after one bus cycle if another device preempts. This situation can "lock out" the CPU to an extent that it can't perform the preparatory setup needed for data transfers or other activity such as memory access that it requires to perform tasks. When this condition is reached the overall system performance deteriorates and, in an extreme case, there may be a system crash. Overcoming the problem of lock out becomes more complicated in systems with memory cache because the main processor may enter a string of cache hits that avoid the need to capture the data bus to accomplish the needed transfer of information. Patent Abstracts of Japan Vol 7, No 48 (p-178) [1193] of 24/02/83 discloses a shared storage device having a priority deciding circuit. A counter is employed to time the period during which access to the device is inhibited in order to avoid the inhibition of access by other CPUs for a long period of time when a CPU responsible for the inhibition of access to the device has a breakdown. In accordance with a first aspect of the invention there is provided a preference circuit for a computer system comprising a data bus and a plurality of masters, one of which is a main processor, and wherein the masters contend for said data bus using bus request signals transmitted over a signal path to arbitration control logic which identifies by a bus signal a current bus owner, said computer system further comprising a memory cache system connected to cooperate with said main processor to provide data from a cache buffer and which includes means to provide a first logic signal for each instance when such data is provided, the preference circuit comprising: means for receiving said bus signal to detect intervals in which the main processor is identified as owner of the data bus and for generating a second logic signal during each such interval; means for receiving and counting said first logic signals and producing a third logic signal when a predefined count is reached; logic connected to receive said second and third logic signals and said bus request signals, which logic blocks said bus request signals when said second logic signal is active and said third logic signal is not active. The invention enables the CPU of such a computer system to be assured a preselected amount of time on for data transfer even during high duty cycle periods for data transfer over the main data bus. Such time is provided without restructuring the arbitration process of the system. Cache transfers of the main processor are therefore, taken into account when allocating bus time to the main processor. The present invention prevents the CPU from being forced off the data bus by creating a controlled preference when the CPU owns the data bus. Such preference is preferably introduced by logic which modifies the bus request signals arriving at the central arbitration controller. By so controlling the bus request line, when the CPU owns the data bus, that request signals may pass to the central arbitration control only after a pattern of successful accesses to memory cache has occurred, the CPU is allowed an opportunity to perform necessary data bus transfers while recognizing the action of the memory cache as an alternative to accessing main memory over the data bus. In a preferred form of the invention the logic detects a bus request signal occurring while said second logic signal is active and cancels the blocking of said bus request signals when a predetermined time interval has elapsed from the detection. By additionally timing out this cancellation of bus requests within a preselected period after the first such request arrives the preferred implementation of the invention is able to assure that the refresh requirements of dynamic main memory may be met. Preferably the predefined count is 2 and if the computer system comprises main memory which requires periodic refresh the predetermined time interval is selected to be shorter than the refresh period of the main memory. The invention further provides a computer system comprising such a preference circuit. The computer system can be in the form of a personal computer which may comprise other components such as a display device, keyboard, communications links and the like. According to another aspect of the invention there is provided a method for controlling bus request signals in a computer system comprising a data bus, a plurality of masters, one of which is a main processor, arbitration control logic for receiving the bus request signals and for identifying by a bus signal a current bus owner, a memory cache connected to cooperate with the main processor to provide data from a cache buffer and to provide a first logic signal for each instance when such data is provided, and wherein the plurality of masters contend for the data bus using the bus request signals, the method comprising: detecting intervals in which the main processor is identified as owner of the data bus and generating a second logic signal during such intervals; receiving and counting the first logic signals and producing a third logic signal when a predefined count is reached; blocking the bus request signals when the second logic signal is active and the third logic signal is not active. Preferably the method comprises detecting a bus request signal occurring while the second logic signal is active; and cancelling the blocking of the bus request signal when a predetermined time interval has elapsed from the detection of the bus request signal. A full understanding of the present invention will be obtained from the detailed description of the embodiment presented below, and the accompanying drawings, wherein: FIG. 1 is a diagrammatic representation, in block form of a system for implementing the invention; FIG. 2 is a diagrammatic representation, in block form, illustrating the control signals applied to the central arbitration control and the DMA controller according to the prior art; FIG. 3 is a diagrammatic representation, in block form, of logic for producing a modified bus request signal according to the invention; Fig. 4 is a diagrammatic representation, in block form, illustrating the application of the modified bus request signal to the central arbitration control and the DMA controller according to the embodiment of the invention; Fig. 5 is a timing diagram illustrating an arbitration sequence according to the embodiment of the invention. Referring to