Patent Publication Number: US-7904896-B2

Title: Program rewriting system, boot loader, storage medium, and electronic control unit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on Japanese Patent Application 2004-358974 filed on Dec. 10, 2004. The descriptions of the Patent Application are therefore all incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to techniques for rewriting application programs while they are being mounted on/in a circuit board installed in an electronic control unit (ECU); these application programs are executable by a computer of the ECU. The electronic control unit will be abbreviated as ECU hereinafter. 
     2. Description of the Related Art 
     ECUs, which are preferably installed in an automobile and operative to control a target, such as an engine installed therein, have been well known, one example of which is disclosed in Japanese Unexamined Patent Publication No. H10-105468. In the ECU disclosed in the Publication, an application program for controlling a target(s) has been stored in a nonvolatile rewritable memory, such as a flash ROM (Reed Only Memory), EEPROM (Electrical Erasable Programmable Rom), or the like. The nonvolatile rewritable memory has been mounted, as a component of a computer, on/in a circuit board of the ECU. The computer of the ECU is operative to rewrite the application program in the nonvolatile rewritable memory without removing it from the circuit board of the ECU. 
     In such an ECU, when rewriting an earlier application program in a rewritable memory, first, an external device, such as a computer device, is prepared to be communicably connected to the ECU through a communication line. Next, a write request is sent from the external device to a computer of the ECU through the communication line as a command; this write request allows the computer to shift its operation mode into a mode (application program rewritable mode) in which the computer can rewrite application programs stored in the rewritable memory. Note that the write request can be transmitted from the external device to the ECU through a signal line different from the communication line. 
     After transmission of the write request, a rewrite control program is sent from the external device to the computer of the ECU; the rewrite control program is designed to cause a computer to: 
     receive a new application program sent from the external device; and 
     update the earlier application program to the received new application program. 
     Specifically, after sending of the rewrite control program, the new application program is sent from the external device to the computer of the ECU. 
     On the other hand, when receiving the write request, the computer of the ECU runs a boot loader (boot program) previously stored in the nonvolatile rewritable memory thereof. The boot loader causes the computer to: 
     receive the rewrite control program to download it in a RAM (Random Access Memory) of the computer; and 
     launch the rewrite control program stored in the RAM. 
     Specifically, as the launching process, the boot loader causes the execution location of the computer to jump to the leading address of the rewrite control program in the RAM. After jump of the computer&#39;s execution location, the computer executes the rewrite control program in the RAM to receive the new application program transferred from the external device, thereby updating the earlier application program to the received new application program in the rewritable memory. 
     These application-program rewriting operations set forth above are disclosed in the aforementioned Unexamined Patent Publication. 
     Specifically, these application-program rewriting operations allow the earlier application program to be rewritten into the new application program in the rewritable memory. For example, an earlier version of the application program can be updated to a new version thereof in the rewritable memory. 
     When no application programs to be updated has been stored in the rewritable memory, the new application program sent from the external device is stored in the rewritable memory. 
     In the Unexamined Patent Publication, the rewrite control program and the new application program have been stored in the external device as different files. The boot loader to be executed by the computer of the ECU serves as download of the rewrite control program into the RAM, and jump of the execution location of the computer to the leading address of the rewrite control program. 
     Such external devices for rewriting application programs in, for example, automobile field use any one of typically three different application-program sending ways (programs, procedures) A to C. A, B, and C types of external devices employ the application-program sending ways A, B and C, respectively. 
     Specifically, the A type of external devices is configured to have stored therein a rewrite control program and a new application program in different files. 
     The B type of external devices is configured to have stored therein a rewrite control program and a new application program in the same single file. 
     The C type of external devices is configured to have stored therein a new application program without storing a rewrite control program. This is because the C type of external devices is designed on the understanding that the rewrite control program has been stored in a nonvolatile memory of a computer of the ECU. 
     In order to rewrite an earlier application program stored in the ECU, in an external device of the A type, the rewrite control program and the new application program are separately sent from the external device of the A type to the ECU. 
     In contrast, in an external device of the B type, the rewrite control program and the new application program have been stored in the same single file, so that, in order to rewrite an earlier application program stored in the ECU, they are sent from the external device of the B type to the ECU in file. For this reason, the boot loader being executed by the computer of the ECU does not cause the computer to identify the end address of the rewrite control program. This results in that download of the rewrite control program into the RAM may come to nothing, making it difficult to accurately update the earlier application program. 
