Abstract:
A memory access system including a memory in which data is organized in pages, each page holding a sequence of data elements; means for receiving a requested address including a requested page address and a requested data element address; logic for accessing a current page from the memory using a current page address; logic for reading out data elements of the current page in the sequence in which they are held in memory; logic for comparing the requested page address with the current page address and for issuing a memory access request with the requested page address when they are not the same; and logic operable when the requested page address is the same as the current page address for comparing a requested data element address with the current address of a data element being read out and returning the data element when the requested data element address matches the current data element address.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/371,192, filed Mar. 8, 2006, which in turn is a continuation of U.S. application Ser. No. 11/186,389, filed Jul. 21, 2005 entitled Memory Access, which application is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method and system for accessing a memory, particularly but not exclusively for directly accessing code for a CPU during booting. 
         [0004]    2. Discussion of the Related Art 
         [0005]      FIG. 1  is a schematic block diagram of a computer system with a direct interface boot arrangement using NOR flash. An integrated circuit  2  has a CPU  4  which is connected via a memory controller  6  to a DRAM  8  for normal operation of the system. In addition, the CPU  4  is connected via a flash memory interface (FMI)  10  to a NOR flash memory  12 . The NOR flash memory  12  holds boot code and operating system code which are used by the CPU for its boot procedure. As a NOR flash memory can be accessed randomly at reasonable speed, the CPU can boot itself directly via the FMI  10  by fetching instructions from the NOR flash memory  12 . After booting, subsequent operations are carried out using the DRAM  8 . 
         [0006]    One of the advantages of providing NOR flash memory to hold boot code is that it is directly interfaceable with the CPU and can be randomly accessed. However, NOR flash memory suffers from disadvantages relative to NAND flash memory in a number of respects. It will be appreciated that the term NOR flash memory relates to a flash memory where the memory cell structure has a NOR structure, such that the memory cells are connected to the bit lines in parallel so that if any memory cell is turned on by the corresponding word line, the bit line goes low. As the logic function is similar to a NOR gate, this cell arrangement is referred to as NOR flash. NAND flash uses a number of transistors in series and the unit cell has a smaller cell area than for NOR flash. Moreover, the erasing and programming times for NAND flash are significantly shorter than for NOR flash. For example, the programming time for NOR flash is typically more than an order of magnitude greater than for NAND flash. Moreover, NAND flash is cheaper and, because of the smaller cell area, has a much higher density. 
         [0007]    The disadvantage of using NAND flash to hold boot code is that it is not randomly accessible. NAND flash memory has no dedicated address lines. It is controlled using an indirect input/output like interface through which commands and addresses are sent via an 8-bit bus to an internal command and address register. The result is that entire pages (or page segments) are read out at once, with bytes in a page only being available in the sequence in which they are stored in the memory. One way of using NAND flash memory for booting is illustrated in  FIG. 2 .  FIG. 2  shows a chip  22  with a CPU  24  and a small embedded ROM  26 . The embedded ROM holds a reset vector and boot loader code. The operating system code is held in a NAND flash memory  28  connected to the chip  22 . After the boot loader code has been executed by the CPU, the operating system code is downloaded from the NAND flash memory  28  into a DRAM  30  which is also connected to the chip  22 . The download is of course on a page-by-page basis, due to the read restriction of the NAND flash memory. Once the operating system code has been downloaded from the NAND flash  28  into the DRAM  30 , it can be executed by the CPU in the normal way. This “duplication” of the operating system code prior to execution is termed “code shadowing”. 
         [0008]    It would be advantageous to be able to make use of the advantages of NAND flash for holding boot code without the need for code shadowing when the boot code is to be executed. 
       SUMMARY OF THE INVENTION  
       [0009]    According to one aspect of the invention there is provided a memory access system comprising: a memory in which data is organized in pages, each page holding a sequence of data elements; means for receiving a requested address comprising a requested page address and a requested data element address; means for accessing a current page from the memory using a current page address; means for reading out data elements of the current page in the sequence in which they are held in memory; means for comparing the requested page address with the current page address and for issuing a memory access request with the requested page address when they are not the same; and means operable when the requested page address is the same as the current page address for comparing a requested data element address with the current address of a data element being read out and returning the data element when the requested data element address matches the current data element address. 
         [0010]    Another aspect provides a method of accessing a memory in which data is organized in pages, each page holding a sequence of data elements, the method comprising: receiving a requested address comprising a requested page address and a requested data element address from a requester; comparing the requested page address with a current page address of a page which is being accessed from the memory and, when the requested page address is not the same as the current page address, issuing a memory access request using the requested page address; reading out data elements of the current page in the sequence in which they are held in memory; and comparing a requested data element address with the current address of a data element being read out and returning to the requester the data element when the requested data element address matches the current data element address. 
         [0011]    Another aspect of the invention provides an integrated circuit comprising: means for receiving a requested address for accessing a memory in which data is organized in pages, each page holding a sequence of data elements, the requested address comprising a requested page address and a requested data element address; means for accessing a current page from the memory using a current page address; means for receiving data elements of a current page in the sequence in which they are held in the memory; means for comparing the requested page address with the current page address and for issuing a memory access request with the requested page address when they are not the same; and means operable when the requested page address is the same as the current page address for comparing a requested data element address with the current address of a data element being read out and returning the data element when the requested data element address matches the current data element address. 
