Abstract:
A memory device comprises an array of bitcells arranged as a plurality of rows of bitcells and a plurality of columns of bitcells, and has a plurality of wordlines and a plurality of readout channels. A control unit is configured to control access to the array of bitcells, wherein in response to a memory access request specifying a memory address the control unit is configured to activate a selected wordline and to activate the plurality of readout channels, and to access a row of bitcells in said array storing a data word and addressed by the memory address. The data word consists of a number of data bits given by a number of bitcells in each row of bitcells. The control unit is further configured to be responsive to a masking signal and, when the masking signal is asserted when said memory access request is received, the control unit is configured to activate only a portion of the selected wordline and a portion of the plurality of readout channels, such that only a portion of the data word is accessed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to memory devices. More particularly, this invention relates to controlling the access to such memory devices. 
     2. Description of the Prior Art 
       FIG. 1  schematically illustrates a known data processing apparatus which is embodied as a system-on-chip (SoC) device  10 . Within this SoC device  10  are provided a processor unit (CPU)  11  and two memory banks  12  and  13 . The CPU  11  uses the memory banks  12  and  13  to store data which it makes use of in its data processing operations. The CPU  11  accesses the memory banks  12  and  13  via a system bus  14  which couples these components together. The CPU  11  is configured to issue a memory access request onto the system bus  14  when it requires access (whether read or write) to data stored in one of the memory banks. As schematically illustrated in  FIG. 1 , the CPU  11  therefore passes an address onto the system bus and sends or receives (depending on whether the memory access request is a read or a write operation) data to/from the system bus. Each memory bank  12 ,  13  is respectively provided with a control unit  15 ,  16  which administers overall control of the respective memory bank, in particular interpreting the address received from the CPU  11  via the system bus  14  to cause the correct storage locations within the memory bank (typically configured as an array of bit cells) to be accessed. 
     The CPU  11  is additionally configured to generate a chip select signal which is also passed via the system bus  14  to the memory banks  12 ,  13 . This chip select signal acts as an overall enable signal with respect to the memory banks, and causes the entire memory bank to be powered up or powered down. The chip select signal may serve to indicate which of the memory banks  12 ,  13  the address passed from the CPU  11  should be applied to, for example in the situation where each memory bank covers the same address space and therefore the chip select signal is required to distinguish between the two. Even if there is no overlap in the memory spaces used by the two memory banks, it is generally desirable in a SoC device to reduce its power consumption as far as possible and accordingly the CPU  11  can make use of the chip select signal to power down a memory bank which is not currently in use. The control units  15 ,  16  of the memory banks  12 ,  13  are therefore configured to respond to the assertion of a chip select signal identifying that particular memory bank by causing it to power up or power down as appropriate. Whilst this technique is advantageous in terms of the power saving advantages it brings, the process of powering up/powering down a memory bank comes at the cost of some delay whilst this is carried out. 
     It would be desirable to provide an improved technique for reducing the power consumption of such memory devices. 
     SUMMARY OF THE INVENTION 
     Viewed from a first aspect, the present techniques provide a memory device comprising: 
     an array of bitcells, each bitcell configured to store a data bit, wherein said array comprises a plurality of rows of bitcells and a plurality of columns of bitcells; 
     a plurality of wordlines, wherein each row of bitcells has an associated wordline; 
     a plurality of readout channels, wherein each column of bitcells has an associated readout channel; and 
     a control unit configured to control access to said array of bitcells, wherein in response to a memory access request which specifies a memory address said control unit is configured to activate a selected wordline and to activate said plurality of readout channels, and to access a row of bitcells in said array storing a data word and addressed by said memory address, wherein said data word consists of a number of data bits given by a number of bitcells in each row of bitcells, 
     wherein said control unit is further configured to be responsive to a masking signal and, when said masking signal is asserted when said memory access request is received, said control unit is configured to activate only a portion of said selected wordline and a portion of said plurality of readout channels, such that only a portion of said data word is accessed. 
     The present techniques recognise that a power saving advantage may be derived in a memory device which stores data words which are accessed in response to memory access request in the situation where only a portion of a given data word needs to be accessed. The inventors of the present technique have for example recognised that it is surprising frequent that a memory access request to a memory device only requires access to a portion of the address data word. For example, in a memory device configured to store 64-bit data words, it may be the case that only, say, the lower 32 bits of a 64-bit data word require accessing (whether reading or writing). 
