Abstract:
An address output apparatus capable of retaining a pre-extension upper compatibility of software post memory extension and of accessing separated RAM areas by consecutive addresses, without needing to alter CPU architecture. The address output apparatus includes an address conversion circuit  20  that allots to a RAM  30  a basic RAM area and a first area, being one of two area obtained by dividing an extension RAM area, allots to a RAM  50  a second area, being an area other than the first area of the extension RAM area, and converts logical address signals designated by a CPU  10  to physical address signals based on a state of the allotting.

Description:
[0001]    This application is based on application no. 2002-089671 filed in Japan, the content of which is hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to technology for accessing memory space in a microprocessor.  
           [0004]    2. Description of the Related Art  
           [0005]    Conventionally, one way of placing a RAM area and a ROM area in a memory address space provided in a microprocessor seeks to shorten instruction codes and instruction execution cycles. This placement method involves placing the RAM area at a head of the memory address space; that is, in a memory address space having a relatively low value. This allows for the utilization of so-called short addressing access, in which the CPU, when accessing the RAM area, designates and accesses only the low order bits of memory addresses, the high order bits having being omitted. Using this access method, it is possible to shorten instruction operands for designating memory addresses, and as a result instruction codes and instruction execution cycles can be shortened.  
           [0006]    More specifically, the RAM area is placed, for example, to include memory addresses x‘00000’ to x‘000FF’ (x‘’ showing that the numerical values within the ‘’ form a hexadecimal memory address) within memory space expressed by 20-bit memory addresses. To access x‘00000’ to x‘000FF’ in the RAM area, it thus becomes possible for the CPU to designate and access only the low order 8 bits, and the high order 12 bits are omitted. Here, the ROM area is placed after the RAM area; that is, in memory address locations having higher values than the memory addresses in which the RAM area is placed.  
           [0007]    Consideration will now be given to placing memory areas in the memory address space of the above microprocessor, when another RAM is newly added and the RAM area is extended. So as to be also able to employ short addressing access after extending, placement of the existing RAM area (referred to as “basic RAM”) must still be at the head of the memory address space after extending. As such, the placement method for the various areas is, in the order from an address head: basic RAM area, ROM area, newly added RAM area (referred to as “extension RAM”), or: basic RAM area, extension RAM area, ROM area.  
           [0008]    When the placement method is in the order, basic RAM area, ROM area, extension RAM area, there is no need to change the pre-extension placement of the basic RAM area and ROM area. Software created for the microprocessor prior to the extension can also be used post-extension, and software compatibility can thus be preserved after extending.  
           [0009]    However with this placement method, memory addresses of the basic RAM area and the extension RAM area are separated by the ROM area and are thus discontinuous. This becomes a problem when memory addresses are accessed sequentially in ascending order in transferring large amounts of data.  
           [0010]    On the other hand, when the placement method is in the order, basic RAM area, extension RAM area, ROM area, consecutive memory addresses can be allotted to the basic RAM area and the extension RAM area. Since the above problem does not arise, this placement method is suitable when large amounts of data are accessed in the memory address order.  
           [0011]    However with this placement method, because the extension RAM area is placed after the basic RAM area, the ROM area has to be shifted back from its pre-extension placement. Since the memory addresses allotted to the ROM area change from pre- to post-extension of the RAM area, the problem arises of not being able to employ pre-extension software created for the microprocessor after extending.  
         SUMMARY OF THE INVENTION  
         [0012]    In view of the above issues, an object of the present invention is to provide an address output apparatus that retains, in a microprocessor, the pre-extension compatibility of software after extending a RAM area, and that allows access designating consecutive memory addresses, with respect to a basic RAM area and an extension RAM area.  
           [0013]    An address output apparatus of the present invention provided to solve the above problems is an address conversion apparatus that outputs address signals for accessing readable/writable first and second memories, and includes: an acquisition unit operable to acquire a logical address value; and an output unit operable to output an address signal for accessing the first memory when the acquired logical address value is included in a first range, to output an address signal for accessing the first memory when the logical address value is included in a third range, which is a section within a second range, the second range being separated from the first range by an interval, and to output an address signal for accessing the second memory when the logical address value is included within the second range and not in the third range.  
