SEMICONDUCTOR MEMORY DEVICE WITH SHIFT REGISTERS FOR HIGH SPEED READING AND WRITING

Abstract: A semiconductor memory device with shift registers used for a video RAM (1), including a memory cell array, bit lines (BL, BL ), and word lines (WL), a pairofshift registers (3, 4), and transfer gate circuits (21 ,22) arranged between the bit lines and the shift registers. Each parallel data transfer cir­ cuit is provided between the shift registers for the parallel data transfer between the shift registers, so that high-speed reading and writing of data for a CRT display is realized.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor memory device, more particularly to a random access memory (RAM) with a shift register for high-speed reading and writing. The RAM according to the present invention can be used for a video RAM. 2. Description of the Related Arts High-speed read/write shift registers are used in a video RAM for storing the picture data for a cathode ray tube (CRT) display in order to treat a large amount of data in a short time. In order to increase the resolution of a picture on a display plane, it is necessary to increase the number of pixels of the picture, which requires the number of memory cells of the video RAM, and accordingly the number of bit lines and word lines, to be increased. If the number of bit and word lines are increased, the number of elements (bits) of the shift register is increased. However, it is very difficult to carry out shifting-in and shifting-out of the data for a shift register having such a large number of bits. There is a need for changing the data stored in a video RAM in order to change the picture displayed on the display plane. It is necessary to read the data from the video RAM and simultaneously to write new data into the video RAM, in the case of the display of a quickly changing picture. SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved semiconductor memory device, with shift registers, capable of being used as a video RAM, in which the writing of data into and the reading of data from the memory cells of the RAM by shift registers can be carried out in an appropriate manner either in a parallel or serial manner. According to the fundamental aspect of the present invention, there is provided a semiconductor memory device comprising : a random-access memory having a memory cell array, a plurality of bit lines, and a plurality of word lines; a pair of shift registers adapted to be connected with said plurality of bit lines and comprised of a shift register for output for carrying out serial reading and a shift register for input for registering data for one word line supplied serially from an external data source; transfer gate means arranged between said plurality of bit lines and the plurality of shift elements of which each of said shift registers is composed; and parallel data transfer means provided between said shift registers for carrying out the parallel data transfer between said shift registers. According to another aspect of the present invention, there is provided a semiconductor memory device comprising : a random-access memory having a memory cell array, a plurality of bit lines, and a plurality of word lines; a shift register for output for carrying out serial reading, data for one word line registered in parallel from said memory cell array into said shift register for output being delivered serially from said shift register for output; a shift register for input for registering data for one word line supplied serially from an external data source; a parallel data transfer means for carrying out logic processing of the data registered in said shift register for output and said shift register for input; an instruction decoder for delivering an instruction for a selected logic operation which should be executed in said parallel data transfer means; means for serially transmitting the data for one word line to be supplied to said shift register for input and an operation code for instructing a selected logic operation to be supplied to said instruction decoder; and means for writing the data for one word line obtained as the result of the processing in said parallel data transfer means according to said instructed selected logic operation into the memory cells of a selected word line in said memory cell array. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, Fig. 1 shows a prior art semiconductor memory device with shift registers; Fig. 2 shows a semiconductor memory device with shift registers according to an embodiment of the present invention; Fig. 3 shows an example of the actual arrangement of the shift register gates in the device of Fig. 2; Figs. 4 and 5 show modified embodiments of the present invention; Fig. 6 shows a further modified embodiment of the present invention; Fig. 7 shows a still further modified embodiment of the present invention; Fig. 8 shows an example of the serial data used in the embodiment of Fig. 7; and Fig. 9 shows the relationship between the operating bits and the instruction. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before entering into the description of the preferred embodiments, a prior art semiconductor memory device with shift registers used for a video RAM is described with reference to Fig. 1. The device of Fig. 1 includes a dynamic RAM 1 of the open bit-line type, groups of transfer gates 21 and 23, and shift registers 3 and 6. The RAM 1 includes sense amplifiers 101, 102, 103, ... 10n, bit lines (BL) 111, 112, 113, ... 11n, bit lines ( BL ) 121, 122, 123, ... 12n, word lines (WL) 131, 132, 133, ... 13n; 141, 142, 143, ... 14n. Memory cells (not shown) are provided at the crosses of the bit lines and the word lines. A pair of data buses (not shown) are provided along the sense amplifiers 101, 102, 103, ... 10n for writing or reading the data. The shift register 3 includes the sequence of elements (bits) 301, 302, 303, ... 