     To take advantage of this kind of situation, the control unit of the memory device according to the present techniques is configured to be responsive to a masking signal. When this masking signal is asserted in association with a memory access request, the control unit is configured only to partially activate the components of the memory device usually associated with the memory access request. In particular, rather than activating an entire wordline corresponding to the memory address specified in the memory access request, the control unit is configured to activate only a portion of that wordline. Similarly, rather than activate the full set of read out channels of the memory device corresponding to the full data word, the control unit is configured to only activate a portion of the read out channels when the masking signal is asserted. This results in a significant saving of dynamic clock power in the memory device when only a portion of the selected data word needs to be accessed in a memory access request. For example, it has been estimated that in a memory device comprising a bank of which only one half need be accessed a saving of approximately 40% of dynamic power can be achieved. Furthermore, by making this modification to the operation of the memory device at the level of wordline portion selection and read out channel portion selection, much faster activation/deactivation of (at least part of) the memory device can be achieved by comparison to the prior art use of chip select signals to power up and power down an entire memory bank. 
     Activation of a portion of a selected wordline could be provided in a number of ways, but in one embodiment each wordline of said plurality of wordlines comprises two partial wordlines, each partial wordline associated with part of each row of bitcells, and wherein said portion of said selected wordline is a selected partial wordline. Thus two partial wordline provide the functionality of one “full” wordline and the partial activation of a selected “full” wordline is provided by the activation of only one of those two partial wordlines. Equally, if both partial wordlines are activated then the effect is the same as if a single wordline covering a full bitcell row had been activated. 
     The activation of a selected wordline, or indeed of a portion of a selected wordline, could be provided in a number of ways, but in one embodiment activation of each partial wordline is dependent on a wordline clock signal and said control unit is configured to generate first and second wordline clock signals, wherein at least one of said first and second wordline clock signals is inactive when said masking signal is asserted. The division of a wordline clock signal into two distinct wordline clock signals thus provides a readily and selectively controllable arrangement by which activation of at least one partial wordline can be suppressed. 
     It should be appreciated that the portion of the data word which is accessed could in principle be any (non-trivial) subset of the full data word, but in one embodiment said portion of said selected wordline and said portion of said plurality of readout channels correspond to a selected half of said data word. 
     The selected half of the data word may in some embodiments comprise the most significant bit half of the data word whilst in other embodiments it may comprise the least-significant-bit half of the data word. For example in a 64-bit data word architecture, the present techniques can allow the memory access request to only be carried out with respect to the upper 32-bit half-word or to the lower 32-bit half-word, whilst saving much of the power consumption that would otherwise occur with respect to the unnecessary access to the other half-word. 
     The masking signal may take a number of forms, but in one embodiment said masking signal is a 2-bit signal, wherein a first bit of said 2-bit signal corresponds to said selected portion of said data word and a second bit of said 2-bit signal corresponds to a remaining portion of said data word. The provision of a bit of the masking signal corresponding to each portion of the data word (for example corresponding to each half-word within that data word) provides an advantageous degree of control over the modification to the memory access request since the activation of the memory device components corresponding to each portion of the data word can be individually controlled. Thus by selected assertion of this two-bit signal, a full (normal) memory access request can be carried out (neither bit asserted), a partial memory access request can be carried out (one bit asserted), or a dummy memory access request may be carried out (both bits asserted). 
     The portions of the data word accessed in response to a two-bit masking signal may, in some embodiments be selected halves of the data word. 
     In some embodiments the memory device is a system-on-chip device. 
     In some embodiments the memory device is further configured to receive a device enable signal, wherein said memory device is configured to transition into an active state in response to assertion of said device enable signal and said memory device is configured to transition into an inactive state in response to de-assertion of said device enable signal. Accordingly, the control unit of the memory device may additionally be responsive to a chip-select style signal which is configured to cause the entire memory device to power up or power down. 
