Abstract:
Disclosed is a method and memory subsystem that allows for speculative issuance of reads to a DRAM array to provide efficient utilization of the data out bus and faster read response for accesses to a single DRAM array. Two read requests are issued simultaneously to a first and second DRAM in the memory subsystem, respectively. Data issued from the first DRAM is immediately placed on the data out bus, while data issued from the second DRAM is held in an associated buffer. The processor or memory controller then generates a release signal if the second read is not speculative or is correctly speculated. The release signal is sent to the second DRAM after the first issued data is placed on the bus. The release signal releases the data issued from the second DRAM to the buffer from the buffer on to the data out bus. Because the data has already been issued when the release signal is received, no loss of time is incurred in issuing the data from the DRAM and only a small clock cycle delay occurs between the first issued data and the second issued data on the data out bus.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field:  
           [0002]    The present invention generally relates to computer memory subsystems, and more particularly to a Dynamic Random Access Memory (DRAM) subsystem. Still more particularly, the present invention relates to a DRAM subsystem that supports speculative reads for efficient utilization of the data out bus.  
           [0003]    2. Description of the Related Art:  
           [0004]    DRAM subsystems are well known in the art. A typical DRAM cell uses the gate capacitance of one or more field-effect transistors to provide the storage of a binary state. The charge on this capacitor will eventually leak and the DRAM cell may change state, causing an incorrect bit to be set or reset in memory. This problem is typically solved by a refresh scheme, which allows the gate of the field-effect transistors to be recharged according to the value stored in the cell, before sufficient discharge has occurred to cause an error.  
           [0005]    The typical DRAM is formed by thousands of individual memory cells arranged in a matrix-like configuration. Each DRAM cell includes a capacitor, which is electrically charged or discharged in a “write” operation. The charge establishes a voltage on the capacitor, and the level of the voltage represents a data bit. The data bit represented by the capacitor charge is determined by comparing the capacitor voltage to a threshold reference. Due to the loss of charge associated with the capacitor, a DRAM typically requires its storage cells to be refreshed after each use.  
           [0006]    The memory cells of the DRAM matrix are addressed by signals supplied on word lines and bit lines. The word lines extend in a horizontal reference direction in the matrix and connect to the memory cells in the horizontal rows and thus intersect vertical columns of memory cells in the matrix. The bit lines extend in a vertical reference direction in the matrix and connect to the memory cells in vertical columns and thus intersect horizontal rows of cells. By energizing a selected word line, the voltage from the memory cells in the horizontal row corresponding to the selected word line are presented on the bit lines extending from each of the cells.  
           [0007]    The DRAM memory array is usually divided into one or more segments, and each of the segments is further divided into bit blocks. Each bit block has a plurality of memory cells, and those memory cells are organized into rows and columns in a matrix. Individual words are selected by addressing the DRAM segments, selecting individual columns in the bit blocks, and selecting the desired word line.  
           [0008]    The individual DRAM components are typically four bits wide and are assembled in parallel in a row to create wider DRAM systems. For example, 32 DRAM&#39;s may be assembled to form a 16 byte wide data bus. The DRAM system would consist of multiple groups of 32 DRAMS that had a common data bus but each having its own unique address bus. In operation, when a read is issued by the processor, the command will be issued on the address bus to a specific DRAM group and the data is provided to the shared data out bus.  
           [0009]    A single data out bus provides the data out for all DRAMs in the DRAM system, while each DRAM has its own address bus. In operation, when a read command is issued by the processor, the command is issued on the address bus to a specific DRAM array and the data is provided to the shared data out bus. If multiple reads are issued, however, with present memory devices, there is no control provided to allow the data to be scheduled to the common data bus, which ultimately leads to situations where there are collisions on the data bus.  
           [0010]    During read operations from different DRAMs, the data issued from one DRAM must not run into the data issued from another DRAM on the data out bus. Additionally, some space is needed between accesses to different groups of DRAMs on a common data bus to prevent switching noise.  
           [0011]    In general there are three stages in a read access of a group of DRAM&#39;S. The first stage is the access stage, during this stage the address is sent to the DRAM&#39;s and the DRAM array is accessed. This stage takes a number of cycles depending on the type and speed of the DRAM&#39;s themselves. The second stage is the data transfer which will be some number of cycles depending on the burst length that is requested. The third stage is the precharge stage, in this stage the DRAM is recovering from the access and returning to a state where another access may be initiated. The length of this stage is also dependent on the DRAM type and speed. In general multiple accesses to the same group of DRAMs will result in large gaps between the data packets. To increase data bus utilization multiple groups of DRAMs will be accessed in a overlapping fashion so that the resulting data packets will fill the gaps on the data bus.  
