an embodiment of the present invention will hereinafter be explained in detail with reference to the accompanying drawings . fig1 is a structural diagram showing a main portion of an embodiment of the present invention comprising storage means and control means therefor . in the drawing , reference numerals 1 , 2 designate memory elements for storing data composed , for example , of sram , the memory elements constituting a main storage apparatus . reference numeral 6 designates a power supply unit for outputting an operation voltage ( a source voltage ) vcc for operating the memory elements 1 , 2 and a data hold voltage v dr for holding data stored in the memory elements . the voltages vcc , v dr are in a relationship expressed by vcc & lt ; v dr . reference numerals 3 , 4 designate mode switching means for switching a mode of the memory elements 1 , 2 , more specifically , switching the same to either of a normal operation mode and a data hold mode . the switching means 3 , 4 are composed of voltage change - over switches 7 , 9 for selectively supplying a power supply terminal vp of the respective memory elements 1 , 2 with the voltage vcc or v dr from the power supply unit 6 and chip select mask gates comprising logical circuits , for example , nand gates 8 , 10 for controlling a chip select terminal cs of the respective memory elements 1 , 2 to access the memory element 1 or 2 . reference numeral 19 designates a microprocessor unit ( mpu ) for accessing the main storage apparatus and so on , 5 control means coupled to the mpu 19 through a system bus 18 as well as to the switching means 34 . the control means 5 monitors access conditions of the memory elements 1 , 2 controlled by the mpu 19 and performs a mode switch for the memory elements 1 , 2 . for example , the control means 5 is operative to set the memory element 1 in a normal operation mode by switching the voltage change - over switch 7 of the switching means 3 to the operation voltage vcc side when the memory element 1 is accessed and to set the memory element 1 in a data hold mode by switching the voltage change - over switch 7 of the switching means 3 to the data hold voltage v dr side when the memory element 1 is in conditions other than the above - mentioned normal operation mode . the control means 5 is further operative to set the memory element 2 in the normal operation mode by switching the voltage change - over switch 9 of the switching means 4 to the operation voltage vcc side when the memory element 2 is accessed and to set the memory element 2 in the data hold mode by switching the voltage change - over switch 9 of the switching means 4 to the data hold voltage v dr side when the memory element 2 is in conditions other than the normal operation mode . the control means 5 outputs mask signals 12 , 14 for masking chip select signals 15 , 16 from a chip select circuit 17 , later referred to , by the gates 8 , 10 . the chip select mask gates 8 , 10 mask the chip select signal 15 from the chip select circuit 17 in accordance with the mask signals 12 , 14 from the control means 15 . for example , the gate 8 masks the select signal 15 in accordance with the mask signal 12 from the control means 5 when the memory element 1 is accessed , while the gate 10 masks the chip select signal 15 in accordance with the mask signal 14 when the memory element 2 is accessed . the outputs from the chip select mask gates 8 , 10 are coupled to a chip select terminal cs ( active low ) of the memory elements 1 , 2 , respectively . the chip select circuit 17 is coupled to the mpu 19 through the system bus 18 . the mpu 19 sets the chip select signal 15 to a high level for accessing the memory element 1 while it sets the chip select signal 16 to a high level for accessing the memory element 2 . next , the operation of the control means 5 will be explained with reference to fig2 and 3 . fig2 shows an example of the structure of the control means 5 , while fig3 shows waveforms of the voltage at the power supply terminal vp of the memory element 1 , the mask signal 12 from the control means 5 , and the chip select signal cs of the memory element 1 shown in fig1 . in fig2 a selector 21 sets and resets an rs flip - flop 22 on the basis of an access request for the memory element 1 or 2 supplied thereto through the system bus 18 . for example , when the memory element 1 is being accessed , the rs flip - flop is in a set state , and accordingly the q output thereof is at a high level . this q output at high level is outputted as a switching signal 11 and supplied to the voltage change - over switch 7 shown in fig1 whereby the switch 7 is switched to the normal operation voltage mode side , thereby supplying the power supply terminal vp of the memory element 1 with the power supply voltage vcc . it should be noted that the q output is delayed by a delay element 23 by a predetermined time period before being supplied to one of input terminals of an and gate 24 . the other input terminal of the and gate 24 is supplied with the q output directly from the rs flip - flop 22 . therefore , the and gate 24 has its output ( the mask signal 12 ) set at a high level after the lapse of a predetermined time period from the time when the q output of the rs flip - flop 22 is set at high level . this output of the and gate 24 is delivered as the mask signal 12 to the chip select mask gate 8 shown in fig1 . this mask signal 12 is operative to prohibit the gate 8 from masking the chip select signal 