Abstract:
A method to operate a computer system bridge circuit includes enqueueing multiple delayed read requests to system memory, where each delayed read request is associated with a different expansion bus devices. The method may also include forwarding a second enqueued read request to the system memory before receiving a response to a first forwarded enqueued read request. The method may further include arbitrating to an expansion bus device (having an enqueued delayed read request) only after read data is received from the system memory in response to a forwarded read request. A computer system incorporating a bridge circuit operated in accordance with the described method is also described.

Description:
BACKGROUND  
         [0001]    The invention relates generally to computer systems, and more particularly to techniques for improving peripheral device to main memory throughput using a delayed read pipeline.  
           [0002]    Personal computer (PC) systems generally employ an expansion bus to handle various data transactions related to input-output devices such as magnetic and/or optical mass storage units and network interface controllers. Typically, expansion buses are coupled to the system bus (to which one or more processors/central processing units are connected) by a bridge circuit.  
           [0003]    In the past, many PCs employed an expansion bus operated in conformance with the Industry Standard Architecture (ISA) standard. The ISA standard defines a 16-bit bus having a maximum transfer rate of 8.33 MBytes/sec. The subsequently defined extended ISA (EISA) bus uses a 32-bit data path to provide a peak transfer rate of 33 MBytes/sec—four times that of the ISA bus. As the operating speed of system components continues to increase, however, ISA, EISA and their descendant bus standards (e.g., the Micro Channel and VESA bus standards) have been unable to provide the necessary operational bandwidth.  
           [0004]    One solution to the low bandwidth capacity of earlier expansion bus architectures is embodied in the Peripheral Component Interconnect (PCI) standard. The PCI standard defines both 32-bit and 64-bit data transfer protocols operating at either 33 MHz or 64 MHz. A key feature of the PCI standard is that it supports burst operations on a transaction by transaction basis, where the length of each burst may be negotiated between the device initiating the transfer (a master) and the device receiving the transfer request (a target). As a result, expansion buses operated in conformance with the PCI specification support transfer rates of 132 MBytes/sec for a 32-bit bus operating at 33 MHz, 264 MBytes/sec for a 32-bit bus operating at 66 MHz or a 64-bit bus operating at 33 MHz, and 528 MBytes/sec for a 64-bit bus operating at 66 MHz. (The current version of the PCI specification (rev. 2.2) is available from the PCI Special Interest Group, 2575 NE. Kathryn Street #17, Hillsboro, Oreg. 97124.)  
           [0005]    While expansion bus operating speeds have continued to increase, many target devices are still not able to respond to master initiated data requests in a timely fashion (i.e., within the bus required latency period). In computer systems employing ISA and EISA expansion buses, for example, the delay in reading data from a slow target device was handled by wait states. That is, when a target (e.g., system memory) could not immediately provide the data requested by a master (e.g., a processor), the target simply marked time using wait states until the data became available. The use of wait states in this manner prevents another device from accessing the bus. Thus, expansion bus bandwidth was effectively limited by the slowest responding device on the bus. To avoid the use of wait states, the PCI standard allows the use of delayed transactions. In a delayed transaction data requests are temporally separated from the delivery of the requested data by other transactions. Wait states are not used—while the originating master waits for the target device to provide the requested information, other bus masters are allowed to use the bus. In accordance with the PCI specification, a delayed transaction progresses to completion in three phases: (1) request by master; (2) completion of the request by the target; and (3) completion of the transaction by the master.  
           [0006]    During phase one, the master generates a transaction on the bus while the target decodes the address, latches the information required to complete the access and terminates the request with a Retry. (“Retry” refers to the condition where a target device issues a transaction termination request before any data is transferred. This condition may occur, for example, because the target device is unable to meet the bus latency requirement, is currently locked by another master, or there is a conflict for a internal resource. Target devices indicate Retry by asserting STOP# and not asserting TRDY# on the initial data phase of a transaction.) The latched request information is referred to as a Delayed Request. The master initiating the Retried transaction must reissue its request until the request completes.  
