Patent Publication Number: US-9412443-B2

Title: Processor system having variable capacity memory

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-053661, filed Mar. 15, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a processor system having a variable capacity memory. 
     BACKGROUND 
     Attempts have been made to realize a power saving processor by utilizing a magnetic random access memory (MRAM) as a cache memory. However, compared to a static random access memory (SRAM) as a conventional cache memory, the MRAM is slow in operation speed. Therefore, a cell configuration for realizing high-speed operation is now being developed. 
     Further, in processors for use in, for example, mobile products, remarkable enhancement in functionality has been recently made in addition to power saving. Because of this, different types of functionality or performance are required for different applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a 1T-1MTJ type STT-MRAM; 
         FIG. 2  is a diagram showing a sense amplifier in  FIG. 1 ; 
         FIG. 3  is a diagram showing a relationship between a read current and an access time; 
         FIG. 4  is a diagram showing a 2T-2MTJ type STT-MRAM; 
         FIG. 5  is a diagram showing a sense amplifier in  FIG. 2 ; 
         FIG. 6  is a diagram showing an entire configuration of a variable capacitance STT-MRAM; 
         FIG. 7  is a diagram showing a switching section in a cell configuration of the variable capacitance STT-MRAM; 
         FIG. 8  is a diagram showing an operation principle of MRAM in  FIG. 7 ; 
         FIG. 9  is a diagram showing a processor system; 
         FIG. 10  is a flowchart of mode change; 
         FIG. 11  is a diagram showing a relationship between a cell configuration and a number of ways; 
         FIG. 12  is a diagram showing a relationship between a data set, a block and a memory macro; 
         FIG. 13  is a diagram showing a principle of optimal capacitance prediction; and 
         FIG. 14  is a diagram showing a hardware structure for an adaptive capacitance control. 
     
    
    