Fig. 1 a preferred system for implementing the invention includes a CPU 100, such as an Intel Corporation 80386 microprocessor, a cache 105, and a cache controller 110 which are connected to other system elements over a set 140 of CPU signal buses including a control bus 125, a data bus 130 and an address bus 135. The CPU signal buses 140 are isolated from a corresponding set of system buses 140′ by a set of buffers 170. The buffers 170 allow the CPU to process instructions and data over the buses 140 independently of the system bus set 140′ when the instructions and data reside in the cache 105. Connected to the bus set 140′ are a main memory 145, which is directly addressable read/write dynamic storage and a memory controller 150 along with an arbitration controller and a DMA controller which serves to control direct memory transfers over the bus set 140′. For systems according to the invention there is preferably provided a connection point 165 permitting bus interface devices 170 such as bus master devices which can control a data bus to be connected to the system bus set 140′. These connections are usually made by inserting circuit boards into slots fitted with card edge connectors (indicated in edge view only in FIG. 1) as is well known in the art. The interface devices 170 may connect to a variety of other devices such as input/output (I/O) devices 180. Such devices 180 may include, for example disc drives or tape units (not shown). Such a system arrangement including interface devices 170, which may be bus masters, and control the data bus 115 is specified for IBM Corporation's Micro Channel architecture described in detail in the "IBM Personal System/2 Hardware Interface Technical Manual". With such a system, a central arbitration controller 155 establishes the priority of devices taking control of the data bus 130′ and a DMA controller 160 coordinates the actual transfers over such bus as is well known. A typical set of arbitration priority level assignments is illustrated in Table 1. ARBITRATION LEVEL PRIMARY ASSIGNMENT -2 MEMORY REFRESH -1 ERROR RECOVERY 0 DMA Port 0 1 DMA Port 1 2 DMA Port 2 3 DMA Port 3 4 DMA Port 4 5 DMA Port 5 6 DMA Port 6 7 DMA Port 7 8 SPARE 9 SPARE A SPARE B SPARE C SPARE D SPARE E SPARE F SYSTEM BOARD PROCESSOR (CPU) Now, referring to Fig. 2, the prior art includes a group of lines 210 forming a part of the control bus 125′(see Fig. 1) which carry signals controlling the ownership of the data bus 130′. A channel 220 carries a +ARB/-GRANT signal that serves to separate periods when arbitration to establish bus ownership is occurring (ARB) and periods when ownership is fixed (GRANT) and the various masters 170 can function respective of that ownership assignment. A channel 240 serves to carry a signal -PREEMPT which indicates when one or more master devices,such as device 170 and including the CPU 100 are waiting to access the data bus 130′. A channel 250 carries the -BURST signal which is asserted by a device 170 when it owns the data bus 130′ to indicate whether it is a device capable of multiple or burst transfers. Also connected to the central arbitration control 155 is an arbitration bus 260 consisting of preferably four lines (0-3) on which arbitration levels are asserted to achieve ownership of the data bus 130′ and then, during the time GRANT is asserted on channel 220, to identify the owner. Now referring to Fig. 3, an AND gate 310 creates a signal +CPUPRE based on the -PREEMPT, +HOLD, the +ARB/GRANT and the ARB BUS signals (1111 = F corresponds to the CPU 100) to indicate the CPU owns the data bus 130′. This signal +CPUPRE triggers a delay timer 315 set to create a signal for a predefined time selected to be within the refresh requirements of the main memory 145 so as not to violate system rules and cause a loss of data. A counter 320 counts cache hits indicated by cache controller 110 and after a predetermined count, preferably two, sets a signal +2HIT. Two hits is selected because it is sufficient to indicate a string of hits is likely and the CPU can then give up the bus 130′. The signal +DELAY and the inverse of +2HIT are ANDed at gate 330 to produce a signal BLK the inverse of which is applied at an AND gate 340 along with the inverse of -PREEMPT to produce a signal G. The signal G is applied at an AND gate 350 along with the inverse of the +CACHEHIT/-MISS signal to produce the signal -P′. The signal -P′ is a modification of the -PREEMPT signal to block requests that occur when the CPU 100 owns the bus 130′ until either two consecutive cache hits occur or the delay period expires after the first request arriving during such ownership. Referring now to Fig. 4, the configuration of Fig. 2 has been modified according to the preferred implementation so that the line 240 carries the signal -P′ which is the version of the request signal -PREEMPT modified as described above. The effect of this modification of the request signal -PREEMPT is seen in the two timing diagrams of Fig. 5, the first illustrating a two cache hit situation and the second illustrate a delay timeout situation. There has been described, in a computer system having a data bus and plural masters, including a main processor, which contend for said data bus using bus request signals transmitted over a signal path to arbitration control logic which identifies by a bus signal a current bus owner, said system having a memory cache system connected to cooperate with said main processor to provide data from a cache buffer and to provide a first logic signal for each instance when such data is provided, a preference circuit for the main processor comprising: means for receiving said bus signal to detect intervals in which the main processor is identified as owner of the data bus and for generating a second logic signal during each such interval; means for receiving and counting said first logic signals and producing a third logic signal when a predefined count is reached; a gate connected to receive said second and third logic signals and said bus request signals, which gate blocks said bus request signals when said second logic signal is active and said third logic signal is not active. Preferably a logic device detects a bus request signal occurring while said second logic signal is active and triggers a fourth logic signal after a predefined delay, which fourth logic is applied to said gate to cancel the blocking of said bus request signals.