30n, the number n being, for example, 256. The elements 301, 302, 303, ... 30n are connected through the transfer gates 211, 212, 213, ... 21n to the bit lines 111, 112, 113, ... 11n of the RAM 1. The elements 601, 602, 603, ... 60n of the shift register 6 are connected through the transfer gates 231, 232, 233, ... 23n to the bit lines 121, 122, 123, ... 12n of the RAM 1. By turning ON the gates 211, 212, 213, ... 21n after the storage of data in the shift register elements 301, 302, 303, ... 30n and the selection of one word line, data are written simultaneously into the memory cells at the crosses of the selected word line and the bit lines. By shifting out the data in the shift register elements 301, 302, 303, ... 30n after the transfer of the data from the memory cells at the crosses of a selected word line and the bit lines through the gates 211, 212, 213, ... 21n to the shift register elements 301, 302, 303, ... 30n, the data in the shift register 3 are delivered from the shift register 3 in a serial form. In order to increase the resolution of a picture on a display plane, it is necessary to increase the number of pixels of the picture, which requires the number of memory cells of a video RAM, and accordingly the number of bit lines and word lines, to be increased. If the number of bit and word lines are increased, the number of bits of the shift register is increased. However, it is very difficult to carry out shifting-in and shifting-out of the data for a shift register having such a large number of bits. There is a need for changing the data stored in a video RAM in order to change the picture displayed on the display plane. It is necessary to read the data from the video RAM and simultaneously to write new data into the video RAM in the case of the display of a quickly changing picture. Under these circumstances, the use of two shift registers, in which one shift register is used for reading of the data from the RAM, while the other shift register is used for writing of the data in the RAM, may be considered. However, no satisfactory devices which meet the above-described requirements have yet been realized. A semiconductor memory device with shift registers according to an embodiment of the present invention is shown in Fig. 2. The device of Fig. 2 includes a dynamic RAM 1 of the open bit-line type, groups of transfer gates 21 and 22, and shift registers 3 and 4. The data are supplied through the input lines 32 and 42 of the shift registers 3 and 4. The data are delivered through the output lines 33 and 43 from the shift registers 3 and 4. If the shift register 3 is used, for example, exclusively for reading, the input line 32 can be omitted. If the shift register 4 is used, for example, exclusively for writing, the output line 43 can be omitted. The RAM 1 includes sense amplifiers 101, 102, 103, ... 10n, bit lines (BL) 111, 112, 113, ... 11n, bit lines ( BL ) 121, 122, 123, ... 12n, word lines (WL) 131, 132, 133, ... 13n; 141, 142, 143, ... 14n. Memory cells (not shown) are provided at the crosses of the bit lines and the word lines. A pair of data buses (not shown) are provided along the sense amplifiers 101, 102, 103, ... 10n for writing or reading the data. The shift register 3 includes the sequence of elements (bits) 301, 302, 303, ... 30n and shift register gates 511, 512, 513, ... 51(n-1). The shift register 4 includes the sequence of elements (bits) 401, 402, 403, ... 40n and shift register gates 521, 522, 523, ... 52(n-1). Between the elements 301, 302, 303, ... 30n of the shift register 3 and the elements 401, 402, 403, ... 40n of the shift register 4, there are provided shift register gates 501, 502, 503, ... 50n. An example of the actual arrangement of the shift register gates 501, 502, 503, ... 50n; 511, 512, 513, ... 51(n-1); 521, 522, 523, ... 52(n-1) is shown in Fig. 3. In the arrangement shown in Fig. 3, the shift register gates are represented as G1 between the adjacent elements BIT(i) and BIT(i+1) of the shift register 3 (shift register A), G2 between the adjacent elements BIT(i) and BIT(i+1) of the shift register 4 (shift register B), G3 between the input terminal of the element BIT(i+1) of the shift register 3 and the output terminal of the element BIT(i) of the shift register 4, and G4 between the output terminal of the element BIT(i) of the shift register 3 and the input terminal of the element BIT(i+1) of the shift register 4. In the case where the shift register 3 is used exclusively for reading, while the shift register 4 is used exclusively for writing, the shift register gates G1 and G2 should be in the ON state, while the shift register gates G3 and G4 should be in the OFF state. Contrary to this, in the case where parallel transfer of data from the shift register 3 to the shift register 4 is carried out, the shift register gates G1 and G3 should be in the ON state, while the shift register gates G2 and G4 should be in the OFF state. Further, in the case where parallel transfer of the data from the shift register 4 to the shift register 3 is carried out, the shift register gates G2 and G4 should be in the ON state, while the shift register gates G1 and G3 should be in the OFF state. The device of Fig. 2 including the circuit of Fig. 3 can be used for a scroll display operation in the upward direction in the cathode ray tube (CRT) display. In this case, the shift register 3 is used for the reading, while the shift register 4 is used for the writing. The data for one word line is transferred first to the shift register 3, and next to the shift register 4. After that, shift operation of the shift register 3 takes place, so that the video signal is supplied to the CRT and the data of the shift register 4 is supplied to the immediately preceding word line for which the scanning has already been completed. This operation is successively repeated. Thus, the scroll