     DETAILED DESCRIPTION 
     In general, in accordance with one embodiment, a processor system comprises: a variable capacity memory; and a processor which instructs a read/write of one bit to the variable capacity memory. The variable capacity memory comprises: a memory cell array including basic units, each of the basic units including one cell transistor and one variable resistance element; a mode selector switching between first and second modes, a read/write of one bit executed in 2 n  basic units (n is an integer) among the basic units in the first mode, the read/write of the one bit executed in 2 m  basic units (m is an integer, m≠n) among the basic units in the second mode; and a control circuit which controls the switching between the first and second modes. 
     Embodiments will be described with reference to the accompanying drawings. 
     1. Outline 
     In the embodiments below, a variable capacity memory capable of selectively changing the memory capacity (or operation speed) will now be proposed. Since a change in memory capacity is realized by changing (reconfiguring) a cell configuration for storing 1 bit, the variable capacity memory can be called a variable cell-configurable memory. The variable capacity memory described below mainly relates to a nonvolatile memory for storing data in a variable resistance element. However, for facilitating the description, an embodiment of a Spin Transfer Torque (STT)-MRAM whose application to a cache memory is most expected, for example, will be described. 
     The STT-MRAM in which the memory capacity (or cell configuration) is reconfigurable, described below, will hereinafter be referred to as a variable capacitance STT-MRAM. The variable capacitance STT-MRAM is characterized in that an advanced perpendicular magnetic tunnel junction (Advanced p-MTJ), for example, is used as a memory element, which enables the device to deal with applications of from a small capacity to a large capacity, using one circuit configuration (one chip), i.e., without redesigning the device. 
     Further, in the embodiments below, adaptive storage control (ASC) cache architecture using the variable capacitance STT-MRAM will be also proposed. The adaptive storage control cache architecture can make the most use of the above-mentioned variable capacitance STT-MRAM. 
     The following three points will now be mainly proposed in the embodiments. 
     Firstly, the design (circuit configuration) of the variable capacitance STT-MRAM will be proposed. 
     In general, in MRAMs, the memory capacity and the operation speed are in trade-off relationship. In contrast, in variable capacitance STT-MRAMs, the memory capacity and the operation speed can be varied selectively in accordance with applications, and hence power saving and high performance can simultaneously be realized. 
     For instance, when the variable capacitance STT-MRAM is applied to an L2 cache memory, an optimal cache capacity is selected by the operating system (OS). Namely, if the application currently executed regards the operation speed (latency) as important, the cache capacity of the variable capacitance STT-MRAM is reduced to thereby enhance the operation speed. In contrast, if the currently executed application regards the cache capacity as important, the cache capacity of the variable capacitance STT-MRAM is increased. 
     Thus, the variable capacitance STT-MRAM exhibits advantage for both the application regarding the operation speed, and the application regarding cache capacity as important. 
     Secondly, adaptive storage control cache architecture for making the most use of the variable capacitance STT-MRAM will be proposed. 
     The variable capacitance STT-MRAM can maximize its performance as a cache memory by selecting an optimal cache capacity in accordance with the currently executed application. Namely, in the application, the variable capacitance STT-MRAM determines an access pattern to a cache memory, and selects the cache capacity based on the determination result. 
     For instance, adaptive storage control cache architecture will be proposed, in which several operation phases are selected from an access pattern to the cache memory, and the cache capacity is dynamically varied in accordance with transition between the operation phases. 
     Thirdly, a method for varying the cache capacity in accordance with addresses in the cache memory, based on the fact that the memory access pattern varies in accordance with variables used in applications, will be proposed. 
     This method has been contrived, considering that when access has concentrated on a predetermined tag address, rush may occur due to thrashing. To avoid this, the way number of the cache memory is made variable for each phase (set) of an application, enabling selection of an optimal way number corresponding to an access pattern for each application phase (set). 
     For instance, in the application phase in which the operation speed (latency) is regarded as important, the cache capacity is reduced to thereby realize high-speed memory access. In contrast, in the application phase in which the cache capacity is regarded as important, the cache capacity is increased. 
     Regarding the operation speed as important means that a latency penalty (reduction of the operation speed), which will occur in the cache memory when the probability (hitting probability) of existence of data to access is high, and data is acquired from a DRAM as a main memory, even if the cache capacity is reduced, is regarded as more important than a miss penalty that no data to access exists in the cache memory. 
     Namely, when hitting probability in a cache memory of a small capacity is high, there is little access to the main memory, and therefore maximizing the operation speed of the cache memory results in high-speed operation of the entire system. 
     Further, regarding the memory capacity (cache capacity) as important means that miss penalty (reduction of the operation speed) indicating that when the cache capacity is reduced, the probability (miss probability) of nonexistence of data to access in the cache memory is high is regarded as more important than the latency penalty that the operation speed of the MRAM as the cache memory is reduced. 
     Namely, when the miss probability of the small-capacity cache memory is high, maximizing the cache capacity even by reducing the operation speed of the cache memory will lead to elimination of access to the main memory, which contributes to high-speed operation of the entire system. 
     2. Simulation of Read Speed 
     In the simulation described below, the circuit configuration and read speed of the STT-MRAM will be described. 
       FIG. 1  is a circuit diagram showing a 1T-1MTJ type STT-MRAM.  FIG. 2  is a circuit diagram showing a sense amplifier used to read data from the memory cell array of  FIG. 1 . 
     As shown, each memory cell MC comprises a cell transistor (FET) T and an MTJ element R. 
     The gate of the cell transistor T is connected to a word line WLi (i=one of 0-n). The memory cell MC is connected between a bit line BLj and a source line SLj (j=one of 0-7). A column selector  1  selects one of eight columns COL 0 -COL 7  based on a column selection signal CSL. The bit line BLj in the selected column COLj is electrically connected to a sense amplifier  2 . 
     The sense amplifier  2  compares the potential of the bit line BLj with a reference potential Reference, thereby determining data in the selected memory cell MC and outputting read data OUT. The sense amplifier  2  is initialized by an initializing circuit (precharge/equalize circuit)  3 . When a control signal SE 1  is at “L,” the sense amplifier  2  is initialized, while when a control signal SE 2  is at “H,” the sense amplifier  2  is activated. 
     In the simulation, a cell transistor T in a reference cell RC is connected to the word line WLi connected to the selected memory cell MC. The reference cell RC comprises an MTJ element R P  in a parallel state (P) and an MTJ element R AP  in an anti-parallel state (AP), and therefore generates the reference potential Ref. 