     Viewed from a second aspect the present techniques provide a computer program storage media storing a memory complier computer program for controlling a computer to generate an instance of a memory device from a memory architecture associated with the memory compiler computer program, the memory architecture specifying a definition of circuit elements and data defining rules for combining those circuit elements, such that said instance generated specifies a memory device accordingly to the first aspect. The computer program storage medium will typically store the memory complier computer program in a non-transient form, as is the case when the computer program is for example stored on a removable storage medium such as a disk or a solid state memory. 
     Viewed from a third aspect the present techniques provide a memory device comprising an array of means for storing data bits, wherein said array comprises a plurality of rows of means for storing data bits and a plurality of columns of means for storing data bits; 
     a plurality of wordlines, wherein each row of means for storing data bits has an associated wordline; 
     a plurality of readout channels, wherein each column of means for storing data bits has an associated readout channel; and 
     means for controlling access to said array of means for storing data bits, wherein in response to a memory access request which specifies a memory address said means for controlling access is configured to activate a selected wordline and to activate said plurality of readout channels, and to access a row of means for storing data bits in said array storing a data word and addressed by said memory address, wherein said data word consists of a number of data bits given by a number of means for storing data bits in each row of means for storing data bits, 
     wherein said means for controlling access is further configured to be responsive to a masking signal and, when said masking signal is asserted when said memory access request is received, said means for controlling access is configured to activate only a portion of said selected wordline and a portion of said plurality of readout channels, such that only a portion of said data word is accessed. 
     Viewed from a fourth aspect the present techniques provide a method of storing data in a memory device wherein said memory device comprises: 
     an array of bitcells, each bitcell configured to store a data bit, wherein said array comprises a plurality of rows of bitcells and a plurality of columns of bitcells; 
     a plurality of wordlines, wherein each row of bitcells has an associated wordline; and 
     a plurality of readout channels, wherein each column of bitcells has an associated readout channel, the method comprising the steps of: 
     receiving a memory access request which specifies a memory address; 
     activating a selected wordline of said plurality of wordlines corresponding to said memory address; 
     activating said plurality of readout channels; and 
     accessing a row of bitcells in said array storing a data word and addressed by said memory address, wherein said data word consists of a number of data bits given by a number of bitcells in each row of bitcells, 
     wherein when a masking signal is received when said memory access request is received, only a portion of said selected wordline and a portion of said plurality of readout channels are activated, such that only a portion of said data word is accessed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described further, by way of example only, with reference to embodiments thereof as illustrated in the accompanying drawings, in which: 
         FIG. 1  is schematically illustrates a prior art system-on-chip device comprising a processor and two memory banks; 
         FIG. 2  schematically illustrates a memory device according to one embodiment; 
         FIG. 3  schematically illustrates the generation of various control signals in a control unit such as that illustrated in  FIG. 2 ; 
         FIG. 4  schematically illustrates a series of steps which are taken in a memory device in one embodiment; 
         FIG. 5  schematically illustrates the generation of a memory instance including modified control circuitry by a memory compiler in one embodiment; and 
         FIG. 6  schematically illustrates a general purpose computing system that may be used to run the memory compiler shown in  FIG. 5 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  schematically illustrates a memory device in one embodiment. This memory device  20  may for example replace one of the memory banks  12 ,  13  shown in the prior art system of  FIG. 1 . Accordingly, the memory device  20  in this example embodiment can represent a memory bank provided in a system-on-chip (SoC) device, in which the memory device  20  is accessible to a processing unit configured to perform data processing operations (such as the CPU  11  shown in  FIG. 1 ). The memory device  20  generally comprises an array of bit cells  22  each configured to store a data bit and which may be accessed under the control of the control unit  24 . As will be familiar to one of ordinary skill in the art, the bit cells  22  are arranged in a matrix of rows and columns, wherein wordline drivers  26  are provided to activate rows of bits cells for reading or writing, whilst in the sections of the memory device generally denoted “data path” in  FIG. 2  readout channels  28  are provided which correspond to each column of bit cells. These readout channels  28  each provide the necessary circuitry (sense amplifiers and so on) to read out the data value stored in a particular bit cell  22  when that bit cell is activated by its corresponding wordline driver, coupled to its respective bit line for readout, and so on. The output of each readout channel  28  is provided at the pins labelled Q on the periphery of the memory device. The architecture of the data processing system of which the memory device  20  forms part is based on an n-bit data word, and hence the output pins in  FIG. 2  are labelled Q[0] to Q[n−1]. 