           [0014]    According to this structure, if the first memory is basic RAM and the second memory is extension RAM, logical address values designated by pre-extension software involve, the same as pre-extension, accessing the first memory, and the compatibility of software is thus secured. Moreover, by dividing the second range into a third range and a range other than the third range, and allotting these ranges to the first and second memories, it is possible access the first and second memories as contiguous address spaces.  
           [0015]    Here, a head address value of the second range may be greater than an end address value of the first range.  
           [0016]    Here, the output unit may include a select signal output subunit operable to specify which range the acquired logical address value is included in, by whether high order bits of the logical address value match high order bits common between the head and end addresses of each range, and to output to the first and second memories a select signal showing which of the first and second memories to access in accordance with a specification result, and an address conversion subunit operable to output to the first and second memories as an address signal, low order bits that exclude the high order bits from the logical address value.  
           [0017]    According to these structures, the address conversion apparatus is able to generate select signals from high order bits of logical address values using a simple circuit structure, and to specify access locations by low order bits of logical address values, as a result of the memories being selected by these select signals.  
           [0018]    The address conversion apparatus may further include a control unit operable to control a validity and invalidity of operations of the output unit depending on whether the second memory is used.  
           [0019]    According to this structure, power consumption is reduced as a result of the address conversion apparatus invalidating operations of the output unit when RAM is not extended. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate specific embodiments of the present invention.  
         [0021]    In the drawings:  
         [0022]    [0022]FIG. 1 shows a structure of a characteristic section of a microprocessor of an embodiment 1;  
         [0023]    [0023]FIG. 2 shows a correspondence relationship between a physical address space and a logical address space;  
         [0024]    [0024]FIG. 3 shows a table of the correspondence between physical addresses and logical addresses;  
         [0025]    [0025]FIG. 4 shows a detailed structure of an address conversion circuit  20 ;  
         [0026]    [0026]FIG. 5 shows a structure of a characteristic section of a microprocessor of an embodiment 2;  
         [0027]    [0027]FIG. 6 shows a relationship between address signals sent by a CPU  10 , signals of the various signal lines, and accessed RAM; and  
         [0028]    [0028]FIG. 7 shows a structure of a characteristic section of a microprocessor of an embodiment 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    The embodiments of the present invention will be described below in detail with reference to the drawings.  
       Embodiment 1  
       [0030]    [0030]FIG. 1 shows a structure of a characteristic section of a microprocessor of the present embodiment.  
         [0031]    A microprocessor  100  in FIG. 1 includes a CPU  10  that conducts calculations and controls, an address conversion circuit  20  that converts logical address signals outputted by CPU  10  into physical address signals, a RAM  30  and a RAM  50  which are writable memories, a ROM  40  which is read-only memory that stores computer programs and the like, and an address decoder  60  that sends select signals to access-targeted memory devices based on physical address signals.  
         [0032]    Omitting RAM  50  and address conversion circuit  20  from this structure result in a structure similar to a basic structure of a conventional microprocessor prior to RAM area being extended. In the present embodiment, RAM  50  is added to this basic structure in order to extend the RAM area, and address conversion circuit  20  is further added in order to resolve problems in the prior art. These prior art problems are, as mentioned above, the compatibility of software before and after extension, and the discontinuity of memory addresses between RAM areas. The correspondence between logical addresses and physical addresses converted by the address conversion circuit is focused on in order to resolve these problems. Here, a physical address is an address actually allotted to a memory device, and a logical address is an address used when CPU  10  designates and accesses a memory address.  
         [0033]    [0033]FIG. 2 shows the correspondence relationship between a physical address space and a logical address space.  
         [0034]    As shown in FIG. 2, in the physical address space is placed a 4-kilobyte RAM  30  in a head section x‘00000’ to x‘00FFF’. The reason for placing RAM  30  at the head like this is, as described in the Related Art, to allow usage of short addressing access.  
