Abstract:
A memory circuit ( 14 ) having features specifically adaptedto permit the memory circuit ( 14 ) to serve as a video frame memory is disclosed. The memory circuit ( 14 ) contains a dynamic random access memory array ( 24 ) with buffers ( 18, 20 ) on input and output data ports ( 22 ) thereof to permit asynchronious read, write and refresh accesses to the memory array ( 24 ). The memory circuit ( 14 ) is accessed both serially and randomly. An address generator ( 28 ) contains an address buffer register ( 36 ) which stores a random access address and an address sequencer ( 40 ) which provides a stream of addresses to the memory array ( 24 ). An initial address for the stream of addresses is the random access address stored in the address buffer register ( 36 ).

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates in general to digital memory circuits. Specifically, the present invention relates to digital memory circuits which have particular advantages when used in connection with video applications.  
         BACKGROUND OF THE INVENTION  
         [0002]    Digital TV, VCR, and related video applications often utilize a frame or field memory that stores pixels which together represent an entire frame of video. Such a frame memory is used in producing a variety of special effects, such as frame freezing, zoom, pan, split screen monitoring, and the like. Although a frame memory may be constructed using conventional discrete integrated circuits, such a frame memory is relatively expensive, dissipates an undesirably large amount of power, and occupies an undesirably large amount of space. When such a frame memory is targeted for use in a commercial product, these problems are major ones. Accordingly, a single integrated circuit, either alone or in combination with as few other integrated circuits as possible, improves upon a frame memory which has been constructed from conventional discrete integrated circuits.  
           [0003]    Prior art integrated circuit devices have attempted to address the frame memory problem. However, such devices fail to provide an architecture which adequately addresses video application needs. For example, devices which include only a few of the typically needed frame memory functions may be used in providing a wide variety of special effects. However, they must be combined with such a large quantity of conventional discrete integrated circuits that little improvement results over constructing a frame memory entirely from conventional discrete integrated circuits. On the other hand, a conventional frame memory integrated circuit may include a random access memory with complete on-chip address calculation. A video application which utilizes such a frame memory accesses the entire frame memory serially. Thus, frame freeze and split screen monitoring special effects are supported. However, zoom and pan functions are either impossible or impractical using such a device.  
           [0004]    Accordingly, the industry feels a need for a frame memory integrated circuit which optimizes circuit architecture to accommodate a wide variety of special effects without requiring a large quantity of surrounding integrated circuits.  
         SUMMARY OF THE INVENTION  
         [0005]    Accordingly, it is an advantage of the present invention that a frame memory circuit is provided which permits limited random access. Consequently, a device constructed according to the teachings of the present invention may be efficiently used to perform a wide variety of special effect video applications.  
           [0006]    Another advantage of the present invention is that a memory circuit is provided which includes a variety of address calculation modes. Thus, a portion of the address calculations for certain special effect functions may be transferred to the memory circuit, and a video application which utilizes such a memory circuit need not allocate processing power to such calculations.  
           [0007]    The above advantages of the present invention are carried out in one form by a memory circuit which stores and provides streams of data. This memory circuit supports both serial access and random access. A data input of a random access memory array couples to a data buffer so that the data buffer may synchronize operation of the memory array with the streams of data. An address input of the random access memory array couples to an address sequencer which generates a sequence of memory addresses that are successively applied to the memory array. An address buffer register also couples to the address sequencer. The address buffer register supplies a random access address to the address sequencer to initialize the sequence of memory addresses supplied by the address sequencer.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the accompanying drawings, in which like reference numbers indicate like features throughout the drawings, and wherein:  
         [0009]    [0009]FIG. 1 illustrates a frame of a video display screen with which the present invention may be used;  
         [0010]    [0010]FIG. 2 shows a block diagram of a memory circuit built according to the teachings of the present invention;  
         [0011]    [0011]FIG. 3 shows a block diagram of a first alternate embodiment of an address generator portion of a memory circuit built according to the teachings of the present invention;  
         [0012]    [0012]FIG. 4 shows a block diagram of a second alternate embodiment of an address generator portion of a memory circuit built according to the teachings of the present invention; and  
         [0013]    [0013]FIG. 5 shows a block diagram of an address sequencer utilized by the address generator portion of a memory circuit built according to the teachings of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0014]    [0014]FIG. 1 illustrates a video frame  10 , such as may appear on a TV tube or other video display terminal. Although frame  10  may appear as a continuous video picture to a viewer, frame  10  may be electrically represented as a multiplicity of digitized pixels  12 . Each one of pixels  12  defines parameters, such as color and relative intensity, for one of a multiplicity of very small areas within the picture of frame  10 . Accordingly, frame  10  may contain a relatively large number of pixels  12 . For example, a frame containing 488 columns of pixels  12  by 488 rows of pixels  12  has a total of 238,144 pixels per frame.  