           [0012]    Due to the requirement that the reads be scheduled to avoid collisions on the common data bus, the memory controller will have to delay read commands that it received in order to line up the data transfer to a hole on the common data bus. By delaying the read command, the memory controller effectively increases the amount of time that the DRAM group is busy for a given read access, thereby lowering the efficiency of the memory subsystem. If the DRAM access could be started as soon as the memory controller had the command and the data transfer delayed until there was space for it on the bus, the DRAM cycle for the read could be completed earlier, and the group of DRAM&#39;s would be available sooner for the next read request.  
           [0013]    Another related problem with DRAM operations is that often a read request is issued speculatively and the processor may later resolve that the read should not be completed or that the read address is incorrect. With current memory systems, once a read has been issued by the processor, there is no way for the processor to prevent the data from being issued. However, as described above, the data is usually not issued until a significant number of clock cycles has elapsed after the read request is sent down the processor pipeline. Given the present inefficient utilization of the data out bus, the issuing of data on the data out bus from a read request that is determined to be incorrectly issued prior to placing the data on the bus is a waste of bus resources. There is presently a read command STOP.for SDRAM components; however, there is no mechanism by which a DRAM issued read can be halted after it has been sent by the memory controller.  
           [0014]    The present invention recognized that it would be desirable to have a method and DRAM system that provides efficient utilization of the data out bus by allowing controlled speculative reads of data from a DRAM subsystem that enables maximum utilization of the data out bus. A method by which the effects of the delay in data issuance on the data bus that is associated with the refresh operation of a DRAM is substantially reduced would be a welcomed improvement. These and other benefits are provided in the present invention.  
         SUMMARY OF THE INVENTION  
         [0015]    Disclosed is a method and memory subsystem that allows for speculative issuance of reads to a DRAM array to provide efficient utilization of the data out bus and faster read response for accesses to a single DRAM array. Two read requests are issued simultaneously to a first and second DRAM in the memory subsystem, respectively. Data issued from the first DRAM is immediately placed on the data out bus, while data issued from the second DRAM is held in an associated buffer. The processor or memory controller then generates a release signal if the second read is not speculative or is correctly speculated. The release signal is sent to the second DRAM after the first issued data is placed on the bus. The release signal releases the data issued from the second DRAM to the buffer from the buffer on to the data out bus. Because the data has already been issued when the release signal is received, no loss of time is incurred in issuing the data from the DRAM and only a small clock cycle delay occurs between the first issued data and the second issued data on the data out bus.  
           [0016]    If the second read request was incorrectly speculated, then no release signal is transmitted to the second DRAM. A next correct read is issued. When a next read is made to the second DRAM, the buffer is overwritten with data issued from the later read command, and the data issued by the incorrectly speculated read is never placed on the data out bus. Additionally, by issuing the second read simultaneously with the first, the second DRAM begins its precharge operation sooner and is ready to process a next read request soon after the first read request issues data on the data out bus. Substantially time savings is realized in the subsequent issuance of data from the DRAMs, and a more efficient utilization of the data out bus is realized.  
           [0017]    The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0019]    [0019]FIG. 1 depicts a block diagram of major components of a data processing system, in which a preferred embodiment of the present invention may be implemented;  
         [0020]    [0020]FIG. 2 depicts a block diagram of a memory subsystem within the data processing system of FIG. 1 within which a preferred embodiment of the present invention may be implemented;  
         [0021]    [0021]FIGS. 3A and 3B illustrate two embodiments of the internal makeup of a memory array with a buffer according to the features of the present invention;  
         [0022]    [0022]FIG. 4 is a flow chart of the process of operating a memory array according to the features provided by the present invention; and  
         [0023]    [0023]FIGS. 5A and 5B are timing diagrams illustrating the contrast between present operation of the memory array and operation according to the preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    With reference now to the figures and in particular with reference to FIG. 1, there is illustrated a high level block diagram of the major component parts of a data processing system that may be utilized to implement the various features of the present invention. Data processing system  100  comprises a Central Processing Unit (CPU)  110 . CPU  110  is coupled to permanent (non-volatile) memory  116 , peripheral input/output (I/O) devices  114 , firmware  124 , and temporary memory (DRAM)  118  via interconnect  120 . CPU  110  is further directly coupled to DRAM  118  via memory interconnect  122 .  
         [0025]    The present invention is preferably implemented in a DRAM subsystem as illustrated in FIG. 2. The DRAM subsystem includes a DRAM array  234 , which may be synonymous with temporary memory  118  of FIG. 1. DRAM array  234  is coupled to a memory controller  235 , which controls all accesses, both read and write, to DRAM array  234 . Requests for reads and writes are received by memory controller  235  from processor  110  as read and write commands, respectively.  
