Abstract:
A method and system for a coherence protocol for buffer memories which covers interventions in multiprocessor data processing units, wherein a bus coupler maps the coherence protocol of a system bus onto a local bus and, in the event that an intervention relates to a buffer line which is in the write register, transfer this buffer line from the write register via the local bus to the intervention register.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The method and system of the present invention relate to the organization of multicomputer systems having a system bus which is common to a number of modules as well as additional bus systems which are local to the modules. 
     2. Description of the Prior Art 
     To improve system performance, multicomputer systems have buffer memories linked to each processor, usually called caches. To simplify programming, various measures, called cache coherence for example as described in the book “Computer Architecture—A Quantitative Approach” by J. L. Hennessy and D. A. Patterson, San Francisco 1995, are taken to ensure that, despite the data copies in the buffer memories, the entire memory of the multicomputer system is accessed uniformly and consistently at any time. 
     In such multicomputer systems, which a hierarchy of bus systems, it is expedient to have additional intermediate stores (registers) for individual cache lines. These registers may conflict with the cache contents, however. In particular, the case in which data in a write register has to be transferred to the system bus, like data from an intervention register, demands appropriate circuitry for both registers. 
     Accordingly, an object of the present invention is to reduce the outlay for dealing with such conflicts. 
     SUMMARY OF THE INVENTION 
     This object is achieved in a method and system wherein the data from the write register is transferred not directly, but indirectly, to the system bus via the local bus and the intervention register. Consequently, the outlay is reduced in that the write register supplies the data via the local bus to the intervention register in the same way as the processors so that the intervention register cannot discern any difference. 
     Accordingly, in an embodiment of the present invention, a system is provided for operating a coherence protocol for buffer memories which covers interventions in multiprocessor data processing units, wherein the system includes: a plurality of processors, each processor having a buffer memory containing a buffer line; at least one local bus connected to the plurality of processors; a system bus; a bus coupler connecting the system bus to the at least one local bus wherein the coherence protocol is mapped from the system bus onto the at least one local bus, the bus coupler including at least one intervention register and at least one write register which are both connected to the at least one local bus for read access and which are both connected to the system bus for write access, the at least one intervention register further including a device for subblock interchange; and wherein the at least one write register is particularly connected to the at least one local bus such that, where an intervention relates to content of the at least one write register, the content can be transferred via the at least one local bus to the at least one intervention register. 
     In another embodiment of the present invention, a method is provided for operating a coherence protocol for buffer memories which covers interventions in multiprocessor data processing units, wherein the method includes the steps of: mapping the coherence protocol from a system bus onto a local bus via a bus coupler, wherein the local bus is connected to processors which respectively have buffer memories containing buffer lines; transferring the buffer lines from the respective buffer memories to an intervention register in the bus coupler via the local bus at the instigation of the bus coupler; transferring the buffer lines from the intervention register to the system bus; writing a first buffer line to a write register in the bus coupler via a buffer memory at the instigation of the respective processor wherein it is specified that the first buffer line is to be transferred via the system bus to a main memory; providing the intervention register with a device for subblock interchange; and transferring the first buffer line from the write register to the intervention register via the local bus in the event that a request is made via the system bus for a buffer line which is in the write register. 
     Additional features and advantages of the present invention are described in, and will be apparent from, the Detailed Description of the Preferred Embodiments and the Drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic representation of a schematic representative of a modular multiprocessor computer system having two bus couplers; 
     FIG. 2 shows a schematic representation of the intervention, write and read registers from one of the bus couplers from FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a modular multicomputer system having a system bus  10 , a number of processor modules  13   a ,  13   c , one of a number of possible memory modules  11  and other modules linked to the system bus  10 , for example an I/O module  12 . 
     Each processor module  13   a ,  13   c  contains a bus system  14   a ,  14   c  which is local to the respective module and is connected to one or more processors  16   a . . . d  via buffer memories  17   a . . . d , respectively. This connection is functionally shown as a series connection of buffer memory  17   a . . . d  and processor  16   a . . . d , respectively, any intention to place a restriction on the actual implementation. Between the local bus  14   a ,  14   c  and the system bus  10 , each module holds a bus coupler  15   a ,  15   c  which converts the signal protocols and, in particular, implements the protocols for maintaining cache coherence. 
     To this end, as indicated in FIG. 2, it is typical for three intermediate registers, namely the intervention register  21 , the write register  22  and the read register  23 , to be used. The read register  23  is used when, for example, the processor  16   a  requests a data word which is not located in its cache  17   a  (nor in the adjacent cache  17   b  on the same local bus  14   a ). The appropriate cache line containing the data word is then requested by the coupler  15   a  on the system bus  10  and, when it is received, is initially temporarily stored in the read register  23  before being transferred via the internal bus  14   a  to the cache  17   a  which made the request. 
     If a cache line in the cache  17   a  is displaced and has to be written back via the system bus  10  to the main memory  11 , it is initially temporarily stored in the write register  22  and then transferred via the system bus  10 . 