     The control unit  24  is configured to receive various input signals, of which only those relevant to the present discussion are illustrated in  FIG. 2 . On the basis of these input signals, the control unit  24  generates various further control signals which operate within the memory device  20  to control its operation. In particular three such control signals are of significance, and discussed further, here namely the wordline signals WL, the sense amplifier enable signals SAE and the write clock signals WRITECLK. Whereas in the prior art these signals would be provided to the entire bank, according to the present techniques these signals are generated in versions which are specific to each half of the bank. Accordingly, as can be seen in  FIG. 2 , these signals are provided as {WL_L, SAE_L and WRITECLK_L} and {WL_R, SAE_R and WRITECLK_R}. In the example wordline driver shown closest to the control unit  24  in  FIG. 2  it can be seen that the wordline signals WL_L and WL_R are generated as the combination (‘AND’) of the signals ROWSEL and ROWCLK_L/ROWCLK_R respectively. ROWSEL is the usual row selection signal derived from part of the ADDRESS signal forming part of the memory access request, whilst the present techniques provide that the usual row clock signal ROWCLK is generated in two parts (ROWCLK_L/ROWCLK_R) for the respective halves of the bank. Similarly, the present techniques provide that the usual sense amplifier enable signals SAE and the write clock signals WRITECLK are generated in two parts (SAE_L/SAE_R and WRITECLK_L/WRITECLK_R) for the respective halves of the bank. The generation of these control signals will be discussed in more details below with reference to  FIG. 3 . 
     One particular input signal of relevance to the present description is the bank mask signal LREN [1:0]. When neither bit of this two-bit signal is asserted, i.e. when LREN[0] is 0 and LREN[1] is 0, the control unit  24  is configured to control the memory device  20  to carry out a “normal” memory access request in dependence on the remaining input signals received. For example, if the input signals define a read memory access request, the control unit  24  is configured to interpret the memory address specified in that memory access request to determine the row of bit cells which corresponds to that memory address. The corresponding wordline driver  26  is then controlled to activate the wordline corresponding to that row of bit cells. The appropriate wordline driver  26  thus activates a wordline across the full width of the array of bit cells, i.e. a full row of bits cells, which according to the labelling shown in  FIG. 2  covers both the “left bank” and the “right bank” note that the array of bits cells  22  of the memory device  20  represents a “bank” within the conventional terminology used to describe memory devices, and the phrases “left” and “right” are used with a particular meaning here, namely corresponding to the least significant bit and most significant bit halves of the data words that make up the full width of the memory array. In coordination with the activation of the appropriate wordline, the read out channels  28  of the memory device  20  are activated under control of the control unit  24  such that the word stored in the bit cells of the selected row can then be read out (via the bit lines which follow each column of bit cells in the usual fashion). Note that for simplicity of illustration only, only four read out channels  28  are explicitly illustrated in the lower portion of the memory device  20  whereas in reality a read out channel is provided for each column of bit cells. The requested data word is then presented at the outputs Q[0] to Q[n−1]. 
     By contrast when at least one bit of the LREN signal is asserted, the control unit  24  is configured to cause a modified memory access to be performed. For example, where the bit LREN[1] is asserted, the control unit  24  modifies the memory access procedure so that only bit cells in the “left bank” are accessed. In particular, when LREN[1] is asserted, only the signals WL_L, SAE_L and WRITECLK_L are generated, whilst the corresponding signals relating to the “right bank” are not generated, namely WL_R, SAE_R and WRITECLK_R. In this situation only the output pins Q[0] to Q[n/2−1] are active (toggle). 
     Conversely, when LREN[0] is asserted in association with the memory access request then generation of the control signals for the left bank are suppressed. Hence, only the signals WL_R, SAE_R and WRITECLK_R are generated, whilst the corresponding signals relating to the “left bank” are not generated, namely WL_L, SAE_L and WRITECLK_L. In this situation only the output pins Q[n/2] to Q[n−1] are active (toggle). 
     It is even possible for both bits of LREN to be asserted, for example for testing purposes, causing a dummy memory access to be carried out in which the wordlines, sense amplifier enable signals and write clock signals on both sides of the memory device are suppressed. These permutations are set out in the following table. 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Q [n − 
                 Q [(n/2) − 
                   