         [0035]    Following RAM  30  is placed a 60-kilobyte ROM  40  from x‘01000’ to x‘0FFFF’.  
         [0036]    The placement of RAM  30  and ROM  40  is the same after extending with RAM  50 , and does not need to be changed. Software compatibility is maintained as a result of the placement being the same.  
         [0037]    A 4-kilobyte RAM  50  is placed from x‘11000’ to x‘11FFF’, so as not to interfere with the placement of RAM  30  and ROM  40  in the physical address space.. Between. ROM  40  and RAM  50  is an address interval equivalent to at least the area of RAM  30 , and nothing is placed in this interval.  
         [0038]    On the other hand, in the logical address space is placed a 4-kilobyte basic RAM area from x‘00000’ to x‘00FFF’, following the basic RAM area is placed a 60-kilobyte ROM area from x‘01000’ to x‘0FFFF’, and following the ROM area is placed a 8-kilobyte extension RAM area from x‘10000’ to x‘11FFF’. This extension RAM area is divided into a first area from x‘10000’ to x‘10FFF’ and a second area from x‘11000’ to x‘11FFF’.  
         [0039]    The correspondence relationship between the logical address space and the physical address space is such that the basic RAM area corresponds to the area of RAM  30 , the ROM area corresponds to the area of ROM  40 , the first area corresponds to the area of RAM  30 , and the second area corresponds to the area of RAM  50 , logical addresses and physical addresses in corresponding areas being in one-to-one correspondence in the arrangement order of the addresses.  
         [0040]    Here, the first area is referred to as a mirror area of the basic RAM area, since the first area and the basic RAM area are both mapped to the area of RAM  30 .  
         [0041]    [0041]FIG. 3 shows a table of the correspondence between physical addresses and logical addresses.  
         [0042]    Address conversion circuit  20 , when logical addresses of the basic RAM area, the ROM area and the second area are inputted by CPU  10 , outputs, as physical addresses, the same values as the inputted logical addresses.  
         [0043]    On the other hand, address conversion circuit  20 , when logical addresses x‘10000’ to x‘10FFF’ are inputted by CPU  10 , converts the inputted to addresses to physical addresses x‘00000’ to x‘00FFF’ and outputs the converted addresses. To give an example, address conversion circuit  20  converts logical address x‘10000’ to physical address x‘00000’, converts logical address x‘10001’ to physical address x‘00001’, converts logical address x‘10002’ to physical address x‘00002’, and converts logical address x‘10FFF’ to physical address x‘00FFF’.  
         [0044]    As a result of these conversions, the specifying of logical addresses and accessing of the basic RAM area and ROM area by CPU  10  equates to the specifying of logical addresses and accessing of the area of RAM  30  and the area of ROM  40  by CPU  10  in a pre-extension microprocessor. As such, it is possible to also employ, in post-extension microprocessor  100 , software for use in a pre-extension microprocessor, and software compatibility is thus preserved.  
         [0045]    Furthermore, consecutive logical addresses are allotted, with the first area corresponding to RAM  30  and the second area corresponding to RAM  50  following one after the other. Since it is possible, as a result, to access an extension RAM area formed from consecutive logical addresses, mass data such as music data and the like can be recorded to extend from the area of RAM  30  to the area of RAM  50 , this structure being suitable, for instance, when specifying and accessing sequential addresses.  
         [0046]    [0046]FIG. 4 shows a detailed structure of address conversion circuit  20 .  
         [0047]    Address conversion circuit  20  in FIG. 4 is structured from a logical address holding unit  21 , an area judgment unit  22 , a conversion unit  23  and a physical address holding unit  24 .  
         [0048]    Logical address holding unit  21  is a buffer that holds logical addresses sent from CPU  10 .  
         [0049]    Area judgment unit  22  judges which area of the physical address space shown in FIG. 2 a logical address held by logical address holding unit  21  belongs to. This judgment is conducted by a logical circuit that compares the size of logical addresses x‘00000’, x‘01000’, x‘10000’, and x‘11000’ at the head of each of the areas in the logical address space with the logical address held by logical address holding unit  21 .  