         [0012]    The integrated circuit can form part of a memory access system by being connected to a memory such as a NAND flash memory. 
         [0013]    In the described embodiment, the data elements are bytes, but it will be appreciated that any size data element could be used with the present invention. The invention is particularly useful where the memory is a NAND flash memory of an existing type, which is subject to the read restrictions which have been described above. In that case, the invention allows the read capabilities of the NAND flash memory to be optimised by only performing a random access to the memory when it is necessary. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0014]    For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings, in which: 
           [0015]      FIG. 1  is a schematic block diagram of a direct interface boot system using NOR flash; 
           [0016]      FIG. 2  is a schematic block diagram of a system using NAND flash to store boot code, which utilizes code shadowing; 
           [0017]      FIG. 3  is a schematic block diagram of one embodiment of the invention; 
           [0018]      FIG. 4  is a transistor diagram of a unit cell in a NAND flash memory; 
           [0019]      FIG. 5  is a schematic diagram of registers in a NAND flash memory; 
           [0020]      FIG. 6  is a diagram of page/byte storage in a NAND flash; 
           [0021]      FIG. 7  is a timing diagram illustrating how the NAND flash is read; and 
           [0022]      FIG. 8  is a flow diagram of a state machine for use in the embodiment of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION  
       [0023]    Reference will now be made to  FIG. 3  to illustrate the principles of one embodiment of the invention. A chip  32  has a CPU  34  which communicates with a NAND flash memory  36  via a flash memory interface  38 . The flash memory interface is connected to the NAND flash memory  36  via an 8-bit bus  40 . The flash memory interface  38  also provides control signals to the NAND flash  36  over control bus  42 . 
         [0024]    The flash memory interface  38  receives a 16-bit address from the CPU over a system bus  44 . The system bus  44  also connects a CPU to other external memory, for example an DRAM as illustrated in  FIG. 1 , though these are not shown in  FIG. 3 . 
         [0025]    The flash memory interface  38  comprises a state machine  46  and address registers  39 ,  41 . A discussion of the operation of the flash memory interface  38  is given later. The NAND flash memory  36  holds boot code and compressed operating system code which is directly executable by the CPU  34  via the FMI  38 . 
         [0026]    For the sake of completeness and although it is known in the art,  FIG. 4  illustrates one possible cell layout of a memory cell of a NAND flash, illustrating the bit line BL, word line WL and source line SL. This will not be discussed in more detail because it is known to a person skilled in the art. 
         [0027]      FIG. 5  is a schematic diagram of the internal registers of the NAND flash memory  36 . The registers include a command register  52 , an address register  54  and a data register  56 . The NAND flash memory also includes a memory array  50  which holds data as pages, for example 512 bytes long. An interface  58  receives 8-bit addresses along the I/O bus  40  and the following control signals over control bus  42 :
       Address Latch Enable (ALE)  62     Command Latch Enable (CLE)  64     Ready/Busy (R/B)  66     Read Enable (RE)  68     Write Enable (WE)  70 .         
         [0033]    The I/O bus  40  also serves the function of transferring data output from the NAND flash memory  36  to the FMI  38 . 
         [0034]      FIG. 6  illustrates how the data is stored in the NAND flash memory  36 . That is, there is a plurality of pages, page 0, page 1 etc., each page holding a plurality of bytes, for example 512 bytes. Byte 0  is labelled as the first byte in a page, and Byte N  is illustrated as a requested byte in a page. The bytes are organized in rows, for example aligned on 32 byte boundaries so that each row comprises a group of 32 bytes: ROW N  included Byte N . 
         [0035]    A typical read sequence for the NAND flash memory  36  will now be described. A read command is written to the command register  52 , and a page address ( 8   b ) is written to the address register  54 . Individual rows/bytes within a page cannot be addressed in a conventional NAND flash memory. The device puts a page of data into the data register  56  and this is read out, byte-by-byte the bytes being read out in the sequence in which they are stored in the page, beginning at the first byte, Byte 0 . In the sequential read mode, once a page of data has been read out of the data register  56 , the next page is loaded in and is ready to be read out. An output data pointer  72  associated with the data register  56  keeps track of which byte is currently being read out. 
         [0036]    A page read operation will now be described in more detail with reference to the timing diagram of  FIG. 7 . It will be noted in the following that a single read mode is described where a whole page is accessed for each read address. In fact, normally there will be two or three read modes for accessing different halves/thirds of the page. 
         [0037]    Command Phase 
         [0038]    A command byte  74  is place on the I/O bus  40  with ALE equal to zero and CLE equal to one. The write enable signal WE is brought low then high and this stores the read mode command into the command register  52 . 
         [0039]    Address Phase 
         [0040]    The address byte Addr 0  is placed onto the input/output bus  40  with CLE equal to zero and ALE equal to one. The write enable signal WE is toggled again to load the address into the address register  54 . 