     In addition, because no rewrite control program is sent from an external device of the C type to the ECU, the boot loader being executed by the computer of the ECU may cause the computer to erroneously download the new application program sent from the external device of the C type into the RAM as the rewrite control program. This may also make it difficult to accurately update the earlier application program. 
     It may be considered that boot loaders (boot programs) are designed exclusively for the types A, B, and C of external devices. This may cause the number of processes for developing the dedicated boot loaders for the types A to C of external devices to increase. In addition, it may be necessary to select one of the boot loaders to match any one of the types of an external device used, causing management of the dedicated boot loaders and the external devices of the types A to C to become complicated. 
     If one of the boot loaders, which does not match the type of an external device used, is installed in the computer of the ECU, earlier application programs, which have been already installed in the computer of the ECU, cannot be updated to new ones. 
     SUMMARY OF THE INVENTION 
     The present invention has been made on the background set forth above. Specifically, at least one preferable embodiment of the present invention provides a program rewriting system and an electronic control unit, which are capable of meeting any different application-program sending ways employed by external devices. 
     According to a first aspect of the present invention, there is provided a system communicably coupled to an external device at least when rewriting, at least in a module, a first application program stored in a first memory of the system into a second application program stored in the external device. The external device is configured to send, to the system, the second application program based on first and second different ways. The first way is to send, to the system, a rewrite control program stored in the external device as write target data, and the second application program next to the rewrite control program as the write target data. The second way is to send, to the system, the second application program as the write target data without sending the rewrite control program. The rewrite control program allows the system to rewrite the first application program into the second application program. The system includes a receiving unit configured to receive an identifier sent from the external device. The identifier decides a sending timing of the second application program from the external device. The system also includes an identification unit configured to identify that the write target data sent from the external device is at least a module of the second application program after the identifier is received by the receiving unit. 
     According to a second aspect of the present invention, there is provided a system communicably coupled to an external device at least when rewriting, at least in a module, a first application program stored in a first memory of the system into a second application program stored in the external device. The external device is configured to send, to the system, the second application program based on first and second different ways. The first way is to send, to the system, a rewrite control program stored in the external device as write target data, and the second application program next to the rewrite control program as the write target data. The second way is to send, to the system, the second application program as the write target data without sending the rewrite control program. The rewrite control program allows the system to rewrite the first application program into the second application program. The system includes a determining unit configured to determine whether to receive an identifier sent from the external device. The identifier decides a sending timing of the second application program from the external device. The system also includes a first identification unit configured to identify that the write target data sent from the external device is at least a module of the rewrite control program until it is determined that the identifier is received. The system further includes a second identification unit configured to identify that the write target data sent from the external device is at least a module of the second application program after it is determined that the identifier is received. 
     According to a third aspect of the present invention, there is provided a program product having a boot loader installed in a system. The system is communicably coupled to an external device at least when rewriting, at least in a module, a first application program stored in a first memory of the system into a second application program stored in the external device. The external device is configured to send, to the system, the second application program based on first and second different ways. The first way is to send, to the system, a rewrite control program stored in the external device as write target data, and the second application program next to the rewrite control program as the write target data. The second way is to send, to the system, the second application program as the write target data without sending the rewrite control program. The rewrite control program allows the system to rewrite the first application program into the second application program. The boot loader includes first means for instructing a computer to determine whether to receive an identifier sent from the external device. The identifier decides a sending timing of the second application program from the external device. The boot loader also includes second means for instructing a computer to identify that the write target data sent from the external device is at least a module of the rewrite control program until it is determined that no identifier is received. The boot loader further includes third means for instructing a computer to identify that the write target data sent from the external device is at least a module of the second application program after it is determined that the identifier is received. 
     According to a fourth aspect of the present invention, there is provided a signal bearing media bearing the boot loader according to the third aspect of the present invention. 
     According to a fifth aspect of the present invention, there is provided a vehicle control unit for controlling a target installed in a vehicle. The vehicle control unit includes a computer and the signal bearing media according to the third aspect of the present invention. The signal bearing media is readable by the computer. 
