MEMORY DEVICE HAVING MULTIPLEXED TWIN I/O LINE PAIRS

Abstract: A memory device, in particular, a dynamic random access memory (DRAM), is comprised of a first (12) and a second (14) input/output (I/O) bus, a first (20) and a second (28) I/O sense amplifier, and a first (24) and a second (30) I/O bus precharge circuit. A control circuit is responsive to the state of a mode control signal (32) for enabling the operation of the I/O busses and the precharge circuits such that in one mode of operation the DRAM operates in a conventional single bit per Column Address Strobe (CAS) cycle page mode. In a second mode of operation a high speed dual bit per CAS cycle page mode is achieved, wherein the I/O busses are alternately enabled, one being enabled when CAS (40) is asserted and the other being enabled when CAS is deasserted. The dual bit mode of operation provides also for precharging the I/O bus which is not enabled during the period when the other bus is enabled. Thus, in the dual bit mode of operation data transfers to or from the DRAM occur both when CAS is asserted and also when CAS is deasserted, thereby doubling the data transfer rate over that of the conventional page mode of operation.

Description: This invention relates to a semiconductor memory device and, more particularly, to a dynamic random access memory (DRAM) having multiplexed twin I/O line pairs for improved page mode speed performance. The invention relates also to a method of increasing the data transfer rate of a memory device. DRAMS are extensively used throughout the computer industry as data storage devices. Their relatively small size, high speed of operation and high storage density render them particularly suitable for the construction of computer memory systems. In general, a DRAM is comprised of a two dimensional array of binary storage cells organized by rows and columns wherein the presence or absence of a stored charge is indicative of a bit of data. In order to select a desired one of the storage cells for the reading or writing of data therein a plurality of address inputs are provided to the DRAM. To minimize the number of input connections to the DRAM, and hence the package size and cost, the address inputs are typically multiplexed. In operation, the least significant address bits are first applied to their respective address inputs followed by the assertion of a row address strobe (RAS) signal. RAS being asserted causes the row address of the storage array to be latched within the DRAM. The most significant address inputs are next applied followed by the assertion of a column address strobe (CAS) signal. The particular combination of the row and column addresses thus applied is decoded within the DRAM by row address and column address decoder circuitry to select one of the binary storage cells for reading or writing and, also to control the operation of the DRAM. As may be appreciated, depending on the particular topology of the two dimensional storage array, the number of address inputs and other factors a variety of different decoder circuits may be employed. Illustrative of some of these decoder circuits of the prior art are the following U.S. Patents, namely, two U.S. Patents 4,156,938 and 4,477,739 to Proelsting et al, two U.S. Patents 4,200,917 and 4,309,629 to Moench, a U.S. Patent 4,309,629 to Kamuro, and a U.S. Patent 4,477,884 to Iwahashi. JA A-58-115683 describes a semiconductor memory device having multiple data line pairs whereby during successive cycles, one data line pair is used for the transfer of data while a second data line pair is precharged ready for use in the next cycle. Only one bit of data is transferred to or from the device per memory cycle. Although the foregoing decoder circuits may be suitable for their intended applications, a problem arises when it is desired to perform multiple data accesses to the DRAM in a high speed manner. Referred to as a page mode of operation, this type of access typically requires maintaining RAS in an asserted state to thereby provide a row address to the array. While RAS is so asserted a plurality of column addresses and associated CAS signals are sequentially provided to the DRAM. Thus, a particular row is selected and a plurality of columns within that row are sequentially accessed. Currently known DRAMS have one I/O line pair, or bus, for connecting the outputs of a plurality of storage array sense amplifiers to an I/O line sense amplifier during the time that CAS is asserted. The use of one I/O line pair requires that some minimum interval of time be allowed between successive assertions of CAS in order to allow for the proper precharging of the I/O line pair. Inasmuch as the I/O line sense amplifier is essentially a differential or cross-coupled type of amplifer, such precharging is required to equalise the voltage potential between the two lines of the I/O line pair, thus