         [0041]    Data Transfer Phase 
         [0042]    With CLE equal to zero and ALE equal to zero, the chip goes busy in preparation for data readout. During the busy period, the ready/busy signal  66  goes low for up to  25  ms while data is being read from the memory array  50  and transferred into the data register  56 . A complete page of data is transferred into the data register  56 . 
         [0043]    Readout Phase 
         [0044]    When the R/B signal  66  goes high again, data is available in the data register for readout. Bytes are read out under the control of the read enable pulses. The first data byte to be output is Byte 0 , and each RE pulse reads out the next byte in the register (which is the next byte in sequence as stored in the page). 
         [0045]      FIG. 8  illustrates a flow chart for the state machine  46  which receives 16-bit random addresses from the CPU  34  and supplies these to the NAND flash  36  in dependence on the protocol to be described to make maximum use of the NAND flash readout facility described above. Each 16-bit address provides an 8-bit page address and an 8-bit byte address. These are referred to herein as the requested page and requested byte. It will be appreciated that in normal operation of the CPU, a group of bytes, for example a row, is normally required. In that case, the 16-bit address supplied by the CPU constitutes the address of the first byte, and the system realizes that it needs to supply the next group of sequential bytes to complete the access which has been requested by the CPU. 
         [0046]    When an address is received from the CPU  34 , it is held in requested address register  39  and the state machine  46  starts at step S 1 . The address of a current access being made to the NAND flash  36  is held in the current address register  41 . The page address in the address which has been received from the CPU  34  is read and is compared with the current page that is being accessed from the NAND flash memory  36  at step S 2 . If the requested page is not the same as the current page, then a decision is made to generate a random read to the NAND flash memory  36  by providing the requested page address over I/O bus  40  (step S 3 ). The page address in the current address register  41  is updated to reflect the new requested page address. The byte address in the current register  41  is updated to read byte zero (or the first byte in the segment of the page which is being addressed when the NAND flash memory is being operated in different read modes). The read operation described above is then completed and data for the addressed page is placed in the data register  56  to be read out byte-by-byte under the control of RE pulses  68 . As each byte is read out, the byte address is updated in the current register  41 . Also, at step S 4 , the current byte address is compared with the requested byte address held in the requested address register  39 . When the current byte address is the same as the requested byte address, this indicates that the group of bytes beginning at that current byte address is the group which has been requested by the CPU. In the present embodiment, this is a row of 32 bytes. Therefore, the RE line  68  is pulsed 32 times to return the next 32 bytes to FMI (step S 5 ). These bytes are Collected and given to the fetch unit of the CPU via the bus  44 . The sequence then ends at step S 6 . 
         [0047]    When the current byte address is not the same as the requested byte address, the state machine continues to read out bytes from the NAND flash  36  and to update the current byte until the determination at step S 4  reaches a positive conclusion. 
         [0048]    Returning to step S 2 , if when the requested page is compared with the current page it is determined that they are the same, then it is necessary to establish whether the requested byte has already been read out of the data register  56  or not (step S 7 ). If the current byte is later in the page than the requested byte, such that the requested byte has already been read out, then it is necessary to return to step S 3  and perform a random read for the same page so that it can begin reading out again at the first byte until it reaches the requested byte. If however the current byte is earlier in sequence than the requested byte, then all that is needed is to carry on reading bytes out of the data register  56  until at step S 4  the requested byte is found to match the current byte. 
         [0049]    While one embodiment of the invention has been described above, it will be clear to a person skilled in the art that there are many variations within the scope of the invention. For example, most existing NAND flash memories operate with different read modes for reading out different segments of the page, because the whole page is too big to be held in the data register  56 . The invention is equally applicable to these situations, the only change being that when a new random read is generated, the read mode is supplied with the page address. 
         [0050]    In addition, the embodiment which has been described has a group (row) of bytes requested by the CPU for each access. It will be clear that the group of bytes requested by the CPU could be any number from one upwards. 
         [0051]    It can be seen that in the above implementation there is no need for a random access memory In the interface which represents a useful advantage, though it will be appreciated that the invention could be implemented with a small cache RAM in the interface if desired. 
         [0052]    Current address register and requested address register have been described as part of the flash memory interface, but it will be apparent that these registers could be located at any suitable place in the system. 
         [0053]    The above-described embodiment of the invention allows the benefits of NAND flash memory to be utilised, particularly during booting. A random access to the NAND flash memory may take about 20 ms, which is much slower than for NOR flash or other equivalent non-volatile memory. However, sequential access of bytes using the read enable signal is about 240 times faster than that, so to the extent that the state machine can establish whether a random access is really needed, a significant time saving can be made. Thus, the invention optimises the use of random access and serial byte access therefore making the best use of the NAND flash memory read capability. 
         [0054]    It will be appreciated that while the figures illustrate a chip  32  having an external boundary communicating with a NAND flash memory  36 , the NAND flash memory could form part of the chip  32  and the principles of the invention could still be applied. Therefore the invention contemplates an integrated circuit having the elements required to communicate with an off-chip memory, or an integrated circuit which incorporates the memory itself. 
         [0055]    Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.