     According to a sixth aspect of the present invention, there is provided a system communicably coupled to an external device at least when rewriting, at least in a module, a first application program stored in a first memory of the system into a second application program stored in the external device. The external device is configured to send, to the system, the second application program based on first and second different ways. The first way is to send, to the system, a rewrite control program stored in the external device as write target data, and the second application program next to the rewrite control program as the write target data. The second way is to send, to the system, the second application program as the write target data without sending the rewrite control program. The rewrite control program allows the system to rewrite the first application program into the second application program. The system includes a receiving unit configured to receive destination information sent from the external device. The destination information indicates an address at which the write target data is written. The system also includes a determining unit configured to determine whether the destination information indicates an address in the first memory. The system further includes an identification unit configured to identify that the write target data sent from the external device is at least a module of the second application program after it is determined that the destination information indicates an address in the first memory. 
     According to a seventh aspect of the present invention, there is provided a system communicably coupled to an external device at least when rewriting, at least in a module, a first application program stored in a first memory of the system into a second application program stored in the external device. The external device is configured to send, to the system, the second application program based on first and second different ways. The first way is to send, to the system, a rewrite control program stored in the external device as write target data, and the second application program next to the rewrite control program as the write target data. The second way is to send, to the system, the second application program as the write target data without sending the rewrite control program. The rewrite control program allows the system to rewrite the first application program into the second application program. The system includes a receiving unit configured to receive destination information sent from the external device. The destination information indicates an address at which the write target data is written. The system also includes a determining unit configured to determine whether the destination information indicates an address in the first memory. The system further includes a first identification unit configured to identify that the write target data sent from the external device is at least a module of the rewrite control program until it is determined that the destination information indicates an address in the first memory. The system still further includes a second identification unit configured to identify that the write target data sent from the external device is at least a module of the second application program after the it is determined that the destination information indicates an address in the first memory. 
     According to an eighth aspect of the present invention, there is provided a program product having a boot loader installed in a system. the system is communicably coupled to an external device at least when rewriting, at least in a module, a first application program stored in a first memory of the system into a second application program stored in the external device. The external device is configured to send, to the system, the second application program based on first and second different ways. The first way is to send, to the system, a rewrite control program stored in the external device as write target data, and the second application program next to the rewrite control program as the write target data. The second way is to send, to the system, the second application program as the write target data without sending the rewrite control program. The rewrite control program allows the system to rewrite the first application program into the second application program. The boot loader includes first means for instructing a computer to determine whether the destination information indicates an address in the first memory. The boot loader also includes second means for instructing a computer to identify that the write target data sent from the external device is at least a module of the rewrite control program until it is determined that the destination information indicates an address in the first memory. The boot loader includes third means for instructing a computer to identify that the write target data sent from the external device is at least a module of the second application program after the it is determined that the destination information indicates an address in the first memory. 
     According to a ninth aspect of the present invention, there is provided a signal bearing media bearing the boot loader according to the eighth embodiment. 
     According to a tenth aspect of the present invention, there is provided a vehicle control unit for controlling a target installed in a vehicle. The vehicle control unit includes a computer and the signal bearing media according to the eighth aspect of the present invention; this signal bearing media is readable by the computer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and aspects of the invention will become apparent from the following description of an embodiment with reference to the accompanying drawings in which: 
         FIG. 1  is a block diagram schematically illustrating an example of the hardware structure of a program rewritable system with an A type external tool according to a first embodiment of the present invention; 
         FIG. 2  is a block diagram schematically illustrating an example of the hardware structure of a program rewritable system with a B type external tool according to a first embodiment of the present invention; 
         FIG. 3  is a block diagram schematically illustrating an example of the hardware structure of a program rewritable system with a C type external tool according to a first embodiment of the present invention; 
         FIG. 4  is a flowchart schematically illustrating the overall procedure of a boot loader according to a first embodiment; 
         FIG. 5  is a flowchart schematically illustrating an algorithm of a transfer determining program included in the boot loader illustrated in each of  FIGS. 1 to 3 ; 
         FIG. 6  is a sequence chart schematically illustrating communication procedures between the A type external tool and the ECU illustrated in  FIG. 1 ; 
         FIG. 7  is a sequence chart schematically illustrating communication procedures between the B type external tool and the ECU illustrated in  FIG. 2 ; 
         FIG. 8  is a sequence chart schematically illustrating communication procedures between the C type external tool and the ECU illustrated in FIG,  3 ; 
         FIG. 9  is a flowchart schematically illustrating the overall procedure of a boot loader according to a second embodiment of the present invention; 
         FIG. 10  is a flowchart schematically illustrating an algorithm of a transfer determining program included in the boot loader illustrated in  FIG. 9 ; 
         FIG. 11  is a sequence chart schematically illustrating communication procedures between each of the A type external tool and the B type external tool and the ECU; and 
         FIG. 12  is a sequence chart schematically illustrating communication procedures between the C type external tool and the ECU. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Embodiments of the present invention will be described hereinafter with reference to the attached figures. 