ensuring the proper operation of the sense amplifier for sensing the state of a data bit appearing on the I/O line pair. This requirement of precharging the single I/O line pair between successive assertions of CAS results in a limitation of the number of bits which may be read from or stored within the DRAM during a given interval of time. It is therefore the object of the invention as characterised in claim 1, to provide a semiconductor memory device, in particular, a dynamic random access memory (DRAM) that is available to substantially all times for storage and retrieval of data. It is a further object of the invention as characterised in claim 9 to provide a method of increasing the data transfer rate of a memory device. The foregoing objects are solved and other advantages are provided by a memory device having one set of circuitry operable for the storage and retrieval of data and a precharge means operable for precharging the circuitry and, in addition, a second set of circuitry similarly operable for the storage and retrieval of data and a second precharge means for precharging the additional circuitry. A decoder, or control circuit, is also provided for selecting the first set of circuitry for the storage and retrieval of data while simultaneously precharging the second set during the assertion of a address strobe signal, and for selecting the second set of circuitry for the storage and retrieval of data while simultaneously precharging the first set during the deassertion of the address strobe signal. In an illustrative embodiment a DRAM comprises two I/O line pairs, or buses, which provide for a dual bit page mode type of access which is substantially twice as fast as that in a conventional DRAM having a single I/O line pair. In the illustrative embodiment, a decoder, or control, circuit is operable for providing two precharge CAS (PCAS) signals which are generated from CAS, each PCAS signal controlling an I/O bus and an associated precharge circuit. The outputs from a plurality of bit line sense amplifiers are alternately connected to the first and second I/O bus. The successive assertion and deassertion, or toggling, of CAS during a page mode access results in the first I/O bus being active when CAS is asserted and the second I/O bus being inactive, thereby being precharged. Similarly, when CAS is deasserted, the first I/O line pair is precharged and the second I/O line pair is active. The decoder circuit is responsive to the state of a control signal to operate in a dual bit per CAS cycle mode or in a conventional single bit per CAS cycle mode. In accordance with a method of the invention, a first and a second I/O bus are provided for coupling a first and a second plurality of bit line sense amplifiers to a first and a second I/O sense amplifier. By controlling the enabling of an individual sense amplifier and its corresponding I/O sense amplifier data is transferred over the associated I/O bus. Simultaneously, the precharge circuit associated with the other I/O bus is enabled for precharging that I/O bus. Typically, one of the I/O buses is transferring data while CAS is asserted and the other bus is transferring data when CAS is deasserted. Thus, the use of the dual bit mode of the invention provides two data bits for each CAS cycle as opposed to only one data bit in the conventional, single bit page mode. Thus, a data transfer rate which is substantially twice as fast as a conventional DRAM is realized. The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawing wherein: Fig. 1 schematically shows a portion of DRAM having an embodiment of the invention; and Fig. 2 illustrates timing waveforms useful in understanding the two types of page mode operation made possible by the invention. In the following description a signal name followed by an asterisk (*) is considered to be asserted, or true, when the signal is at a logic zero state, or low. Conversely, a signal name not followed by an asterisk is considered to be asserted when the signal is at a logic one state, or high. Also, a high applied to the gate electrode of a metal-oxide-semiconductor field effect transistor (MOSFET) of n-type conductivity is operable to activate, or turn on the transistor while a low applied to the gate is operable to turn off the transistor. Conversly, a low applied to the gate of a MOSFET having p-type conductivity is operable to turn on the MOSFET while a high applied to the gate is operable to turn off the transistor. With reference to Fig. 1, a portion of a dynamic random access memory (DRAM) is shown to be comprised of a plurality of bit line sense amplifiers (SA) 10a, 10b, 10c and 10d. In accordance with the invention, SA's 10 are alternately connected to a first and a second input/output (I/O) line pair, or bus I/O₁ BUS 12 and I/O₂ BUS 14 respectively, through a plurality of n-type MOSFET switches 16a through 16h. Although