     Apart from very simple coherence protocols, the present invention provides the possibility of the only valid copy of a cache line being located in one of the cache memories, for example  17   a , and not in the main memory  11 . If a processor, for example  16   c  in the module  13   c , requests a data item from a cache line in the module  13   a , then the module  13   a  undertakes the data transfer on the system bus  10  instead of the main memory  11 . This is called “intervention”. In this instance, protocols on the system bus  10  are preferably used in which the address phase and the associated data phase may have other address or data phases inserted between them. In response to any necessary intervention, the cache line is therefore initially transferred to the intervention register  21  via the internal bus  14   a  and then sent to the receiver using the data phase associated with the initiating address phase on the system bus  10 . 
     The registers  21 ,  22  and  23  are usually necessary simply because the internal bus  14   a  and the system bus  10  are of different design. In particular, the internal bus  14   a  is easily capable of simultaneously transferring a cache line of 64 bytes because, from a technical point of view, it is possible to produce an internal bus having 512 lines on a printed circuit board designed using multilayer technology. For a system bus with plug-in modules, smaller data widths, for example 64 bits, are customary. In particular, the system bus is usually only capable of a lower speed because the speed is restricted not only by the poor electrical properties of the connectors but also by the relatively long lines which are determined by the size of the backplane. Therefore, in order to be able to freely select the frequency on the internal bus for a specified frequency on the backplane, it is expedient to decouple the bus systems via registers. When the data is transferred, for example from the registers to the system bus, or vice versa, space-division multiplexing is converted to time-division multiplexing, or vice versa a. In this case, a cache line is called a block and, in particular, the quantity of data which can be transferred on the system bus in parallel is called a subblock. 
     In order to reduce the waiting time, when a data request is made by a cache, until the data word which is needed first from a cache line is received, subblock interchange is used. For this, the subblock is not transferred in the order of increasing addresses of the data words but, rather, it starts with the data word requested in each case, which even may be in the middle of a block. Subblocks whose addresses have been interchanged in this manner may appear in the intervention register  21  and in the read register  23 , but not in the write register  22 , whose transfer to the main memory is, after all, a filing operation which may be done in any, order. 
     The introduction of a local bus  14   a ,  14   c  creates additional conflicts for cache coherence. In particular, the situation may arise in which a displacement in the cache  17   a  means that the cache  17   a , or its processor  16   a , has output a cache line via the internal bus  14   a ,  14   c  with the aim of writing this cache line back to the main memory  11 . Hence, when transfer on the internal bus  14   a  has finished, the copy in the write buffer register  22  is the sole valid one in the system (otherwise, the coherence protocol would not have specified anything being written back to the main memory  11  at all). The coupler  15   a  thus tries to allocate an appropriate address cycle to the system bus in order to transfer the content of the write register  22  via the system bus  10  to the main memory  11 . Before this is accomplished, however, another processor may request data from this very cache line, which therefore has to come from the write register  22 . The coupler circuit is designed to satisfy such inquiries from the intervention register  21 , however. Disregarding the inefficient solution of terminating the inquiry in the hope that, by the time it is repeated, the write operation might have taken place, the data therefore has to be transferred from the write register  22  to the system bus  10 . At first glance, this appears to be possible without any difficulty, as the write register  22  has a data link  24  to the system bus anyway. 
     In practice, however, a problem arises inasmuch as the data in the write register  22  now has to be passed to the system bus with subblock interchange, even though this would not be necessary for writing. The write register would therefore have to be supplemented by a multiplexer which is able to effect the desired subblock interchange for transfer to the system bus. 
     The present invention makes use of the fact that the intervention register  21  already has this device and is usually unoccupied. An additional data path  25  from the write register  22  to the internal bus  14   a  is therefore introduced, and the content of the write register  27  is transferred via the internal bus to the intervention register  21  and, from there, to the system bus with the correct subblock interchange. 
     Although this solution appears unfavorable and complex at first glance, it has turned out to be efficient and very simple to implement. Any intervention in which the data is located in the write register  21  instead of in a cache  17   a . . . d  remains unchanged throughout control of the intervention register  21 . Thus, considerably reduces the complexity of control in comparison with previous solutions. It is merely necessary to activate the additional data link  25  instead of the relevant cache  17   a . . . d  in order to place the data onto the internal bus. In so doing, during an intervention, a cache  17   a . . . d  places the data onto the internal bus as an entire cache line anyway. This new data path  25  is less complex than a multiplexer, which places the data onto the system bus in a different order, and has the further benefit of the lower control complexity. As the lower bandwidth of the system bus generally means that it requires more data cycles than the internal bus to transfer an entire cache line, the roundabout way via the internal bus does not have much of a delaying effect. In addition, the write register  22  has become free again as a result of the transfer to the intervention register  21 . 
     In the description, the main memory  11  is depicted as a logic unit. It may be formed by one or more memory modules connected to the system bus. Alternatively, or in addition, the processor modules  13   a ,  13   c  may also hold a memory behaving as an independent memory  1 l from the viewpoint of each of the other modules. 
     The present invention also may be broadened to the extent that a number of intervention registers are provided and the resultant control allows additional parallel work. As such, a further intervention register is filled via the internal bus while the first intervention register continues to transfer its data via the system bus. 
     Although the present invention has been described with reference to specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the spirit and scope of the invention as set forth in the hereafter appended claims.