     First Embodiment 
     Referring to  FIG. 1 , a program rewriting system RS according to the first embodiment includes an ECU  1  preferably installed in an automobile and operative to control a target, such as an engine installed therein. The program rewriting system RS also includes an external tool  5 , such as an external device consists of a computer-implemented communication device. At least when rewriting an earlier application program, which has been stored in the ECU  1 , communication between the external tool  5  and the ECU  1  through a communication line  3  is established. 
     The ECU  1  includes a microcomputer  7  and a communication driver  9  for allowing the microcomputer  7  to communicate with the external tool  5  through the communication line  3 . The microcomputer  7  includes a CPU  11  operative to run programs, a flash ROM  13  as an example of nonvolatile rewritable memories, and a RAM  15  as an example of volatile memories, which serves as a main working memory of the microcomputer  7 . For example, the elements of the microcomputer  7  are mounted on/in a circuit board of the ECU  1 . 
     The flash ROM  13  has stored therein an earlier application program AP 1  and a boot loader BL. The boot loader BL causes the CPU  11  to, when a write request is sent from the external tool  5  to the microcomputer  7 , launch a rewrite control program RP, described hereinafter, stored in the RAM  15 . The write request allows the CPU  11  to shift its operation mode into a mode in which the CPU  11  can rewrite application programs stored in the flash ROM  13 . The rewrite control program causes the CPU  11  to update the earlier application program AP 1  to a new application program. The boot loader BL can be stored in a ROM of the microcomputer  7 . In addition, the write request is configured to be sent from the external tool  5  to the ECU  1  when a predetermined operation is executed in the external tool  5 . 
     In the first embodiment, as the external tool  5 , any one of three types A, B, and C of external tools  5   a  to  5   c  can be used. The three types A to C of external tools  5   a  to  5   c  employ application-program sending ways A to C, respectively. 
     Specifically, the A type external tool  5   a  is configured to have stored therein the rewrite control program RP and a new application program AP 2  in different files F 1  and F 2  (see  FIG. 1 ). 
     The B type external tool  5   b  is configured to have stored therein the rewrite control program RP and the new application program AP 2  in the same single file F 3  (see  FIG. 2 ). 
     The C type external tool  5   c  is configured to have stored therein the new application program AP 2  as a file F 2  without storing the rewrite control program RP. This is because the C type external tool  5   c  is designed on the understanding that the rewrite control program RP has been stored in the microcomputer  7  of the ECU  1 . 
     As illustrated in  FIGS. 1 and 3 , for example, a ROM identification code CD is located at the top (beginning) of the whole data (information) stored in the file F 2 ; this ROM identification code CD allows the CPU  11  to identify that the following data (information) from the identification code CD is a new application program. Each of the external tools  5   a  and  5   c  is therefore operative to send the ROM identification code CD to the ECU  1  before sending the new application program AP 2 . 
     Similarly, as illustrated in  FIG. 2 , for example, the ROM identification code CD is located at the boundary between the rewrite control program RP and the new application program AP 2 ; this ROM identification code CD allows the CPU  11  to identify that the following data (information) from the identification code CD is a new application program. The external tool  5   b  is therefore operative to send the ROM identification code CD to the ECU  1  before sending the new application program AP 2 . 
     Each of the external tools  5   a  to  5   c  is configured to sequentially send, to the ECU  1  through the communication line  3 , data in a predetermined unit quantity of data, such as in 2 bytes. 
     Specifically, when the ECU  1  has a function capable of communicating with the C type of external tools, the ECU  1  has stored the rewrite control program RP in the flash ROM  13  at a predetermined first address area thereof. Note that the rewrite control program RP can have been stored in another nonvolatile memory, such as a ROM of the microcomputer  7 . 
     Next, operations of each of the external tools  5   a  to  5   c  and of the ECU  1  will be described hereinafter when rewriting the earlier application program AP 1 . 
     Independently of which external tool is ready to communicate with the ECU  1 , when the write request is sent from any one of the external tools  5   a  to  5   c  to the ECU  1 , the CPU  11  of the microcomputer  7  shifts its execution location from the earlier application program AP 1  in the flash ROM  13  to the leading address of the boot loader BL therein. 