         [0015]    Pixels  12  are typically transmitted or otherwise processed in a predetermined order to preserve the spatial relationships between pixels  12 . For example, in a conventional raster scan application, pixels  12  may be transmitted to a memory device or a video display in successive order beginning with a pixel  12   a , that represents the pixel  12  in the first column of the first row of frame  10 , and continuing in successive order to a pixel  12   b , which represents the pixel  12  in the last column of the first row of frame  10 . Immediately following the transmission of pixel  12   b  and sync information (not shown), a pixel  12   c , which represents the pixel  12  in the first column of the second row, may be transmitted followed in successive order by the remaining pixels  12  contained in the second row of frame  10 . Transmission of pixels  12  continues in this fashion until a pixel  12   d , which represents the pixel  12  in the last column of the last row of frame  10 , has been transmitted. Thus, any processing device which knows the timing relationship between a pixel  12  and beginning pixel  12   a  also knows or can easily calculate the spatial location of such pixel  12  within frame  10 .  
         [0016]    A digital TV, VCR, or the like may contain a large frame or field memory which is capable of storing all of pixels  12  within frame  10 . Pixels  12  collectively appear as a serial data stream to the frame memory. Except for special effects, the relative order of pixels  12  in this serial data stream must generally be preserved when read from the frame memory to preserve the spatial relationships between pixels  12 . However, various special effects do not require this preserved order, and valuable computation time may be wasted by precisely preserving the order of pixels  12  as pixels  12  are being read from the frame memory.  
         [0017]    One such special effect is a zoom effect wherein a small portion of a frame is expanded to fill an entire video display. For example, if frame  10  in FIG. 1 represents an entire video display, then an area within frame  10  bounded by rows i and j and columns m and n is expanded in a zoom special effect to fill the entire frame  10 . Thus, in the zoom special effect all of pixels  12  residing within frame  10  outside of the area bounded by rows i and j and columns m and n are inactive and may be discarded. In other words, these inactive ones of pixels  12  need not be stored or read from the frame memory. Consequently, the pixel  12  located at column m and row i will be utilized as first pixel  12   a  in the zoom special effect. Active pixels  12  may be duplicated to complete an entire row of frame  10 , and rows may be duplicated to complete the vertical component of the zoom effect.  
         [0018]    In a split screen special effect, an entire frame  10  may be shrunk into a small area of a screen, such as that bounded by row j and the last row of frame  10 , and column n and the last column of frame  10 . This special effect is accomplished by utilizing only one of pixels  12  out of each of a predetermined number of pixels  12  from an entire frame  10  of pixels  12 , and ignoring the intervening inactive ones of pixels  12  (i.e. skipping pixels). For the example depicted in FIG. 1, the shrunken frame is formed using only pixels  12  from one of every three columns and one of every three rows of frame  10 .  
         [0019]    The present invention provides a memory circuit which series as a frame memory and permits these and other special effects to be performed efficiently. FIG. 2 shows a block diagram of a memory circuit  14  built according to the teachings of the present invention. In general, the preferred embodiment of memory circuit  14  represents a single chip integrated circuit that contains 2 20  or 1,048,576 bits of memory storage organized as 262,144 four bit wide words. Accordingly, a sufficient quantity of words are provided to buffer or store an entire 488×488 frame of pixels  12  (see FIG. 1). If more than four bits of precision are required to accurately describe each pixel, then additional ones of memory circuit  14  may be used to store such additional bits.  