     In the STT-MRAM described above, it is necessary to reduce read power in order to reduce dynamic power. However, if a read current Ir is reduced to reduce the read power, the read speed is inevitably reduced (read delay is increased). 
     To check the relationship between the read current and the read delay, simulation was carried out using the circuit shown in  FIGS. 1 and 2 .  FIG. 3  shows the simulation result. 
     In  FIG. 3 , the horizontal axis indicates read current, and the vertical axis indicates read access time. The read current is that used to read out data from an MTJ element in an anti-parallel state. The read access time indicates the time required for the sense amplifier to output data after the potential of the selected word line reaches half (Vdd/2) a power supply potential Vdd. 
     Further, the number (bit number) of bit cells connected to a bit line was employed as a parameter. In the simulation, the number (array structure) of memory cells connected to a bit line was set to 128 bits, 256 bits, 512 bits and 1024 bits. 
     According to the simulation based on a circuit level using SPICE, to realize a read access time of 2 ns, read currents of 8 μA, 13 μA, 20 μA and 50 μA were required for the four array structures of 128 bits, 256 bits, 512 bits and 1024 bits. 
     It can be understood from this simulation result that for a greater array structure, a greater amount of read current is required to maintain a predetermined read speed. This means that in order to maintain the predetermined read speed, there is an upper limit for the number of memory cells that can be connected to a bit line. This leads to area penalty. 
     In an MRAM with a memory macro having a capacity of several megabits, area efficiency is an important design factor. Therefore, in order to enhance the area efficiency, it is important to design each central transistor as small as possible under the design rule. 
       FIG. 4  shows an example of the memory cell array of a 2T-2MTJ type STT-MRAM, and  FIG. 5  shows an example of a sense amplifier used to read data from the memory cell array of  FIG. 4 . 
     As shown, each memory cell MC comprises two cell transistors (FET) T 0  and T 1 , and two MTJ elements R 0  and R 1 . 
     The gates of the cell transistors T 0  and T 1  are connected to one of the word lines WLi (i=one of 0-n). Each memory cell MC is connected between the bit line BLj/bBLj and the source line SLj/bSLj (j=one of 0-3). A column selector  1  selects one of the four columns COL 0 -COL 3 , based on a column selection signal CSL. The bit line BLj in the selected column COLj is electrically connected to the sense amplifier  2 . 
     The sense amplifier  2  compares the potential of the bit line BLj with the potential of the bit line bBLj, thereby outputting read data OUT. The sense amplifier  2  is initialized by an initializing circuit (precharge/equalize circuit)  3 . When a control signal SE 1  is at “L,” the sense amplifier  2  is initialized, while when a control signal SE 2  is at “H,” the sense amplifier  2  is activated. 
     In the simulation, complementary data are stored in the two MTJ elements R 0  and R 1  in one memory cell MC. Namely, one of the two MTJ elements R 0  and R 1  is set in a parallel state (e.g., a low resistance state), and the other is set in an anti-parallel state (e.g., a high resistance state). As a result, during reading, a differential reading in which the potential of the bit line BLj is compared with the potential of the bit line bBLj is enabled, thereby realizing a high-speed reading operation compared to the array structure shown in  FIGS. 1 and 2 . 
     However, the memory cell size required to store 1 bit is twice the cell size of the array structure shown in  FIGS. 1 and 2 . This is disadvantageous to increase the cache capacity. Further, it is necessary to guide read currents to the two MTJ elements R 0  and R 1  during reading, although the read access time can be enhanced because a read margin is enhanced. 
     3. Basic Structure 
       FIG. 6  shows the entire configuration of the variable capacitance STT-MRAM. 
     As shown, a memory cell array  10  comprises an array of MTJ elements (memory cells). A row decoder  11   a  and a column decoder  11   b  randomly access the MTJ elements in the memory cell array  10 , based on an address signal Add. 
     A column selector  1 A serves to electrically connect the memory cell array  10  to the sense amplifier  2  based on a signal from the column decoder  11   b . A mode selector  1 B serves to change the basic unit (cell configuration) for performing reading/writing of 1 bit (data). The sense amplifier  2  is used to read data Out from the memory cell array  10 . 
     A control circuit  14  controls the operations of the row decoder  11   a , the column decoder  11   b , the mode selector  1 B and the sense amplifier  2 . For instance, the control circuit  14  executes adaptive storage control described below under the control of a cache controller (CPU). 
     The positional relationship between the column selector  1 A and the mode selector  1 B may be opposite. 
       FIG. 7  shows a cell configuration switching section incorporated in the variable capacitance STT-MRAM. 
     As shown, the memory cell array comprises central transistors T 0 , T 1 , T 2  and T 3 , and MTJ elements R 0 , R 1 , R 2  and R 3 . 
     The gates of the cell transistors T 0  and T 1  are connected to a word line WL 0 , and the gates of the cell transistors T 2  and T 3  are connected to a word line WL 1 . The central transistors T 0 , T 1 , T 2  and T 3  are connected in series to the MTJ elements R 0 , R 1 , R 2  and R 3 , respectively. 
     The cell transistors T 0  and T 2  are connected to a bit line BL 0 , and the MTJ elements R 0  and R 2  are connected to a source line SL 0 . The central transistors T 1  and T 3  are connected to a bit line BL 1 , and the MTJ elements R 1  and R 3  are connected to a source line SL 1 . 
     The bit lines BL 0  and BL 1  are connected to the mode selector  1 B (SEL 0 , SEL 1 ) via the column selector  1 A. 
     A reference cell RC comprises a central transistor Td, an MTJ element R P  in a parallel state (P) and an MTJ element R AP  in an anti-parallel state (AP), the MTJ element R P  and the MTJ element R AP  being connected in parallel. The central transistor Td is connected to a dummy bit line DBL, and the MTJ element R P  and the MTJ element R AP  are connected to a dummy source line DSL. 
     The dummy bit line DBL is connected to the mode selector  1 B (SEL 0 , SEL 1 ). 
     Based on a control signal φ 0 , the mode selector SEL 0  transfers the potential V BL0  no of the bit line BL 0  or the reference potential Ref of the dummy bit line DEL to the sense amplifier  2 . Similarly, based on a control signal φ 1 , the mode selector SEL 1  transfers the potential V BL1  of the bit line BL 1  or the reference potential Ref of the dummy bit line DEL to the sense amplifier  2 . 
     The sense amplifier  2  compares the output of the mode selector SEL 0  with the output of SEL 1 , thereby determining data in the selected memory cell MC and outputting read data OUT. The sense amplifier  2  is initialized by an initializing circuit (precharge/equalize circuit)  3 . When a control signal SE 1  is at “L,” the sense amplifier  2  is initialized, while when a control signal SE 2  is at “H,” the sense amplifier  2  is activated. 
     In the above-described variable capacitance STT-MRAM, the configuration (operation mode) of each memory cell MC can be selected from (a) a 1T-1MTJ cell mode, (b) a 2T-2MTJ cell mode, and (c) a 4T-4MTJ cell mode, as is shown in  FIG. 8 . 
     Since the memory cell size required for storing 1 bit differs between the three operation modes, the read speed (read delay) differs therebetween. Table 1 below shows differences between the modes. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Read 
                   
