Abstract:
A memory system including a non-volatile flash memory and a method for simultaneously selecting a plurality of memory blocks are disclosed. The memory system is organized into multiple main blocks each having multiple smaller blocks, emulating a disk drive. Control lines activate a number of modes. In a first mode, high-order address lines select only one block, while in a second mode, user-specified multiple blocks are selected. Blocks are selected by loading registers with selection bits or by using some of the address lines directly as selection bits. Each bit specifies one of the blocks, and each bit is independent of the others. The memory system also includes a predecoder and a controller which controls the predecoder and the registers so as to select at least two blocks of memory cells. In a third mode, all of the blocks are selected, and in a fourth mode, all blocks are deselected. Selecting multiple blocks allows simultaneous erasing, writing, and reading of multiple bytes stored in the memory.

Description:
[0001]    This application is a divisional of U.S. Ser. No. 09/496,759 filed Feb. 3, 2000 which is a continuation of U.S. Ser. No. 08/739,266 filed Oct. 29, 1996, now issued as U.S. Pat. No. 6,047,352 on Apr. 4, 2000. These applications are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention pertains to a memory system having an array of memory cells (e.g., a flash memory system which includes an array of flash memory cells and emulates a magnetic disk drive). More specifically, the invention pertains to a method and system for simultaneously selecting two or more blocks of cells of a memory cell array, so that data can be written to (or read from) the selected blocks simultaneously.  
           [0004]    2. Description of Related Art  
           [0005]    It is conventional to implement a memory system as an integrated circuit which includes an array of flash memory cells (or other non-volatile memory cells) and circuitry for independently erasing selected blocks of the cells, programming selected ones of the cells (i.e., writing data to selected ones of the cells), and reading data from selected ones of the cells. FIG. 1 is a simplified block diagram of a flash memory system (flash memory system  3 ) which is designed to emulate a magnetic disk drive system. Although system  3  can be implemented as a single integrated circuit, it is not necessarily implemented as a single integrated circuit, and the following description of system  3  will not assume that it is an integrated circuit.  
           [0006]    As shown in FIG. 1, system  3  includes memory cell array  16  which comprises rows and columns of flash memory cells (each row of cells connected along a different wordline, and each column of cells connected along a different bitline), predecoding circuit or predecoder  49 , row decoder circuit (X address decoder)  12 , and Y-decoder circuit  13 . Row decoder circuit  12  includes two physically separated sets of wordline drivers: a first set of wordline drivers  12 A (positioned physically nearest to the bitline on the left side of array  16 ), and a second set of wordline drivers  12 B (positioned physically nearest to the bitline on the right side of array  16 ).  
           [0007]    The wordlines of array  16  will be referred to as being numbered consecutively from top to bottom of array  16 , so that the wordlines are: wordline  0  (or “WLO”), wordline  1  (or “WL 1 ”), wordline  2 , . . . , wordline X- 1 , and wordline X (where X is an integer).  
           [0008]    Typically, each memory cell is implemented by a floating-gate N-channel transistor. All the cells in a particular column have their drain regions connected to a common bitline (one of bitlines BL 0  through BLN) and all the cells in a particular row have their control gates connected to a common wordline (one of wordlines WLO through WLX). All of the cells have their sources connected to a common source line SL. Alternatively, it is possible to arrange the cells into array segments having separate source lines that can be sequentially accessed during an erase cycle (e.g., to reduce the maximum erase current).  
           [0009]    The cells of array  16  are typically arranged in column pairs, with the cells of each pair sharing a common source region. The drain region of each cell is connected to the bitline (one of BL 0  through BLN) associated with the column in which the cell is located.  
           [0010]    The wordlines of array  16  are driven by two physically separated sets of wordline drivers: a first set of wordline drivers  12 A (positioned physically nearest to bitline BL 0  on the left side of the array), and a second set of wordline drivers  12 B (positioned physically nearest to bitline BLN on the right side of the array). Each of the control gates of each of the cells connected along the even-numbered wordlines (wordlines WL 0 , WL 2 , etc.) is driven by a driver circuit within set  12 A (i.e., each driver circuit within set  12 A asserts an appropriate control voltage to each such control gate). Each of the control gates of each of the cells connected along the odd-numbered wordlines (wordlines WL 1 , WL 3 , etc.) is driven by a driver circuit within set  12 B.  
           [0011]    The drivers comprising set  12 A are positioned along the left side of array  16  and are connected to the control gates of each of the flash memory cells of array  16  that are connected along the even-numbered wordlines of array  16 , and the drivers comprising set  12 B are positioned along the right side of array  16  and connected to the control gates of each of the cells connected along the odd-numbered wordlines of array  16 . This arrangement of drivers  12 A and  12 B provides most efficient use of the area of system  3 , allowing system  3  to be implemented with a smaller overall size than if all of drivers  12 A and  12 B were positioned on the same side of array  16 .  
           [0012]    In variations on system  3 , array  16  is implemented so that each of two or more integrated circuits contains a different portion of array  16 .  
           [0013]    To enable a conventional flash memory system such as system  3  to implement the present invention, its predecoder circuit would need to be modified to become capable of asserting multiblock selection bits, so that in response to each set of multiblock selection bits, the system is capable of simultaneously selecting two or more selected blocks of cells of array  16  (in a manner to be explained below).  
           [0014]    For convenience throughout this disclosure, we use the following notation to describe address bits. “A(Y:Z)” denotes a set of (Y−(Z−1)) address bits, consisting of binary bits A Y , A y−1 , A Z+1 , and A z . For example, A(8:0) denotes the following nine address bits: A 8 , A 7 , A 6 , A 5 , A 4 , A 3 , A 2 , A 1 , and A 0 .  
           [0015]    With reference again to FIG. 1, memory system  3  also includes control engine (or “controller”)  29 , output buffer  10 , input buffer  11 , and host interface  4 . Host interface  4  asserts data from output buffer  10  (e.g., data read from array  16 ) to an external device (not shown), and asserts input data from the external device to input buffer  11  (so that such input data can be written to array  16 ). Alternatively, where host interface  4  includes input and output data buffers, buffers  10  and  11  can be eliminated and the data buffers within interface  4  used in place of them.  
           [0016]    Host interface  4  also includes an address buffer for receiving external address bits from the external device, and is configured to send buffered address bits (including bits identifying cylinder, head, and sector addresses) to controller  29  in response to receiving external address bits from the external device. Host interface  4  also generates control signals in response to external control signals received from the external device and asserts the control signals to controller  29 .  
           [0017]    Where the external device is a host processor having a standard disk operating system (DOS) with a Personal Computer Memory Card International Association (PCMCIA)-AT Attachment (ATA) interface for communicating with a magnetic disk drive system, interface  4  should also comply with the PCMCIA-ATA standard so that it can communicate with the standard PCMCIA-ATA interface of the external device.  
           [0018]    The column multiplexer (Y multiplexer) circuitry of system  3  comprises above-mentioned Y-decoder circuit  13 , and one subset of Y Multiplexer circuitry for each main block of array  16  (e.g., circuit YMuxA for main block  16 A, circuit YMuxB for main block  16 B, and circuit YMuxJ for main block  16 J).  
           [0019]    In response to receiving the above-mentioned address bits (including bits identifying cylinder, head, and sector addresses) from interface  4 , control engine  29  generates translated address bits A(22:0) and asserts the translated address bits to predecoding circuit (“predecoder”)  49 . In response to the translated address bits (and to control signals from control engine  29 ), predecoder  49  asserts wordline and bitline selection bits to row decoder  12  and Y decoder circuit  13 . In response to the selection bits (and to below-discussed address bit AX and control signals from control engine  29 ), circuits  12  and  13  select cells of array  16  to which data is to be written or from which data is to be read.  
           [0020]    For example, where address bits A18, A17, and A16 determine the erase block of the target cells (and where array  16  includes eight erase blocks per main block), predecoder generates an 8-bit set of selection bits XC(7:0) (sometimes referred to as “erase block enable” bits) as follows, in response to each set of address bits A(18:16):  
                                                           A18   A17   A16   XC(7:0)                           0   0   0   00000001           0   0   1   00000010           0   1   0   00000100           0   1   1   00001000           1   0   0   00010000           1   0   1   00100000           1   1   0   01000000           1   1   1   10000000                      
 
           [0021]    The single bit having value “one” in each set of selection bits XC(7:0) selects a different erase block (within a single selected main block). Bits XC(7:0) consist of XCO which selects the first erase block, XC1 which selects the second erase block, XC2 which selects the third erase block, XC3 which selects the fourth erase block, XC4 which selects the fifth erase block, XC5 which selects the sixth erase block, XC6 which selects the seventh erase block, and XC7 which selects the eighth erase block.  