the number of SA's 10 is shown as four, it is understood that the actual number will be dependent on the size of the DRAM data storage array. For example, if the array has a size of 16,384 storage elements, 128 SA's would be required. I/O₁ BUS 12 is further connected to a pair of input terminals 18a and 18b of a differential or cross-coupled type of first I/O sense amplifer I/O₁ SA 20. I/O₁ SA 20 is operable to sense a difference in potential between each line of the I/O₁ BUS 12, the difference in potential being due to one of the SA's 10a or 10c driving the lines 12 in response to a data bit present on a plurality of associated bit line inputs 22. In order that the potential between lines 31 and 33 of I/O₁ BUS 12 be substantially equalized before being driven by SA 10a or 10c a first precharge circuit (I/O₁ PRECHG) 24 is provided. I/O₁PRECHG 24 is typically comprised of a pair of p-type MOSFETS (not shown) each having a source electrode connected to a positive voltage potential, a drain electrode connected to a respective one of the lines of I/O₁ BUS 12, and a gate electrode connected to an enabling precharge signal. I/O₂ BUS 14 is similarly connected to a pair of input terminals 26a and 26b of a second I/O sense amplifier (I/O₂ SA) 28. In addition, each line 35 and 37 of I/O BUS 14 is connected to a second precharge circuit (I/O₂ PRECHG) 30, the operation of which is similar to that of I/O₁ PRECHG 24. In order to control the operation of the aforementioned I/O buses, sense amplifiers and precharge circuits the invention provides a decoder circuit, also referred to herein as a control circuit. The operation of the control circuit will now be described. In accordance with the invention, the dual bit per CAS cycle page mode of operation is enabled by the assertion of a dual bit control (DBC) 32 signal. DBC 32 being high provides an enabling input to NAND gate 34, the other enabling input 36 being provided through inverter 38 during the time that CAS* 40 is asserted, or low. The assertion of DBC 32 further disables NAND gates 42, 44 and 46 via the outputs 48 and 50 of inverters 52 and 54, respectively. The disabling of gates 42, 44 and 46 is required to enable the dual bit mode and disable the conventional single bit mode of operation, the operation of which will be discussed in detail hereinafter. During the time that DBC 32 is asserted, gate 42 will provide an enabling input 56 to NAND gate 58, a second input 60 to gate 58 being provided by gate 34. The signal appearing at input 60 will be indicative of the state of CAS* 40 because of the double inversion provided by inverter 38 and gate 34. Thus, the output 62 of gate 58 will be the inversion of CAS* 40, which output 62 is inverted once more by inverter 64 such that the output 66 of inverter 64 is in phase with CAS* 40, that is, when CAS* 40 is low output 66 will be low. The output 36 of inverter 38 is also provided as an input to the inverter chain comprised of inverters 68, 70, 72 and 74 such that the output 76 of inverter 74 is 180 degrees out of phase with output 66, that is, when output 66 is low output 76 will be high. Thus the output 76 is also 180 degrees out of phase with CAS* 40. The use of multiple inverters in the aforementioned inverter chain is to substantially equalize the signal delays due to circuit propagation delays experienced by both outputs 66 and 76. Outputs 66 and 76 are applied as inputs to NAND gates 78 and 80, respectively. Gates 78 has as a second input the output 82 of gate 46, which output 82 is high during the dual bit mode of operation. Gate 78 further has as an input the output 84 of inverter 86. The input to inverter 86 is the row sense amp latch (RSAL*) 88 signal. RSAL* 88 is asserted by other circuitry (not shown) in the DRAM row circuits soon after the row address strobe (RAS*) 178 signal is asserted (Fig. 2). The assertion of RSAL* 88 indicates that a row sense amp latching action has been completed and that the DRAM is ready to respond to the assertion of CAS* 40. Thus, it may be seen that the output signal Precharge CAS2 (PCAS2*) 90 of gate 78 is substantially the inversion of CAS* 40. The output signal Precharge CAS1 (PCAS1*) 92 of gate 80 is substantially identical to CAS* 40, gate 80 being enabled by output 84 and also, in the dual bit mode, by the output of gate 44. In operation, when CAS* 40 is asserted during the dual bit mode of operation the low on PCAS1* 92 is buffered by buffer 94 to provide a low on output 96 for enabling the operation of I/O₁ PRECHG 24, thereby precharging I/O₁ BUS 12. Simultaneously, the low on PCAS1* 92 is applied as an input to NOR gates 98, 100 and other, unillustrated NOR gates, each of which has an output for controlling the MOSFET switches 16 which couple alternate SA's 10 to I/O₂ BUS 14. A second input to NOR gate 98 is provided by the output 102 of inverter 104. The input to inverter 104 is