         [0020]    Memory circuit  14  generally operates in a serial access mode but has particular features which permit random access of memory circuit  14  on a limited scale. Those skilled in the art will understand that serial access refers to a mode of storing and reading data in which data must be read out from a memory in the same order in which it was stored into the memory. Furthermore, random access refers to the ability to write, read, or otherwise access any location in a memory array by suppiying a unique address which corresponds to such memory location.  
         [0021]    Specifically, memory circuit  14  includes a serial pixel data input  16   a , which in the preferred embodiment supplies four bits of data. Serial pixel data input  16   a  couples to an input port of a write serial latch  18   a , and an output port of write serial latch  18   a  couples to an input port of a write register  20   a . An output port of write register  20   a  couples to a data input port  22   a  of a memory array  24 . in the preferred embodiment, memory array  24  is a dynamic random access memory (DRAM) array containing 2 18  or 262,144 four bit memory locations. A data output port  22   b  of memory array  24  couples to a data input port of a read register  20   b , and a data output port of read register  20   b  couples to a data input port of a read serial latch  18   b . A data output port of read serial latch  18   b  couples to a serial pixel data output  16   b , which in the preferred embodiment provides four bits of data.  
         [0022]    A serial write clock terminal  26   a  couples to a write address generator  28   a , an arbitration and control circuit  30 , and a clock input of write serial latch  18   a . Similarly, a serial read clock terminal  26   b  couples to a read address generator  28   b , arbitration and control circuit  30 , and a clock input of read serial latch  18   b . A refresh address and timing circuit  32  has an output which couples to an input of arbitration and control circuit  30 , and outputs from arbitration and control circuit  30  couple to a clock input of write register  20   a , a clock input of read register  20   b , a control input of memory array  24 , and an address input of memory array  24 .  
         [0023]    As shown in FIG. 2, address generators  28   a  and  28   b  are structurally similar to one another in the preferred embodiment. thus, a write control data terminal  34   a  couples to a serial data input of an address buffer register  36   a  in write address generator  28   a . A read control data terminal  34   b  couples to a serial data input of an address buffer register  36   b  in read address generator  28   b . Likewise, a write control strobe terminal  38   a  couples to a clock input of address buffer register  36   a , and a read control strobe terminal  38   b  couples to a clock input of address buffer register  36   b . A data output of address buffer register  36   a  couples to a data input of an address sequencer  40   a , and a data output of address buffer register  36   b  couples to a data input of an address sequencer  40   b . A write reset terminal  42   a  couples to a clear input of address sequencer  40   a , and a write transfer terminal  44   a  couples to a preset input of address sequencer  40   a . A read reset terminal  42   b  couples to a clear input of address sequencer  40   b , and a read transfer terminal  44   b  couples to a preset input of address sequencer  40   b . Terminal  26   a  couples to a clock input of address sequencer  40   a  within address generator  28   a , and terminal  26   b  couples to a clock input of address sequencer  40   b  within address generator  28   b . An output  46   a  of address sequencer  40   a  presents the output signal from address generator  28   a  and couples to an input of arbitration and control circuit  30 . Likewise, an output  46   b  of address sequencer  40   b  presents the output signal from address generator  20   b  and couples to arbitration and control circuit  30 . Memory circuit  14  may be provided in a 20 pin integrated circuit package.  
         [0024]    As discussed above, memory circuit  14  may be operated in either a serial or a limited random access mode. In addition, the storing or writing of data into memory circuit  14  may occur asynchroniously with the reading or providing of data from memory circuit  14 . Memory circuit  14  may be written into serially by activating write reset signal on terminal  42   a  to clear address sequencer  40   a . Then, a four bit wide stream of serial data may be stored in memory circuit  14  by applying the four bit data nibbles at data input  16   a  while asserting a serial write clock signal at terminal  26   a.  One assertion of the serial write clock signal causes write serial latch  18   a  to temporarily store or buffer one four bit data nibble. Write serial latch  18   a  operates as a four bit wide shift register. Thus, subsequent four bit nibbles from the data stream of serial pixel data applied at data input  16   a  are shifted into serial latch  18   a  upon subsequent assertions of the serial write clock signal.  
         [0025]    In addition, each assertion of the serial write clock signal causes address sequencer  40   a  of write address generator  28   a  to supply a new a random access address to arbitration and control circuit  30 . In other words, address sequencer  40   a  provides a stream of addresses to arbitration and control circuit  30  which corresponds to the stream of data being stored in write serial latch  18   a.    