           [0022]    Each of the cells (storage locations) of memory array circuit  16  is indexed by a row index (an “X” index determined by decoder circuit  12 ) and a column index (a “Y” index determined by Y decoder circuit  13 ). Each column of cells of array  16  comprises “X” memory cells (where X is an integer), with each cell implemented by a single floating-gate N-channel transistor.  
           [0023]    In one embodiment in which array  16  includes ten main blocks ( 16 A through  16 J), each main block has 1024 rows of cells, each row has 4352 cells (and thus there are 4352 columns of cells), and array  16  includes a total of 4352×10,240 cells. Each column of cells is connected along a single bitline, each column comprises 10,240 cells, and circuit  33  includes a set of eight sense amplifiers provided for reading eight cells in parallel (each cell connected along a different bitline). Each bitline extends through all ten main blocks.  
           [0024]    In variations on the embodiment described in the previous paragraph, each column of cells consists of several groups of cells (with the cells in each group being connected along a different bitline) and each bitline is entirely within a main block (no bitline extends through more than one main block). In one such variation, for example, array  16  comprises 10,240 wordlines and 10×4352=43,520 bitlines (with 1024 cells connected along each bitline, 1024 rows per main block, and 4352 cells per row). Circuit  33  can include a separate set of sense amplifiers for reading each main block of cells (e.g., eighty sense amplifiers are provided within circuit  33 , of which eight sense amplifiers are used to read eight cells of each main block in parallel, each of these cells being connected along a different bitline). Alternatively, circuit  33  could include one set of sense amplifiers (e.g., eight sense amplifiers for reading eight cells in parallel, each of these cells being connected along a different bitline), and multiplexing circuitry for coupling this set of sense amplifiers to bitlines in any selected one of the main blocks.  
           [0025]    The drains of all transistors of a column are connected to a bitline, the control gate of each of the transistors is connected to a different wordline, and the sources of the transistors are held at a source potential (which is usually ground potential for the system during a read or programming operation). Each memory cell is a nonvolatile memory cell since the transistor of each cell has a floating gate capable of semipermanent charge storage. The current drawn by each cell (i.e., by each of the N-channel transistors) depends on the amount of charge stored on the cell&#39;s floating gate. Thus, the charge stored on each floating gate determines a data value that is stored “semipernanently” in the corresponding cell. Where each of the N-channel transistors is a flash memory device, the charge stored on the floating gate of each is erasable (and thus the data value stored by each cell is erasable) by appropriately changing the voltage applied to the gate and source (in a well known manner). In memory systems comprising an array of non-volatile memory cells other than flash memory cells, such nonvolatile cells are erased using other techniques which are well known.  
           [0026]    As noted, system  3  emulates a conventional magnetic disk drive system. Accordingly, the cells of array  16  are addressed in a manner emulating the manner in which conventional magnetic disk storage locations are addressed. System  3  can be mounted on a card for insertion into a computer system. Alternatively, variations on system  3  (which lack array  16  and instead include a flash memory interface for interfacing with one or more separate memory array circuits) can be implemented as part of a card (for insertion into a computer system), where the card has a chip set mounted thereon, and the chip set includes a controller chip and several memory chips controlled by the controller chip. Each memory chip implements an array of flash memory cells.  
           [0027]    The dominant computer operating system known as “DOS” (Disk Operating System) is essentially a software package used to manage a disk system. DOS has been developed by IBM Corporation, Microsoft Corporation, and Novell as the heart of widely used computer software. The first generation of the “Windows”® (trademark of Microsoft Corp.) operating system software was essentially a continuation of the original DOS software with a user friendly shell added for ease of use.  
           [0028]    The DOS software was developed to support the physical characteristics of hard drive structures, supporting file structures based on heads, cylinders and sectors. The DOS software stores and retrieves data based on these physical attributes. Magnetic hard disk drives operate by storing polarities on magnetic material. This material is able to be rewritten quickly and as often as desired. These characteristics have allowed DOS to develop a file structure that stores files at a given location which is updated by a rewrite of that location as information is changed. Essentially all locations in DOS are viewed as fixed and do not change over the life of the disk drive being used therewith, and are easily updated by rewrites of the smallest supported block of this structure. A sector (of a magnetic disk drive) is the smallest unit of storage that the DOS operating system will support. In particular, a sector has come to mean 512 bytes of information for DOS and most other operating systems in existence. DOS also uses clusters as a storage unit. Clusters, however, are nothing more than the logical grouping of sectors to form a more efficient way of storing files and tracking them with less overhead.  
           [0029]    The development of flash memory integrated circuits has enabled a new technology to offer competition to magnetic hard drives and offer advantages and capabilities that are hard to support by disk drive characteristics and features. The low power, high ruggedness, and small sizes offered by a solid state flash memory system make such a flash memory system attractive and able to compete with a magnetic hard disk drive system. Although a memory implemented with flash memory technology may be more costly than a hard disk drive system, computers and other processing systems are being developed that require (or benefit greatly from) use of flash memory features.  
           [0030]    Thus, flash memory systems have been developed that emulate the storage characteristics of hard disk drives. Such a flash memory system is preferably structured to support storage in 512 byte blocks along with additional storage for overhead bits associated with mass storage, such as ECC (error correction code) bits. A key to this development is to make the flash memory array respond to a host processor in a manner that looks like a disk so the operating system can store and retrieve data in a known manner and be easily integrated into a computer system including the host processor.  
           [0031]    In some flash memory systems that emulate the storage characteristics of hard disk drives, the interface to the flash memory is identical to a conventional interface to a conventional magnetic hard disk drive. This approach has been adopted by the PCMCIA standardization committee, which has promulgated a standard for supporting flash memory systems with a hard disk drive protocol. A flash memory card (including one or more flash memory array chips) whose interface meets this standard can be plugged into a host system having a standard DOS operating system with a PCMCIA-ATA (or standard ATA) interface. Such a flash memory card is designed to match the latter standard interface, but must include an onboard controller which manages each flash memory array independent of the host system.  
           [0032]    Since system  3  of FIG. 1 emulates a magnetic disk drive, above-mentioned address bits A(22:0) determine cylinder, sector, and packet addresses of the type conventionally used in magnetic disk drive systems. In a preferred implementation, array  16  of FIG. 1 has 544 bytes per row of flash memory cells each byte consisting of eight bits, and each memory cell is capable of storing one bit). Each row of cells is equivalent to a magnetic disk “sector” (512 bytes of data plus 32 bytes of “overhead”).  
           [0033]    In such an implementation, array  16  is partitioned into ten large “decode” blocks (sometimes referred to as “main” blocks) of cells (schematically indicated in FIG. 1). The decode blocks are physically isolated from one another. This partitioning of blocks allows defects in one decode block to be isolated from the other decode blocks in the array, allows defective decode blocks to be bypassed by a controller, and allows for high usage of die and enhances overall yield of silicon produced (driving down the cost of flash mass storage systems).  
           [0034]    Array  16  of FIG. 1 includes ten decode blocks (blocks  16 A,  16 B,  16 C,  16 D,  16 E,  16 F,  16 G,  16 H,  16 I, and  16 J, which are also referred to herein as “main blocks,” and of which only blocks  16 A,  16 B, and  16 J are shown in FIG. 1). Y-select gate circuitry is provided for each decode block of array  16 . Specifically, Y-select gate circuitry YMuxA is provided for selecting columns of decode block  16 A in response to indices received from circuit  13 , Y-select gate circuitry YMuxB is provided for selecting columns of decode block  16 B in response to indices received from circuit  13 , Y-select gate circuitry YMuxJ is provided for selecting columns of decode block  16 J in response to indices received from circuit  13 , and seven other subsets of Y-select gate circuitry (not separately shown) are provided for selecting columns of the other decode blocks (blocks  16 C,  16 D,  16 E,  16 F,  16 G,  16 H, and  16 I) in response to indices received from circuit  13 .  