display operation in the upward direction is carried out. Modified embodiments of the present invention are shown in Figs. 4 and 5. In the devices of Figs. 4 and 5, logic gates 5A and 5B are provided between the shift registers 3 and 4. The display of a negative picture based on a positive picture can be carried out quickly by using the devices of Figs. 4 and 5. In the devices of Figs. 4 and 5, the logic gates 5A and 5B consist of exclusive OR gates. The exclusive OR gates operate as "through" elements when one of the input signals is "0" and operate as inverter elements when one of the input signals is "1". After registering one word line data of RAM 1 in the shift register 3, the logic gate 5A, if applied with "1" at each one input of each of its exclusive OR gates, inverts the data in the shift register 3 and transfers the thus-inverted data to the shift register 4 in parallel, and the data are written in the memory cells of the selected word line through the shift register 4. Alternatively, the data can be read out through the shift register 4 to the exterior. By carrying out this operation over all of the picture of the display, inversion between the positive picture and negative picture is realized quickly. If selection of a word line is changed, displacement of the picture is also realized. The device shown in Fig. 5 having the shift register 3' (shift register C) is suitable for supplying a control input signal to one of the input terminals of the exclusive OR gates. By supplying the mask data to the shift register 3', a partial-inversion and remainder-through operation can be realized. In another modified embodiment in which three shift registers are provided, it is possible to carry out data exchanges between two of the three shift registers. In another embodiment in which two shift registers and storage units are provided, the number of the storage units being equal to the number of elements of the shift register, it is possible to carry out data exchange between two of the three shift registers. In another embodiment in which the RAM is divided into two portions and each of two shift registers is associated exclusively with one of the portions of the RAM, it is possible to carry out data exchange between the two shift registers. A further modified embodiment of the present invention is shown in Fig. 6. The system of the RAM in the device shown in Fig. 6 includes the memory cell array 71 having bit lines (BL), word lines (WL), and memory cells (not shown); the shift register 73 for input; the shift register 74 for output; the parallel data transfer circuit 751; the operation code (OP code) decoder 752; the input/output gate 76; the column decoder 77; and the word decoder 78. The system of the RAM is connected with the central processor unit (CPU) 70. Serial data are transmitted from the CPU 70 through the transmission line to the shift register 73 for input and the operation code decoder 752. The shift register 73 for input registers serial data for one word line. The parallel data transfer circuit 751 carries out the logic processing of the data registered in the shift register 73 for input and the shift register 74 for output to generate new data for one word line. The parallel data transfer circuit 751 has logic functions such as AND and OR. The selection of the logic function of the parallel data transfer circuit 751 is designated by the output of the operation code decoder 752. The operation code for designating the logic processing to be selected is transmitted together with the data for one word line from the CPU 70 through the transmission line to the operation code decoder 752, where the sent operation code is decoded to be supplied to the parallel data transfer circuit 751. The operation code decoder 752 and the parallel data transfer circuit 751 are connected through n signal lines, n being the number of kinds of logic processing. The designation of the selected kind of logic processing is carried out by turning HIGH the potential of the signal line corresponding to the selected kind of logic processing. The data for one word line which is to be supplied to the shift register 73 for input is determined by the logic processing which is to be applied to the picture data stored in the memory cell array 71. If the OR logic is determined, the result of the OR logic processing with regard to the new picture data supplied to the shift register 73 for input is the superposition of the old and new pictures. When blue line data stored in the memory cell array 71 is to be changed into red line data, the parallel data transfer circuit 751 acts as a circuit for detecting the blue line data and changing the detected blue line data into red line data. The data supplied to the shift register 73 for input is data such as mask data, which instructs in which portion of the word lines the processing in question should be executed. A still further modified embodiment of the present invention is shown in Fig. 7. The device of Fig. 7 is designed for the case where one pixel is represented by 8 bits. It should be noted that one pixel of a color picture consists of at least 3 bits, that is, R, G, and B, and one pixel of a monochromic picture consists of plural bits if the monochromic picture has a varieties of color densities. Such a situation can be dealt with by providing as many of the memory cell array 81, the shift register 83 for input, the shift register 84 for output, and parallel data transfer circuit 85 as the number of bits for one pixel. In the device of Fig. 7, the parallel data transferring circuit 85 consists of the assemblies of the parallel data transferring circuits 85(1), 85(2), 85(3), and so on. An example of the serial data used in the device of Fig. 7 is shown in Fig. 8. In Fig. 8, the beginning 8 bits are the operating bits, that is, the operating code. Each of the former 4 bits (BIT 0 to BIT 3) and the