                   
                 Sense 
               
               
                   
                 Mode 
                 mechanism 
                 Current 
                 Capacity 
                 margin 
               
               
                   
                   
               
             
            
               
                   
                 1T-1MTJ 
                 With 
                 1 
                 ×4 
                 Low 
               
               
                   
                   
                 reference 
               
               
                   
                 2T-2MTJ 
                 Without 
                 1 
                 ×2 
                 Middle 
               
               
                   
                   
                 reference 
               
               
                   
                 4T-4MTJ 
                 Without 
                 2 
                 ×1 
                 High 
               
               
                   
                   
                 reference 
               
               
                   
                   
               
            
           
         
       
     
     In the 1T-1MTJ cell mode, the reference potential Ref (With reference) is used as a read mechanism. In contrast, in the 2T-2MTJ and 4T-4MTJ cell modes, the reference potential Ref is not used because differential read is employed as the read mechanism (Without reference). 
     Table 1 shows the comparison results obtained when the read current in the 1T-1MTJ cell mode is set to 1, and the memory capacity (this is inversely proportional to the cell size) in the 4T-4MTJ cell mode is set to 1. 
     In the 1T-1MTJ cell mode, the memory cell size is minimum, and the memory capacity is four times that in the 4T-4MTJ cell mode. However, since the read current is small, the sense margin is low (Low). 
     In the 4T-4MTJ cell mode, since the memory cell size is maximum and the read current is large, the sense margin is high (High). However, the memory capacity is ¼ of that in the 1T-1MTJ cell mode, since the memory cell size is large. 
     Further, in the 2T-2MTJ cell mode, the intermediate properties between the 1T-1MTJ cell mode and the 4T-4MTJ cell mode are exhibited. Namely, the memory capacity is twice that in the 4T-4MTJ cell mode and the sense margin is middle (Middle). 
     Switching of these modes is executed by, for example, a cache controller in accordance with a currently executed application. As a result, both power saving and high performance can be realized in the MRAM cache. 
     Although the basic structure employs three cell configurations (operation modes), i.e., (a) the 1T-1MTJ cell mode, (b) the 2T-2MTJ cell mode, and (c) the 4T-4MTJ cell mode, it is not limited to this. For instance, other xT-xMTJ (x is an even number) cell configurations (modes), such as a 6T-6MTJ cell mode or an 8T-8MTJ cell mode, may be employed. Further, the number of modes is not limited to 3. It is sufficient if the number of modes is two or more. 
     The operations executed in the above-mentioned three modes will be described in detail. 
     (a) 1T-1MTJ Cell Mode 
     In the 1T-1MTJ cell mode, during read, for example, the word line (selected word line) WL 0  is set to “H,” and the word line (non-selected word line) WL 1  is set to “L.” Similarly, the bit line (selected bit line) BL 0  is set to “H,” and the bit line (non-selected bit line) BL 1  is set to “L.” Further, the source lines SL 0  and SL 1  are both set to “L.” 
     The mode selector SEL 0  selects the potential V BL0  of the bit line BL 0 , and transfers the potential V BL0  to the sense amplifier  2 . The mode selector SEL 1  selects the reference potential Ref of the bit line BL 0 , and transfers the reference potential Ref to the sense amplifier  2 . The read current Ir is guided into one MTJ element R 0  in the selected memory cell MC. 
     The 1T-1MTJ cell mode is the mode where the cell density is highest between the three modes. In this mode, the read operation is executed by, for example, amplifying the difference between the read current Ir guided into a memory cell (selected memory cell) to access, and the reference current guided into a reference cell. 
     As aforementioned, the reference cell has a structure in which, for example, a low-resistance MTJ element R P  having a spin in a parallel state (P) and a high-resistance MTJ element R AP  having a spin in an anti-parallel state (AP) are connected in parallel. Namely, the resistance r ref  resulting from the low-resistance MTJ element R P  and the high-resistance MTJ element R AP  is half (r P +r AP )/2 the sum of the resistance r P  of the low-resistance MTJ element R P  and the resistance r AP  of the high-resistance MTJ element R AP . 
     The read margin is determined from a magnetoresistance ratio (MR ratio) RA, i.e., (r AP −r P )/r P . However, since in the 1T-1MTJ cell mode, the resistance r P  or r AP  of the MTJ element in the selected memory cell is compared with the resistance (r P +r AP )/2 of the MTJ element in the reference cell, the substantial MR ratio is half RA. 
     Accordingly, in this mode, the read delay is significantly great, compared to those in SRAMs. In contrast, regarding the cache capacity (memory capacity), the cell density is approx. four times that in SRAMs, and hence a cache capacity approx. four times that of each SRAM can be realized. 
     (b) 2T-2MTJ Cell Mode 
     In the 2T-2MTJ cell mode, during read, for example, the word line (selected word line) WL 0  is set to “H,” and the word line (non-selected word line) WL 1  is set to “L.” The bit lines (selected bit lines) BL 0  and BL 1  are both set to “H.” Further, the source lines SL 0  and SL 1  are both set to “L.” 
     The mode selector SEL 0  selects the potential V BL0  of the bit line BL 0 , and transfers the potential V BL0  to the sense amplifier  2 . The mode selector SEL 1  selects the potential V BL1  of the bit line BL 1 , and transfers the potential V BL1  to the sense amplifier  2 . The read current Ir is guided into two MTJ elements R 0  and R 1  in the selected memory cell MC. 
     In the 2T-2MTJ cell mode, a 1-bit datum is stored as a complementary datum in the two MTJ elements R 0  and R 1 . Namely, the sense amplifier  2  executes comparison (differential read) between the current flowing through one (in the high resistance state) of the two MTJ elements R 0  and R 1 , and the current flowing through the other one (in the low resistance state), thereby determining the data stored in a selected memory cell MC. Therefore, no reference cell is required. 
     Although in this mode, the cache capacity (memory capacity) is half that in the 1T-1MTJ cell mode, it is still approx. twice that in SRAMs. Further, since a read operation is executed without a reference cell, a read margin twice that in the 1T-1MTJ cell mode can be realized, and higher-speed read can be executed. Furthermore, since no reference cell is used, the dynamic power required for read is smaller than in the 1T-1MTJ cell mode. 
     (c) 4T-4MTJ Cell Mode 
     In the 4T-4MTJ cell mode, during read, for example, the word lines (selected word lines) WL 0  and WL 1  are both set to “H.” Similarly, the bit lines (selected bit lines) BL 0  and BL 1  are both set to “H.” Furthermore, the source lines SL 0  and SL 1  are both set to “L.” 
     The mode selector SEL 0  selects the potential V BL0  of the bit line BL 0 , and transfers the potential V BL0  to the sense amplifier  2 . The mode selector SEL 1  selects the potential V BL1  of the bit line BL 1 , and transfers the potential V BL1  to the sense amplifier  2 . The read current Ir is guided into four MTJ elements R 0 , R 1 , R 2  and R 3  in the selected memory cell MC. 
     The 4T-4MTJ cell mode is used to realize high-speed read. To execute high-speed read, 1-bit data is stored using the four MTJ elements R 0 , R 1 , R 2  and R 3 . During read, two word lines WL 0  and WL 1  are activated. The read current Ir is guided from the sense amplifier  2  to the source lines SL 0  and SL 1  via four current paths. 
     Accordingly, in this mode, the read current is twice that in the 2T-2MTJ cell mode, and hence high-speed read can be executed. However, the read current is increased, which inevitably increases the dynamic power used for read. In addition, since 1-bit data is stored using the four MTJ elements R 0 , R 1 , R 2  and R 3 , the cell density is ¼ of that in the 1T-1MTJ cell mode. Namely, the capacitor capacity is substantially the same as that in SRAMs. 
     4. Operation Mechanism (Updating of a Tag Memory) 
     In this section, a description will be given of the operation mechanism functioning when the above-described variable capacitance STT-MRAM is provided in an L2 cache memory. 
       FIG. 9  shows a processor system. 
     A processor  4  temporarily stores, in an MRAM  5 , the data used in a currently executed application. The processor  4  comprises a tag memory (RAM)  6  configured to store address information indicating where in the MRAM  5  as a cache memory the data is stored, and history data associated with this data. 
     When requesting data from the MRAM  5 , the processor  4  refers to the tag memory  6  to confirm that the requested data is stored in the MRAM  5  (to hit the data in the MRAM), and then reads the data from the MRAM  5 . If it is determined referring to the tag memory  6  that the requested data is not stored in the MRAM  5  (the data is missed), the processor  4  reads data from, for example, a DRAM as a main memory. 
     In the above-described variable capacitance STT-MRAM, an optimal mode for the currently executed application is selected from a plurality of cell configurations (operation modes). Accordingly, the processor  4  configured to control data writing/reading to/from the MRAM  5  needs to detect in which one of the plurality of modes each datum is stored in the MRAM  5 . 
     To this end, mode information, which is associated with the data stored in the MRAM  5  and indicates in which one of the above-mentioned modes, i.e., (a) the 1T-1MTJ cell mode, (b) the 2T-2MTJ cell mode, and (c) the 4T-4MTJ cell mode, each datum is stored, is additionally stored in the tag memory  6  of the processor  4 . 
     The mode information is updated when the mode switching has been executed. 
     Table 2 below shows examples of information items stored in the tag memory  6 . 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Tag bits 
                 Description 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 18 
                 bits 
                 Address information 
               
               
                 1 
                 bit 
                 Security information 
               
               
                 1 
                 bit 
                 Valid/Invalid information 
               
               
                 1 
                 bit 
                 Dirty information 
               
               
                 2 
                 bits 
                 Mode information 
               
               
                 4 
                 bits 
                 LRU position information 
               
               
                   
               
            
           
         
       
     