a first decoded column address (DCA₁) 106 signal provided by column address decoder circuitry (not shown). In operation, DCA₁ 106 will be asserted for a first specific combination of column addresses, thereby selecting a specific column of the memory storage array. The inversion of DCA₁ 106 appearing on output 102, in conjunction with PCAS1* 92 being low, will cause the output 108 of NOR 98 to be high, thereby enabling MOSFET switches 16g and 16h to couple the storage array column connected to SA 10d to I/O₂ BUS 14. PCAS1* 92 being low will further cause the output 110 of inverter 112 to be high. Output 110 is delayed by a delay element 114 a sufficient amount of time to permit the switches 16g and 16h to couple SA 10d to I/O₂ BUS 14, afterwhich the high appears on line 116 and activates transistor 118 to latch the state of the I/O₂ BUS 14. A further delay is introduced by a delay element 120, afterwhich switches 122 and 124 are enabled to couple I/O SA₂ 28 to the DRAM data bus lines 126 and 128. During a data storage, or write cycle, the data bit to be written is driven into the data bus lines 126 and 128 by a pair of well known write drivers (not shown), the direction of data flow thereafter being through switches 122 and 124, through the I/O₂ SA 28 and onto the I/O₂ BUS 14. Depending upon which of the plurality of switches 16 are selectively enabled, the data will be applied to a storage cell for storage. Thus the use of the invention is equally advantageous for performing both read and write high speed, or pipelined, transfers. As may be appreciated, delay elements 114 and 120 may be comprised of a selected number of gates, such as two serially connected inverters, such that the summation of the propagation delays through the individual gates provides a desired amount of signal delay. The signal DCA₂ 130 is another output of the column address decoder circuitry (not shown) and is high for a second specific combination of column address signals. DCA₂ 130 being asserted causes the output 132 of inverter 134 to provide an enabling signal to NOR gate 100 which, in conjunction with PCAS1* 92 being low, enables, via output 136, the switches 16c and 16d for coupling storage array column connected to SA 10b to I/O₂ BUS 14. It should be realized that there will be 2 n number of DCA signals (DCA₁ - DCA n ) where n equals the number of column address signals less one, the remaining column address signal being the most significant column address (MSCA) 168 which is applied to gates 44 and 46 for selectively enabling one of the BUSES 12 and 14 in the single bit mode of operation, as will be described in detail below. The operation of PCAS2* 90 is similar to that of PCAS1* 92, that is, it provides for enabling the operation of one I/O BUS while simultaneously precharging the other I/O BUS. As may be seen, PCAS2* 90 being low causes the output 138 of buffer 140 to be low, thereby enabling I/O₂ PRECHG 30 to precharge I/O₂ BUS 14. Simultaneously, NOR gates 142 and 144 have one input enabled, the other input being the inversion of one of the DCA signals. Depending on whether DCA₁ 106 or DCA₂ 130 is asserted, either the output 146 of gate 142 or the output 148 of gate 144 is high. Thus, either SA 10a or SA 10c coupled to I/O₁ BUS 12 by means of MOSFET pairs 16a and 16b or 16e and 16f, respectively. PCAS2* 90 being low further causes the output 150 of inverter 152 to be high, which high is delayed by element 154 before appearing on line 156 to enable switch 158 to latch I/O₁ SA 20. The high on line 156 is further delayed by element 160 before enabling switches 162 and 164 to couple I/O₁ SA 20 to the lines 126 and 128 of the DRAM data bus. In accordance with the invention, the signal DBC 32 being deasserted, or low, causes the DRAM to be placed in the conventional single data bit per CAS cycle page mode of operation. DBC 32 being low disables NAND gate 34 and enables one input to each of the NAND gates 42, 44 and 46 via inverters 52 and 54. The other input to NAND gate 42 is the output 166 of inverter 68, which output 66 is in phase with CAS* 40. Thus, both the outputs 66 and 76 are made to be in phase with one another and 180 degrees out of phase with CAS* 40. As has been previously mentioned, MSCA 168 is utilized in the single bit mode to select either the I/O₁ BUS 12 or the I/O₂ BUS 14 to be coupled to the lines 126 and 128 of the DRAM data bus. MSCA 168 is applied as an input to gate 44 and the inversion of MSCA 168 is applied to gate 46 via inverter 170. In operation, when CAS* 40 and RSAL* 88 are both asserted during a DRAM access, and DBC 32 is low thereby selecting the single bit mode, the state of MSCA 168 determines whether PCAS1* 92 or PCAS2* 90 will be asserted. If MSCA 168 is high the output 94 of gate 44 will be low, thereby causing the output of gate 80 (PCAS1* 92) to be deasserted. MSCA 168 being high further causes the