Claims: A preference circuit for a computer system comprising a data bus (140) and a plurality of masters, one of which is a main processor (100), and wherein the masters contend for said data bus (130, 130') using bus request signals transmitted over a signal path to arbitration control logic (155) which identifies by a bus signal a current bus owner, said computer system further comprising a memory cache system (105, 110) connected to cooperate with said main processor to provide data from a cache buffer (105) and which includes means to provide a first logic signal for each instance when such data is provided, the preference circuit comprising: means (310) for receiving said bus signal to detect intervals in which the main processor (100) is identified as owner of the data bus (130, 130') and for generating a second logic signal during each such interval; means (320) for receiving and counting said first logic signal and producing a third logic signal when a predefined count is reached; logic (315, 330, 340, 350) connected to receive said second and third logic signals and said bus request signals, which logic (315, 330, 340, 350) blocks said bus request signals when said second logic signal is active and said third logic signal is not active. A preference circuit as claimed in claim 1 wherein the logic (315, 330, 340, 350) detects a bus request signal occurring while said second logic signal is active and cancels the blocking of said bus request signals when a predetermined time interval has elapsed from the detection. A preference circuit as claimed in Claim 1 or 2 wherein said predefined count is two. A computer system comprising a preference circuit as claimed in any preceding claim comprising a data bus (140) and a plurality of masters, one of which is a main processor (100), and wherein the masters contend for said data bus (130, 130') using bus request signals transmitted over a signal path to arbitration control logic (155) which identifies by a bus signal a current bus owner, said computer system further comprising a memory cache system (105, 110) connected to cooperate with said main processor to provide data from a cache buffer (105) and which includes means to provide a first logic signal for each instance when such data is provided. A computer system as claimed in Claim 4 comprising main memory (145) which requires periodic refresh and the predetermined time interval is selected to be shorter than the refresh period of said main memory (145). A method for controlling bus request signals in a computer system comprising a data bus, a plurality of masters, one of which masters being a main processor, arbitration control logic for receiving the bus request signals and for identifying by a bus signal a current bus owner, a memory cache connected to cooperate with the main processor to provide data from a cache buffer and to provide a first logic signal for each instance when such data is provided, and wherein the plurality of masters contend for the data bus using the bus request signals, the method comprising: detecting intervals in which the main processor is identified as owner of the data bus and generating a second logic signal during such intervals; receiving and counting the first logic signals and producing a third logic signal when a predefined count is reached; blocking the two request signals when the second logic signal is active and the third logic signal is not active. A method as claimed in claim 6 comprising: detecting a bus request signal occurring while the second logic signal is active; and cancelling the blocking of the bus request signal when a predetermined time interval has elapsed from the detection of the bus request signal. A method as claimed in claim 6 or claim 7 wherein the predetermined count is 2. A method as claimed in any of claims 6 to 8 wherein the computer system comprises main memory which requires periodic refresh and the predetermined time interval is shorter than the refresh period of the main memory.

Inventors: SMITH BRUCE ALAN, TRAN LOC TIEN, SMITH, BRUCE ALAN, TRAN, LOC TIEN
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G06F13
IPC codes: G06F13, G06F12