               
               
                   
                   
                 1:n/2] 
                 1:0] 
               
               
                 LREN 
                 Oper- 
                 Right-side 
                 Left-side 
               
               
                 [1:0] 
                 ation 
                 bits 
                 bits 
                 Comments 
               
               
                   
               
             
             
               
                 00 
                 Read/ 
                 Toggle 
                 Toggle 
                 Full read/write operation. 
               
               
                   
                 Write 
                   
                   
                 Word-lines toggle on both the 
               
               
                   
                   
                   
                   
                 left &amp; right banks. Max read/ 
               
               
                   
                   
                   
                   
                 write dynamic power. 
               
               
                 01 
                 Read/ 
                 No-change 
                 Toggle 
                 Partial read/write. Word-lines/ 
               
               
                   
                 Write 
                   
                   
                 SAE on the left-bank (LSB) are 
               
               
                   
                   
                   
                   
                 inactive. Word-lines/SAE on 
               
               
                   
                   
                   
                   
                 the right-bank (MSB) are active. 
               
               
                   
                   
                   
                   
                 ~40% less dynamic power 
               
               
                   
                   
                   
                   
                 compared to full read/write. 
               
               
                 10 
                 Read/ 
                 Toggle 
                 No-change 
                 Partial read/write. Word-lines/ 
               
               
                   
                 Write 
                   
                   
                 SAE on the left-bank (LSB) are 
               
               
                   
                   
                   
                   
                 active. Word-lines/SAE on the 
               
               
                   
                   
                   
                   
                 right-bank (MSB) are inactive. 
               
               
                   
                   
                   
                   
                 ~40% less dynamic power 
               
               
                   
                   
                   
                   
                 compared to full read/write. 
               
               
                 11 
                 Read/ 
                 No-change 
                 No-change 
                 Dummy read/write. Neither left- 
               
               
                   
                 Write 
                   
                   
                 bank nor right-bank word-lines/ 
               
               
                   
                   
                   
                   
                 SAE are active in this cycle. 
               
               
                   
                   
                   
                   
                 Negligible dynamic power 
               
               
                   
                   
                   
                   
                 compared to full read/write. 
               
               
                   
               
             
          
         
       
     