           [0035]    Each decode block is subdivided into a number (e.g., eight) of independently erasable blocks, sometimes referred to herein as “erase blocks.” In a preferred implementation of the FIG. 1 system, each erase block consists of rows of flash memory cells, each row being capable of storing seventeen “packets” of binary bits, each packet consisting of 32 bytes (each byte consisting of eight binary bits). Thus, each row (capable of storing 544 bytes) corresponds to one conventional disk sector (comprising 544 bytes), and each row can store 512 bytes of data of interest as well as 32 ECC bytes for use in error detection and correction (or 32 “overhead” bytes of some type other than ECC bytes, or a combination of ECC bytes and other overhead bytes).  
           [0036]    Each erase block is divided into two blocks of cells known as “cylinders” of cells (in the sense that this expression is used in a conventional magnetic disk drive), with each cylinder consisting of 256K bits of data organized into 64 sectors (i.e. 64 rows of cells). Thus, each erase block in the preferred implementation of the FIG. 1 system consists of 128 sectors (i.e., 128 rows of cells).  
           [0037]    Each erase block can be independently erased in response to control signals supplied from controller  29  to circuits  12  and  13 . All flash memory cells in each erase block are erased at the same (or substantially the same) time, so that erasure of an erase block amounts to erasure of a large portion of array  16  at a single time.  
           [0038]    The individual cells of array  16  of FIG. 1 are addressed by address bits A(22:0) and AX, with the four highest order address bits (A22, A21, A20, and A19) determining the main block, the three next highest order address bits (A18, A17, and A16) determining the erase block, the next address bit (A15) determining the cylinder, the next six address bits (A(14:9)) determining the sector, the next four address bits (A(8:5)) and bit AX determining the packet (within the sector), and the five lowest order address bits (A(4:0)) determining the byte within the packet. Address bits A(22:9) are used by predecoder  49  to generate selection bits which are processed by circuit  12  to select the row (sector) of array  16  in which the target byte is located, and the remaining nine address bits A(8:0) and bit AX are used by predecoder  49  to generate selection bits which are processed by Y decoder circuit  13  to select the appropriate columns of array  16  in which the target byte is located. In the preferred implementation, address bit AX is asserted (by controller  29 ) to predecoder  49  and is used by circuit  49  for selecting a packet consisting of overhead bits (such as ECC check bits and redundancy bits). More specifically, seventeen packets are stored per sector, including sixteen packets of ordinary data (any one of which can be selected by address bits A(8:5)) and one packet of overhead bits (which can be selected by address bit AX).  
           [0039]    System  3  executes a write operation as follows. Control engine  29  asserts appropriate ones of address bits A(22:0) and AX to predecoder  49 , and the selection bits output by predecoder  49  are asserted to decoder circuits  12  and  13 . Control engine  29  also asserts appropriate control signals to other components of the system, including buffer  11  and circuits  12  and  13 . In response to the selection bits, circuit  12  selects one sector (row) of cells and circuit  13  selects eight of the columns of memory cells of array  16 . Address bits A(22:0) and AX thus together select a total of eight target cells in one selected row (for storing one byte of data). In response to a write command (a control signal) supplied from controller  29 , a signal (indicative of an eight-bit byte of data) present at the output of input buffer  11  is asserted through the relevant Y multiplexer circuitry (e.g., through circuit YMuxJ, where the data is to be written to target cells in block  16 J) to the eight target cells of array  16  determined by the row and column address (e.g., to the drain of each such cell). Depending on the value of each of the eight data bits, the corresponding target cell is either programmed or it remains in an erased state.  
           [0040]    System  3  executes a read operation as follows. Control engine  29  asserts address bits A(22:0) and AX to predecoder  49 , and the selection bits output by predecoder  49  are asserted to circuits  12  and  13 . Control engine  29  also asserts appropriate control signals to other components of the system, including circuits  12  and  13 . In response to the selection bits, circuit  12  selects one row (sector) of cells, and circuit  13  selects eight of the columns of memory cells of array  16 . Address bits A(22:0) and AX thus together determine a total of eight target cells in one selected row (for reading one byte of data). In response to a read command (a control signal) supplied from control unit  29 , a current signal (a “data signal”) indicative of a data value stored in one of the eight target cells of array  16  is supplied from the drain of each of the target cells through the bitline of the target cell and then through the relevant Y multiplexer circuitry (e.g., through circuit YMuxJ, where the data is stored in cells within block  16 J) to sense amplifier circuitry  33 . Each data signal is processed in sense amplifier circuitry  33 , buffered in output buffer  10 , and finally asserted through host interface  4  to an external device.  
           [0041]    Circuits  12 ,  13 ,  33 , and the described Y multiplexer circuitry (including the YMuxA, YMuxB, and YMuxJ circuitry) are sometimes referred to herein collectively as “array interface circuitry.” 
           [0042]    System  3  also includes a pad (not shown) which receives a high voltage V pp  from an external device, and a switch connected to this pad. During some steps of a typical erase or program sequence (in which cells of array  16  are erased or programmed), control unit  29  sends a control signal to the switch to cause the switch to close and thereby assert the high voltage V pp  to various components of the system including wordline drivers within X decoder  12  (or the source line within array circuit  16 .  
           [0043]    When reading a selected cell of array  16 , if the cell is in an erased state, the cell will conduct a first current which is converted to a first voltage in sense amplifier circuitry  33 . If the cell is in a programmed state, it will conduct a second current which is converted to a second voltage in sense amplifier circuitry  33 . Sense amplifier circuitry  33  determines the state of the cell (i.e., whether it is programmed or erased corresponding to a binary value of 0 or 1, respectively) by comparing the voltage indicative of the cell state to a reference voltage. The outcome of this comparison is an output which is either high or low (corresponding to a digital value of one or zero) which sense amplifier circuitry  33  sends to output buffer  10 .  
           [0044]    It is important during a write operation to provide the wordline of each selected cell with the proper voltage and the drain of each selected cell with the appropriate voltage level (the voltage determined by the output of input buffer  11 ), in order to successfully write data to the cell without damaging the cell.  
           [0045]    Controller  29  of system  3  controls detailed operations of system  3  such as the various individual steps necessary for carrying out programming, reading, and erasing operations. Controller  29  thus functions to reduce the overhead required of the external processor (not depicted) typically used in association with system  3 .  
           [0046]    It would be desirable to improve existing memory system technology to allow simultaneous selection of two or more blocks of cells (e.g., erase blocks or main blocks) of a memory cell array, in an efficient and controllable manner. This would allow manipulation of data in several blocks simultaneously (i.e., writing of data to, reading of data from, or erasing of several blocks simultaneously). This capability would be particularly useful during test mode operation of a memory system (e.g., a flash memory system) in order to reduce the time required to execute typical tests of memory cells of the system.  
         SUMMARY OF THE INVENTION  
         [0047]    The memory system of the invention includes an array of memory cells (which are flash memory cells or other non-volatile memory cells in preferred embodiments), and a predecoding circuit operable in a mode in which it asserts multiblock selection bits (for selecting two or more blocks of the cells simultaneously) in response to control signals. Preferably, the predecoding circuit is operable in a selected one of a first mode in which it asserts single block selection bits in response to address bits (where each set of address bits determines one or more cells in a single block of the array) and a second mode in which it asserts multiblock selection bits in response to control signals. Preferably, the system includes registers in which at least some of the multiblock selection bits are stored, the predecoding circuit receives the stored multiblock selection bits from selected ones of the registers and asserts the received multiblock selection bits in response to specific control signals, and the system can replace the stored multiblock selection bits by loading replacement bits into each register at desired times.  
           [0048]    In a write mode of a preferred embodiment of the system, each set of address bits is associated with a data byte to be written to cells in a single row of one block, each set of multiblock selection bits is associated with cells in a row of each of two or more blocks, and the system writes the same data byte to multiple sets of cells (each set of cells in a different block) in response to each set of multiblock selection bits. In a read mode of the preferred embodiment, each set of address bits identifies cells in a single row of one block from which a data byte is to be read, each set of multiblock selection bits identifies cells in a single row of each of two or more blocks from which a data byte is to be read, and the system reads data from multiple sets of cells (each set of cells in a different block) in response to each set of multiblock selection bits.  
