Abstract:
A method and system for performing byte-writes are described, where byte-writes involve writing only particular bytes of a multiple byte write operation. Embodiments include mask data that indicates which bytes are to be written in a byte-write operation. No dedicated mask pin(s) or dedicated mask line(s) are used. In one embodiment, the mask data is transmitted on data lines and store in response to a write_mask command. In one embodiment, the mask data is transmitted as part of the write command.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/359,809, filed Feb. 22, 2006, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/733,280 filed Nov. 2, 2005, entitled Error Detection in High-Speed Asymmetric Interfaces, and further claims the benefit of U.S. Provisional Patent Application Ser. No. 60/735,731 filed Nov. 10, 2005, also entitled Error Detection in High-Speed Asymmetric Interfaces. The entirety of each application being incorporated by reference as if fully set forth herein. 
    
    
     TECHNICAL FIELD 
     The invention is in the field of data transfer in computer and other digital systems. 
     BACKGROUND 
     As computer and other digital systems become more complex and more capable, methods and hardware to enhance the transfer of data between system components or elements continually evolve. Data to be transferred includes digital signals representing data, commands, addresses, or any other information. System components or elements can include different functional hardware blocks on a single integrated circuit (IC), or on different ICs. The different ICs may or may not be on the same printed circuit board (PCB). System components typically include an input/output (I/O) interface specifically designed to receive data from other system components and to transmit data to other system components. 
     One consistent trend as computing systems become more capable is an increase in the amount of data to be transferred per time period. Some applications that require high data rates include game consoles, high definition television (HDTV), personal computers (PCs) main memory, graphics processors, and various consumer devices not already mentioned. In response to the demand for increased data rates, double data rate (DDR) standards have been developed to standardize the behavior of hardware and software using high data rates. Several generations of graphics DDR (GDDR) standards have been developed specifically for graphics processing and video processing, which typically demand the capability to transfer and process very large amounts of data. 
     A conventional DDR dynamic random access memory (DRAM) protocol specifies a 256-bit write command according to which 256 bits are transferred as bytes over a 32-bit interface and written in response to a single write command. In some instances, it is necessary to write only a portion of the 256 bits (some but not all of the bytes). This will be referred to as a byte-write operation herein. One conventional byte-write method is to read the 256 bits that are in the DRAM, modify the portion to be changed, and write all of the 256 bits back. This method is commonly used in central processing units (CPUs), and is referred to as a read-modify-write operation. Unfortunately, for high-speed applications, such as graphics applications, read-modify-write operations are both too slow and too complex to implement. One reason is that graphics processors often use a complex operation reordering schemes in order to optimize the memory interface. So historically DRAMs have instead implemented a data mask which is transmitted on an additional, dedicated data mask signal pin for every 8-bit byte. The mask signal on the data mask pins is a multi-bit signal, and each bit indicates whether a corresponding byte should be written or not. This data mask method is referred to in existing DRAM specifications. One disadvantage of this method is it results in one extra pin for every eight data bit pins, thus increasing the pin count. 
     In addition, the existing data mask method in existing DRAM specifications is not applicable in high speed interfaces which are susceptible to data bit errors. One reason is that the errors on the data mask pins are fatal. This is due to the fact that an error on the data mask pin may result in a byte write that was not intended. The data in that byte would be destroyed (overwritten) and could not be retried. 
       FIG. 1A  is a block diagram of components of a prior art digital system  100 . The components include a processor  102  and a memory component  104 . The processor  102  controls the memory component  104  by communicating over an interface that includes address/command lines, data lines, and separate, dedicated data mask lines. The memory component  100  is a DDR memory that communicates with the processor  102  over a DDR interface. 