           [0007]    During phase two, the target independently completes the request using the latched information from the Delayed Request. If the Delayed Request corresponds to a read operation, the target obtains the requested read data and completion status. If the Delayed Request corresponds to a write transaction, the target delivers the write data and obtains the completion status. The result of completing the Delayed Request produces a Delayed Completion (consisting of the latched information of the Delay Request, the completion status and, possibly, data). The target stores the Delayed Completion until the master repeats its initial request.  
           [0008]    During phase three, the master successfully rearbitrates for the bus and reissues the original request. The target decodes the request and gives the master the completion status (and data if the transaction is a read transaction). At this point, the Delayed Completion is retired and the transaction has completed.  
           [0009]    In accordance with conventional PCI bus to system memory control devices (e.g., the 440GX chip from Intel Corporation), only a single PCI device to system memory delayed read transaction may be accepted for processing at a time. Thus, each delayed read by a PCI device to system memory incurs the full memory access latency. Thus, it would be beneficial to provide a mechanism to reduce the read latency (thereby improving bandwidth utilization of the PCI bus) associated with PCI device to system memory read operations.  
         SUMMARY  
         [0010]    In one embodiment the invention provides a method to operate a computer system bridge circuit. The method includes enqueueing multiple delayed read requests to system memory, wherein each delayed read request is associated with a different expansion bus devices. The method may also include forwarding a second enqueued read request to the system memory before receiving a response to a first forwarded enqueued read request. The method may further include arbitrating to an expansion bus device (having an enqueued delayed read request) only after read data is received from the system memory in response to a forwarded read request.  
           [0011]    In another embodiment, the invention provides a computer system including a system memory, an expansion bus, a plurality of devices coupled to the expansion bus, and a bridge circuit having a queue and a control circuit. The control circuit is adapted to enqueue a plurality of delayed read requests to the system memory from the expansion bus devices (each enqueued delayed read request being associated with a different expansion bus device). The control circuit may be further adapted to transmit a plurality of read requests to the system memory (each read request corresponding to an enqueued delayed read request), and to receive read data from the system memory in response to the transmitted read requests. The bridge circuit may further include an arbiter circuit adapted to arbitrate to that expansion bus device associated with that delayed read request for which the read data was received only after the read data is received from system memory. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 shows a block diagram of a computer system incorporating a delayed read request queue in accordance with one embodiment of the invention.  
         [0013]    [0013]FIG. 2A shows how a conventional peripheral component interconnect (PCI) interface processes one delayed read request at a time.  
         [0014]    [0014]FIG. 2B shows how a PCI interface in accordance with the invention overlaps (pipelines) the processing of multiple delayed read requests at once.  
         [0015]    [0015]FIG. 3 shows an expanded block diagram of the PCI interface of FIG. 1.  
     
    
     DETAILED DESCRIPTION  
       [0016]    Techniques to reduce the read latency associated with Peripheral Component Interconnect (PCI) device to system memory read operations are described. The following embodiments, described in conjunction with the above described figures, are illustrative only and are not to be considered limiting in any respect.  
         [0017]    Referring to FIG. 1, computer system  100  having PCI bus interface  102  incorporating delayed read queue  104  in accordance with one embodiment of the invention is shown. Illustrative computer system  100  also includes one or more host processors (only one is shown for convenience,  106 ), system memory  108 , a plurality of PCI devices (e.g.,  110 ,  112  and  114 ) coupled to PCI bus  116 , and bridge circuit  118 . Bridge circuit  118 , in turn, incorporates memory interface circuit  120  and the aforementioned PCI bus interface circuit  102  and delayed read queue  104 . Illustrative processors (e.g., processor  106 ) include the PENTIUM processor from Intel Corporation and the AMD-K6 processor from Advanced Micro Devices Corporation. An illustrative system memory  108  comprises dynamic random access memory (DRAM) such as static DRAM or double data rate DRAM. Illustrative PCI devices (e.g., devices  110 ,  112  and  114 ) include network control adapters, audio control adapters, video control adapters, small computer system interface (SCSI) control adapters, and graphics control adapters.  
         [0018]    Delayed read queue  104  provides a structure through which a delayed read transaction pipeline (from PCI bus  116  to system memory  108 ) may be implemented. Delayed read queue  104  allows PCI interface  102  to accept multiple, simultaneously pending read requests from PCI bus devices (e.g., devices  110 ,  112  and  114 ) targeting system memory  108 . Each accepted delayed request may be forwarded to system memory  108  (via memory interface  120 ) in turn. The ability to store multiple delayed read requests allows PCI interface  102  to submit subsequent delayed read requests to memory interface  120  before receiving the read data associated with a previously submitted read request.  