         [0026]    Refresh Control  238  provides refresh requests to the DRAM array  234 . Processor  110  and Direct Memory Access (DMA) Device  232 , provide requests and data for write operations to memory controller  235 . Refresh control  238  provides refresh requests to memory controller  235 . Memory controller  235  accesses DRAM array  234  and provides data to processor  110  and DMA Device  232  on read operations. DRAM Array  234 , is refreshed by the action of the control logic through Memory Controller  235 , which will further generate CAS (Column Address Select) and/or RAS signals (Row Address Select) signals within the DRAM array  234 , depending on the particular refresh technique that the DRAM array  234  requires.  
         [0027]    These signals are well known names for signals within the art of DRAM&#39;s, but should not be construed to limit the application of these circuits and techniques. DRAM array  234  can be memory that is external to the processor integrated circuit or can constitute DRAM within a processor such as L 1  or L 2  cache memory provided in a microprocessor or other program execution device.  
         [0028]    The preferred embodiment of the invention provides a method for controlled issuance of read data from the DRAM subsystem on the data out bus and speculative read operations on DRAMs as illustrated in FIG. 3 and described below. The invention provides a mechanism for pre-fetching read operations to DRAM to allow early read access of read data that has not passed system snoop protocols without reducing the effective utilization of the memory data bus. The invention finds applicability with large memory subsystems with multiple address buses but a common data bus as illustrated in FIG. 3.  
         [0029]    [0029]FIGS. 3A and 3B are block diagrams illustrating two embodiments of the parallel banks of DRAMs within a DRAM subsystem as provided in FIG. 2 in accordance with the present invention. As shown in FIG. 3A, a number of DRAM components  303 A- 303 N are lined in parallel, each having a separate address bus  305 A- 305 N. In the illustrated embodiment, DRAM components  303 A- 303 N each comprise a buffer  307 A- 307 N that is in turn connected to a shared data out bus  309 . Buffer  307 A- 307 N may be a latch capable of storing one line of data from its respective DRAM component  303 A- 303 N. Each address bus  305 A- 305 N is capable of receiving/transmitting both a read request that comprises an associated address and a release signal that controls the release of data from the respective buffer  307 A- 307 N. According to one embodiment, read request also comprises a hold bit which may be set by the memory controller to indicate that data issued by the read request is to be held in the respective buffer  307 A- 307 N until a release signal is transmitted to the DRAM component  303 A- 303 N.  
         [0030]    Although illustrated as an associated component part of the DRAM component  303 A- 303 N, the buffer  307 A- 307 N may be a separate component. Also, the buffer  307 A- 307 N may be shared among several DRAM components (illustrated in FIG. 3B). With a shared-buffer configuration, the invention provides that the second set of issued data is loaded into the shared buffer  307 A- 307 N and the memory controller may signal the shared buffer  307 A- 307 N directly (i.e., not the individual DRAM component) to release the data to the data out bus. FIG. 3B further illustrates multiple banks of memory components. Bank A (DRAM A 1   303 A to DRAM N 1   303 N and corresponding address buses  305 A- 305 N) to Bank N (DRAM AN  304 A to DRAM NN  304 N and corresponding address buses  306 A- 306 N) have a shared buffer configuration with a single shared buffer  307 A- 307 N connecting the data outputs from each bank of DRAMs, respectively.  
         [0031]    The invention thus modifies and/or adds a command to the DRAM address bus that initiates a read of data from the DRAM component but holds the issued data temporarily in a buffer until another command is issued to transfer the data from the buffer to the data bus. In one embodiment, the buffer is implemented as a simple latch, which opens and closes when a specific command is received. Thus, the hold command may comprise a single bit issued to the latch to close the latch and the subsequent release signal may also be a single bit issued by the memory controller that opens the latch. In the shared-buffer embodiment, the signals may be transmitted directly to the buffer via a signal line connecting the buffer to the memory controller or the DRAM component&#39;s address bus. The utilization of these commands allows the memory controller to issue multiple successive DRAM read commands to memory, prior to the resolution of speculative read or system snoop operations, and then basically cancel the commands by not sending a signal to transfer the data from the buffer to the memory data bus. The memory controller may also overwrite the buffer in the DRAM with the next read command.  
         [0032]    In the preferred embodiment, speculative or simultaneously release data is held in the buffer until the memory controller decides to released the data by sending a release signal. In another embodiment, data is automatically release from the buffer on to the data out bus after a predefined number of clock cycles. In this embodiment, the number of cycles for holding each set of data is determined by the processor and sent within the read request. The number may be calculated as: the number of cycles for the previous data to be issued to the data out bus+1. The additional 1 cycle provides a barrier to switching noise and prevents overlap on the data bus. This allows the data to be issued on the data out bus in a pre-selected order and/or within a number of clock cycles of each other. The pre-selected order may be overridden by transmitting a release or hold signal prior to release of data from a particular buffer. Thus, data out bus utilization may be efficiently maximized.  