     In the operation mechanism, address information, security information, valid/invalid information, dirty information, mode information and least recent update (LRU) information are stored in the tag memory  6 . 
     The address information indicates that data is stored in the MRAM  5 . In this embodiment, the address information is of 18 bits. However, the number of bits will be varied in accordance with the size of one way in the memory cell array. 
     The security information (1 bit) indicates whether the data in the MRAM is protected by the OS. The valid/invalid information (1 bit) indicates whether the add information is valid or invalid. Namely, address information being valid means data is actually stored in the MRAM  5 . Address information being invalid means that the data stored in the MRAM  5  is no more stored at present as a result of ejection from a cache line (history information or ghost information). 
     The dirty information (1 bit) indicates that the data updated in the MRAM  5  as a cache memory is reflected in a DRAM as a main memory. If the updated data is not reflected in the main memory, processing for reflecting the updated data in the main memory is executed at a predetermined time. 
     The mode information is newly provided to validate the above-described variable capacitance STT-MRAM. For instance, when the variable capacitance STT-MRAM has the above-mentioned three cell configurations (operation modes)—(a) the 1T-1MTJ cell mode, (b) the 2T-2MTJ cell mode and (c) the 4T-4MTJ cell mode, mode information of 2 bits is provided as shown in Table 2. 
     The bit number of the mode information varies in accordance with the number of reconfigurable cell configurations. If the number of reconfigurable cell configurations is 2 y  or less, it can be detected, based on mode information of y bits, in which mode of the 2 y  modes, each datum is stored in the MRAM  5 . 
     The LRU position information is used to detect whether the data stored in the MRAM  5  is old or new. Since the MRAM  5  as the cache memory always stores new data, it is necessary to sequentially erase (eject), from the MRAM  5 , old data, the frequency of use of which is low. To this end, LRU position information is used to detect the relative positional relationship between old and new data. 
     The data stored in the MRAM  5  earlier will hereinafter be referred to as LRU side data, and the data stored in the MRAM  5  later be referred to as MRU (most recent update) side data. 
     A cache controller (CPU)  7  incorporated in the processor  4  instructs access of data to the MRAM  5 , based on the information stored in the tag memory  6 , e.g., information indicating the cell configuration or mode set for each data set incorporated in the memory cell array in the MRAM  5 . 
     Further, a counter  8  in the processor  4  counts the number of times of access to each bit as a determination condition for, for example, adaptive storage control, i.e., mode selection. 
       FIG. 10  is a flowchart of processing executed to access data in the MRAM. 
     Firstly, if requested data does not exist in an L1 cache memory (L1 miss), it is confirmed referring to the tag memory whether the data exists in the MRAM as the L2 cache memory (step ST 1 : L2 tag access). 
     If it is determined that the requested data exists in the MRAM as the L2 cache memory (i.e., if the requested data is hit), it is determined, based on the mode information in the tag memory, in which mode the data was stored in the MRAM (steps ST 3  and ST 4 ). 
     Subsequently, based on the mode information, data is read from the MRAM as the L2 cache memory by switching the read method as shown in  FIG. 8  (step ST 5 : L2 data access). 
     In contrast, if it is determined that the MRAM as the L2 cache memory does not contain the requested data (L2 miss), processing (Ghost hit) for checking whether there is data indicating that the data was once stored in the MRAM is executed, and then the data is read from the main memory. 
     5. Adaptive Storage Control (ASC) 
     In this section, adaptive storage control for making best use of the above-described variable capacitance STT-MRAM will be described. In the adaptive storage control, the cache capacity of the variable capacitance STT-MRAM is varied in accordance with a currently executed application, using a counter configured to predict a pattern of access to the variable capacitance STT-MRAM. 
     In this control, a two-level cache hierarchy is assumed. As an L21 cache memory, an SRAM is used, and as an L2 cache memory, the variable capacitance STT-MRAM is used. In the variable capacitance STT-MRAM, the memory capacity (cache capacity) and the operation speed (latency) are in a tradeoff relationship. 
     Accordingly, if the memory capacity is increased, the operation speed is reduced. In this case, however, since the memory capacity is large, the probability of non-existence of data in the L2 cache memory, i.e., miss ratio, is reduced. In contrast, if the memory capacity is reduced, the operation speed is increased. Since in this case, the memory capacity is small, the probability of non-existence of data in the L2 cache memory, i.e., miss ratio, is increased. 
     If a cache miss has occurred in a last level cache LLC, it is necessary to access the main memory as an off-chip memory to acquire data therefrom. This is a great penalty in view of both power saving and latency. In the MRAM as the L2 cache memory, such a penalty can be avoided and the cache performance be maximized if an optimal cache capacity is selected. 
       FIG. 11  shows a relationship between the cell configuration (operation mode) and the number of ways. 
     The cell configuration and the way number are controlled by, for example, the cache controller (CPU). The way number means the number of cache lines made to correspond to one block size of the MRAM cache. The larger the way number, the smaller the number of times of data competition, and the higher the ratio of hitting. Namely, miss penalty is reduced. 
     If 4 ways are adopted in the 1T-1MTJ cell mode, two ways are adopted in, for example, the 2T-2MTJ cell mode, and one way is adopted in the 4T-4MTJ cell mode. 
     By thus performing adaptive storage control, the performance of the cache memory can be maximized. 
     Alternatively, the way number in the MRAM (cache memory) may be made variable for each block in a memory macro as shown in  FIG. 12 , thereby enabling selection of an optimal way number corresponding to an access pattern for each block. 
     6. Optimal Storage Prediction for Adaptive Storage Control 
     In adaptive storage control, an optimal L2 cache size is automatically determined in accordance with, for example, a currently executed application. To predict the tendency of access of the application to an MRAM cache, a counter, for example, is utilized. 
     Mode switching is executed between two processes of “Expanding of cache capacity” and “Shrinking of cache capacity.”  FIG. 13  shows the flow of the mode switching. The condition for executing “Expanding of cache capacity” is that the advantage of expanding the cache capacity is greater than the disadvantage of the same. 
     An index, “Avoidable misses×L2 miss penalty,” for example, can be used as that indicating the degree of advantage of expanding the cache capacity. Similarly, an index, “Current hits×Expanding hit penalty,” can be used as that indicating the degree of disadvantage of expanding the cache capacity. 
     The term “Avoidable misses” represents the number of misses that can be avoided by the expansion of the cache capacity. The term “L2 miss penalty” represents the operation delay and energy that will be caused by access to the main memory due to misses in the L2 cache memory (MRAM). 
     Further, the term “Current hits” represents the number of hits resulting from the expansion of the cache capacity. The term “Expanding hit penalty” represents the operation delay and energy that will be caused by each hit resulting from the expansion of the cache capacity. 
     The factors “L2 miss penalty” and “Expanding hit penalty” are varied in accordance with the processor structure. 
     Table 3 shows an example of the processor structure. 
     
       
         
           
               
               
             
               
                 TABLE 3 
               
               
                   
               
             
            
               
                 MRAM 
                 512 KB/1 MB/2 MB, 4-/8-/16-way set associative, 
               
               
                 (L2 cache) 
                 64B block size, 13-/15-/18-cycle hit latency 
               
               
                   
                 (MRAM write latency is 16 cycles), write-back 
               
               
                 Main memory 
                 2 GB, 100-cycle latency 
               
               
                   
               
            
           
         
       
     
