Abstract:
A technique to arbitrate computer memory request signals includes selecting a first memory request signal (associated with a first requestor), associating an access count value with the first memory request signal, and allowing the first memory requestor to access the computer memory access count value consecutive times.

Description:
BACKGROUND 
     The invention relates generally to computer system memory access operations and, more particularly, to the allocation of memory access bandwidth based on an access count priority scheme. Each device requesting access to system memory may be assigned an access count—the value of which determines the number of consecutive memory access cycles the device may use before a different device is allowed an opportunity to access memory. 
     Many current computer systems employ memory sharing architectures in which a plurality of devices share access to, and use of, a common system memory resource. For example, the system memory of a personal computer (PC) is typically shared by one or more central processing units (CPUs), one or more Accelerated Graphics Port (AGP) devices, one or more Peripheral Component Interconnect (PCI) devices, one or more Universal Serial Bus (USB) devices, and one or more embedded devices such as bus-to-bus bridge circuits and digital signal processors. 
     In some prior art computer systems, memory access is controlled by a memory control device which arbitrates between various requestors (i.e., devices seeking access to system memory) in a round-robin fashion. In these systems, a first requester is granted a single access followed by a second requester and so on. When all requesters have been granted access once, the process repeats. A drawback to conventional round-robin based arbitration schemes is that it may take a unacceptably long time to completely service/satisfy a requester having a multiple memory access transaction. In some other prior art computer systems, memory access is controlled by a memory control device which arbitrates between various requestors based on a requestor&#39;s assigned priority. In these systems, higher priority requesters are favored over lower priority requestors. A drawback to conventional priority based arbitration schemes is that high priority requesters may block lower priority requesters from gaining access to system memory for an unacceptably long time. 
     As the number of devices issuing memory access requests increases, it becomes ever more important to allocate memory bus bandwidth (i.e., share system memory) in an efficient manner. Thus, there is a need for a memory access control technique that efficiently services requestors issuing multiple access transactions without denying access to those requestors issuing single access transactions and/or low priority requesters for an unacceptably long time. 
     SUMMARY 
     In one embodiment the invention provides a method to arbitrate computer memory request signals. The method includes selecting a first memory request signal (associated with a first requestor), associating an access count value with the first memory request signal, and allowing the first memory requester to access the computer memory access count value consecutive times. The method may be stored in any media that is readable and executable by a programmable control device. 
     In another embodiment, the invention provides a computer memory access control device comprising a memory controller to access a computer memory, a storage element adapted to receive an access count value, and an arbiter coupled to the memory controller and the storage element, the arbiter adapted to selectively couple one of a plurality of memory request signals to the memory controller for N consecutive memory access operations, wherein N is based on the access count value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of a computer system in accordance with one embodiment of the invention. 
     FIG. 2 shows an expanded block diagram of a memory interface circuit in accordance with FIG.  1 . 
     FIG. 3 shows an expanded block diagram of the configuration register and counters of FIG.  2 . 
     FIG. 4 shows a flowchart of memory interface processing in accordance with one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Techniques (including methods and devices) to allocate memory access bandwidth based on an access count priority scheme are described. Each device/process capable of requesting system memory access (hereinafter referred to as a requestor) may be assigned an access count value. A requestor&#39;s access count value determines the number of consecutive memory access cycles it may use before a different device is allowed to access memory. The following embodiments of the invention are illustrative only and are not to be considered limiting in any respect. 
     FIG. 1 shows a block diagram of computer system  100  incorporating system controller  102  and memory interface  104  in accordance with one embodiment of the invention. As shown, system controller  102  couples processor  106 , accelerated graphics port (AGP) device  108 , system bus  110  and devices connected thereto (e.g., devices  112  and secondary bus bridge circuit  114 ) to system memory  116  (via memory bus  118 ). In turn, secondary bus bridge circuit  114  provides a mechanism to couple secondary bus  120  and attached devices to system memory  116 . For example, non-volatile memory  122 , and other secondary bus devices (e.g., device  124 ) such as floppy disks may be coupled directly to secondary bus  122  while one or more intelligent drive electronics (IDE) devices may be coupled to system  100  via IDE interface  126 , and one or more universal serial bus (USB) devices may be coupled to system  100  via USB interface  128 . 