latter 4 bits (BIT 4 to BIT 7) has the value of 0 to 9, A, B, C, D, E, or F in the hexadecimal number system. An example of the instructions represented by the operating bits is shown in Fig. 9. The data bits for one word line follow the operating bits BIT 0 to BIT 7 shown in Fig. 7. Each 8 bits, such as BIT 8 to BIT 15 and BIT 16 to BIT 23, correspond to one pixel. Thus, by including the operating bits in the serial data series, which is the serial data series for the data bits, the input line 731 (Fig. 6) can be used both for the operating code and the data so that the number of the external terminals of the system of RAM need not by increased. The device of Fig. 7 has the usual read/write mode using a data bus, the serial read mode using a shift register for output, and the serial write mode using a shift register for input. In this serial write mode, serial data S(IN) having the format of Fig. 8 are delivered from the CPU 70 through the S(IN) transmission line. Upon receipt of the serial data S(IN), the operation code decoder 752 decodes the beginning 8 bits, which is the operating code, and the output of the operation code decoder 752 carries out the selection of the processing in the parallel data transfer circuit 751 according to the instructions shown in Fig. 9. The data bits of the serial data S(IN) for one word line are registered in the shift register 73 for input. The operating code is automatically shifted out from the shift register 73 for input. The processing in the parallel data transfer circuit 751 is started after the data bits for one word line are registered in the shift register 73 for input. The new data resulting from the processing are written in parallel in the memory cell array connected to the selected word line in the memory cell array 71. In the case where the word line to which the writing is carried out is the word line from which the data is read for the shift register 74 for output, the operation is the updating of the picture data. In the case where the word line to which the writing is carried out is the line next to the above-mentioned read word line, the operation includes scrolling of the picture in addition to the updating of the picture data. Parallel transmission lines are provided between the shift register 74 for output and the memory cell array 71. Hence, the new data resulting from the processing are first registered in the shift register 74 for output and subsequently written in the corresponding word line in the memory cell array 71. When the data in the shift register 74 for output are all 0 without the reading of the data from the memory cell array 71 to the shift register 74 for output, and the writing is carried out under the OR logic of the parallel data transfer circuit 751, the data stored in the memory cell array 71 are changed into the data registered in the shift register 73 for input, so that high-speed writing is realized.

Claims: A semiconductor memory device comprising : a random-access memory (1) having a memory cell array, a plurality of bit lines (111, 112, ..., 11n; 121, 122, ..., 12n), and a plurality of word lines (131, 132, 133, ...; 141, 142, 143, ...); a pair of shift registers (3, 4) adapted to be connected with said plurality of bit lines and comprised of a shift register for output (e.g. 3) for carrying out serial reading and a shift register for input (e.g. 4) for registering data for one word line supplied serially from an external data source; transfer gate means (21, 22) arranged between said plurality of bit lines and the plurality of shift elements (BIT1, ..., BITn) of which each of said shift registers (3, 4) is composed; and parallel data transfer means (G1, G2, G3, G4; 5A, 5B; 85(1), 85(2), 85(3)) provided between said shift registers (3, 4) for carrying out the parallel data transfer between said shift registers. A device according to claim 1, wherein said parallel data transfer means comprises logic gates (5A; 5B) for effecting logic processing of the data to be transferred. A device according to claim 2, wherein said logic gates (5A; 5B) consist of exclusive OR gates. A device according to claim 3, wherein an additional shift register (3') is provided at the input side of said logic gates (5B). A semiconductor memory device comprising : a random-access memory having a memory cell array (71, 81), a plurality of bit lines (BL), and a plurality of word lines (WL); a shift register for output (74, 84) for carrying out serial reading, data for one word line registered in parallel from said memory cell array into said shift register for output (74, 84) being delivered serially from said shift register for output; a shift register for input (73, 83) for registering data for one word line supplied serially from an external data source; a parallel data transfer means (751, 85) for carrying out logic processing of the data registered in said shift register for output (74) and said shift register for input (73, 83); an instruction decoder (752) for delivering an instruction for a selected logic which should be executed in said parallel data transfer means (751); means (731) for serially transmitting the data for one word line to be supplied to said shift register for input (73, 83) and an operation code for instructing a selected logic to be supplied to said instruction decoder (752); and means for writing the data for one word line obtained as the result of the processing in said parallel data transfer means (751, 85) according to said instructed selected logic into the memory cells of a selected word line in said memory cell array (71, 81). A device according to claim 5, wherein said parallel data transfer means (85) consists of assemblies of parallel data transferring circuits (85(1), 85(2), 85(3)).

Inventors: OGAWA, JUNJI, TAKEMAE, YOSHIHIRO
Applicants: FUJITSU LIMITED
Application year: 1985
Main classification: G11C7
IPC codes: G11C7, H01L27