         [0050]    Conversion unit  23 , when judged as a result of the judgment by area judgment unit  22  that the logical address belongs to one of the basic RAM area, the ROM area, and the second area, outputs, as a physical address, the same value as the logical address, and when judged that the logical address belongs to the first area, converts the logical address to a physical address corresponding to the area of RAM  30  and outputs the converted address. This conversion involves converting a value of the 4 th  bit from a most significant bit in the logical address from 1 to 0.  
         [0051]    Physical address holding unit  24  is a buffer that holds physical addresses outputted from conversion unit  23 , and outputs the held physical addresses to an address bus  70 .  
         [0052]    The physical addresses outputted from physical address holding unit  24  are inputted into each of RAM  30 , ROM  40  and RAM  50 , and the high order 8 bits are inputted into address decoder  60 . Address decoder  60  selects which memory device to access out of RAM  30 , ROM  40  and RAM  50 , from a comparison of a value of the inputted high order 8 bits of the physical address with a range of the physical addresses of each area in the physical address space, and outputs a select signal to the selected memory device. The memory device that receives this select signal recognizes the inputted physical address as an address for itself, and accesses that memory address.  
         [0053]    As a result of the above structure, the placement of pre-expansion address spaces is not changed, and thus in addition to being able to employ, in post-extension microprocessor  100 , software for use in a pre-extension microprocessor, microprocessor  100  is able to access RAM  30  and RAM  50  placed separately in the physical address space using consecutive physical addresses.  
       Embodiment 2  
       [0054]    [0054]FIG. 5 shows a structure of a characteristic section of a microprocessor of an embodiment 2.  
         [0055]    In a microprocessor  200  in FIG. 5, structural elements having the same numbering as microprocessor  100  in FIG. 1 are the same. Although in FIG. 5 depiction of ROM  40  is omitted, logical addresses outputted by the CPU are accessed as physical addresses in ROM  40  in an unaltered state. The following description centers on the differences with embodiment 1. Here, the memory mapping in the present embodiment is the same as FIGS. 2 and 3 of embodiment 1.  
         [0056]    The high order 8 bits of address signals sent from CPU  10  are transferred to a RAM access control circuit  80  by a signal line  90   a  of the address bus, and the low order 12 bits are transferred to RAM  30  and RAM  50  by a signal line  90   b  of the address bus.  
         [0057]    RAM access control circuit  80  is a structural element that replaces address conversion circuit  20  and address decoder  60 , and sends, on a signal line  110   a  and a signal line  110   b , a select signal showing which of RAM  30  and RAM  50  is selected based on the high order 8 bits of an address signal. When RAM  30  is selected, the signal on signal line  110   a  is “1” and the signal on signal line  110   b  is “0”, and vice versa when RAM  50  is selected.  
         [0058]    RAM  30  accesses an address shown by the low order 12 bits of an address signal of signal line  90   b  when the select signal of signal line  110   a  is “1”.  
         [0059]    RAM  50  accesses an address shown by the low order 12 bits of an address signal of signal line  90   b  when the select signal of signal line  110   b  is “1”.  
         [0060]    [0060]FIG. 6 shows a relationship between address signals sent by CPU  10 , the signals of the signal lines, and the accessed RAM.  
         [0061]    As shown in FIG. 6, when the value of signal line  90   a  is x‘00’ and x‘10’, the signals of signal lines  110   a  and  110   b  are respectively “1” and “0”, resulting in RAM  30  being accessed, and when the value of signal line  90   a  is x‘11’, the signals of signal lines  110   a  and  110   b  are respectively “0” and “1”, resulting in RAM  50  being accessed.  
         [0062]    In other words, RAM access control circuit  80  specifies which range an address signal belongs to, by whether the high order 8 bits common between the head address and end address of the pre-conversion addresses in each range in FIG. 6, are the same value as the high order 8 bits of an address signal output by CPU  10 , and outputs to RAM  30  and RAM  50  a select signal showing which of RAM  30  and RAM  50  to access in accordance with the specification result. RAM  30  or RAM  50  selected by the select signal then specifies a location to be accessed from the low order 12 bits of the address signal.  