       FIG. 1B  is a timing diagram illustrating a write operation  106  with a data mask, as performed by the prior art components of  FIG. 1A . In this illustration, the interface is a 64-bit interface. 64 bits are transmitted in a single operation in eight bursts. The top waveform is the clock (CLK) waveform. The waveform below the CLK waveform shows the commands on the address/command lines. Three write commands are shown, write  108 , write  110  and write  112 . The data waveform is shown below the address/command waveform. In each of the valid data periods labeled 1-8, eight bits of data are transmitted. A data mask waveform is shown below the data waveform. The data mask waveform indicates the values being transmitted on the data mask lines. For write  108 , the data mask line is high during the entire write. In this illustration, a high data mask bit, or a 1 logic level on a data mask line, indicates that the corresponding byte is to be written. So, for write  108 , all of the bytes  108  are to be written. 
     Write  110  differs from write  108 . Write  110  is a byte-write in which only particular bytes of the eight bytes are to be written. Referring to the data mask waveform for write  110 , it can be seen that the data mask has a value of 10110111, and bytes 1, 3, 4, 6, 7, and 8 are to be written. Therefore, data mask pins and lines are required to be available for each write operation, even if byte-write operations make up a small percentage of write operations, as is often the case in many modern applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram of components of a prior art digital system. 
         FIG. 1B  is a timing diagram illustrating a write operation with a data mask, as performed by the prior art components of  FIG. 1A . 
         FIG. 2  is a block diagram of components of a digital system, according to an embodiment. 
         FIG. 3  is a flow diagram of a method for performing byte writes as performed by the components of  FIG. 2 , according to another embodiment. 
         FIG. 4  is a timing diagram illustrating a write operation with a data mask, as in  FIG. 3 , according to an embodiment. 
         FIG. 5  is a block diagram of components of a digital system, according to an embodiment. 
         FIG. 6  is a flow diagram of a method for performing byte writes as performed by the components of  FIG. 5 , according to an embodiment. 
         FIG. 7  is a timing diagram illustrating a write operation with a data mask, as in  FIG. 6 , according to an embodiment. 
     
    
    
     In the drawings, the same reference numbers identify identical or substantially similar elements or acts. To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number refer to the Figure number in which that element is first introduced (e.g., element  102  is first introduced and discussed with respect to  FIG. 1 ). 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of a write data mask method and system are described herein. A data mask indicates which bytes of a multiple byte data transmission over a high-speed interface are to be written in a byte-write operation. In one embodiment, the data mask is stored in a register. In one embodiment, the data mask is transmitted over data lines of the interface for storage. In another embodiment, the data mask is transmitted over address/command lines of the interface as a portion of a write command. The embodiments described eliminate data mask pins and data mask lines. 
       FIG. 2  is a block diagram of components  200  of a digital system, according to an embodiment. The components  200  include a processor  202  and a memory  206 . In an embodiment, the processor  202  includes a memory controller functionality for communicating with the memory  206 . In embodiments that are described herein, the memory  206  is a DDR memory, including a graphics DDR (GDDR) memory, but the embodiments are not so limited. 
     The processor  202  includes a set of commands  204  for the memory controller functionality, including a write_nomask command, a write_mask command, and a write_with_mask command, as further described below. The memory  206  includes a write data mask register  208  for storing a write data mask (also referred to herein as a data mask). In the embodiments described, the interface (which includes the address/command lines and the data lines as shown) is a 256-bit interface that transmits data in bursts of 8 bits over 32 pins, but the embodiments are applicable to any comparable configuration. 
       FIG. 3  is a flow diagram of a method  300  for performing byte writes as performed by the components of  FIG. 2 , according to another embodiment. As described herein, a byte-write is a write operation in which only particular ones of the bytes transferred are to be written. At  302 , it is determined whether the upcoming operation is a byte-write operation. If the operation is not a byte-write operation, a write command is transmitted over the interface from the processor to the memory at  304 , and all bytes are written to the memory at  306 . 
     If the operation is a byte-write operation, a data mask is written to the write data mask register at  308 . The data mask includes one bit for each byte in a write data transaction. In embodiments in which the interface is a 256-bit interface, for example, the data mask is 32 bits. A write_with_mask command is transmitted at  310 . This command indicates that the data mask is to be referred to for determining which bytes to write. The write data mask register is read at  312 , and the indicated bytes are written to the memory at  314 . 