     Note that one input signal received by the control unit  24  is a chip select signal CEN. A global timing pulse (GTP) employed within the control unit  24  is generated in dependence on this chip select signal and the received clock signal CLK, such that the global timing pulse can only be generated within the memory device when the chip-select-signal CEN is asserted. Additionally the control unit  24  is configured to power down the memory device  20  when the chip-select-signal CEN is not asserted. This for example involves causing the headers  35  to be turned off. Whilst the de-assertion of the chip enable signal is an effective means of reducing power consumption of the memory device  20 , several clock cycles are required to enter (and exit) this power saving mode. Furthermore, the entire memory device becomes inaccessible when this power saving mode (chip select off) is engaged. 
       FIG. 3  schematically illustrates how various control signals are generated within the control unit  24  shown in  FIG. 2 . As mentioned above, the global timing pulse (GTP) requires both the clock signal (CLK) received by the control unit to be active and the chip select signal CEN. The sense amplifier enable signal for each half of the memory device (labelled SAE_X in  FIG. 3 ) requires the assertion of the general sense amplifier enable signal ISAE (generated within the control unit  20  in the usual fashion) and the non-assertion of corresponding bank mask signal LREN_X (where X is L or R as appropriate and it is understood that LREN[0] is equivalent to LREN_L and LREN[1] is equivalent to LREN_R). The row clock signals ROWCLK_X require the assertion of the global timing pulse GTP, the address selection signal ADDR_SEL (derived from the input signal ADDRESS) and the non-assertion of corresponding bank mask signal LREN_X. Finally the write clock signals WRITECLK_X require the assertion of the global timing pulse GTP, the global write enable signal GWEN (one of the input signals) and the non-assertion of corresponding bank mask signal LREN_X. 
       FIG. 4  schematically illustrates a sequence of steps which may be taking in one embodiment, in particular by the control circuitry of a memory device such as that illustrated in  FIG. 2 . Here “control circuitry” should be understood to mean not only the control unit  24 , but also the wordline drivers  26  and components in the read out channels  28  within the data paths. When a memory access is received at step  100  it is thereafter determined at step  102  if the LSB of the LREN signal (i.e. LREN[0]) is set to 0. If it is not, i.e. if this bit is asserted, then the flow proceeds to step  104  and the control signals generated by the control unit  24  with respect to the left bank of the array (i.e. ROWCLK_L, WRITE CLK_L and SAE_L) are inactive. Next at step  108  it is determined if the MSB of the LREN signal (i.e. LREN[1]) is set to 0. If it is not, i.e. if this bit is asserted, then the control signals generated by the control unit  24  with respect to the right bank of the array (i.e. ROWCLK_R, WRITECLK_R and SAE_R) are also inactive. Accordingly, in this configuration both halves of the bit cell array are inactive and a fully dummy read/write operation is carried out. By contrast if at step  108  it is determined that LREN[1] is 0 then (step  116 ) the above-mentioned control signals with respect to the right bank of the array are active. Accordingly, in this configuration a partial read/write operation is carried out. 
     Returning to step  102 , if it is determined that LREN[0] is set to 0 then (step  106 ) the control signals associated with the left bank of the array will be active. At step  110  it is determined if LREN[1] is asserted. If it is not then (step  112 ), the control signals associated with the right bank of the array are also active and a full read/write operation with respect to the bit cell array is carried out. If however at step  110  it is determined that LREN[1] is asserted then (step  114 ) the control signals associated with the right bank of the array are inactive. Accordingly, a partial read/write operation carried out. 
     It should be understood that the steps shown in  FIG. 4  are not taken in sequential order, but are merely illustrated in this fashion for ease of discussion. In reality the determination of the assertion of the LREN bits (steps  102 ,  108  and  110 ) takes place simultaneously, and the final state of the left bank (steps  104 ,  106 ) and right bank (steps  112 ,  114 ,  116 ,  118 ) thus result in parallel with one another. 
       FIG. 5  schematically illustrates how a memory instance including modified control (decode) circuitry and write driver circuitry in accordance with the above described embodiments may be created from a memory compiler  700  with reference to a memory architecture  710 . The memory architecture  710  specifies a definition of circuit elements and data defining rules for combining those circuit elements in order to create a memory instance. Particular requirements for the memory instance are entered into the memory compiler  700  as input parameters via a graphical user interface (GUI). As will be appreciated by those skilled in the art, such input parameters can specify various aspects of the desired memory instance, for example defining the size of the memory array, the multiplexing arrangements of the memory array, selection of various optional features such as power gating features, built-in-self-test (BIST) modes to be supported, etc. 
     The memory compiler  700  then generates the required memory instance based on the input parameters and the memory architecture  710 . In accordance with one embodiment, the memory compiler modifies the control circuitry (i.e. control unit circuitry and write driver circuitry) so that the above discussed “left bank” and “right bank” specific control signals (WL_L/R, ROWCLK_L/R, SAE_L/R and WRITECLK_L/R) within the memory instance are provided to enable the configurations described with reference to  FIGS. 2-4  above. 
       FIG. 6  schematically illustrates a general purpose computer  800  of the type that may be used to implement the above described memory compilation operation in order to generate a memory instance. The general purpose computer  800  includes a central processing unit  802 , a random access memory  804 , a read only memory  806 , a network interface card  808 , a hard disk drive  810 , a display driver  812  and monitor  814  and a user input/output circuit  816  with a keyboard  818  and mouse  820  all connected via a common bus  822 . In operation the central processing unit  802  will execute computer program instructions that may be stored in one or more of the random access memory  804 , the read only memory  806  and the hard disk drive  810  or dynamically downloaded via the network interface card  808 . The results of the processing performed may be displayed to a user via the display driver  812  and the monitor  814 . User inputs for controlling the operation of the general purpose computer  800  may be received via the user input output circuit  816  from the keyboard  818  or the mouse  820  (and hence for example the input parameters used to determine certain properties of the required memory instance can be entered via this mechanism). It will be appreciated that the computer program could be written in a variety of different computer languages. The computer program may be stored and distributed on a recording medium or dynamically downloaded to the general purpose computer  800 . When operating under control of an appropriate computer program, the general purpose computer  800  can perform the above described memory compiler operation and can be considered to form an apparatus for performing the above described memory compiler operation. The architecture of the general purpose computer  800  could vary considerably and  FIG. 6  is only one example. 
     Although particular embodiments of the invention have been described herein, it will be apparent that the invention is not limited thereto, and that many modifications and additions may be made within the scope of the invention. For example, various combinations of the features of the following dependent claims could be made with the features of the independent claims without departing from the scope of the present invention.