Abstract:
Semaphore operation manages exclusive access to a memory that is shared by a plurality of processing elements. Semaphore reservation status for exclusive access by a processing element is monitored by a memory controller. To clear an obsolete reservation status, a command signal is transmitted for a write operation to the memory while prohibiting update of the contents of a memory. The reservation status at the controller is changed from a reservation state to a non-reservation state in response to receipt of the command signal.

Description:
BACKGROUND  
       [0001]     1. Field  
         [0002]     The present disclosure relates to processing systems having shared memory, more particularly to method and apparatus for clearing semaphore reservations.  
         [0003]     2. Background  
         [0004]     Computers have revolutionized the electronics industry by enabling sophisticated processing tasks to be performed with just a few strokes of a keypad. These sophisticated tasks involve an incredibly high number of complex components that communicate with one another in a fast and efficient manner using a bus. A bus is a channel or path between components in a computer.  
         [0005]     Many buses resident in a computer have traditionally been implemented as shared buses. A shared bus provides a means for any number of components to communicate over a common path or channel. In recent years, shared bus technology has been replaced to a large extent by point-to-point switching connections. Point-to-point switching connections provide a direct connection between two components on the bus while they are communicating with each other. Multiple direct links may be used to allow several components to communicate at the same time. A bus arbiter may be used to manage communications over the bus.  
         [0006]     A computer implementing a bus architecture may include a number of processing components connected to memory. The memory may be divided into regions either by the system designer or dynamically during operation. Each processing component may have its own dedicated memory region. “Shared memory regions,” on the other hand, are memory regions that may be accessed by multiple processing. A semaphore may be used to manage access to the shared memory regions. A “semaphore” is a hardware or software flag, residing in a shared memory location, that indicates the accessibility of a shared resource. A processing component that needs access to the shared memory region may read the semaphore to determine the accessibility of the shared memory region. If the semaphore indicates that the shared memory region is available, then the processing component may set the semaphore to indicate that the shared memory region is locked, and proceed to access the memory.  
         [0007]     When the processing component reads the semaphore, the memory creates a “reservation” for the processing component to the shared memory region. The reservation is cleared once the processing component sets the semaphore to lock the shared memory region. However, if the processor is interrupted before it has a chance to set the semaphore, the reservation must be cleared. An efficient means for clearing the semaphore reservation is desirable.  
       SUMMARY  
       [0008]     A method of clearing a semaphore reservation for exclusive access to a shared memory is disclosed. The method includes transmitting a write request to a semaphore memory region associated with the shared memory, and prohibiting update of the contents of the semaphore memory region in response to the write request.  
         [0009]     A processing system is disclosed. The processing system includes a plurality of processing components, a memory comprising a shared memory region that can be shared by the processing components and a semaphore memory region that is indicative of exclusive write access availability of the shared memory portion, and a controller that maintains semaphore reservation status, wherein a reserved status of said controller is cleared in response to a write request to the semaphore memory region from one of the processing components without data being written to the semaphore memory region.  
         [0010]     Another processing system is disclosed. The processing system includes a plurality of processing components, a memory comprising a shared memory region that can be shared by said plurality of processing components and a semaphore memory region that is indicative of exclusive write access availability of the shared memory portion, and a controller that maintains semaphore reservation status, wherein each of the processing components is configured to transmit a write request to the semaphored memory region that clears a semaphore reserved status without writing to the semaphore memory region.  
         [0011]     This disclosed technique provides advantages in that the semaphore reservation can be cleared without occupying additional memory and avoids memory accessing steps. Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawing and in which like reference numerals refer to similar elements and in which:  
         [0013]      FIG. 1  is a functional block diagram illustrating an example of a processing system.  
         [0014]      FIG. 2  is a more detailed partial block diagram of elements of the processing system.  
         [0015]      FIG. 3  is a flow diagram illustrating the operation of a processing system. 
     
    
     DETAILED DESCRIPTION  
       [0016]      FIG. 1  is a functional block diagram illustrating an example of a processing system. The processing system may be a computer, or resident in a computer, and capable of processing, retrieving and storing information. The processing system may be a stand-alone system, or embedded in a device, such as a wired or wireless phone, personal digital assistant (PDA), laptop, desktop computer (PC), pager, modem, game console, digital camera, video conferencing equipment, broadcast equipment, or any other suitable device that requires data processing capability. Memory  100  is coupled to one or more processing components  102   a  and  102   b  through memory controller  104  and system bus interconnect and bus arbiter  106 . Each processing component may be any type of bus mastering component including, by way of example, a microprocessor, a digital signal processor (DSP), a bridge, programmable logic, discrete gate or transistor logic, or any other information processing component. Although only two processing components are shown for simplicity of illustration, a greater number may be employed.  