     Next, as illustrated in  FIG. 1 , when the A type external tool  5   a  has sent the write request to the ECU  1 , the A type external tool  5   a  sends the rewrite control program RP (the file F 1 ) to the ECU  1  after sending the write request thereto. 
     When the rewrite control program RP is sent from the A type external tool  5   a , the CPU  11  of the microcomputer  7  runs the boot loader BL. The boot loader B 1  causes the CPU  11  to receive the rewrite control program RP to sequentially download it in the RAM  15  from a predetermined address (leading address) thereof. After completion of the rewrite control program download, the boot loader BL causes the CPU  11  to shift its execution location to the leading address of the rewrite control program RP. 
     After completion of sending the rewrite control program RP, the external tool  5   a  sends, to the ECU  1 , the new application program AP 2  (the file F 2 ). 
     On the other hand, after shift of the CPU&#39;s execution location to the leading address of the rewrite control program RP, the CPU  11  runs the rewrite control program RP from the leading address thereof. Specifically, the rewrite control program RP causes the CPU  11  to execute rewrite (update) operations for updating the new application program AP 2  sent from the external tool  5   a  in the flash ROM  13 . The rewrite operations permit the earlier application program AP 1  stored in the flash ROM  13  is rewritten into the new application program AP 2 . If no application programs to be updated has been stored in the flash ROM  13 , the new application program AP 2  sent from the external tool  5   a  is stored in the flash ROM  13  in a new entry. 
     As illustrated in  FIG. 2 , when the B type external tool  5   b  has sent the write request to the ECU  1 , the B type external tool  5   b  sends the rewrite control program RP and the new application program AP 2  (the file F 3 ) to the ECU  1  after sending the write request thereto. 
     When the file F 2  is sent from the B type external tool  5   b , the CPU  11  of the microcomputer  7  runs the boot loader BL. The boot loader BL causes the CPU  11  to receive the file F 3  (the rewrite control program RP and the new application program AP 2 ) to sequentially download them in the RAM  15  from a predetermined address (leading address) thereof. After completion of the file F 3  download, the boot loader BL causes the CPU  11  to shift its execution location to the leading address of the file P 3  (rewrite control program RF 1 . 
     Subsequently, the CPU  11  runs the rewrite control program RP from the leading address thereof. Specifically, the rewrite control program RP causes the CPU  11  to execute rewrite (update) operations for updating the new application program AP 1  sent from the external tool  5   a  in the flash ROM  13 . 
     On the other hand, as illustrated in  FIG. 3 , when the C type external tool  5   c  has sent the write request to the ECU  1 , the C type external tool  5   c  sends the new application program AP 2  (the file F 2 ) to the ECU  1  after sending the write request thereto. 
     When the file F 2  is sent from the C type external tool  5   c , the CPU  11  of the microcomputer  7  runs the boot loader BL. The boot loader BL causes the CPU  11  to receive the new application program AF) to sequentially download it in the RAM  15  from a predetermined address (leading address) thereof. After completion of the new application program AP 2  download, the boot loader BL causes the CPU  11  to copy the rewrite control program RP previously stored in the flash ROM  13  into the RAM  15  from a predetermined address (leading address) thereof. After completion of the rewrite-control program copy, the boot loader BL causes the CPU  11  to shift its execution location to the leading address of the rewrite control program RP. 
     Subsequently, the CPU  11  runs the rewrite control program RP from the leading address thereof. Specifically, the rewrite control program RP causes the CPU  11  to execute rewrite (update) operations for updating the new application program AP 1  sent from the external tool  5   a  in the flash ROM  13 . 
     Next, the specific instructions (procedures) of the boot loader BL according to the first embodiment will be described hereinafter with reference to  FIGS. 4 and 5 . Specifically the boot loader BL according to the first embodiment is able to be executed by the ECU  1 , to meet any types of external tools  5   a  to  5   c , and to realize the rewrite operations in cases of using any one of the external tools  5   a  to  5   c.    
       FIG. 4  schematically illustrates the overall procedure of the boot loader BL. 
     Specifically as set forth above, the CPU  11  of the microcomputer  7  of the ECU  1  shifts its execution location from the earlier application program AP 1  to the boot loader BL when the write request is sent from the external tool  5  to the ECU  1 . For example, the earlier application program AP 1  includes a predetermined procedure to detect that the communication driver  9  receives the write request sent from the external tool  5 . As a result, when the CPU  11  executes the predetermined procedure while executing the earlier application program AP 1 , the CPU  11  executes the procedures of the boot loader BL illustrated in  FIG. 4 . 