     In the optimal storage prediction, the MRAM (L2 cache memory) selects one of the three cell modes—the 1T-1MTJ cell mode, the 2T-2MTJ cell mode and the 4T-4MTJ cell mode. The way number can be set for each set (set associative), the write latency is 16 cycles, and the block size is 64 bytes. Further, a write back system, in which even when data has been updated in the MRAM cache, this update is not directly reflected in the main memory, is employed. 
     &lt;1T-1MTJ cell mode (Slow mode)&gt; 
     Memory capacity: 2 Mbytes; 
     Way number: 16; 
     Read latency (hit latency): 18 cycles 
     &lt;2T-2MTJ cell mode (Middle mode)&gt; 
     Memory capacity: 1 Mbytes; 
     Way number: 8; 
     Read latency (hit latency): 15 cycles 
     &lt;4T-4MTJ cell mode (Fast mode)&gt; 
     Memory capacity: 512 Kbytes; 
     Way number: 4; 
     Read latency (hit latency): 13 cycles 
     In contrast, the DRAM as the main memory is constructed such that, for example, the memory capacity is 2 Gbytes and the hit latency is 100 cycles. 
     The hit latencies (operation delays) 18, 15 and 13 in the MRAM cache are included in “Expanding hit penalty.” Further, the latency (operation delay) of 100 cycles in the main memory is included in “L2 miss penalty.” 
     An example of determining an optimal L2 cache size will be described referring to  FIG. 13 . 
     In this example, N hit-A , N hit-B  and N hit-C  are counted using a counter, to thereby select one of the three cell modes, i.e., 1T-1MTJ cell mode (Slow mode), 2T-2MTJ cell mode (Middle mode) and 4T-4MTJ cell mode (Fast mode). 
     In  FIG. 13 , areas A, B, C 1  and C 2  indicate basic units for storing 1 datum. 
     N hit-A  represents the frequency of access to the area A nearest to the MRU (Most recent update) side among the four areas A, B, C 1  and C 2 . N hit-B  represents the frequency of access to the area B second nearest to the MRU (Most recent update) side among the four areas A, B, C 1  and C 2 . N hit-C  represents the frequency of access to the areas C 1  and C 2  on the LRU (Least recent update) side among the four areas A, B, C 1  and C 2 . 
     The frequency of access means, for example, the number of times of access within a predetermined period. Further, N hit-C  represents the average or sum of the frequencies of access to the two areas C 1  and C 2 . 
     Switching from the 1T-1MTJ Cell Mode to the 2T-2MTJ Cell Mode 
     In the 1T-1MTJ cell mode, different data are stored in the four areas A, B, C 1  and C 2 . In this state, if, for example, N hit-C &lt;α×(N hit-A +N hit-B ), the cache controller (CPU) switches the cell mode from the 1T-1MTJ cell mode to the 2T-2MTJ cell mode. Namely, the cache capacity (memory capacity) is shrunk. 
     The above conditional inequation means that since the sum (α×(N hit-A +N hit-B )) of the frequencies of access to the two areas A and B is greater than the frequency N hit-C  of access to the other two areas C 1  and C 2 , the degree of the advantage of shrinking the cache capacity (decrease of latency of data stored in the two areas A and B) is greater than that of the disadvantage of shrinking the cache capacity (increase of the access time required to access the data, stored in the two areas C 1  and C 2 , from the main memory). 
     α in the above conditional inequation is an index for determining which has a greater degree between the advantage and the disadvantage. Specifically, this index is used to determine the difference between the sum of the frequencies of access to the two areas A and B and the frequency of access to the two areas C 1  and C 2 . This index is determined based on, for example, the processor system, or the application executed in the system. 
     Switching from the 2T-2MTJ Cell Mode to the 4T-4MTJ Cell Mode 
     In the 2T-2MTJ cell mode, different data are stored in substantially two areas A and B. For instance, 1 datum (=two complementary bits) is stored over two areas A and C 1 , and 1 datum (=two complementary bits) is stored over two areas B and C 2 . “1 datum” means one datum that provides one meaning. 
     Namely, in the 2T-2MTJ cell mode, the data stored in the two areas C 1  and C 2  in the 1T-1MTJ cell mode are erased. However, information associated with the data stored in the two areas C 1  and C 2  in the 1T-1MTJ cell mode is stored as history information (ghost information) in the tag memory. 
     In this state, if, for example, N hit-B &lt;β×(N hit-A ), the cache controller (CPU) switches the cell mode from the 2T-2MTJ cell mode to the 4T-4MTJ cell mode. Namely, the cache capacity (memory capacity) is shrunk. 
     This conditional inequation means that since the frequency (β×N hit-A ) of access to one area A is greater than the frequency N hit-B  of access to one area B, the degree of the advantage of shrinking the cache capacity (decrease of latency of the datum stored in the one area A) is greater than that of the disadvantage of shrinking the cache capacity (increase of the access time required to access the datum, stored in the one area B, from the main memory). 
     β in the above conditional inequation is an index for determining which has a greater degree between the advantage and the disadvantage. Specifically, this index is used to determine the difference between the frequency of access to the one area A and the frequency of access to the one area B. This index is determined based on, for example, the processor system, or the application executed in the system. 
     Switching from the 4T-4MTJ Cell Mode to the 2T-2MTJ Cell Mode 
     In the 4T-4MTJ cell mode, 1 datum is stored in substantially one area A. For instance, 1 datum (=two complementary bits) is stored over two areas A and C 1 , and the same datum (=the same two complementary bits) is stored over two areas B and C 2 . 
     Namely, in the 4T-4MTJ cell mode, the data stored in the two areas C 1  and C 2  in the 1T-1MTJ cell mode are erased, and the datum stored in the one area B in the 2T-2MTJ cell mode is erased. However, information associated with the data stored in the three areas B, C 1  and C 2  is stored as history information (ghost information) in the tag memory. 
     In this state, if, for example, N hit-B &gt;β×(N hit-A ), the cache controller (CPU) switches the cell mode from the 4T-4MTJ cell mode to the 2T-2MTJ cell mode. Namely, the cache capacity (memory capacity) is expanded. 
     This conditional inequation means that since the frequency (β×N hit-A ) of access to one area A is lower than the frequency N hit-B  of access to one area B, the degree of the advantage of expanding the cache capacity (decrease of the access time due to the fact that the data stored in the two areas A and B becomes accessible from the cache memory) is greater than that of the disadvantage of expanding the cache capacity (increase of latency of the data stored in the two areas A and B). 
     β in the above conditional inequation is an index for determining which has a greater degree between the advantage and the disadvantage. Specifically, this index is used to determine the difference between the frequency of access to the one area A and the frequency of access to the one area B. This index is determined based on, for example, the processor system, or the application executed in the system. 
     Switching from the 2T-2MTJ Cell Mode to the 1T-1MTJ Cell Mode 
     In the 2T-2MTJ cell mode, different data are stored in substantially two areas A and B. For instance, 1 datum (=two complementary bits) is stored over two areas A and C 1 , and 1 datum (=two complementary bits) is stored over two areas B and C 2 . 
     Namely, in the 2T-2MTJ cell mode, the data stored in the two areas C 1  and C 2  in the 1T-1MTJ cell mode are erased. However, information associated with the data stored in the two areas C 1  and C 2  is stored as history information (ghost information) in the tag memory. 
     In this state, if, for example, N hit-C &gt;α×(N hit-A +N hit-B ), the cache controller (CPU) switches the cell mode from the 2T-2MTJ cell mode to the 1T-1MTJ cell mode. Namely, the cache capacity (memory capacity) is expanded. 