         [0026]    Arbitration and control circuit  30  receives addresses from address generators  28   a - 28   b  and refresh address and timing circuit  32 . Circuit  30  monitors these inputs and various timing signals to decide which of the addresses provided on these inputs should be transferred to memory array  24 . Arbitration and control circuit  30  includes conventional logic circuits for controlling the timing operation of the dynamic memories which comprise memory array  24 . Thus, arbitration and control circuit  30  passes an address generated by address generator  28   a  onto memory array  24  so that data may be written into memory array  24 , but a delay may occur due to refresh operations or read accesses of memory array  24 . Accordingly, arbitration and control circuit  30  may additionally contain storage devices so that addresses generated by address generators  28   a - 28   b  are not lost when immediate access to memory array  24  is blocked. When arbitration and control circuit  30  identifies a time at which the serial pixel data may be written into memory array  24 , such data is transferred from write serial latch  18   a  into write register  20   a  and then written into memory array  24 . Accordingly, write serial latch  18   a  and write register  20   a  together represent a double buffering scheme which permits asynchronious operation of memory array  24  with the storing of serial pixel data into memory circuit  14 .  
         [0027]    The reading of data from memory array  24  occurs in a manner similar to that described above for the storing of data into memory array  24 . Thus, an address generated by address generator  28   b  is transferred through arbitration and control circuit  30  at an appropriate time to cause data from memory array  24  to be read into read register  20   b . Thereafter, this data is transferred into read serial latch  18   b  so that such data may be provided at data output terminal  16   b  through the application of a serial read clock signal at terminal  26   b . Serial data is provided at output  16   b  asynchroniously with the operation of memory array  24  and asynchroniously with the storing of serial pixel data into memory circuit  14  at terminal  16   a.    
         [0028]    The limited random access feature of memory circuit  14  is provided through address generators  28   a - 28   b . In the embodiment of memory circuit  14  shown in FIG. 2, write address generator  28   a  and read address generator  28   b  are structurally and operationally identical, except that write address generator  28   a  provides write addresses while read address generator  28   b  provides read addresses. Accordingly, both of address generators  28   a - 28   b  are described below by reference only to write address generator  28   a . Those skilled in the art will recognize that read address generator  28   b  operates identically in the preferred embodiment.  
         [0029]    A random access address may be serially loaded into address buffer register  36   a  by applying such address to control data terminal  34   a  in a sequential manner and activating a control strobe signal applied at terminal  38   a  when valid data appear at terminal  34   a . Thus, in the embodiment shown in FIG. 2, address buffer register  36   a  represents a serial shift register. The use of a serial shift register conserves the number of external pins needed for constructing memory circuit  14  in an integrated circuit when compared to a parallel loaded register. After the random access address has been entered into address buffer register  36   a , it may be transferred to data sequencer  40   a  by the application of a write transfer signal at terminal  44   a . In the preferred embodiments of the present invention, address sequencer  40   a  may represent a presetable, binary counter or other presetable sequencing circuit. Thus, the transferred address initiates a sequence of addresses which are subsequently generated by address generator  28   a . If address sequencer  40   a  represents a binary counter, then subsequent addresses will increment or decrement starting with this preset value.  
         [0030]    If memory array  24  contains 2 four bit words of memory, then address buffer register  36   a  may advantageously represent an 18 bit register, and address sequencer  40   a  may represent an 18 bit counter, or other sequencing circuit. On the other hand, address buffer register  36   a  and address sequencer  40   a  may contain fever bits, such as nine bits for example. In the nine bit situation, the random access address provided by address buffer register  36   a  could access the beginning of memory pages or rows wherein each page or row contains 2 9  or 512 words of memory.  