     When the CPU  11  launches the procedures of the boot loader BL, in step S 110 , the CPU  11  receives a unit quantity of the data sent from the external tool  5  and received by the communication driver  9 . Next, in step S 120 , the CPU  11  analyzes the received unit quantity of data to determine whether the received unit quantity of data is the write request based on the analyzed result. 
     If it is determined that the write request has been received (the determination in step S 120  is YES), the CPU  11  goes to step S 130  to execute an operation required to send, to the external tool  5 , a response to the write request, and thereafter returning to the step S 110 . The response to the write request is a response message indicating, to the external tool  5 , that the write request is normally received. 
     When it is determined that the write request has not been received (the determination in step S 120  is NO), the CPU  11  goes to step S 140 . In step S 140 , the CPU  11  analyzes the received unit quantity of data in step S 110  to determine whether the received unit quantity of data is a data download request based on the analyzed result. 
     The data download request is a command to indicate, to the ECU  1 , that the external tool  5  starts to send data to be written except for commands; this data to be rewritten specifically is the rewrite control program RP or the new application program AP 2 . The data download request also includes download-data size information indicating the total volume of the data to be written. The data to be written will be referred to as write target data or write data hereinafter. 
     When it is determined that the data download request has been received (the determination in step S 140  is YES), the CPU  11  goes to step S 150  to store the download-data size information in the RAM  15 . Subsequently, in the next step S 160 , the CPU  11  executes an operation required to send, to the external tool  5 , a response to the data download request, and thereafter returning to the step S 110 . The response to the data download request is a response message indicating, to the external tool  5 , that the data download request is normally received. 
     When it is determined that the data download request has not been received (the determination in step S 140  is NO), the CPU  11  goes to step S 170 . In step S 170 , the CPU  11  analyzes the received unit quantity of data in step S 110  to determine whether the received unit quantity of data is a part of the write target data based on the analyzed result. Note that, in step S 170 , it is determined that the ROM identification code CD belongs to the write target data. 
     When it is determined that the write target data has not been received (the determination in step S 170  is NO), the CPU  11  returns to step S 110 . When it is determined that the write target data has been received (the determination in step S 170  is YES), the CPU  11  goes to step S 180 . In step S 180 , the CPU  111  executes a transfer determining program, such as a subroutine, illustrated in  FIG. 5  hereinafter, and thereafter returns to step S 110 . The transfer determining program is a part of (a module of) the boot loader BL. 
     When launching the transfer determining program, the CPU  11  analyzes the received data in step S 110  to determine whether the received data is the ROM identification code CD based on the analyzed result in step S 210  of  FIG. 5 . 
     When it is determined that ROM identification code CD has not been received (the determination in step S 210  is NO), the CPU  11  goes to step S 220 . In step S 220 , the CPU  11  sets a rewrite control program download flag DF in its register; this flag DF indicates that the download of the rewrite control program RP has been completed. Next, the CPU  11  writes the current received data, that is, the write target data, into the RAM  15  at a predetermined address thereof as data constituting the rewrite control program RP in step S 230 . 
     In the next step S 240 , the CPU  11  determines whether received data volume is equivalent to the total volume indicated by the download-data size information stored in the RAM  15  in step S 150 . In other words, the CPU  11  determines whether the download-data size of the write target data has been entirely written in the RAM  15  in step S 240 . 
     When it is determined that the download-data size of the write target data has not been entirely written in the RAM  15  (the determination in step S 240  is NO), the CPU  11  exits from the transfer determining program, returning to step S 110 . 
     In step S 240 , when it is determined that the download-data size of the write target data has been entirely written in the RAM  15  (the determination in step S 240  is YES), the CPU  11  goes to step S 250 . In step S 250 , the CPU  11  executes an operation required to send, to the external tool  5 , a response to the write data transfer, and thereafter exits from the transfer determining program, returning to step S 110 . The response to the write data transfer is a response message indicating, to the external tool  5 , that the download-data size of the write target data has been entirely written in the RAM  15 . 
     On the other hand, in step S 210 , when it is determined that ROM identification code CD has been received (the determination in step S 210  is YES), the CPU  11  goes to step S 260 . In step S 260 , the CPU  11  determines whether the rewrite control program download flag DF is set to its register. 
     When it is determined that the rewrite control program download flag DF is set to its register (the determination in step S 260  is YES), the CPU  11  goes to step S 270 . In step S 270 , the CPU  11  shifts its execution location to the leading address of the rewrite control program RP in the RAM  15 . 
