The present invention relates to dynamic random access memory (DRAM), and more specifically, to a method and apparatus for controlling data transfers to and from a dynamic random access memory.
Dynamic random access memory (DRAM) components, such as those illustrated in FIG. 1A, provide an inexpensive solid-state storage technology for today""s computer systems. Digital information is maintained in the form of a charge stored on a two-dimensional array of capacitors. One such capacitor is illustrated in FIG. 1B.
FIG. 2 illustrates a prior art memory system including DRAM with the corresponding control, address and data wires which connect the DRAM to the processor or memory controller component. In synchronous DRAMs, a write access is initiated by transmitting a row address on the address wires and by transmitting row address strobe (RAS) signal. This causes the desired row to be sensed and loaded by the column amplifiers. The column address is transmitted on the address wires and the column address strobe (CAS) signal is transmitted along with the first word of the write data WData(a,1). The data word is then received by the DRAM and written into the column amplifiers at the specified column address. This step can be repeated xe2x80x9cnxe2x80x9d times in the currently loaded row before a new row is sensed and loaded. Before a new row is sensed, the old row must be restored back to the memory core and the bit lines of the DRAM precharged.
FIG. 3A illustrates synchronous write timing. In the figure, a, b . . . represent a row address; 1, 2 . . . n represent a column address, WData[row, col] represents the DRAM address of data words, the row address strobe (RAS) is a control signal for initiating a sense operation, and WRlTE(CAS) initiates the write operation on the column amplifiers. In the present example, the row column address delay timing parameter is equal to two clock cycles. After the row address is asserted at the first clock cycle, column addresses and write data are asserted after the delay to write the data into the DRAM array.
FIG. 3B illustrates synchronous read timing. A processor initiates a read access by transmitting a row address on the address wires and by transmitting the row address strobe (RAS) signal. This causes the desired row to be sensed by the column amplifiers. The column address is then transmitted on the address wire and the column address strobe (CAS) signal is transmitted. The first word of the read data RData (a,1) is then transmitted by the DRAM and received by the processor. This step can be repeated xe2x80x9cnxe2x80x9d times in the currently loaded row before a new row is sensed and loaded. Before a new row is sensed, the old row must be restored back to the memory array.
Various attempts have been made to improve the performance of conventional DRAMs. Such attempts have resulted in DRAM architectures that deviate in varying degrees from conventional DRAM architectures. Various alternative DRAM architectures are described in detail in NEW DRAM TECHNOLOGIES, by Steven A. Przybylski, published by MicroDesign Resources, Sebastopol, Calif. (1994). Some of those architectures are generally described below.
The prior art includes Extended Data-Out (EDO) memory systems. In EDO DRAMs, the output buffer is controlled by signals applied to output enable (OE) and column address stobe (CAS) control lines. In general, data remains valid at the output of an EDO DRAM longer than it does for conventional DRAMs. Because the data remains valid longer, the transfer of the data to the latch in the memory controller can be overlapped with the next column precharge. As a result, burst transfers can be performed in fewer clock cycles.
The prior art also includes Synchronous DRAM (SDRAM) memory systems. The interface of an SDRAM includes a multiplexed address bus and a high-speed clock The high speed clock is used to synchronize the flow of addresses, data, and control on and off the DRAM, and to facilitate pipelining of operations. All address, data and control inputs are latched on the rising edge of the clock. Outputs change after the rising edge of the clock. SDRAMs typically contain a mode register. The mode register may be loaded with values which control certain operational parameters. For example, the mode register may contain a burst length value, a burst type value, and a latency mode value. The burst length value determines the length of the data bursts that the DRAM will perform. The burst type value determines the ordering of the data sent in the bursts. Typical burst orders include sequential and sub-block ordered. The latency mode value determines the number of clock cycles between a column address and the data appearing on the data bus. The appropriate value for this time interval depends largely on the operating frequency of the SDRAM. Since the SDRAM cannot detect the operating frequency, the latency mode value is programmable by a user.
The prior art also includes memory systems in which data transfer operations are performed by DRAMs in response to transfer requests issued to the DRAMs by a controller. Referring to FIG. 4, it illustrates a memory system in which data transfers are made in response to transfer requests. The request packet format is designed for use on a high speed multiplexed bus for communicating between master devices, such as processors, and slave devices, such as memories. The bus carries substantially all address, data, and control information needed by the master devices for communication with the slave devices coupled to the bus. The bus architecture includes the following signal transmission lines: BusCtl, BusData [8:0], BusEnable, as well as clock signal lines and power and ground lines. These lines are connected in parallel to each device.
The processors communicate with the DRAMs to read and write data to the memory. The processors form request packets which are communicated to the DRAMs by transmitting the bits on predetermined transmission lines at a predetermined time sequence (i.e. at predetermined clock cycles). The bus interface of the DRAM receiver processes the information received to determine the type of memory request and the number of bytes of the operation. The DRAMs then perform the memory operation indicated by the request packet.
FIG. 5 illustrates command control information 500 that is sent in a data transfer request according to a prior art protocol. In the illustrated example, the command control information 500 is sent over a BusCtl line and a nine-bit data bus (BusData[8:0]) in six clock cycles. The command control information 500 includes groups of bits 501, 502, 504, 506 and 508 that constitute an address, an operation code consisting of six bits 510, 512, 514, 516, 518 and 520, and groups of bits 522, 524 and 528 that specify a count. The address identified in the command control information 500 specifies the target DRAM and the beginning location within the DRAM of the data on which the operation is to be performed. The count identified in the command control information 500 specifies the amount of information on which the operation is to be performed.
One object of the present invention is to provide a mechanism to decouple control timing from data timing.
Another object of the present invention is to provide mechanisms that use minimal bandwidth to determine data timing while minimizing the latency from signaling that the data transfer should terminate to the transmission of the final data packet.
Another object of the present invention is to provide mechanisms for arbitrarily long data transfers following a command. This may include simultaneous provision of a new column address for each data packet transferred.
Another object of the present invention is to provide a means to signal simultaneously with termination of the data transfer that a precharge operation should be performed.
Another object of the present invention is to provide mechanisms and methods for interleaving control and data information in such a fashion that pin utilization is maximized without placing latency requirements upon the DRAM core that are difficult or expensive to satisfy.
Another object of the present invention is to provide a mechanism for interleaving control and data information that minimizes bandwidth consumed for signaling the beginning and ending of data transfers.
Another object of the present invention is to provide for devices that do not always interpret the information presented at their pins. Each command provides sufficient information that all further control information related to the command can be easily determined even in the presence of control information related to previous command transfers.
Another object of the present invention is to provide a mechanism for optionally sequencing a series of core operations prior to data transmission and, optionally, a final core operation after data transmission is terminated
Another object of the present invention is to provide a DRAM core which allows a single high current RAS operation at any one time in order to minimize the cost and complexity of the DRAM.
Another object of the present invention is to provide an encoding of the command such that decoding space and time is minimized and functionality is maximized.
The present invention provides a method and apparatus for performing data transfers within a computer system. The method includes causing a controller to transmit control information on a bus. The control information specifies a data transfer operation and a beginning location of data to be transferred. The controller determines, after transmitting the control information on the bus, a desired amount of data to be transferred in the data transfer operation. The controller transmits over the bus a terminate indication at a time that is based on the desired amount of data and a beginning time of the data transfer operation. A memory device reads the control information on the bus. The memory device performs the specified data transfer operation on data stored at the beginning location. The memory device continues to perform the specified data transfer operation until detecting the terminate indication on the bus. The memory device ceases to perform the data transfer operation at a time that is based on the time at which the terminate indication is detected.
Other objects, features, and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.