           [0049]    Preferably, the predecoding circuit asserts a selected one of several different sets of multiblock selection bits in response to each of several different sets of control signals. For example, where the memory array is organized into main blocks of cells, each main block consisting of erase blocks, and each erase block consisting of rows of cells, the predecoder is preferably controllable to assert one of: a set of multiblock selection bits which selects all erase blocks in a single main block, a second set of multiblock selection bits which selects the same erase block (or the same combination of two or more erase blocks) in all main blocks (or in any selected combination of two or more main blocks), a third set of multiblock selection bits which selects all the erase blocks in all the main blocks, and a fourth set of multiblock selection bits which selects any combination of erase blocks in one main block.  
           [0050]    Preferably, the memory cells of the inventive system are flash memory cells. Other embodiments of the invention are methods implemented by any of the embodiments of the inventive system during operation.  
           [0051]    The invention allows tests to be performed on memory cells more rapidly (by erasing multiple blocks of cells simultaneously) than such tests could be performed if blocks of the cells could only be erased sequentially. The step of erasing each block of cells is very time-consuming, and thus it is useful to select multiple blocks of cells in accordance with the invention and to simultaneously erase the selected blocks. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0052]    [0052]FIG. 1 is block diagram of a conventional nonvolatile (flash) memory system (implemented as an integrated circuit).  
         [0053]    [0053]FIG. 2 is block diagram of a preferred embodiment of the inventive flash memory system (implemented as an integrated circuit).  
         [0054]    [0054]FIG. 3 is a block diagram of a portion of a preferred embodiment of predecoder  50  of FIG. 2.  
         [0055]    [0055]FIG. 4 is a block diagram of a second portion of the preferred embodiment of predecoder  50  of FIG. 2.  
         [0056]    [0056]FIG. 5 is a block diagram of the decoder portion (XCDEC circuit  52 ) of the FIG. 3 circuit.  
         [0057]    [0057]FIG. 6 is a schematic diagram of gate circuit  53  (CGATE2) of the FIG. 5 circuit.  
         [0058]    [0058]FIG. 7 is a schematic diagram of gate circuit  55  (CGATE3) of the FIG. 4 circuit.  
         [0059]    [0059]FIG. 8 is a schematic diagram of multiplexer circuit  54  (AMUX) used in both the FIG. 3 circuit and the FIG. 4 circuit.  
         [0060]    [0060]FIG. 9 is a block diagram of another preferred embodiment of the inventive flash memory system.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0061]    Throughout the disclosure, including in the claims, the term “block” (of memory cells) is used to denote a P-row subset of an N row×M column array of memory cells, where M, N, and P are integers, P is less than N, and each “row” and “column” is a one-dimensional (linear) array of cells. Thus, the term “block” assumes an N×M array consisting of cells organized in rows and columns, with a “block” of the cells being a P×M subset of the array. Typically, the cells will be connected along bitlines and wordlines, with each row of cells connected along a single wordline and each column of cells connected along a single bitline. However, the term “row” is not intended to be limited to denote an array of cells connected along a wordline, and the term “column” is not intended to be limited to denote an array of cells connected along a bitline.  
         [0062]    Throughout the disclosure, including in the claims, the term “bits” (as in “multiblock selection bits”) is used to denote signals indicative of bits of information (e.g., signals indicative of ones and zeros). For example, “multiblock selection bits” denotes signals indicative of a set of binary bits (or other bits of information), where the set of binary bits (or other bits of information) determines two or more selected blocks of memory cells.  
         [0063]    A preferred embodiment of the system of the invention will be described with reference to FIGS.  2 - 8 . One such embodiment is flash memory system  30  shown in FIG. 2 which includes array  16  of flash memory cells. Memory system  30  of FIG. 2 is identical to system  3  of FIG. 1, except in three respects: system  30  includes controller or control engine  129  (rather than controller  29  of FIG. 1); system  30  includes predecoder circuit  50  (rather than conventional predecoder  49  of FIG. 1); and system  30  includes registers  40  and  41 . Controller  129  can be designed and programmed identically to controller  29  of FIG. 1, except that in accordance with the invention it has the additional capability to load registers  40  and  41  and to control predecoder  50  (in a manner to be explained below) to assert multiblock selection bits. In a preferred implementation, register  40  has capacity to store eight bits of data (bits E(7:0)) and can output these eight bits in parallel, and register  41  has capacity to store ten bits of data (bits M(9:0)) and can output these ten bits in parallel.  
         [0064]    Those elements of system  30  of FIG. 2 that are identical to corresponding elements of system  3  of FIG. 1 are identically numbered in FIGS. 1 and 2, and the foregoing description of them (with reference to FIG. 1) will not be repeated below. Although system  30  can be implemented as a single integrated circuit, it is not necessarily implemented as a single integrated circuit, and the following description of system  30  will not assume that it is an integrated circuit.  
         [0065]    In a preferred implementation, array  16  of system  30  has capacity to store forty megabits of ordinary data (plus two and a half Megabits of overhead data), and includes ten main blocks ( 16 A through  16 J) as indicated in FIG. 2). Main blocks  16 A through  16 J are preferably organized in the same manner as are the above-described preferred implementations of blocks  16 A through  16 J of FIG. 1 (with a set of bitlines for each main block, of which none of the bitlines extend through more than one main block). In the preferred implementation, memory system  30  of FIG. 2 is designed to emulate a magnetic disk drive system (as is system  3  of FIG. 1), with each row of cells of array  16  corresponding to a sector of a magnetic disk drive system.  
         [0066]    In an alternative implementation, array  16  of system  30  has capacity to store thirty-two Megabits of ordinary data (plus two Megabits of overhead data), and comprises only eight main blocks of the type described above with reference to the alternative implementation of system  3  of FIG. 1.  
         [0067]    In the preferred implementation of system  30  of FIG. 2, array  16  has 544 bytes per row of flash memory cells. Each byte consists of eight bits, each memory cell is capable of storing one bit, each row of cells is equivalent to a magnetic disk “sector” (512 bytes of data plus 32 bytes of “overhead”), and the array is partitioned into ten main blocks of cells ( 16 A through  16 J). The main blocks are decode blocks (of the type mentioned above) and are physically isolated from one another. Each main block consists of 1024 rows of cells. Each row consists of 4352 cells connected along a common wordline. Each of the cells in a row is connected along a different bitline. Each row is capable of storing seventeen “packets” of bits, each packet consisting of 32 eight-bit bytes. Thus, each row (capable of storing 544 bytes) corresponds to one conventional magnetic disk sector (comprising 544 bytes). Each row can store 512 bytes of data of interest as well as 32 ECC bytes for use in error detection and correction (or 32 “overhead” bytes of some type other than ECC bytes, or a combination of ECC bytes and other overhead bytes).  
         [0068]    Each main block is subdivided into eight independently erasable erase blocks. Each erase block consists of 128 of the described rows of flash memory cells, and thus has capacity to store 128×4352 bits. Each erase block is divided into two blocks of cells known as “cylinders” of cells, each cylinder having capacity to store 278,528 bits of data organized into 64 sectors (i.e. 64 rows).  
         [0069]    The individual cells of the preferred implementation of array  16  (of FIG. 2) are addressed by address bits A(22:0) and AX, in the same manner as are the cells of the above-described preferred implementation of array  16  of FIG. 1. For example, in a write mode of a preferred embodiment of the FIG. 2 system (with predecoder  50  operating in a first mode in which it performs the same functions as predecoder  49  of FIG. 1), each set of address bits A(22:0) and AX is associated with a data byte to be written to cells in a single row of one erase block of one main block. In response to bits A(22:0) and AX, predecoder  50  (in its first mode of operation) asserts wordline and bitline selection bits to row decoder  12  and Y decoder circuit  13  (and circuits  12  and  13  then select the cells to which the data byte is to be written, in response to the selection bits).  