       FIG. 4  is a timing diagram illustrating a write operation with a data mask, as in  FIG. 3 , according to an embodiment. In the illustration of  FIG. 4 , for ease of illustration, the interface is a 64-bit interface rather than a 256-bit interface. The waveform of  FIG. 4  is a clock (CLK) waveform. Below the CLK waveform is an address/command waveform showing particular commands being transmitted. A write_nomask command  402 , a write_mask command  404 , a write_with_mask command  406 , and another write_nomask command  408  are shown. Below the address/command waveform is a data waveform showing the periods of valid data corresponding to the commands on the address/command waveform. With the write_nomask command  402 , data to be written is transmitted on the data lines. With the write_mask command  404 , data mask that has the value 10110111 is transmitted in each burst as shown. In this example, it shows that the same data mask value is transmitted in 8 consecutive bursts, but embodiments are not limited to 8 bursts. The following command is the write_with_mask command  406 , during which the bytes corresponding to the 1s of the data mask are written to the memory. The bytes corresponding to the 0s of the data mask are not written. Following the write_with_mask command  406  is the other write_nomask command  408 . 
       FIG. 5  is a block diagram of components  500  of a digital system, according to another embodiment. The components  500  include a processor  502  and a memory  506 . In an embodiment, the processor  502  includes a memory controller functionality for communicating with the memory  506 . In embodiments that are described herein, the memory  506  is a DDR memory, including a graphics DDR (GDDR) memory, but the invention is not so limited. 
     The processor  502  includes a set of commands  504  for the memory controller functionality, including a write_nomask command, and a write_including_mask command, as further described below. In the embodiments described, the interface (which includes the address/command lines and the data lines as shown) is a 256-bit interface that transmits data in bursts of 8 bits over 32 pins, but the embodiments are applicable to any comparable configuration. 
       FIG. 6  is a flow diagram of a method for performing byte writes as performed by the components of  FIG. 5 , according to an embodiment. At  602  it is determined whether an upcoming write operation is a byte-write operation. If the upcoming operation is not a byte-write operation, a write command is transmitted at  604 , and all bytes are written at  606 . If the upcoming operation is a byte-write operation, a command portion of a write command is transmitted at  608 . A mask portion of the same write command is transmitted at  610 . In various embodiments, the order of  608  and  610  may be reversed. In an embodiment, the single write command is a write_including_mask command that has two parts, a command part and a data mask part. At  612 , the bytes indicated by the data mask part of the write_including_mask command are written to the memory. 
       FIG. 7  is a timing diagram illustrating a write operation  700  with a data mask, as in  FIG. 6 , according to an embodiment. In the illustration of  FIG. 7 , for ease of illustration, the interface is a 64-bit interface rather than a 256-bit interface. The waveform at the top of  FIG. 7  is a clock (CLK) waveform. Below the CLK waveform is an address/command waveform showing particular commands being transmitted. A write_nomask command  702 , a write_cmd_a  704 A, a write_cmd_b  704 B, and another write_nomask command  706  are shown. In an embodiment, the write_cmd_a  704 A and the write_cmd_b  704 B make up one write_including_mask command  704 . Below the address/command waveform is a data waveform showing the periods of valid data corresponding to the commands on the address/command waveform. The write_cmd_a  704 A is the command portion of the write_including_mask command. The write_cmd_b  704 B is another portion of the write_including_mask command that includes the data mask. With the write_cmd_b  704 B, a data mask transmitted on the address command lines, and the bytes corresponding to the 1s of the data mask are written to the memory. The bytes corresponding to the 0s of the data mask are not written. Following the write_cmd_b  704 B is another write_nomask command  706 . 
     High-speed interfaces to which the described embodiments are applicable are usually more susceptible to data bit errors than slower interfaces. Data bit errors may be avoided in several ways. However, conventional methods such as ECC are not practical for DDR interfaces. One reason is that in many DDR applications, the most likely errors are multi-bit errors (affecting more than one bit in a word) that are due to noise or timing issues. ECC is not well adapted to detect and correct such multi-bit errors. 