     This conditional inequation means that since the sum (α×(N hit-A  N hit-B )) of the frequencies of access to the two areas A and B is lower than the frequency N hit-C  of access to the other two areas C 1  and C 2 , the degree of the advantage of expanding the cache capacity (decrease of the access time due to the fact that the data stored in the four areas A, B, C 1  and C 2  becomes accessible from the cache memory) is greater than that of the disadvantage of expanding the cache capacity (increase of latency of the data stored in the four areas A, B, C 1  and C 2 ). 
     α in the above conditional inequation is an index for determining which has a greater degree between the advantage and the disadvantage. Specifically, this index is used to determine the difference between the sum of the frequencies of access to the two areas A and B and the frequency of access to the two areas C 1  and C 2 . This index is determined based on, for example, the processor system, or the application executed in the system. 
     As described above, mode selection/switching (change of cache capacity) is executed based on the past pattern of access to the MRAM cache, i.e., the relationship in magnitude between N hit-A , N hit-B  and N hit-C . 
     7. Hardware Configuration of Adaptive Storage Control 
       FIG. 14  shows an example of hardware configuration for adaptive storage control. 
     The tag memory (tag array) is formed of, for example, an SRAM, and a data memory (data array) is formed of, for example, a variable capacitance STT-MRAM (MRAM cache). As aforementioned, the tag memory stores mode information indicating in which mode each datum is stored in the variable capacitance STT-MRAM, in addition to normal tag data. 
     In the tag memory, the areas A, B, C 1  and C 2  indicate memory areas for storing tag information included in a datum stored in the MRAM cache. “n” indicates one of the areas A, B, C 1  and C 2 . 
     “Normal” means an area storing effective data, or an area that is included in a plurality of areas storing the effective data and has stored the effective data so far. Further, “Ghost” means one or more areas that are included in the areas storing the effective data and once stored data other than effective data. 
     In the hardware configuration, three areas B, C 1  and C 2  can be used as ghost areas. 
     In the MRAM cache, areas w, x, y and z indicate memory areas each for storing 1 datum. “bx” indicates storage of a datum obtained by inverting the datum stored in the area x, and “by” indicates storage of a datum obtained by reversing the datum stored in the area y. 
     In the MRAM cache, numerical values 0 to 15 mean data. b 0 , b 1 , b 2 , b 3 , b 4 , b 5 , b 6  and b 7  mean inverted data of data 0, 1, 2, 3, 4, 5, 6 and 7, respectively. In the tag memory, numerical values 0 to 15 mean tag data. 
     The variable capacitance STT-MRAM mode can be varied simultaneously in the entire memory cell array (the entire memory area), or be varied for each block in the memory cell array (see  FIG. 12 ). 
     In the 1T-1MTJ cell mode, data 0 to 15 are stored in sixteen areas within the data memory (MRAM). In accordance with this, tag data 0 to 15 corresponding to the data 0 to 15 are stored in sixteen areas in the tag memory. 
     In the 2T-2MTJ cell mode, data 0 to 7 and b 0  to b 7  are stored in sixteen areas within the data memory (MRAM). Namely, 1 datum (=two complementary bits) is stored over two areas, 1 datum (two bits) being, for example, 0 and b 0 , 1 and b 1 , 2 and b 2 , 3 and b 3 , 4 and b 4 , 5 and b 5 , 6 and b 6 , or 7 and b 7 . 
     In accordance with the above, tag data 0 to 7 corresponding to the data 0 to 7 are stored as normal data in the 8 areas in the tag memory. Further, tag data 8 to 15 corresponding to data 8 to 15 are stored as ghost data in the 8 areas in the tag memory. 
     Namely, in the 2T-2MTJ cell mode, although the data 8 to 15 are erased from the data memory, the tag data 8 to 15 corresponding to the data 8 to 15 are held as ghost data in the tag memory. 
     In the 4T-4MTJ cell mode, for example, data 0 to 3 and b 0  to b 3  are stored in the sixteen areas in the data memory (MRAM). Namely, 1 datum (=four complementary bits) is stored over four areas, the 1 datum being, for example, (0 and b 0 )×2, (1 and b 1 )×2, (2 and b 2 )×2, or (3 and b 3 )×2. 
     In accordance with this, tag data 0 to 3 corresponding to the data 0 to 3 are stored in four areas in the tag memory, and tag data 4 to 15 corresponding to data 4 to 15 are stored in twelve areas in the tag memory. 
     Namely, in the 4T-4MTJ cell mode, although the data 4 to 15 are erased from the data memory, the tag data 4 to 15 corresponding to the data 4 to 15 are held as ghost data in the tag memory. 
     Thus, the tag memory always holds all tag data 1 to 16 corresponding to all sixteen data 1 to 16 stored in the sixteen areas, regardless of the operation mode during adaptive storage control. Accordingly, N hit-A , N hit-B  and N hit-C  shown in  FIG. 13  can be counted at any time by referring to the tag data. 
     In the above-described hardware configuration, data mapping is executed in the four areas w, x, y and z of two rows x two columns, thereby realizing three modes (variable capacitance operation). Further, one word line is shared between the memory cells in the two areas w and y, and one word line is shared between the two areas x and z. 
     By virtue of this structure, memory capacity control can be realized for each data set (each block) incorporated in the memory cell array. 
     To camouflage overhead in read delay in the MRAM cache and reduce cache miss penalty, the above-described adaptive storage control is very effective. This is because the pattern of access to the MRAM cache varies in accordance with a currently executed application, and even the same benchmark varies in accordance with the phase of the application, too. 
     In particular, if a state (Dirty state) in which the data in the cache memory is not reflected in the main memory has continued for a long time because of, for example, an unexpected threshing address pattern or a streaming access pattern, the amount of data in the cache memory is increased, which may cause a lot of cache misses. 
     Since in the above-described adaptive storage control, the cache capacity of the variable capacitance STT-MRAM can be made variable for, for example, each data set (block), the above-mentioned cache misses can be reduced to thereby prevent excessive increase in latency. 
     8. Data Switching Policy During Mode Change 
     In adaptive storage control, mode change may cause overhead in both power and performance. When expanding the cache size (cache capacity), the cache controller can smoothly execute mode change, since it is sufficient if redundant data is invalidated. However, when shrinking the cache size, serious performance degradation may occur since it is necessary to retreat effective data from the cache memory. 
     To avoid this, when shrinking the cache size, it is necessary to determine which data should be retreated from the cache memory. As a policy to be taken when shrinking the cache size (at the time of mode change), the following two policies are possible: 
     Policy 1: Round-Robin Type Data Selection Policy 
     The MRU-side data included in all data of the block as a data shuffle target are left, and the LRU-side data therein are discarded. Although in this policy, the penalty due to mode change is greater than in the tournament type polity described below, the hit number of the L2 cache can be maintained at a large number. 
     Discarding of data is executed when effective data to be maintained in a block has been accessed after it is determined that the cache size should be shrunk. Namely, discarding of data is executed by overwriting, at the time of the access, the effective data on the data to be discarded. 
     Policy 2: Tournament Type Data Selection Policy 