         [0070]    An important advantage of the FIG. 2 system over the FIG. 1 system is that predecoder  50  is also operable in a second mode (rather than the first mode mentioned in the previous paragraph) in which predecoder  50  asserts multiblock selection bits to circuit  12 . To enable the system to write a data byte simultaneously to two or more blocks (with predecoder  50  operating in its “second” mode), predecoder  50  asserts multiblock selection bits to row decoder circuit  12  and Y decoder circuit  13 , and in response to the multiblock selection bits, circuits  12  and  13  select cells (in each of two or more blocks) to which the data byte is to be written. The system then writes the same data byte to multiple sets of selected cells (each set of selected cells in a different block).  
         [0071]    The preferred embodiment of predecoder  50 , to be described with reference to FIGS.  3 - 8 , is designed to operate with the preferred implementation of array  16  which comprises ten main blocks of cells (each determined by address bits A(22:19)) and eight erase blocks within each main block (each erase block determined by address bits A(18:16)).  
         [0072]    In this preferred embodiment, predecoder  50  includes erase block predecoder circuit  50 A (shown in FIG. 3) and main block predecoder circuit  50 B (shown in FIG. 4). Predecoder  50 A operates in response to control signals C1 and C2 from controller  129  and address bits A (18:16) to assert in parallel at its output a set of eight selection bits XC (7:0), and is coupled to register  40  so that it can read an eight-bit set E (7:0) stored in register  40 . Predecoder  50 B operates in response to control signals C3, C4, and C5 from controller  129  and address bits A (22:19) to assert in parallel at its output a set of ten selection bits BS (9:0), and is coupled to register  41  so that it can read the ten-bit set M (9:0) stored in register 41.  
         [0073]    Each of control signals C1 and C2 is a bit which controls operation of circuit  50 A according to the following truth table (in which “x” denotes “don&#39;t care”):  
                                           TABLE A                                   C1   A18   A17   A16   C2   XC(7:0)                           0   0   0   0   0   00000001           0   0   0   1   0   00000010           0   0   1   0   0   00000100           0   0   1   1   0   00001000           0   1   0   0   0   00010000           0   1   0   1   0   00100000           0   1   1   0   0   01000000           0   1   1   1   0   10000000           1   x   x   x   x   E(7:0)           0   x   x   x   1   11111111                      
 
         [0074]    All the erase blocks (in each selected main block) can be taken low via the wordlines (i.e., all wordlines can be deselected, which effectively deselects all the erase blocks in each selected main block). If a main block is deselected, all the erase blocks in that main block are automatically deselected.  
         [0075]    When predecoder  50 A operates in a first mode (in response to each of control signals C1 and C2 having the value “0”), each set of bits XC(7:0) output therefrom is a set of single erase block selection bits (which selects only one erase block in each selected main block). In each such set of single erase block selection bits, the single bit having value “one” selects a different erase block (a single erase block within each selected main block) determined by the current values of address bits A(18:16).  
         [0076]    When predecoder  50 A operates in a second mode (in response to control signal C1 having the value “1”, regardless of the value of C2), each set of bits XC(7:0) output from predecoder  50 A is a set of block selection bits E(7:0) which has been retrieved from register  40  by predecoder  50 A. If two or more bits of a set of bits E(7:0) have the value “1,” then that set is a set of multiblock selection bits (in response to which the system selects two or more erase blocks in each selected main block). An example of such a set of multiblock selection bits is the following: E7=1, E6=1, E5=0, E4=0, E3=0, E2=0, E1=0, and E0=0. Control engine  129  preferably is capable of loading register  40  with bits E(7:0) having any possible combination of values.  
         [0077]    When predecoder  50 A operates in a third mode (in response to control signal C1 having the value “0” and control signal C2 having the value “1”), each set of bits XC(7:0) output from predecoder  50 A is a set of multiblock selection bits XC7=1, XC6=1, XC5=1, XC4=1, XC3=1, XC2=1, XC1=1, and XC0=1. In response to this set, the system selects all eight erase blocks in each selected main block).  
         [0078]    Predecoder  50 B operates in response to control signals C3, C4 and C5 from controller  129 , and in response to address bits A(22:19), to assert in parallel at its output a set of ten selection bits BS(9:0), and is coupled to register  41  so that it can read a ten-bit set M(9:0) stored in register  41 .  
         [0079]    Each of control signals C3, C4, and C5 is a bit which controls operation of circuit  50 B according to the following truth table (in which “x” denotes “don&#39;t care”):  
                                           TABLE B                       C3   A22   A21   A20   A19   C4   C5   BS(9:0)                   0   0   0   0   0   0   1   0000000001       0   0   0   0   1   0   1   0000000010       0   0   0   1   0   0   1   0000000100       0   0   0   1   1   0   1   0000001000       0   0   1   0   0   0   1   0000010000       0   0   1   0   1   0   1   0000100000       0   0   1   1   0   0   1   0001000000       0   0   1   1   1   0   1   0010000000       0   1   0   0   0   0   1   0100000000       0   1   0   0   1   0   1   1000000000       1   x   x   x   x   x   x   M(9:0)       0   x   x   x   x   1   x   1111111111       0   x   x   x   x   0   0   0000000000                  
 
         [0080]    When predecoder  50 B operates in a first mode (in response to each of control signals C3 and C4 having the value “0” and control signal C5 having the value “1”), each set of bits BS(9:0) output from predecoder  50 B is a set of single block selection bits (which selects only one main block). In each such set of single block selection bits, the single bit having value “one” selects a different main block determined by the current values of address bits A(22:19).  
         [0081]    When predecoder  50 B operates in a second mode (in response to control signal C3 having the value “1”, regardless of the values of C4 and C5), each set of bits BS(9:0) output from predecoder  50 B is a set of block selection bits M(9:0) which has been retrieved from register  41  by predecoder  50 B. If two or more bits of a set of bits M(9:0) have the value “1,” then that set is a set of multiblock selection bits (in response to which the system selects two or more main blocks of cell array  16 ). An example of such a set of multiblock selection bits is the following: M9=1, M8=1, M7=0, M6=0, M5=0, M4=0, M3=0, M2=0, M1=0, and M0=0. Control engine  129  preferably is capable of loading register  41  with bits M(9:0) having any possible combination of values.  
         [0082]    When predecoder  50 B operates in a third mode (in response to control signal C3 having the value “0” and control signal C4 having the value “1”), each set of bits BS(9:0) output from predecoder  50 B is a set of multiblock selection bits BS9=1, BS8=1, BS7=1, BS6=1, BS5=1, BS4=1, BS3=1, BS2=1, BS1=1, and BS0=1. In response to this set, the system selects all ten main blocks.  
         [0083]    When predecoder  50 B operates in a fourth mode (in response to all three of the control signals C3, C4, and C5 having the value “0”), each set of bits BS(9:0) output from predecoder  50 B is a set of deselection bits BS9=0, BS8=0, BS7=0, BS6=0, BS5=0, BS4=, BS3=0, BS2=0, BS1=0, and BS0=0. In response to this set, the system does not select any of the main blocks (and thus, no data can be written to or read from array  16 ).  
         [0084]    In the described embodiment, array  16  has eighty erase blocks, but registers  40  and  41  store only eighteen bits. These eighteen bits can be routed through predecoder  50  for use in simultaneously selecting any desired combination of erase blocks (i.e., any selected set of two or more of the erase blocks). More generally, preferred embodiments of the invention include an array comprising N erase blocks, and M registers coupled to a predecoder wherein the registers store a total of no more than X multiblock selection bits, where N, M, and x are integers, and X is less than N. For example, N can equal 80, M can equal one (or two), and X can equal eight, ten, or eighteen.  
         [0085]    U.S. patent application Ser. No. 08/563/505, filed Nov. 28, 1995, now U.S. Pat. No. 5,615,159 (and assigned to the assignee of the present application), describes a method and apparatus for storing control bits in registers (of a type which can be used to implement registers  40  and  41 ) and for using such registers to store control bits so that the control bits are accessible to address bit processing circuitry which selectively inverts address bits. The text of U.S. patent application Ser. No. 08/563,505, now U.S. Pat. No. 5,615,159 is incorporated herein by reference.  