         [0017]     The memory may be divided into regions either by a system designer or dynamically during operation. A memory region may be dedicated to each processing component, while other regions of the memory may be shared for access by multiple processing components. The term “memory region” refers to any memory device, any portion of a memory device, a memory map region, or the like. A shared common memory resource may be any type of bus slave component including, by way of example, a register, memory, a bridge, or any other component capable of retrieving and storing information.  
         [0018]     Typically, the bus arbiter is coupled to the bus in order to manage memory access in an effective manner. For example, processing component  102   a , seeking access to a shared memory region, may broadcast a read or write request to the bus arbiter through the bus. The bus arbiter may then grant the requesting processing component  102   a  access to the shared memory region to perform the read or write operation. In the event that multiple read and/or write requests from one or more processing components contend at the bus arbiter, the bus arbiter may then determine the sequence in which the read and/or write operations will be performed and dynamically configure bus interconnection according to a bus arbitration scheme. The bus arbitration scheme may vary depending on the specific application and overall design constraints. Access to memory may be based on a balance between priority criteria and equitable distribution considerations.  
         [0019]     A processor may broadcast a read request by placing an address on an address bus. A write request may be made in a similar fashion, with the addition of data being placed on a data bus. The address bus and data bus may be separate buses, or a single time-shared bus. Sideband signaling may be employed to provide the appropriate control signals to the memory controller  104 . Alternatively, the control signals can be multiplexed with the address and data, either on separate buses or a single time-shared bus. The control signals may include, by way of example, a read/write strobe, an exclusive or non-exclusive operation, payload size signal, and in the case of a write operation, write byte enables. “Write byte enables” may be used to indicate which byte lanes on the bus will be used to broadcast the data for the write operation. By way of example, a 2-byte payload broadcast on a 32-bit data bus may use 2 of the 4 byte lanes. The write byte enables may be used to indicate to the memory controller which of the 2 byte lanes on the data bus will be used to broadcast the payload.  
         [0020]     Processing applications often require exclusive access to write to a shared memory region during an uninterrupted sequence of operations. The processing system must provide appropriate memory access management to processors that require such exclusive access.  FIG. 2  is a more detailed block diagram of the memory  100  and memory controller  104 . Exclusive accessibility to shared memory is governed by “semaphore” operation. Memory  100  is divided into regions. Memory regions  110  and  112 , respectively, are dedicated to processors  102   a  and  102   b . Memory region  114  is a memory region that is shared by multiple processors. If appropriate to particular system usage, this region can be further divided into portions that are each shared by specific processors of a multiple processor system. Region  116  is dedicated to semaphore storage.  
         [0021]     Memory controller  104  includes exclusive state machine monitors  119  and  120 . Monitor  119  is an exclusive monitor associated with processor  102   a , to monitor and control exclusive access operations of that processor to shared memory  114 . Monitor  120  is an exclusive monitor associated with processor  102   b.    
         [0022]     As explained earlier in the background portion of the disclosure, a semaphore is a flag that resides in semaphore memory region  116  and is indicative of accessibility or non-accessibility of the shared memory resource  114 . A processing component that seeks exclusive access to the shared memory region can read the semaphore memory region, via the associated memory controller monitor, to determine the accessibility of the shared memory region or memory region portion. If the semaphore indicates that the shared memory region is available for access, then the memory controller may set a flag in the semaphore region to indicate that the shared memory region is “locked,” and proceed to access the memory for writing into the shared memory region  114  by the associated processor. During the locked state of the semaphore, no other processing component is able to access the shared memory. Upon completion of the processing operations, the flag is removed by the memory controller and the semaphore returns to an unlocked state, indicative of accessibility.  