         [0033]    The invention issues reads when they are received and do not wait for a previously issued read to clear the data out bus. The reads are issued at the same time because each DRAM has an individual address bus and the issued data may be held in the buffers and issued without overlapping each other. The invention thus provides a mechanism that monitors when two or more DRAMS receive either simultaneous reads or reads whose issued data overlaps on the data bus, and then withholds the issuance of the data from one or more of the DRAMs until an appropriate time to transmit the data without overlapping other data traveling on the data bus.  
         [0034]    Thus, with speculatively issued read requests, data is in effect pre-fetched, albeit speculatively, and issued when the read request is determined to be correctly speculated. The processor signals the memory controller, which in turn generates the release signal and transmits the release signal to cause the DRAM (or the buffer) to release the data to the data bus.  
         [0035]    [0035]FIG. 4 is a flow chart illustrating the process of reading data from DRAM subsystem according to the methods of the invention. The process begins at block  401 , and following a pair of read requests are issued by the processor as shown at block  403 . The memory controller issues both read request to their respective DRAMs simultaneously as indicated at block  405 . The DRAM receiving the first read request in the processing sequence issues the data on to the data out bus as indicated at block  407 . Concurrently, the second DRAM issues its requested data to a buffer, which temporarily stores the data as provided at block  409 . A determination is then made at block  411 , whether the second read was correctly speculated.  
         [0036]    If the read was correctly speculated, a signal is generated from the memory controller at block  412 . A determination is then made at block  413  whether all of the previously issued data has been issued to the data out bus. When all of the previously issued data has been issued, the release signal is transmitted to the buffer as shown at block  414 . The signal causes the buffer to release the data on to the data out bus as indicated at block  415 . Following, a determination is made at block  416  whether the memory controller has received a next read request to the second DRAM. According to the invention, a subsequent read request to the second DRAM is immediately processed by the DRAM because the array has had time to refresh while the above processes were completed. The second DRAM thus issues the next set of data directly on to the data out bus as shown at block  417 .  
         [0037]    Returning to block  411 , if the second read was incorrectly speculated, no signal is sent and the data remains in the buffer until it is overwritten by data from a next speculative read as shown at block  419 . The process then ends as indicated at block  421 . When multiple reads are issued, the invention operates to schedule access to the data bus by sending signals to each DRAM to release data from respective buffers in a timed sequence that prevents overlap of data on the data bus while maximizing the use of the data bus.  
         [0038]    [0038]FIGS. 5A and 5B provide a set of timing diagrams for current DRAM operation and DRAM operation according to the present invention. During standard operation, a read command is followed several cycles later by the issuing of the data. Following, DRAM cell is pre-charged before it can receive another read command.  
         [0039]    Each timing diagram tracks processor clock cycles  503 . The first three rows track the read request and issuing of corresponding data from the DRAM (A or B). Data is assumed to begin issuing on to the data bus three (3) clock cycles after the read request is sent to the DRAM. The final row illustrates the relative timing of data issuance on the data bus. As represented in timing diagram  501 , read request A issued to DRAM A is followed two cycles later by a read request B issued to DRAM B. Data from read request A is issued on the data out bus followed by a space of a processor clock cycle, and then the data of read request B is issued. Read B+1 request is then not sent to DRAM B until seven cycles after the Read B because it has to wait until DRAM B completes its pre-charge operation of two cycles.  
         [0040]    In timing diagram  513 , in contrast, both of the first two reads are allowed to proceed simultaneously. The data from the read request A and from the read request B are issued at the same time; however data from read request B is held in a buffer for two cycles before it is issued on the data out bus. This allows the data from read request A to complete issuing on the data out bus. Because the read request B was issued earlier than before, i.e., simultaneously with the read request A, the DRAM B completes its pre-charge operation at an earlier clock cycle and is thus able to process read B+1 command four clock cycles earlier. Then, accordingly, the data from each of the three reads can be issued on the data bus with a minimum number of clock cycles separating them to reduce switching noise on the bus. Therefore, a more efficient utilization of the data out bus is achieved.  
         [0041]    The invention allows requests to be sent early to memory thus reducing system latency on the memory access without sacrificing memory data bus bandwidth if the command is canceled. The mechanism allows multiple request to run in parallel, freeing up the DRAM earlier for future requests and thereby allowing the memory controller better control in scheduling data to the common data bus. The overall effect is that the overall system data bus utilization is increased.  
         [0042]    While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.