     In one embodiment of the invention, memory interface  104  determines an access count value for each requester that may access system memory  116 . In another embodiment, memory interface  104  determines an access count value for each type of requestor that may access system memory  116 . For example, all processors (e.g., processor  106 ) may be assigned a common access count value as may all AGP devices (e.g., device  108 ). In yet another embodiment, some devices (e.g., a specific processor or a specific primary bus device) may have specifically assigned access count values while other devices may have assigned access count values based on their type. Regardless of which approach is taken to assign access count values with a particular requester, once a device is granted access to memory  116  by memory interface  104  (based on any selected arbitration scheme—priority based or round-robin, for example), a requestor may access system memory  116  up to N consecutive times (where N equals the requestor&#39;s assigned access count value) before another requestor is granted access. 
     Generally speaking, requesters may be assigned access count values commensurate with their natural memory transaction size. For example, requests initiated by processor  106  are typically for a cache line&#39;s worth of data (e.g., 256 bits or 32 bytes). Thus, a processor requestor may be assigned a access count value sufficient to transfer 32 bytes of data between itself and system memory. Thus, if system memory  116  is accessed via 64-bit (32-bit) words, a processor requestor may be assigned an access count value of 4 (8). Similarly, block devices such as graphics devices (e.g., AGP device  108  and one or more system bus devices  112 ) may naturally initiate memory transactions of 8 kilobytes (KB). These devices may be assigned access count values sufficient to transfer 8 KB. 
     Referring again to FIG. 1, illustrative processors (e.g., processor  106 ) include the PENTIUM® processor and 80×86 families of processors from Intel Corporation. An illustrative system bus (e.g., bus  110 ) is one operated in conformance with the Peripheral Component Interconnect (PCI) specification. Illustrative primary bus devices  112  include audio, network interface, video and graphics accelerator PCI expansion card devices. An illustrative secondary bus bridge circuit (e.g., bridge circuit  114 ) is the PIIX4 PCI-to-ISA/IDE accelerator chip from Intel Corporation. Illustrative secondary buses (e.g., bus  120 ) include those bus structures operated in conformance with the Low Pin Count (LPC) interface, Industry Standard Architecture (ISA) and Extended Industry Standard Architecture (EISA) standards. Illustrative non-volatile memory devices (e.g., NVRAM  126 ) include read only memory (ROM), electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), flash memory, and complementary metal oxide semiconductor (CMOS) memory. Illustrative secondary bus devices (e.g., device  128 ) include input-output device controllers, audio and modem devices. It will be recognized that each of the elements of FIG. 1 are typically connected, directly or indirectly, to a printed circuit board often referred to as a motherboard. It will be further recognized that motherboards often include embedded devices (a digital signal processor, for example) that may communicate with system memory  116  via system controller  102 . 
     FIG. 2 shows system controller  102  and an expanded block diagram of memory interface  104  in accordance with one embodiment of the invention. As indicated, switch  200  may be used to route memory request signals  202  through  206  between requesting devices and memory interface  104 . Each request signal represents a signal path between system controller  102  and a requesting device (e.g., processor  108  and secondary bus device  124 ). It will be recognized that memory request signals (e.g.,  202 ) typically encode information identifying the device issuing the memory request, the amount of memory being requested (to read or write), a memory transaction address, and other information as needed by other system controller functions. 
     Memory interface  104  includes memory controller  208 , arbiter  210 , configuration register  212 , and one or more counters  214 . Memory controller  208  processes selected memory access request signals, transferring data into and out of memory  116  in accordance with conventional memory controller operations. Arbiter  210  determines which request signal (via select signal  216 ) is routed by switch  200  to memory controller  208  for processing. In one embodiment, arbiter  210  implements a grant-request handshake protocol wherein each device that may access memory  116  has a corresponding grant/request signal pair  218 . Using a chosen arbitration scheme (e.g., a priority-based or round-robin scheme), arbiter  210  monitors grant/request signals  218  and, in combination with configuration registers  214  and counters  216  selectively passes one request signal at a time between switch  200  and memory controller  208 . Configuration registers  212  may be used to record access count values for each requestor (or class of requester), and counters  214  may be used by arbiter  210  to track the number of memory access operations remaining for a selected requestor. 