         [0063]    As a result of the above structure, microprocessor  200  is able to exhibit the same effects as microprocessor  100  using smaller scale circuitry than microprocessor  100 .  
       Embodiment 3  
       [0064]    [0064]FIG. 7 shows a structure of a characteristic section of a microprocessor of embodiment 3.  
         [0065]    A microprocessor  300  in FIG. 7 is different from microprocessor  200  in FIG. 2 in that it includes, instead of RAM access control circuit  80 , a RAM access control circuit  130  having an enable signal input terminal, and has added thereto a RAM capacity judgment circuit  120  that outputs an enable signal.  
         [0066]    RAM capacity judgment circuit  120  judges whether the RAM capacity has been extended, sends an enable signal of “1” or “0” in accordance with the judgment, and thus controls the operation or suspension of the RAM access control circuit. The judgment as to whether the RAM capacity has been extended is conducted, for instance, by judging whether operator guidance by a user, a computer program or the like has been received, or whether RAM  50  is connected to microprocessor  300 . As a result of this enable signal, microprocessor  300  is able to switch between a mode that uses both the basic RAM area and the extension RAM area, and a mode that uses only the basic RAM area.  
         [0067]    When the enable signal is “1”, RAM access control circuit  130  operates in the same manner as RAM access control circuit  80  of embodiment 2.  
         [0068]    When the enable signal is “0”, RAM access control circuit  130  outputs a select signal of “1” on signal line  110   a  and a select signal of “0” on signal line  110   b , and the operation of RAM access control circuit  130  is suspended. Due to this suspension, when the low order 12 bits of an address signal outputted by CPU  10  appears on the address bus, it is possible to access RAM  30  using this low order 12 bit signal.  
         [0069]    As a result of this structure it is possible to reduce power consumption, since the operation of RAM access control circuit  130  is suspended and the high order 8 bits on the address bus are no longer used when RAM extension is not conducted with respect to microprocessor  300 .  
         [0070]    Although embodiments of the present invention have been described above, the present invention is not limited to these embodiments. The present invention may, for example, be structured as follows.  
         [0071]    Although address conversion circuit  20  is structured so as to judge which area of the logical address space an inputted logical address belongs to and to conduct address conversion in accordance with the judged area, address conversion circuit  20  may have a very simple circuit structure that judges whether an inputted logical circuit belongs to the first area, conducts address conversion if it does, and outputs the logical address in an unaltered state without conducted address conversion if it does not.  
         [0072]    Although in FIG. 3 the value of logical addresses in the basic RAM area, the ROM area and the second area are the same as corresponding physical addresses, they need not be the same. In sum, it is sufficient that the basic RAM area and the first area correspond to RAM  30 , that the basic RAM area and the first area are separated with an interval opened up therebetween, and that the second area is mapped to correspond to RAM  50 .  
         [0073]    In FIG. 3, the second area may be mapped to correspond to RAM  30  and the first area may be mapped to correspond to RAM  50 .  
         [0074]    Furthermore, although in FIG. 3 a structure of the extension RAM area is divided into a first area and a second area, it is not limited to this structure, and may be structured such that a section of the extension RAM area is determined as a first area, and a remaining section of the extension RAM area that is not part of the first area is determined as a second area. That is, the second area may be divided in two and sandwich the first area top and bottom, or conversely, the first area may be divided in two and sandwich the second area top and bottom.  
         [0075]    Although in the embodiments, values are allotted so that the head address in the extension RAM area has a larger value than then the end address in the basic RAM area, values may be allotted such that the head address in the basic RAM area has a larger value than then the end address in the extension RAM area.  
         [0076]    Although in the embodiments address conversion circuit  20 , address decoder  60 , RAM access control circuit  80 , RAM capacity judgment circuit  120  and RAM access control circuit  130  are constituted by hardware, they may be structured by software that realizes the same functions using a computer program.  
         [0077]    Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.