     Thereafter, the CPU  11  runs the rewrite control program RP from the leading address thereof. Specifically, the CPU  11  executes the rewrite operations to update the new application program AP 2  sent from the external tool  5  in the flash ROM  13 . After completion of the new application program update, the CPU  11  sends, to the external tool  5 , the response to the write data transfer. 
     In step S 260 , when it is determined that the rewrite control program download flag DF is not set to its register (the determination in step S 260  is NO), the CPU  11  determines that the rewrite control program RP sent from the external tool  5  has not been stored in the RAM  15 , going to step S 280 . In step S 280 , the CPU  11  copies the rewrite control program RP previously stored in the flash ROM  13  at the first address area into the RAM  15  from the leading address thereof. After completion of the rewrite control program copy, the CPU  11  shifts to the step S 270 . 
     In this case, the CPU  11  executes the rewrite control program RP copied from the flash ROM  13  into the RAM  15  in step S 280 . 
     Thereafter, the CPU  11  runs the rewrite control program RP from the leading address thereof. Specifically, the CPU  11  executes the rewrite operations to update the new application program AP 2  sent from the external tool  5  in the flash ROM  13 . After completion of the new application program update, the CPU  11  sends, to the external tool  5 , the response to the write data transfer. 
     Next, individual operations of each of the external tools  5   a  to  5   c - 23  and of the ECU  1  will be described hereinafter with reference to  FIGS. 6 to 8 . 
     First,  FIG. 6  illustrates a sequence chart representing communication procedures between the A type external tool  5   a  and the ECU  1 . 
     As illustrated by the reference numeral ( 1 ) in  FIG. 6 , when the predetermined operation is executed in the external tool  5   a  so that the write request RS is sent therefrom to the ECU  1 , the execution location of the CPU  11  is shifted from the earlier application program AP 1  to the boot loader BL. Thereafter, the boot loader BL is executed by the CPU  11 . These operations of the CPU  11  are represented by the reference character  100  in  FIG. 6 . 
     In the boot loader BL, the affirmative determination is executed by the CPU  11  in step S 120 , and the response to the write request is sent therefrom to the A type external tool  5   a  in step S 130 , which is illustrated by the reference numeral ( 2 ) in  FIG. 6 . 
     When receiving the response to the write request, the A type external tool  5   a  sends, to the ECU  1 , the data download request, as a preparation for sending the rewrite control program RP, which is illustrated by the reference numeral ( 3 ) in  FIG. 6 . The data download request includes the download-data size information indicating the total volume of the rewrite control program RP, that is, the total volume of the file F 1 . 
     In the boot loader BL, the affirmative determination is executed by the CPU  11  in step S 140 . Next, the download-data size information is stored in the RAM  15  by the CPU  11  in step S 150 , and the response to the data download request is sent therefrom to the A type external tool  5   a  in step S 160 , which is illustrated by the reference numeral ( 4 ) in  FIG. 6 . 
     When receiving the response to the data download request, the A type external tool  5   a  sends (downloads), to the ECU  1 , the rewrite control program RP stored as the file F 1  as the write target data, which is illustrated by the reference numeral ( 5 ) in  FIG. 6 . In more detail, the A type external tool  5   a  downloads, to the ECU  1 , data constituting the rewrite control program RP as the write target data. In  FIGS. 6 to 8 , the write target data is described as “write data”. 
     In the boot loader BL, the affirmative determination is executed by the CPU  11  in step S 170 . Next, the transfer determining program is executed by the CPU  11  in step S 180  illustrated in  FIG. 5 . 
     In the transfer determining program, when the rewrite control program RP is sent from the A type external tool  5   a , because the ROM identification code CD has not been sent therefrom, the negative determination is executed by the CPU  11  in step S 210 . Next, the rewrite control program download flag DF is set in the register by the CPU  11  in step S 220 , and a unit quantity of the rewrite control program RP is written in the RAM  15  by the CPU  11  in step S 230 . 
     In the boot loader BL, every time a unit quantity of the rewrite control program RP is sent from the A type control tool  5   a , the affirmative determination is executed by the CPU  11  in step S 220  and the operations in step S 230  are executed thereby. As a result, when the total volume of the rewrite control program RP has been entirely written in the RAM  15 , the affirmative determination is executed by the CPU  11  in step S 240 . Next, the response to the write data transfer is sent from the CPU  11  to the A type external tool  5   a  in step S 250 , which is illustrated by the reference numeral ( 6 ) in  FIG. 6 . 