     Two data as one pair are selected from all data in a block as a data shuffle target. The MRU-side datum included in the two data is left, and the LRU-side datum is discarded. In this case, the penalty resulting from mode change is smaller than in the above-mentioned round-robin type, but the hit number of the L2 cache may be degraded. 
     As in the round-robin type, discarding of data is executed when effective data to be maintained in a block has been accessed after it is determined that the cache size should be shrunk. Namely, discarding of data is executed by overwriting, at the time of the access, the effective data on the data to be discarded. 
     9. Static Adaptive Control 
     In this control, a description will be given of the performance obtained when the variable capacitance STT-MRAM has been statically controlled. 
     The variable capacitance STT-MRAM can select which should be preferred between the memory capacity (increase in capacity) and the operation speed (increase in operation speed). In a simple way of use of the variable capacitance STT-MRAM, the memory access pattern of each application planned to be executed is pre-analyzed, and the operation mode of the variable capacitance STT-MRAM is changed based on the analysis result. 
     For instance, for the application sensitive to L2 cache hit latency, the overhead of the hit latency of the variable capacitance STT-MRAM can be suppressed by selecting the mode in which the access latency is shortest. Further, for the application sensitive to the number of L2 cache misses in which no data exists in the L2 cache, the number of cache misses can be reduced by selecting the mode in which the cache size is enlarged. 
     It is assumed here that the operating system (OS) can request the cache controller (CPU) to change the operation mode of the variable capacitance STT-MRAM, based on the pattern of access to the L2 cache. 
     In this case, the capacity of the L2 cache can be changed to, for example, 512 Kbytes in the 4T-4MTJ cell mode, or 1 Mbytes in the 2T-2MTJ cell mode, or 2 Mbytes in the 1T-1MTJ cell mode. 
     In most applications, the hit ratio is enhanced and the IPC (instruction per cycle) of the CPU is also enhanced, if the capacity of the L2 cache is increased. However, in the case of some benchmarks, the IPC of the CPU may be degraded because the hit latency of the L2 cache becomes long. 
     In such a case, the performance of the L2 cache can be improved if the mode requiring a small cache capacity is selected, using adaptive storage control. Further, in most benchmarks, the number of times of access to a main memory (e.g., a DRAM) as an off chip can be reduced by increasing the capacity of the L2 cache, thereby reducing the energy per instruction (EPI) of the CPU. 
     10. Dynamic Adaptive Control 
     In order to further effectively utilize a feature that the pattern of access to the L2 cache varies in accordance with a currently executed application, a description will now be given of dynamic (adaptive) change of the variable capacitance STT-MRAM mode. 
     In association with the dynamic mode change, the following two policies are possible: 
     Policy 1: Mode Change Based on Temporal Granularity 
     In each benchmark, the pattern of access to the L2 cache varies momentarily. In this policy, the cache controller (CPU) additionally includes, for example, a function of confirming the number of times of access to each of the areas in the memory macro. The number of times of access to each area is counted using, for example, a counter. 
     After that, mode selection is executed on, for example, a memory macro basis greater than a block basis, utilizing the aforementioned optimal storage prediction for adaptive storage control. 
     Policy 2: Mode Change Based on Address (Spatial) Granularity 
     In this policy, the capacity of the variable capacitance STT-MRAM is controlled on, for example, a block basis. Optimal storage prediction for adaptive storage control is executed for each block to change the operation mode for each block. As a result, the cache capacity of a block, which is included in all blocks of the L2 cache and is concentrically accessed, can be increased by mode change (switching). Further, in a block in which no access concentration occurs, a mode for realizing high-speed access, i.e., reduction of the operation delay (latency), can be selected. 
     Also, in this policy, finer control (mode change) than in the above-mentioned policy 1 is possible, although overhead due to the mode change is increased. 
     According to a simulation result obtained when dynamic adaptive storage control was executed, it is desirable to execute mode change (switching) for each block to thereby enhance the IPC of the CPU, in association with a benchmark, such as “perl script,” “bzip2 compression,” “gcc compile” or “Ethernet network controller,” wherein the addresses at which data access concentration occur are biased. 
     Furthermore, if data requiring write-back to a main memory (such as a DRAM) as an off chip exist, it is desirable to write back data to the main memory whenever the mode of the STT-MRAM as the L2 cache is changed. 
     For instance, in a benchmark, such as “quantum mechanical calculation,” wherein the operation phase of an application is frequently changed, mode change is often executed within a short time. In this case, data is written back to the main memory when mode switching is executed. 
     This is because if updated data is reflected in the main memory by write-through whenever mode switching is executed, increase in energy will be inevitably involved, which degrades the average energy EPI of the CPU. 
     11. Applied Examples 
     The magnetic random access memory according to the above-described embodiment can be used as, for example, a cache memory for a power-saved processor. 
     The processors installed in mobile devices represented by smartphones have recently been made to exhibit high performance. Although these processors are mobile processors, multi-core processors have come to be mainly used as the processors. In such mobile processors of high performance, a problem that the duration of their battery has become short is now conspicuous. Thus, the power consumption of the processors has come to be an important index, as well as the processing performance. Namely, increases in power consumption resulting from increases in chip area have become problematic. 
     Because of the above, not only “maximum performance” but also “minimum power consumption” are now required for the high-performance mobile processors. The “maximum performance” includes operability with no stress, a graphics function of high performance, etc. The “minimum power consumption” includes a thin body, a low temperature and a long battery duration, etc. 
     In order to effectively reduce the power consumption of the processor, attention has recently been paid to researches that aim to use a nonvolatile memory, such as a spin transfer torque (STT) MRAM, as a cache memory for the processor. The nonvolatile memory can hold data even when power supply is interrupted, and hence can make, “zero,” the power consumption in the standby state. 
     As well as MRAMs, Phase-change RAMs (PCRAMs), Resistive RAMs (ReRAMs), NAND flash memories, etc., have currently been researched and developed as nonvolatile memories. Among these nonvolatile memories, attention has been paid to, in particular, STT-MRAMs as nonvolatile memories that are closest to practical use. This results from their fast operation (&lt;10 ns) comparable to SRAMs, their high programmable resistance (&gt;10 15  times), and their high affinity for a low-power CMOS logic process. 
     Because of the above, the magnetic random access memory according to the above-described embodiments is effectively applicable to a high-performance mobile processor required to simultaneously realize high performance and power saving. 
     12. Conclusion 
     The above-described embodiments can realize a magnetic random access memory reconfigurable in memory capacity and operation speed. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.