         [0086]    In another class of embodiments, registers  40  and  41  are implemented as volatile memories (with register  40  capable of storing eight bits in volatile fashion, and register  41  capable of storing ten bits in volatile fashion). In such embodiments, each register preferably includes a decoding circuit which receives register control bits from control engine  129  and generates write control bits (or read control bits) from the register control bits, and latch circuitry. The latch circuitry receives the write control bits from the decoding circuit, temporarily stores a set of input data bits (a set of eight bits E(7:0) in the case of register  40 , and a set of ten bits M(9:0) in the case of register  41 ) in response to the write control bits, and asserts the set of stored data bits to predecoder  50  (predecoder  50  uses the bits asserted by the latch circuitry in some but not all of its modes of operation). In response to the read control bits, the latch circuitry also asserts the data bits stored therein to output buffer  10 , from which they can be asserted to an external drive through interface  4 .  
         [0087]    In a class of preferred embodiments (useful only for implementing simultaneous erasures of multiple blocks; not simultaneous writes to or reads from multiple blocks), registers  40  and  41  are not included in the inventive memory system (or are not used if they are included). One such embodiment is that shown in FIG. 9 (to be described below). Rather, control engine  129  asserts bits E(7:0) directly to predecoder  50  in place of address bits A(17:10). In response, predecoder  50  asserts bits E(7:0) to decoder  12  (rather than block selection bits generated by processing address bits A(18:16)). Or, control engine  129  asserts bits M(9:0) directly to predecoder  50  in place of address bits A(9:0) and in response, predecoder  50  asserts bits M(9:0) to decoder  12  (rather than block selection bits generated by processing address bits A(22:19)). Preferably, control engine  129  asserts both bits M(9:0) and E(7:0) to predecoder  50  in place of address-bits A(17:0), and predecoder  50  asserts all of them to decoder  12 .  
         [0088]    [0088]FIG. 3 is a diagram of a preferred embodiment of portion  50 A of predecoder  50  of FIG. 2. FIG. 5 is a diagram of a preferred implementation of decoder circuit  52  (also denoted as the “XCDEC” circuit) of the preferred embodiment of circuit  50 A. As shown in FIG. 5 (and in FIG. 3), the preferred embodiment of predecoder  50 A includes eight identical gate circuits  53  connected as shown, and eight identical multiplexer circuits  54  connected as shown. Each of circuits  53  receives control bit C2 and three of address bits A(18:16) and the inverses of such address bits, and operates in response thereto (in a manner to be described with reference to FIG. 6, which is a diagram of a preferred implementation of circuit  53 ). More specifically, the top circuit  53  in FIG. 5 receives bits A18, A17, and A16, the next circuit  53  from the top (in FIG. 5) receives bits A18, A17, and A — 16, the next circuit  53  from the top receives bits A18, A — 17, and A16, the next circuit  53  from the top receives bits A18, A 17, and A — 16, the next circuit  53  from the top receives bits A — 18, A17, and A16, the next circuit  53  from the top receives bits A — 18, A — 17, and A — 16, the next circuit  53  from the top receives bits A — 18, A — 17, and A16, and circuit  53  at the bottom of FIG. 5 receives bits A — 18, A — 17, and A — 16.  
         [0089]    Each of circuits  54  receives the inverted output of a corresponding one of circuits  53  (at its “A0” terminal), and receives a different one the bits E(7:0) from register  40  (at its “A1” terminal), and outputs a selected one of these two input bits in response to control bit C1 (and the inverse of bit C1 7 ) in a manner to be described with reference to FIG. 8. When the FIG. 5 circuit is controlled to output the bits E(7:0) by passing them through circuits  54 , the FIG. 5 circuit effectively reads the bits E(7:0) from register  40  and asserts them to the array interface circuitry of the system (which includes circuits  12  and  13  and Y multiplexer circuits YMuxA through YMuxJ).  
         [0090]    As shown in FIG. 8, each multiplexer circuit  54  includes two pass transistor switches SW 1  and SW 2 . Each of pass transistor switches SW 1  and SW 2  consists of a PMOS transistor and an NMOS transistor connected as shown. In response to assertion of control bit C1 with a high value to the control gate of the NMOS transistor of SW 1  and the control gate of the PMOS transistor of SW 2  (and assertion of inverse C1_ of bit C1 with a low value to the control gate of the PMOS transistor of SW 1  and the control gate of the NMOS transistor of SW 2 ), switch SW 2  is in an open state (preventing assertion of the inverted output of the corresponding circuit  53  to output terminal A) and switch SW 1  is in a closed state (so that it passes the relevant one of bits E(7:0) received at input terminal A1 through to output terminal A). In response to assertion of control bit C1 with a low value to the control gate of the NMOS transistor of SW 1  and the control gate of the PMOS transistor of SW 2  (and assertion of C1_with a high value to the control gate of the PMOS transistor of SW 1  and the control gate of the NMOS transistor of SW 2 ), switch SW 2  is in a closed state (so that it passes the inverted output of the corresponding circuit  53  through to output terminal A) and switch SW 1  is in an open state (preventing assertion of the relevant one of bits E(7:0) to output terminal A). Control bits C1 and C1_are received, respectively, at terminals S 1  and S 0  of each FIG. 8 circuit embodying one of circuits  54  of FIG. 5.  
         [0091]    With reference to FIGS. 5 and 6, each circuit  53  receives address bit A18 (or its inverse A — 18) at its “C” terminal, address bit A17 (or its inverse A — 17) at its “B” terminal, address bit A16 (or its inverse A — 16) at its “A” terminal, and control bit C2 at its “T” terminal. The FIG. 6 implementation of each circuit 53 consists of eight transistors connected as shown in FIG. 6. When C2 is low (and C1 is also low), circuits 53 assert at their “Output” terminals the inverses of the output bits XC(7:0) which are specified in one of the first eight rows of Table A (each circuit  53  asserts a different one of the inverted bits XC(7:0) in the row of Table A determined by the values of bits A(18:16)). As mentioned above, when control bit C1 is low (while C2 is low), the inverses of the outputs of circuits  53  pass through multiplexers  54 , and thus the output bits XC(7:0) of circuit  52  are the bits specified in one of the first eight rows of Table A (where the row is determined by the values of bits A(18:16)).  
         [0092]    When control bit C2 is high, the output of each of circuits  53  is grounded (i.e., has a low level). As mentioned above, when control bit C1 is low (while C2 is high), the inverses of the outputs of circuits  53  pass through multiplexers  54 , and thus the output bits XC(7:0) of circuit  52  all have a high value.  
         [0093]    [0093]FIG. 4 is a block diagram of a preferred implementation of portion  50 B of predecoder  50  of FIG. 2. As shown in FIG. 4, predecoder  50 B operates in response to control signals C3, C4, and C5 from controller  129 , address bits A(22:19), and the inverses A_(22:19) of these address bits, to assert in parallel at its output a set of ten selection bits BS(9:0). The FIG. 4 embodiment of circuit  50 B is also coupled to register  41  so that it can read the ten-bit set M(9:0) stored in register 41.  
         [0094]    As shown in FIG. 4, the preferred embodiment of predecoder  50 B includes ten identical gate circuits  55  connected as shown, ten identical multiplexer circuits  54  connected as shown, and NAND gates  56  and  57  connected as shown. Each of circuits  55  receives control bit C4 and four bits selected from the set of address bits A(21:19), the inverses of such address bits, and the inverted outputs of NAND gates  56  and  57 , and operates in response thereto (in a manner to be described with reference to FIG. 7, which is a diagram of a preferred implementation of circuit  55 ). More specifically, the two inputs to NAND gate  56  are address bit A22 and control bit C5, and the two inputs to NAND gate  57  are inverted address bit A — 22 and control bit C5. Thus, the inverted output of gate  56  is a logical AND of A22 and C5, and the inverted output of gate  57  is a logical AND of A — 22 and C5. The circuit  55  at the left side of FIG. 4 receives bits A — 19, A — 20, A — 21, and the inverted output of gate  57 ; the next circuit  55  from the left (in FIG. 4) receives bits A19, A — 20, A — 21, and the inverted output of gate  57 ; the next circuit  55  from the left receives bits A — 19, A20, A — 21, and the inverted output of gate  57 ; the next circuit  55  from the left receives bits A19, A20, A — 21, and the inverted output of gate  57 ; the next circuit  55  from the left receives bits A — 19, A — 20, A21, and the inverted output of gate  57 ; the next circuit  55  from the left receives bits A19, A — 20, A21, and the inverted output of gate  57 ; the next circuit  55  from the left receives bits A — 19, A20, A21, and the inverted output of gate  57 ; the next circuit  55  from the left receives bits A19, A20, A21, and the inverted output of gate  57 ; the next circuit  55  from the left receives bits A — 19, A — 20, A — 21, and the inverted output of gate  56 ; and the circuit  55  on the right side of FIG. 4 receives bits A19, A — 20, A — 21, and the inverted output of gate  56 .  