         [0019]    This benefit may be readily understood by comparing the latency associated with processing delayed read requests in a conventional PCI bus interface (i.e., one not using a delayed read queue) with that of a PCI interface incorporating a delayed read queue in accordance with the invention. As shown in FIG. 2A, conventional PCI interface circuits can only initiate the processing of one delayed read request at a time. As a result, each delayed read transaction suffers the full read latency (T L ) of system memory. In contrast, a PCI bus interface incorporating delayed read queue  104  may initiate prefetch operations for as many delayed read requests to system memory  108  as memory interface  120  is capable of handling (see FIG. 2B). For example, if a prior art PCI bus interface takes eight time units to complete two delayed read transactions (see FIG. 2A), three delayed read transactions may be completed and a fourth begun in the same eight time units (see FIG. 2B). Pipelining delayed read requests in this manner “hides” much of the latency inherent in each memory access. The reduced read latency may contribute significantly to the improved performance of PCI bus devices.  
         [0020]    In one embodiment, delayed read queue  104  is sufficiently deep (i.e., has sufficient storage) to allow at least N simultaneously pending delayed read requests, where N is chosen to so that read data is likely to be received from system memory (in response to a first submitted delayed request) before all delayed completions are submitted. As shown in FIG. 2B, the ability to queue and submit multiple delayed requests to system memory  108  in accordance with the invention substantially reduces the effective read latency associated with all delayed requests (except the first) which has the effect of increasing the effective PCI bus to system memory bandwidth.  
         [0021]    Referring now to FIG. 3, an expanded block diagram of PCI bus interface  102  is shown. As indicated, PCI bus interface  102  includes a bus master circuit  300 , a bus target circuit  302  and an arbiter circuit  304 . Bus master circuit  300  is responsible for initiating transactions on PCI bus  116 . Bus target circuit  302  is responsible for processing transactions initiated by PCI bus masters (e.g., device  110 ). Because bus target circuit  302  mediates communication between a bus device and system memory  108 , it includes delayed read queue  104 . Arbiter circuit  304  selectively grants access to PCI bus  116  to one device at a time (e.g., bus master circuit  300 , bus target circuit  302 , or a bus device).  
         [0022]    Bus target circuit  302  incorporates that circuitry needed to perform conventional bus target functions (as required by the PCI specification). In addition, bus target circuit  302  includes the necessary control circuitry to enqueue delayed read requests into queue  104  from PCI bus  116 , forward delayed read requests to memory interface  120 , identify read data received from memory interface  120  as being associated with a pending delayed read, and return received read data to the appropriate bus device. To satisfy PCI transaction ordering requirements, bus target  302  should also be capable of accepting and processing a system memory write transaction from a bus device while there are pending delayed read requests. Implementation of these functions, while complex, would be a routine undertaking for one of ordinary skill in the art of PCI interface and/or computer system bridge circuit design having the benefit of this disclosure.  
         [0023]    In one embodiment, arbiter  304  selectively enables (arbitrates) to PCI bus devices in a cyclic or round-robin fashion. In another embodiment, arbiter  304  does not arbitrate to a PCI bus device having an enqueued delayed read request in queue  104 . This latter technique may avoid wasting PCI bus cycles by ignoring those devices which would only initiate a retry operation. In yet another embodiment, arbiter  304  immediately arbitrates to a device whose delayed read transaction is ready to complete (that is, on receipt of read data from system memory  108 ).  
         [0024]    Various changes in the materials, components and circuit elements of the described embodiments are possible without departing from the scope of the claims. For instance, delayed read queue  104  may be incorporated within PCI interface  102  as shown in FIGS. 1 and 3 or it may be incorporated within memory interface  120  (as part of a memory control circuit). Further, PCI bus interface  102  (and its constituent elements, see FIGS. 1 and 3) may be implemented as a custom designed state machine (or part thereof) comprising discrete logic, integrated circuits, or specially designed application specific integrated circuits (ASICs).  
         [0025]    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 sprit and scope of the invention.