15 . more specifically , the chip select signal 15 ( at a high level ) is inverted ( at a low level ) and inputted to the cs terminal of the memory element 1 . the above relationship corresponds to a region 31 shown in fig3 . in this region 31 , the memory element 1 is set in the normal operation mode . at this time , the q output of the flip - flop 22 or a switching signal 13 is at a low level , while a mask signal 14 outputted through a delay element 25 and an and gate 26 is also at a low level . next , when an access to the memory element 2 is started , the selector 21 shown in fig2 resets the rs flip - flop 22 , whereby the q output of the rs flip - flop 22 goes low . at this time , the switching signal 11 also goes low , whereby the voltage change - over switch 7 of the switching means 3 is switched to the data hold mode side to supply the power supply terminal vp of the memory element 1 with the power supply voltage v dr . also , the mask signal 12 immediately goes low . thus , the chip select mask gate 8 shown in fig1 masks the chip select signal 15 , which results in setting the cs terminal of the memory element 1 at a high level ( non - select state ). the above relationship corresponds to a region 32 shown in fig3 . in this region , the memory element 1 is switched to the data hold mode , wherein the power consumption is reduced . subsequently , when an access to the memory element 1 is started , the selector 21 shown in fig2 sets the rs flip - flop 22 to immediately return the switching signal 11 to high level . however , the mask signal 12 returns to high level after a delay of a predetermined time period t r provided by the delay element 23 . this delay t r is equal to an operation recovery time period necessary to change from the data hold mode to the normal operation mode . after this operation , the cs terminal of the memory element 1 goes low , whereby an access to the memory element 1 is started . the above relationship corresponds to regions 33 and 34 shown in fig3 wherein the region 33 indicates the operation recovery time period ( non - select state ) and the region 34 the normal mode . the foregoing explanation has been made in view of the mode switching for the memory element 1 . the mode switching for the memory element 2 is also performed by means of the q output of the rs flip - flop , the delay element 25 and the and gate 26 shown in fig2 . incidentally , the selector 21 and the flip - flop 22 may be replaced by registers which can store arbitrary data . this structure is useful when more than three memory elements are included . as mentioned above , a switching operation from the data hold mode to the normal operation mode requires the operation recovery time period t r therebetween . therefore , frequent mode switching operations are not preferable in respect to the performance . to cope with this problem , a memory map of the main storage apparatus may be divided into two areas i . e ., upper and lower areas , as shown in fig9 wherein the upper area is assigned to the memory element 1 and the lower area to the memory element 2 , by way of example . this is because programs and data are generally located in a particular area of the memory , so that the memory element 1 or 2 may be continually accessed with a high possibility , which results in reducing the frequency of the switching operation from the data hold mode to the normal operation mode . incidentally , fig9 shows a memory map where the capacity of the main storage apparatus is 512 kb and the upper 256 kb area is assigned to the memory element 1 while the lower 256 kb is assigned to the memory element 2 . fig4 shows another example of the structure of the control means 5 , wherein the rs flip - flop 22 is replaced with timers 41 , 42 . a selector 21 shown in fig4 resets the timer 41 each time the memory element 1 is accessed . the timer 41 starts measuring the time from a reset time and maintains its output at high level for a time period previously set by the user . in other words , the timer 41 places and maintains the memory element 1 in the normal operation mode during the previously set time period . when the set time period has elapsed , the timer 41 sets its output at low level to switch the memory element 1 to the data hold mode . if the timer 41 is reset during a time measurement , the timer 41 resumes the time measurement from the reset time . the timer 42 is also reset each time the memory element 2 is accessed , and operates in a manner similar to the timer 41 . as described above , an accessed memory element is maintained in the normal operation mode at least during the previously set time period . it is therefore possible that both the memory elements 1 and 2 are simultaneously set in the normal operation mode , different from the case where the control means shown in fig2 is employed . a long time period set in the timers may result in reducing the frequency of switching to the data hold mode . nevertheless , the frequency of switching from the data hold mode to the normal operation mode is also reduced . on the contrary , setting a short time period in the timers will cause an increase in the frequency of switching to the data hold mode , however , the frequency of switching from the data hold mode to the normal operation mode is also increased . therefore , the user can previously set a time period appropriate to respective use modes in consideration of the power consumption and the performance . next , the operation