         [0095]    Each of circuits  54  receives the inverted output of a corresponding one of circuits  55  (at its “A0” terminal), and receives a different of one of the bits M(9:0) read from register  41  (at its “A1” terminal), and outputs a selected one of these two input bits in response to control bit C3 (and the inverse C3_of bit C3) in the manner described above with reference to FIG. 8 (except that control bits C3 and C3_ are received, respectively, at terminals S 1  and S 0  of each FIG. 8 circuit embodying one of circuits 54 of FIG. 4, while bits C1 and C1_ are received at terminals S 1  and S 0  of each FIG. 8 circuit embodying one of circuits  54  of FIG. 5 as described above with reference to FIG. 8). When the FIG. 4 circuit is controlled to output the bits M(9:0) by passing them through circuits  54 , the FIG. 4 circuit effectively reads the bits M(9:0) from register  41  and asserts them to the array interface circuitry of the system.  
         [0096]    [0096]FIG. 7 is a preferred implementation of circuit  55  of FIG. 4, which consists of ten transistors connected as shown in FIG. 7. With reference to FIGS. 4 and 7, each circuit  55  receives address bit A19 (or its inverse A — 19) at its “A” terminal, address bit A20 (or its inverse A — 20) at its “B” terminal, address bit A21 (or its inverse A — 21) at its “C” terminal, the inverse of the output of gate  56  (or 57) at its “D” terminal, and control bit C4 at its “T” terminal. When C4 is low (and C5 is high), circuits  55  assert at their “Output” terminals the inverses of the output bits BS(9:0) which are specified in one of the first ten rows of Table B (each circuit  55  asserts a different one of the inverses of bits BS(9:0) in a row of Table B determined by the values of bits A(22: 19)). Thus, the inverted outputs of circuits  55  pass through multiplexers  54 , and thus the output bits BS(9:0) of circuit  50 B are the bits specified in one of the first eight rows of Table A (where the row is determined by the values of bits A(22:19)).  
         [0097]    When both of control bits C4 and C5 are low, the output of each of circuits  55  is high, and the inverted outputs of circuits  55  pass through multiplexers  54 , and thus the output bits BS(9:0) of circuit  50 B are all “low” bits (zeroes).  
         [0098]    When control bit C4 is high, the output of each of circuits  55  is grounded (i.e., has a low level). When control bit C3 is low (while C4 is high), the inverses of the outputs of circuits  55  pass through multiplexers  54 , and thus the output bits BS(9:0) of circuit  50 B are all “high” bits (ones).  
         [0099]    When control bit C3 is high, multiplexers  54  pass through bits M(9:0), regardless of the outputs of circuits  55 , and thus the output bits BS(9:0) of circuit  50 B are bits M(9:0).  
         [0100]    In a typical write mode of operation of system  30  of FIG. 2, each set of address bits A(22:0) is associated with a data byte to be written to cells in a single row of one erase block (within one main block) of array  16 , and each set of multiblock selection bits XC(7:0) and BS(9:0) is associated with cells in a row of each of two or more erase blocks (or main blocks), and the system writes the same data byte to multiple sets of cells (each set of cells in a different block) in response to each set of multiblock selection bits. The multiblock selection bits XC(7:0) and BS(9:0) determine each erase block and each main block to which the data byte is to be written. Address bits A(15:0) determine the line within each such erase block to which the data byte is to be written and the cells within each such line to which the data byte is to be written, and selection bits generated from address bits A(1 5:0) in circuits  12  and  13  select the line (within each target erase block in each target main block) to which the data byte is to be written and the cells within each such line to which the data byte is to be written.  
         [0101]    In a typical read mode of operation of system  30  of FIG. 2, each set of address bits A(22:0) identifies cells in a single row of one erase block within one main block of array  16  from which a data byte is to be read, and each set of multiblock selection bits XC(7:0) and BS(9:0) identifies cells in a single row of each of two or more erase or main blocks of array  16  from which a data byte is to be read. If all the selected cells were programmed (so that they are all expected to conduct no more than negligible current when read by a sense amplifier), the system can simultaneously read data from multiple selected sets of cells (each set of selected cells in a different block) in response to each set of multiblock selection bits, in the sense that it can verify whether or not all the selected cells are in the expected (programmed) state. However, where each bitline extends through all the main blocks, and each sense amplifier in circuit  33  is coupled (during a read operation) to cells connected along one bitline, they system cannot distinguish between the case that one selected cell along a single bitline has changed state (from a programmed to an erased state), and the case that more that one selected cell along the bitline has changed state (from a programmed to an erased stat). In an alternative embodiment of FIG. 2 system in which each bitline is contained within a single main block, circuit  33  is implemented to include multiple blocks of sense amplifiers (each block including sense amplifiers for reading cells in a different main block), and each sense amplifier is coupled (during a read operation) to cells connected along one bitline within a single main block. Such multiple blocks of sense amplifiers could simultaneously read cells in two or more erase blocks of array  16  (where each of the erase blocks is in a different main block, and all the cells are simultaneously selected in accordance with the invention).  
         [0102]    In any of the embodiments in the previous paragraph, multiblock selection bits XC(7:0) and BS(9:0) determine each erase block and each main block from which a data byte is to be read. Address bits A(15:0) determine the line within each such erase block from which a data byte is to be read and the cells within each such line from which the data byte is to be read, and selection bits generated from address bits A(15:0) in circuits  12  and  13  select the line (within each target erase block in each target main block) from which a data byte is to be read and the cells within each such line from which the data byte is to be read.  
         [0103]    In a typical erase mode of operation (in a test mode) of system  30  of FIG. 2, address bits AX and A(15:0) select all cells of all rows, and multiblock selection bits XC(7:0) and BS(9:0) select a desired combination of at least two erase blocks (in a desired combination of one or more main blocks). The system simultaneously erases all cells in all selected erase blocks.  
         [0104]    In either a read mode or a write mode of the system, predecoder  50  asserts a selected one of different sets of multiblock selection bits XC(7:0) and BS(9:0) in response each combination control bits C1, C2, C3, C4, and C5 that it receives from controller  129 . For example, predecoder  50  asserts a set of multiblock selection bits (including a set of bits BS(9:0) consisting of nine bits having value “0” and one bit having value “1”, and bits XC(7:0) having the following values: XC7=1, XC6=1, XC5=1, XC4=1, XC3=1, XC2=1, XC1=1, and XC0=1) which selects all erase blocks in a single main block in response to control bits having the following values: C1=0, C2=1, C3=0, C4=0, and C5=1. For another example, predecoder  50  asserts another set of multiblock selection bits (including a set of bits XC(7:0) consisting of seven bits having value “0” and one bit having value “1”, and bits BS(9:0) having the following values: BS9=1, BS8=1, BS7=1, BS6=1, BS5=1, BS4=1, BS3=1, BS2=1, BS1=1, and BS0=1) which selects the same erase block in all main blocks, in response to control bits having the following values: C1=0, C2=0, C3=0, and C4=1. For another example, predecoder  50  asserts another set of multiblock selection bits (including bits XC(7:0) having the values XC7=1, XC6=1, XC5=1, XC4=1, XC3=1, XC2=1, XC1=1, and XC0=1, and bits BS(9:0) having the values BS9=1, BS8=1, BS7=1, BS6=1, BS5=1, BS4=1, BS3=1, BS2=1, BS1=1, and BS0=1) which selects all the erase blocks in all the main blocks, in response to control bits having the following values: C1=0, C2=1, C3=0, and C4=1.  
         [0105]    In other examples, predecoder 50 asserts a set of multiblock selection bits (including a set of bits BS(9:0)=M(9:0) including two or more bits having value “1 ”) which selects two or more main blocks in response to control bit C3 having the value C3=1. In other examples, predecoder  50  asserts a set of multiblock selection bits (including a set of bits XC(7:0)=E(7:0) including two or more bits having value “1”) which selects two or more erase blocks in response to control bit C1 having the value C1=1.  