output 82 of gate 46 to be high, due to inverter 170. Output 82 being high enables gate 78, thereby causing PCAS2* 90 to be asserted. Alternately, MSCA 168 being low causes PCAS1* 92 to be asserted and PCAS2* 90 to be deasserted. Thus, in the single bit mode of operation the state of MSCA 168 selects either I/O₁ BUS 12 or I/O₂ BUS 14 for coupling a selected SA 10 to the lines 126 and 128 of the DRAM data bus. Referring now to Fig. 2 there is shown a timing diagram illustrative of the above described operation of the circuitry of Fig. 1. In Fig. 2 there is shown generally as A the conventional single bit output page mode cycle (DBC 32 low) and, generally as B, the dual bit output page mode cycle (DBC 32 high). As can be seen, the A cycle is initiated by the assertion of RAS* 178, followed by the assertion of CAS* 40, as indicated by 182a. MSCA 168 is shown as high during the assertions 182a and 182b of CAS* 40, and low during the assertion 182c, thereby causing PCAS2* 90 to be asserted twice and PCAS1* 92 to be asserted once. Assuming that DCA₁ 106 (Fig. 1) is asserted during the cycle A the output 146 will be asserted twice in response to the assertions of PCAS2* 90. Each such assertion of output 146, as indicated by 184a and 184b, causes SA 10c to be coupled to the I/O BUS 12 via switches 16e and 16f. The subsequent assertion 186 of output 108 in response to the assertion of PCAS1* 92 causes SA 10d to be coupled to the I/O BUS 14 via switches 16g and 16h. Thus, three bits of data may be read from or written to the DRAM storage array, one bit for each assertion of CAS* 40. The B cycle is initiated in part by the assertion of RAS* 178 and the assertion 190 of DBC 32. Inasmuch as the assertion of DBC 32 results in the disabling of gates 44 and 46 via inverter 54 (Fig. 1), the state of MSCA 168 during the B cycle is immaterial. The subsequent assertion 192a of CAS* 40 results in the assertion of PCAS1* 92 and the assertion 196a of output 108. The deassertion 194a of CAS* 40 results in the deassertion of PCAS1* 92 and the assertion of PCAS2* 90, which further results in the assertion 198a of output 146. As can be seen, for each successive assertion (192b, 192c) of CAS* 40, the output 108 is asserted (196b, 196c). Similarly, for each successive deassertion (194b, 194c) of CAS* 40, the output 146 is asserted (198b, 198c). Thus, a total of six bits of data may be written to or read from the storage array, resulting in a data transfer rate which is twice as great as the transfer rate of cycle A, as described above. It may be appreciated that the use of the invention confers several distinct advantages upon a DRAM. Firstly, the data access rate in the page mode of operation may be doubled, as has been previously mentioned. Secondly, such a DRAM may be tested twice as rapidly as a DRAM having conventional single bit per CAS cycle page mode, resulting in substantial time and money savings in a high production environment. Thirdly, the ability to select between conventional and dual bit page modes of operation allows such a DRAM to be utilized in a variety of applications without the requirement of performing expensive retrofitting or circuit modifications. The use of the invention is especially advantageous in a DRAM fabricated with complementary metal oxide semiconductor (CMOS) technology, wherein the address buffers and column decoder circuits are constructed using CMOS static circuits to realize fast circuit speeds and low power consumption. The use of the invention is, however, not limited to DRAMS constructed with CMOS circuitry, but may be advantageously employed in DRAMS fabricated with N channel MOS (NMOS) and other technologies. In addition, the invention may be advantageously employed in any type of semiconductor memory wherein a precharge means is required to periodically precharge a portion of the memory circuitry or buses. For example, in a semiconductor memory wherein precharging of circuits is required, the inclusion of an additional bus or buses, control circuitry and precharge means allows for the active use of the device during the period of time which would, in a device of the prior art, be devoted to precharging. In addition, within a DRAM there are other circuits which require precharging, such as the row address bus when the device is used in a row access mode. The use of the invention similarly allows for increasing the data access rate by providing an additional row address bus and precharge means therefore. It may also be realized that a DRAM which is internally partitioned into a plurality of storage blocks may make use of the invention by performing a partial device selection whereby one or more blocks may be precharged while another block is actively transferring data. Such a partitioned device would thereby be accessible at all times inasmuch as a periodic, total device refresh cycle would not be required.