         [0031]    The inclusion of address buffer register  36   a  to provide a limited random access feature permits memory circuit  14  to be efficiently utilized in a zoom special effect. For example, a zoom effect may be accomplished by writing an entire frame of memory into memory array  24  using a serial access mode. A beginning pixel address, such as the address of a pixel located at row i column m, in FIG. 1, may then be loaded into read address buffer register  36   b  and transferred to address sequencer  40   b . A first row, such as row i, of the portion of frame  10  which is to be expanded into an entire frame may then be read from memory array  24  in a serial or sequential mode until a pixel corresponding to, for example, row i, column n appears at output terminal  16   b . A row may be repeated as often as necessary to achieve vertical zoom by transferring the random access address from address buffer register  36   b  to address sequencer  40   b . An address corresponding to the pixel located at row i+1 and column m may then be loaded into address buffer register  36   b  and transferred to address sequencer  40   b . This process continues until a final pixel for the frame to be expanded has been output from memory array  24 . Due to this feature, a video system need not start accesses of memory circuit  12  at an initial address, such as pixel  12   a  (shown in FIG. 1) and access inactive pixels stored within memory array  24 . Faster operation results.  
         [0032]    The present invention contemplates alternate embodiments of address generators  28   a - 28   b . A first alternate embodiment of address generators  28   a - 28   b  is shown in FIG. 3. FIG. 3 shows only one of address generators  28 . The address generator  28  shown in FIG. 3 may serve as either write address generator  28   a  or read address generator  28   b  (see FIG. 2).  
         [0033]    In this first alternate embodiment of an address generator  28 , address buffer register  36  may be loaded both serially and in parallel. Thus, control data terminal  34 , which may represent either write control data terminal  34   a  or read control data termin,al  34   b , as discussed above in connection with FIG. 2, couples to the serial data input of address buffer register  36 . Control strobe terminal  38  couples to the serial clock input of address buffer register  36  and a serial clock input of an address offset register  48 . The parallel data output of address buffer register  36  couples to a first input of an adder  50  and the data input of address sequencer  40 . A parallel data output of address offset register  46  couples to a second input of adder  50 . An output of adder  50  couples to a parallel data input of address buffer register  36 , and transfer terminal  44  couples to a parallel clock input of address buffer  36  and the preset input of address sequencer  40 . A most significant bit from the parallel data output or a serial output bit, of address buffer register  36  couples to a serial data input of address offset register  48 . Serial clock terminal  26  couples to the clock input of address sequencer  40 , and reset terminal  42  couples to a clear input of address sequencer  40 . A data output of address sequencer  40  couples to address generator output  46 .  
         [0034]    Address buffer register  36  and address sequencer  40  operate in this first alternate embodiment similarly to their above-described operation in connection with address generator  28   a - 28   b  of FIG. 2. However, in this first alternate embodiment, the control data provided at terminal  34  is used to load both address buffer register  36  and address offset register  48 . Thus, additional bits of control data are loaded into memory circuit  14  without requiring additional integrated circuit pins. Moreover, a most significant bit, or a serial output bit  51 , from address offset register  48  may advantageously be routed to the control data input for the other one of read and write address generators  28   a  and  28   b  (see FIG. 1). In addition, the control strobe signal applied at terminal  38  may be routed to the other one of control strobe terminals  38   a  and  38   b  of FIG. 2. These two connections between address generators  28   a  and  28   b  eliminate two integrated circuit pins from the structure shown in FIG. 2.  
         [0035]    In this first alternate embodiment of the present invention, the control data contained in address offset register  48  is added to a current initial address value contained in address buffer register  36  to provide a new initializing random access address value. This new initializing value is loaded into address buffer register  36  when the current address value is transferred into address sequencer  40 .  
         [0036]    Referring additionally to FIG. 1, the first alternate embodiment of the present invention may be advantageous in performing, for example, the zoom special effect. Thus, the address offset value loaded into address offset register  48  may represent the quantity of inactive pixels occurring between column n of one row and column m of the next row. At the end of each frame row a transfer signal may be asserted on terminal  44 , and the random access address of the next active pixel, corresponding to column n of the next row, is automatically calculated and stored in address buffer register  36  to initiate another sequence of sequential accesses to memory circuit  16 . Complexity of a video system employing memory circuit  14  decreases because components external to memory circuit  14  need not calculate this address.  