         [0106]    A variation on the FIG. 2 embodiment will next be described with reference to FIG. 9. System  300  of FIG. 9 is identical to system  30  of FIG. 2, except in that it lacks registers  40  and  41 . In operation, control engine  129  can operate in a mode in which it sends address bits A(22:0) and AX to predecoder  50 , and in response, predecoder  50  asserts single block selection bits to circuits  12  and  13  (to select one or more cells in a single selected erase block of array  16 ). In another mode of operation, control engine  129  sends address bits A(22:18) and AX and above-described bits M(9:0) and E(7:0) to predecoder  50 . In response, predecoder asserts multiblock selection bits (including bits M(9:0) and E(7:0)) to circuits  12  and  13  (to select all cells of a selected combination of at least two erase blocks). System  300  then simultaneously erases all the cells of all the erase blocks determined by these multiblock selection bits.  
         [0107]    The FIG. 9 embodiment thus operates in a mode in which address bits (bits A(17:0)) sent by controller  129  to predecoder  50  function as mask bits (bits M(9:0) and E(7:0)), to enable the system to implement simultaneous erasure of selected combinations of two or more erase blocks in accordance with the invention. In other modes of operation of the FIG. 9 system, the address bits A(17:0) sent by controller  129  to predecoder  50  function in a conventional manner as address bits for selecting an erase block, cylinder, sector, and cells within such sector.  
         [0108]    Other aspects of the invention are methods (which can be implemented by the above-described memory system  30  or system  300 , or variations thereon) which select at least one cell of each of two or more blocks of an array of memory cells organized in rows and columns.  
         [0109]    One such method assumes that the rows are organized into N erase blocks of cells, where N is an integer, and includes the steps of:  
         [0110]    (a) prestoring multiblock selection bits in M registers, wherein the registers store a total of no more then X of the multiblock selection bits, wherein M is an integer equal to at least one, and X is an integer less than N;  
         [0111]    (b) reading the multiblock selection bits from at least one of the registers in response to control signals; and  
         [0112]    (c) simultaneously selecting a combination of blocks of the cells, in response to the multiblock selection bits.  
         [0113]    In a class of embodiments of the method of the previous paragraph, step (b) includes the step of receiving a first set of Z multiblock selection bits from a first register (in which the Z bits have been prestored) and receiving a second set of Y multiblock selection bits from a second register (in which the Y bits have been prestored), and asserting the Z+Y=X received multiblock selection bits (where Z and Y are integers, for example Z=8 and Y=10). In some embodiments in this class, the first set of multiblock selection bits read from the first register determines an erase block in at least one main block of the array, and the second set of multiblock selection bits read from the second register determines at least one main block of the array.  
         [0114]    Another embodiment of the inventive method is a method for selecting at least two blocks of cells of an array of memory cells, where the array is organized in rows and columns of the cells, including the steps of:  
         [0115]    (a) generating multiblock selection bits by asserting control bits to a predecoder and processing the control bits in logic circuitry in the predecoder; and  
         [0116]    (b) simultaneously selecting a combination of the blocks of cells, in response to the multiblock selection bits.  
         [0117]    In variations on this embodiment, step (a) includes the steps of generating a first subset of the set of multiblock selection bits by processing address bits in response to a first set of the control bits, and generating a second subset of the set of multiblock selection bits by processing a second set of the control bits in the logic circuitry. In other variations on this embodiment, step (a) includes the steps of reading a first subset of a set of multiblock selection bits from at least one register in response to a first set of the control bits, and generating a second subset of the set of the multiblock selection bits by processing a second set of the control bits in the logic circuitry.  
         [0118]    Another embodiment of the inventive method is a method for selecting multiple blocks of cells of an array of memory cells, where the array is organized in rows and columns of the cells, including the steps of:  
         [0119]    (a) asserting a first set of multiblock selection bits, in response to processing a first set of control bits in logic circuitry in a predecoder;  
         [0120]    (b) selecting a first selected combination of blocks of the cells simultaneously, in response to the first set of multiblock selection bits;  
         [0121]    (c) after step (a), asserting a second set of multiblock selection bits, in response to processing a second set of control bits in the logic circuitry; and  
         [0122]    (d) selecting a second selected combination of blocks of the cells simultaneously, in response to the second set of multiblock selection bits.  
         [0123]    In some implementations of the embodiment of the previous paragraph, step (a) includes the steps of generating a first subset of the first set of multiblock selection bits by processing address bits in response to a first subset of the first set of control bits, and generating a second subset of the first set of multiblock selection bits by processing a second subset of the first set of control bits in the logic circuitry, and step (c) includes the steps of generating a first subset of the second set of multiblock selection bits by processing address bits in response to a first subset of the second set of control bits, and generating a second subset of the second set of multiblock selection bits by processing a second subset of the second set of control bits in the logic circuitry. In other implementations of the embodiment of the previous paragraph, step (a) includes the step of receiving at least one bit of the first set of multiblock selection bits from a register in which said at least one bit of the first set of multiblock address bits has been prestored, and step (c) includes the step of receiving at least one bit of the second set of multiblock address bits from a register in which said at least one bit of the second set of multiblock address bits has been prestored.  
         [0124]    Another embodiment of the inventive method is a method for selecting multiple blocks of cells of an array of memory cells, where the array is organized in rows and columns of the cells, and the rows are organized into N erase blocks of the cells, where N is an integer, including the steps of:  
         [0125]    (a) prestoring multiblock selection bits in M registers, wherein the registers store a total of no more then X of the multiblock selection bits, wherein M is an integer equal to at least one;  
         [0126]    (b) asserting a first set of multiblock selection bits in response to a first set of control bits, including by reading at least a subset of the first set of multiblock selection bits from at least one of the registers;  
         [0127]    (c) selecting a first selected combination of the erase blocks simultaneously, in response to the first set of multiblock selection bits;  
         [0128]    (d) after step (b), asserting a second set of multiblock selection bits in response to a second set of control bits, including by reading at least a subset of the second set of multiblock selection bits from at least one of the registers; and  
         [0129]    (e) selecting a second selected combination of the erase blocks simultaneously, in response to the second set of multiblock selection bits.  
         [0130]    Another embodiment of the inventive method is a method for writing data to multiple selected blocks of cells of an array of memory cells, where the array is organized in rows and columns of the cells, including the steps of:  
         [0131]    (a) asserting multiblock selection bits in response to control bits by asserting control bits to a predecoder and processing the control bits in logic circuitry in the predecoder;  
         [0132]    (b) selecting a combination of blocks of the cells simultaneously in response to the multiblock selection bits, and selecting at least one cell in each of the blocks in response to address bits; and  
         [0133]    (c) simultaneously writing a set of data bits to said at least one cell in said each of the blocks.  
         [0134]    Another embodiment of the inventive method is a method for reading data from multiple selected blocks of cells of an array of memory cells, where the array is organized in rows and columns of the cells, including the steps of:  
         [0135]    (a) asserting multiblock selection bits in response to control bits, by asserting control bits to a predecoder and processing the control bits in logic circuitry in the predecoder;  
         [0136]    (b) selecting a combination of blocks of the cells simultaneously in response to the multiblock selection bits, and selecting at least one cell in each block of said combination of blocks in response to address bits; and  
         [0137]    (c) simultaneously reading data from said at least one cell in each block of said combination of blocks.  
         [0138]    Another embodiment of the inventive method is a method for erasing multiple selected blocks of cells of an array of memory cells, where the array is organized in rows and columns of the cells, including the steps of:  
         [0139]    (a) asserting multiblock selection bits in response to a first set of control bits, by asserting the first set of control bits to a predecoder and processing the first set of control bits in logic circuitry in the predecoder;  
         [0140]    (b) selecting a combination of blocks of the cells simultaneously in response to the multiblock selection bits; and  
         [0141]    (c) simultaneously erasing all the blocks in said combination of blocks.  
         [0142]    Preferred embodiments of the invention have been described with reference to FIGS.  2 - 9 . Although these embodiments have been described in some detail, it is contemplated that changes from these embodiments can be made without departing from the spirit and scope of the invention as defined by the appended claims.