         [0023]     A semaphore reservation scheme is typically invoked for managing exclusive access to shared memory. As a prerequisite to gaining exclusive write access to shared memory, a reservation must be established for the requesting processor in its associated memory controller monitor. The controller sets a reservation state when a semaphore read operation takes place. As an example, an instruction code sequence may require exclusive access by processor  102   a  to write to locations in the shared memory region  114 . In order to execute a write exclusive request by the processor, the semaphore must first be read to determine whether region  114  can be accessed by processor  102   a . The semaphore read operation initiates the controller to set a reservation state for processor  102   a  in monitor  119 .  
         [0024]     If the shared memory region is not locked, processor  102   a  may immediately gain access or wait to later in the code sequence. Processor  102   b  may also establish a reservation state in monitor  120  if the semaphore is not locked. If both processors concurrently have reservation status, the first processor to have its request win arbitration to the memory controller will gain exclusive access to the shared memory region.  
         [0025]     Upon accessing region  114 , the semaphore flag is set and processor  102   a  gains exclusive access. In response to the write exclusive command, monitor  119  activates the controller to clear the reservation state stored in monitor  119 , as well as any reservation to the same address that may have been stored in monitor  120 . The locked status of the semaphore is released when the processor that owns the lock writes the semaphore location indicating that the semaphore is no longer locked.  
         [0026]     After establishing the reservation, processor  102   a  may have aborted processing the code sequence, or the code sequence may have been superceded, before a writing operation to the shared memory region has taken place. If the shared memory has not been accessed by any of the processors, the reservation will not have been cleared. As the memory controller monitor is unable to distinguish between whether the reservation was associated with an earlier or later code sequence, protocol requires that an obsolete reservation be cleared before a new reservation can be obtained for subsequent exclusive write access. Thus, if no exclusive write operation for a reservation is to take place, provision must be made to clear the reservation.  
         [0027]     In at least one embodiment of the processing system, the semaphore reservation may be cleared by broadcasting a write request to the semaphore memory region  116 , but at the same time, prohibiting the processor from writing to the semaphore memory region  116 . This may be achieved in a variety of ways. By way of example, the write byte enables may be used to disable all the byte lanes on the data bus during the request by the processor to write to the semaphore memory region  116 . As a result, the reservation for the processor is cleared, but the semaphore memory region  116  is not updated, and thereby, remains unlocked for use by another processor.  
         [0028]     Semaphore clearance operation is illustrated in the flow chart of  FIG. 3 . At step S 20 , processor  102   a  initiates processing of a code sequence. Determination is made at step S 22  as to whether an exclusive write operation to a shared memory address is required by the processor. If not, processing continues until the code sequence processing is completed. If it is determined in step S 22  that an exclusive write is required, the processor issues a read command at step S 24  to the semaphore address. This command sets a reservation in memory controller monitor  119  that is associated with processor  102   a.    
         [0029]     At step S 26 , the semaphore address is read and it is determined whether exclusive access to the shared memory region is available. If not, the reservation is cleared at step S 28  and processing flow returns to step S 20  to continue processing the code string. If the semaphore does not indicate a locked state, determination is made at step S 28  whether the code sequence associated with the reservation has been aborted. If the code sequence is still to be processed, an exclusive write command is issued by processor  102   a . Memory controller  104 , cognizant of the reservation status of processor  102   a  set in monitor  119 , provides access to the processor to the shared memory region  114  at step S 30 . The memory controller writes a lock flag to the semaphore memory and clears any existing semaphore reservations. Processor  102   a  continues to execute the code sequence with exclusive access to the shared memory region. At step S 32 , the processor has completed its operation sequence and relinquishes exclusive access. Memory controller  104  clears the semaphore flag to indicate that the shared memory is available for access. A new code sequence can be processed at step S 20 .  
         [0030]     If it is determined at step S 28  that the code sequence has been aborted, the reservation must be cleared from the memory controller. At step S 34 , the processor issues an exclusive write command. The processor sets the write byte enables to disable all byte lanes on the data bus. In response to the write command, the memory controller clears the reservation state in monitor  119  and, since there are no bytes lanes enabled, does not write to the semaphore memory region. A new code sequence can be processed at step S 20 .  
         [0031]     The various illustrative logical blocks, modules, circuits, elements, and/or components described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic component, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing components, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.  
         [0032]     The methods or algorithms described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. A storage medium may be coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor.  
         [0033]     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein, but is to be accorded the full scope consistent with the claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” All structural and functional equivalents to the elements of the various embodiments described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”