     As shown in FIG. 3, configuration register  212  may have a plurality of fields, each field adapted to store an access count value for a single requestor. For example, field-1  300  may record an access count value for processor  106 , field-2  302  may record an access count value for AGP device  108  and field-N  304  may store an access count value for secondary bus device  124 . Counters (e.g.,  306 ,  308  and  310 ) are used by arbiter  210  to track the number of memory access operations that remain allocated to a selected requestor. Each access by a requestor causes its associated counter value to be decremented. When the counter reaches 0 (zero), or the requester deasserts its request signal (on grant/request signal pairs  218 ), arbiter  210  performs an arbitration cycle—selecting a different requestor or the same requestor (depending upon the arbitration scheme). In one embodiment (see FIG.  3 ), the number of counters may equal the number of access count value fields in configuration register  212 . In another embodiment, there are fewer counters than there are access count value fields in configuration register  212 . For example, counter  214  may comprise a single counter which is loaded with the appropriate configuration field value when a device is initially selected, decremented each access thereafter, and reloaded when another device is selected by arbiter  210 . 
     A flowchart of memory interface processing in accordance with one embodiment of the invention is shown in FIG.  4 . Arbiter  210  initially chooses one device from the plurality of devices that may access memory  116  (block  400 ). Arbiter  210  may, for example, implement a round-robin arbitration mechanism in which requestors are chosen during block  400  in a predetermined and cyclic pattern. If the chosen device does not have a pending memory request (the ‘no’ prong of diamond  402 ), the next requestor is chosen and processing continues at block  400 . If the chosen device has a pending memory request (the ‘yes’ prong of diamond  402 ), arbiter  210  selects the chosen device by loading the access count value associated with the selected device from configuration register  212  to counter  214 , and causing switch  200  to couple the selected requestor (request signal  204 , for example) to memory  116  via memory controller  208  (block  404 ). Memory controller  208  performs a single memory access operation in accordance with the selected memory request (block  406 ), the requestor&#39;s associated counter value is decremented (block  408 ), and a check is made to determine if the selected device has completed all of its allocated consecutive accesses (diamond  410 ). If the selected device&#39;s counter value is zero, or the selected device deasserts it memory request signal (the ‘yes’ prong of diamond  410 ), processing continues at block  406 . If the selected associated device&#39;s counter value greater than zero (the ‘no’ prong of diamond  410 ), processing continues at block  406 . 
     In one embodiment, configuration register  212  is initialized during computer system power on self-test (POST) operations by basic input-output system (BIOS) routines  130  (see FIG.  1 ). For example, if system bus  110  is a PCI bus, configuration register  212  may reside in PCI configuration address space. (Techniques to read and write to PCI configuration space registers are well-known to those of ordinary skill in the art of system controller and memory controller design.) In another embodiment, requester access count values may be determined dynamically at computer system start up and/or modified during system operations. For example, access count values may be based on requestor operating speed, wherein faster devices are allocated larger access count values. Alternatively, system controller  102  or memory interface  104  may monitor (snoop) memory bus  118  activity and empirically set and adjust requestor access count values (i.e., modify values stored in configuration register  212 )—wherein the more frequently a requester seeks to access memory  116  (relative to other requesters), the larger its access count value is set. 
     Various changes in the materials, components, circuit elements, as well as in the details of the illustrated operational method are possible without departing from the scope of the claims. For instance, the illustrative system of FIGS. 1 and 2 show memory interface  104  as being one element within system controller  102 . While many current personal computer systems do employ integrated system and memory controllers (often referred to as a “north bridge”), a memory interface in accordance with the invention may be implemented as a stand-alone circuit. That is, memory interface  104  may be implemented as one or more custom designed state machines, embodied in a hardware device such as a printed circuit board comprising discrete logic, integrated circuits, or specially designed application specific integrated circuits (ASICs). In addition, a requestor may have different access count values for read and write memory transactions. That is, configuration register  212  may provide two fields per requestor—one field adapted to store a read operation access count value and another field adapted to store a write operation access count value. 
     While the invention has been disclosed with respect to a limited number of embodiments, numerous modifications and variations will be appreciated by those skilled in the art. It is intended, therefore, that the following claims cover all such modifications and variations that may fall within the true spirit and scope of the invention.