     Another reason conventional methods such as ECC are not practical for DDR interfaces is that ECC requires extra pins for parity bits. Additional pins for error detection may also be multiplied further when one system component, such as a processor, must interface with many other components, necessitating error detection and correction for each data path. 
     In one embodiment, bit errors in the data mask are minimized or eliminated by sampling the mask bits toward the middle of the transmission period. Referring to  FIG. 4 , for example, it can be seen that the entire transmission of 64 bits is not required to transmit the 8 bits of the mask. Therefore, sampling is done when the transmission lines have stabilized and data bit errors caused by physical phenomena are least likely to occur. This is an improvement over the traditional ECC methods and also does not require dedicated ECC pins. 
     In another embodiment, data bit errors in the data mask are minimized or eliminated by storing a signature for the processor and a signature for the memory. The signatures are compared to determine whether a command was executed accurately. If there was an error in executing a command, the command is retried. To concentrate more complex logic in the processor, rather then in the memory, logic for comparison of the signatures may be present in the memory, but embodiments are not so limited. Therefore, the memory is not required to have additional logic present to support this signature capability. 
     Aspects of the embodiments described above may be implemented as functionality programmed into any of a variety of circuitry, including but not limited to programmable logic devices (PLDs), such as field programmable gate arrays (FPGAs), programmable array logic (PAL) devices, electrically programmable logic and memory devices, and standard cell-based devices, as well as application specific integrated circuits (ASICs) and fully custom integrated circuits. Some other possibilities for implementing aspects of the embodiments include microcontrollers with memory (such as electronically erasable programmable read only memory (EEPROM)), embedded microprocessors, firmware, software, etc. Furthermore, aspects of the embodiments may be embodied in microprocessors having software-based circuit emulation, discrete logic (sequential and combinatorial), custom devices, fuzzy (neural) logic, quantum devices, and hybrids of any of the above device types. Of course the underlying device technologies may be provided in a variety of component types, e.g., metal-oxide semiconductor field-effect transistor (MOSFET) technologies such as complementary metal-oxide semiconductor (CMOS), bipolar technologies such as emitter-coupled logic (ECL), polymer technologies (e.g., silicon-conjugated polymer and metal-conjugated polymer-metal structures), mixed analog and digital, etc. 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in a sense of “including, but not limited to.” Words using the singular or plural number also include the plural or singular number, respectively. Additionally, the words “herein,” “hereunder,” “above,” “below,” and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. When the word “or” is used in reference to a list of two or more items, that word covers all of the following interpretations of the word, any of the items in the list, all of the items in the list, and any combination of the items in the list. 
     The above description of illustrated embodiments of the system and method is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the system and method are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. The teachings of the disclosure provided herein can be applied to other systems, not only for systems including graphics processing or video processing, as described above. The various operations described may be performed in a very wide variety of architectures and distributed differently than described. In addition, though many configurations are described herein, none are intended to be limiting or exclusive. 
     In other embodiments, some or all of the hardware and software capability described herein may exist in a printer, a camera, television, a digital versatile disc (DVD) player, a handheld device, a mobile telephone or some other device. The elements and acts of the various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the system and method in light of the above detailed description. 
     In general, in the following claims, the terms used should not be construed to limit the system and method to the specific embodiments disclosed in the specification and the claims, but should be construed to include any processing systems and methods that operate under the claims. Accordingly, the system and method is not limited by the disclosure, but instead the scope of the method and system is to be determined entirely by the claims. 
     While certain aspects of the method and system are presented below in certain claim forms, the inventors contemplate the various aspects of the method and system in any number of claim forms. For example, while only one aspect of the system and method may be recited as embodied in computer-readable medium, other aspects may likewise be embodied in computer-readable medium. Accordingly, the inventors reserve the right to add additional claims after filing the application to pursue such additional claim forms for other aspects of the system and method.