of the voltage change - over switch 7 and the chip select mask gate 8 will be explained with reference to fig5 and 6 . fig5 shows an example of the structure of the voltage change - over switch constituting the switching means shown in fig1 . in the drawing , the voltage change - over switch 7 is composed of mos transistors 51 , 52 and an inverter 53 . when the switching signal 11 is at a high level , the mos transistor 52 is in the on state while the mos transistor 51 is in the off state , whereby the power supply voltage vcc ( the operation voltage ) is supplied to the power supply terminal vp of the memory element 1 . when the switching signal 11 is at a low level , the mos transistor 52 is in the off state while the mos transistor 51 is in the on state , whereby the power supply voltage v dr ( the data hold voltage ) is supplied to the power supply terminal vp . fig6 shows another example of the structure of the voltage change - over switch 7 , wherein a zener diode 61 for voltage drop is interposed between the mos transistor 51 and a power supply line coupled to the power supply voltage vcc . when the switching signal 11 is at a low level , the power supply voltage vcc is dropped by the zener diode 61 to the data hold voltage which is supplied to the power supply terminal vp of the memory element 1 through the mos transistor 51 in the on state . thus , the voltage change - over switch 7 shown in fig6 differs from that of fig5 in that the data hold voltage v dr is not supplied from outside the voltage change - over switch but is generated internally . as shown in fig5 and 6 , the voltage change - over switch 7 is constituted of a simple transistor circuit , thereby making it possible to integrate the whole switching means including the chip select mask gate 8 in a single chip together with the memory elements . the above embodiment of the present invention has been explained giving a case where the main storage apparatus includes two memory elements , but , can be implemented in a similar manner in a case where the main storage apparatus includes three or more memory elements . next , another embodiment of the present invention will be explained with reference to fig7 and 8 . the present embodiment is an example which is applied to a case where dynamic ram ( hereinafter referred to as &# 34 ; dram &# 34 ;) is employed for memory elements . in fig7 a data bus 80 has a 32 bit width ( the respective bits are designated d0 - d31 ), and memory elements 71 - 74 are used to read and write data on portions d0 - d7 , d8 - d15 , d16 - d23 and d24 - d31 of the data bus 80 , respectively . in other words , 32 bit data is divided into four bytes and managed by the respective memory elements . a controller 76 is coupled to a mpu 19 through a system bus 18 and outputs dram access signals ( a ras signal 78 and a cas signal 79 ) and a refresh signal ( a ref signal 77 ) to all the memory elements on the basis of a dram access request generated by the mpu 19 . a control means 75 is also coupled to the mpu 19 through the system bus 18 and outputs switching signals 91 - 94 indicating corresponding memory elements to respective switching means 81 - 84 on the basis of a byte enable signal ( indicating which byte of the 32 bit data is to be accessed ) generated by the mpu 19 . for example , if bytes corresponding to the memory elements 71 , 72 are to be accessed , the control means 75 sets the switching signals 91 , 92 at a high level . the switching means 81 , 82 , upon receiving the switching signals 91 , 92 , supply the output signals of the controller 76 to the respective memory elements 71 , 72 . more specifically , a ras terminal and a cas terminal of the respective memory elements 71 , 72 are made active ( set to a low level ). on the other hand , the switching signals 93 , 94 go low , which has the switching means 83 , 84 set the ras terminal and the cas terminal of the respective memory elements 73 , 74 at a high level , whereby the memory elements 73 , 74 are placed in a low power consumption state ( a non - select state ). fig8 illustrates an example of the structure of the switching means 81 shown in fig7 . it should be noted that the other switching means 82 , 83 and 84 are constituted in the same manner . in the drawing , when the switching signal 91 is at a high level , a nand gate 97 does not mask the ras signal 78 . likewise , a nand gate 96 does not mask the cas signal 79 . therefore , when the ras signal 78 and the cas signal 79 are both active ( at a high level ), the ras terminal and the cas terminal of the memory element are also active ( at a low level ). incidentally , when the ref signal 77 is active ( at a low level ) for a data hold , or when a refresh request is generated , the ras terminal is made active ( set at a low level ) through an and gate 98 , irrespective of the level of the switching signal 91 . the present embodiment utilizes a non - select state with a low power consumption in the normal operation mode and therefore is effective in use for a memory element which does not have a data hold mode . according to the present invention as explained above , upon accessing the main storage apparatus , all memory elements constituting the same are not set in the normal operation mode , wherein memory elements assumed to be accessed with a low possibility are switched to the data hold mode with a low power consumption , thereby producing effects in reducing the power consumption of the main storage apparatus .