         [0037]    A second alternate embodiment of address generators  28   a - 28   b  from FIG. 2 is shown in FIG. 4. The FIG. 4 embodiment illustrates that random access addresses may be loaded into address buffer register  36  in a parallel fashion, which may be more compatible with conventional microprocessor integrated circuits. However, the number of integrated circuit pins needed to implement this embodiment increases over the embodiments discussed above in connection with FIGS. 2 and 3. In addition, FIG. 4 shows the inclusion of an alternate address buffer register  52  in addition to address buffer register  36 . Specifically, control data terminals  34  may advantageously provide an eight bit microprocessor data bus which couples to data inputs of individual eight bit portions  54   a ,  54   b , and  54   c  of address buffer register  36 . In addition, control data terminals  34  couple to data inputs of individual eight bit portions  56   a ,  56   b , and  56   c  of alternate address buffer register  52 . Data outputs of individual portions  54   a - 54   c  together form a 24 bit bus which couples to a first data input of a multiplexer  58 . Likewise, data outputs of individual portions  56   a - 56   c  form a 24 bit bus which couples to a second data input of multiplexer  58 . A data output of multiplexer  58  couples to a data input of a binary counter which serves as address sequencer  40  in this second alternate embodiment. Of course, those skilled in the art will recognize that the number of subregisters included within address buffer register  36  and alternate address buffer register  52  and the number of bits contained within the buses described above are subject to a substantial variation in accordance with specific application requirements.  
         [0038]    In addition, microprocessor address input terminals  60   a ,  60   b , and  60   c , couple to address inputs of a decoder  62  and an address input terminal  60   d  couples to an enable input of decoder  62 . The control strobe terminal  38 , discussed above, couples to an enable input of decoder  62 . Outputs O 1 -O 6  of decoder  62  couple to clock inputs of individual address buffer register portions  54   a - 54   c  and clock inputs of individual alternate address buffer register portions  56   a - 56   c , respectively. An output  07  from decoder  62  couples to a clock input of a flip flop  64  which is configured to toggle upon the activation of the clock input. An output of flip flop  64  couples to a select input of multiplexer  58 . An output O 8  of decoder  62  couples to a preset input of binary counter  40 . The serial clock  26  couples to a clock input of binary counter  40 , and reset terminal  42  couples to a clear input of flip flop  64  and a clear input of binary counter  40 . An output of binary counter  40  couples to output  46  of address generator  28 .  
         [0039]    In this second alternate embodiment of address generator  28 , one initializing random access address may be stored in address register  36  while an alternate initializing random access address is stored in alternate address buffer register  52 . A microprocessor (not shown) may store these addresses in memory circuit  14  through conventional memory or I/O write operations to addresses specified by signals applied on terminals  60   a - 60   c . An address input bit applied at terminal  60   d  may advantageously distinguish between a write address generator  28   a  and a read address generator  28   b  (see FIG. 1). By applying an active signal to reset terminal  42 ; flip flop  64  and binary counter  40  may be initialized to a cleared state. At this point, address generator  28  operates substantially as described above in connection with FIG. 2. However, an alternate random access address stored in alternate address buffer  52  may selectively preset binary counter  40 . A microprocessor write operation which toggles flip flop  54 , followed by a microprocessor write operation that transfers data into binary counter  40 , presets binary counter  40  with an alternate random access address. Flip flop  64  may be toggled by performing a write operation to the address which activates output O 7  of decoder  62 . A transfer operation from the selected one of address buffer registers  36  and  52  occurs by writing to the address which activates the output O 8  of decoder  62 .  
         [0040]    Alternate address buffer register  52  may advantageously be used by a video system to efficiently buffer a line within a frame of data. Since memory circuit  14  of the preferred embodiment contains a sufficient quantity of memory to accommodate 2 18  or 262,144 pixels, memory circuit  14  has unused memory locations when used to store a single frame of data which contains, for example, 480 pixel columns by 480 pixel rows. Accordingly, a random access address in this unused portion of memory may be loaded in alternate address buffer register  52 . A single line of a frame may be efficiently stored in memory circuit  14  by transferring this alternate address value to binary counter  40 , then sequentially storing such line of pixels into the otherwise unused portion of memory circuit  14 .  
         [0041]    In addition, the present invention contemplates alternative embodiments for address sequencer  40 . As shown in FIG. 4, address sequencer  40  may represent a conventional presetable, clearable, binary counter. Such circuits are well known to those skilled in the art and need not be described in detail herein. However, address sequencer  40  may alternatively represent a circuit which increments or decrements by a variable step value which may differ from the value of one. Such a circuit is shown in FIG. 5.  