Claims: A semiconductor memory device comprising: first (16a,b,e,f;162,164) and second (16c,d,g,h;122,124) circuits including respective first and second I/O busses (12;14) for coupling a selected one of a plurality of memory cells within the device to a data bus (126,128); first (24) and second (30) precharge means operable for periodically charging the respective first (12) and second (14) I/O busses, the first and second circuits being inoperable for the transfer of data between data bus (126,128) and memory cells during charging of the respective first (12) and second (14) I/O busses; control means having: a first output state operable for selectively activating substantially simultaneously with one another the first circuit and the second precharge means (30) whereby the second I/O bus (14) is precharged while the first circuit couples a selected memory cell to the data bus (126,128); and a second output state operable for selectively activating substantially simultaneously with one another the second circuit and the first precharge (24) means whereby the first I/O bus (12) is precharged while the second circuit couples a selected memory cell to the data bus (126,128); the control means being responsive to the assertion of an address strobe signal (CAS*) to select the first output state and responsive to a deassertion of the address strobe signal (CAS*) to select the second output state whereby two units of data are transferred between data bus (126,128) and memory cells during a single address strobe signal cycle. The memory device of claim 1, the first and second circuits further including: respective first (10a,10c) and second (10b,10d) pluralities of bit line sense amplifiers coupled by means of the first and second I/O busses to respective first (20) and second (28) I/O sense amplifiers. and wherein in the first output state the control means enables an individual one of the first plurality of bit line sense amplifiers and the corresponding first I/O sense amplifier, and in the second output state, the control means activates an individual one of the second plurality of bit line sense amplifers and the corresponding second I/O sense amplifier. The memory device of claim 2, wherein the control means is responsive to the states of at least a mode control signal (32), the column address strobe signal (40) and a decoded column address signal (106,130) to enable the bit line sense amplifier (10), the I/O sense amplifier (20,28) and the precharge means (24,30). The memory device of claim 2 or 3, wherein each of the first (10a,10c) and the second (10b,10d) plurality of bit line sense amplifiers is coupled to a column of memory cells and is further couplable to respective first (20) and second (28) I/O sense amplifiers by an individual one of respective first (16a,b,e,f) and second (16c,d,g,h) plurality of switching means whereby a selected memory cell is coupled to the data bus (126,128). The memory device of claim 4, wherein in its first output state (90) the control means activates the first I/O sense amplifier (20), one of the first plurality of switching means (16a,b,e,f), and the second precharge means (30) and wherein in its second output state the control means activates the second I/O sense amplifier (28), one of the second plurality of switching means (16c,d,g,h), and the first precharge means (24). The memory device of claim 5, wherein the control means alternately selects first and second output states during successive assertions and deassertions of the address strobe signal in response to a second state of the mode control signal and wherein in response to a first state of the mode control signal (32,A), the control means has the first output state (90) or the second output state (92) only during a time when the column address strobe signal (40) is asserted. The memory device of any one or more of the preceeding claims wherein the device is a dynamic random access memory. A method of increasing the data transfer rate of a semiconductor memory device comprising the steps of : providing first (16a,b,e,f;162,164) and second circuits (16c,d,g,h;122,124) including respective first (12) and second (14) I/O busses for coupling a selected one of a plurality of memory cells within the device to a data bus (126,128); providing first (24) and second (30) precharge means operable for periodically charging the respective first (12) and second (14) I/O busses to a voltage potential, the first and second circuitry being inoperable for the transfer of data between data bus (126,128) and memory cells during charging of the respective first (12) and second (14) I/O busses; enabling the first circuit to couple a selected memory cell to the data bus (126,128) while simultaneously enabling the second precharge means (30) to precharge the second I/O bus (14) during the time that a column address strobe signal (CAS*) is asserted and further enabling the second circuit to couple a selected memory cell to the data bus (126,128) while simultaneously enabling the first precharge means (24) to precharge the first I/O bus (12) during the time that a column address strobe signal (CAS*) is deasserted.

Inventors: CHIN, DAEJE, HWANG, WEI, LU, NICKY CHAU-CHUN
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: G11C7
IPC codes: G11C11, G11C7