         [0042]    Accordingly, in FIG. 5 the address sequencer data input couples to a first input of a multiplexer  66 , the address sequencer&#39;s preset terminal couples to a select input of multiplexer  66 . An output of multiplexer  66  couples to a data input of a register  68 , and the clock input of address sequencer  40  couples to a clock input of register  68 . Likewise, the reset terminal  42  couples to a clear input of register  68 . A data output of register  68  provides the data output of address sequencer  40  and additionally couples to a first input of an adder  70 . An output of adder  70  couples to a second input of multiplexer  66 . The control data terminals  34 , discussed above in connection with FIGS.  2 - 4 , couple to a data input of a register  72 . Additionally, the control strobe terminal  38 , discussed above in connection with FIGS.  2 - 4 , couples to a clock input of register  72 . A data output of register  72  couples to a second input of adder  70 .  
         [0043]    In this FIG. 5 embodiment of address sequencer  40 , register  72  may represent either a parallel or a serially loaded register, as discussed above in connection with FIGS.  2 - 4 . Additionally, if register  72  represents a serially loaded register, then register  72  may represent one register out of many coupled together in a long chain of serially loaded registers, as discussed above in connection with FIG. 3. The data loaded into register  72  is intended to represent a increment step by which address sequencer  40  generates successive addresses at output  46  of address generator  28 . A current output of address sequencer  40  is added to this step increment value in adder  70 , and routed through multiplexer  66  back to register  68 . Thus, a subsequent address generated by address sequencer  40  equals the previous address plus the address step increment contained in register  72 . This address step increment need not equal the value of one but may equal any positive or negative value. Furthermore, if the number of bits contained in the buses which couple register  72 , adder  70 , multiplexer  66 , and register  68  together is greater than the number of bits provided at the output of address sequencer  40 , then subsequent addresses may be incremented in fractional steps.  
         [0044]    Address sequencer  40  may be preset, or initialized, with a random access address by applying an active signal on the preset terminal, supplying data at the data input terminals, and clocking the clock signal of address sequencer  40 . Thus, this initializing random access value is loaded directly into register  68 . In addition, address sequencer  40  may be cleared, or reset, by applying a reset signal to the clear input terminal.  
         [0045]    Referring additionally to FIG. 1, the address sequencer  40  depicted in FIG. 5 is useful in performing the split screen special effect where an entire frame is displayed in only a small portion of a video screen, such as the lower right hand portion shown in FIG. 1. With this special effect, if memory circuit  14  has every pixel  12  of a frame  10  stored therein, then only one out of every group of a predetermined number of stored pixels is active in constructing the shrunken screen. Address sequencer  40  shown in FIG. 5 allows memory circuit  14  to provide only the active pixels by supplying a sequence of addresses which omits inactive pixel addresses.  
         [0046]    In summary, the present invention provides a memory circuit which allows a video system to efficiently perform special effects. Specifically, the inclusion of various limited random accessing features allows memory circuit  14  to store and/or provide only active pixels for a given special effect and not inactive pixels.  
         [0047]    Consequently, active pixels may be retrieved from memory circuit  14  much quicker than occurs with the use of prior art frame memory circuits.  
         [0048]    The foregoing description uses preferred embodiments to illustrate the present invention. However, those skilled in the art will recognize that changes and modifications may be made in these embodiments without departing from the scope of the present invention. For example, read address generator  28   b  need not precisely resemble write address generator  28   a . Additionally, although the embodiments depicted in FIGS. 3-5 are mentioned above as being alternative embodiments, nothing prevents one skilled in the art from combining the teachings from more than one of these alternate embodiments into a single frame memory circuit  14 . Moreover, those skilled in the art will recognize that additional address processing cap abilities may be built into frame memory circuit  14 . Such additional address processing capabilities may include the addition of a signal which indicates the end of a frame line, a signal which indicates the end of a frame, and the automatic transferring of random access addresses to an address sequencer upon the occurrence of the end of line and end of frame signals. Furthermore, although specific frame and memory array dimensions have been presented herein to aid in teaching the present invention, it is intended that the present invention not be limited to any particular dimensions. These and other modifications obvious to those skilled in the art are intended to be included within the scope of the present invention.