     When receiving the response to the response to the rewrite data, the A type external tool  5   a  sends, to the ECU  1 , the data download request as a preparation for sending the rewrite control program RP, which is illustrated by the reference numeral ( 7 ) in  FIG. 6 . The data download request includes the download-data size information indicating the total volume of the new application program Ap 2 , that is, the total volume obtained by subtracting the volume of the ROM identification code CD from the total volume of the file F 2   
     In the boot loader BL, the affirmative determination is executed by the CPU  11  in step S 140 . Next, the download-data size information is stored in the RAM  15  by the CPU  11  in step S 150 , and the response to the data download request is sent therefrom to the A type external tool  5   a  in step S 160 , which is illustrated by the reference numeral ( 8 ) in  FIG. 6 . 
     When receiving the response to the data download request, the A type external tool  5   a  sends (downloads), to the ECU  1 , a first unit quantity of the new application program AP 2  stored in the file F 2  and the write target data, which is illustrated by the reference numeral ( 9 ) in  FIG. 6 . That is, the A type external tool  5   a  downloads, to the ECU  1 , the ROM identification code CD located at the top of the file F 2 . 
     In the boot loader BL, the affirmative determination is executed by the CPU  11  in step S 170 . Next, the transfer determining program is executed by the CPU  11  in step S 180  (see  FIG. 5 ). 
     In the transfer determining program, because the ROM identification code CD has been received by the CPU  11 , the affirmative determination is executed by the CPU  11  in step S 210 . In addition, because the rewrite control program download flag DF has been set in the register by the CPU  11  in step S 220  so that the rewrite control program RP has been stored in the RAM  11 , the affirmative determination is executed by the CPU  11  in step S 260 . As a result, the execution location of the CPU  11  jumps to the leading address of the rewrite control program RP in the RAM  15  by the CPU  11  in step S 270 . 
     Thereafter, a unit quantity of the new application program AP 2  is sequentially sent from the A type external tool  5   a  to the ECU  1 , which is illustrated by the reference numeral ( 10 ) in  FIG. 6 . As a result, in the microcomputer  7 , the rewrite control program RP downloaded to be stored in the RAM  15  from the A type external tool  5   a  is executed by the CPU  11 . The execution of the rewrite control program RP by the CPU  11  allows the earlier application program AP 1  stored in the flash ROM  13  to be sequentially updated into the new application program AP 2  sent from the A type external tool  5   a  every sent unit of the program AP 2 , which is illustrated by the reference numeral  150  in  FIG. 6 . 
     The rewrite operations by the CPU  11  in accordance with the rewrite control program RP are carried out, for example, as follows. Specifically, a predetermined second address area of the flash ROM  13  in which the earlier application program AP 1  is stored is cleared, and each sent unit of the program AP 2  is sequentially stored in the second address area of the flash ROM  13 . 
     After all units of the new application program AP 2  has been stored in the storage area of the flash ROM  13 , the response to the write data transfer is sent from the CPU  1  to the A type external tool  5   a  based on the rewrite control program RP, which is illustrated by the reference numeral ( 11 ) in  FIG. 6 . The response to the write data transfer indicates, to the external tool  5 , that the download-data size of the new application program AP 2  has been entirely stored in the RAM  15  so that the application program rewrite operations have been completed 
     Secondly,  FIG. 7  illustrates a sequence chart representing communication procedures between the B type external tool  5   b  and the ECU  1 . 
     In  FIG. 7 , the communication procedures up to the reference numeral ( 4 ) are substantially identical those up to the reference numeral ( 4 ) in  FIG. 6  except for the procedure indicated by the reference numeral ( 3 ). In  FIG. 7 , as the procedure indicated by the reference numeral ( 3 ), the B type external tool  5   b  sends, to the ECU  1 , the data download request. The data download request includes the download-data size information indicating the total volume of the rewrite control program RP and the new application program AP 2 , that is, the total volume obtained by subtracting the volume of the ROM identification code CD from the total volume of the file F 3 . The descriptions of the communication procedures up to the reference numeral ( 4 ) are therefore omitted. 
     When receiving the response to the data download request, the B type external tool  5   b  sends (downloads), to the ECU  1 , the data stored in the file F 3  as the write target data, which is illustrated by the reference numeral ( 5 ) in  FIG. 7 . In more detail, the B type external tool  5   b  downloads, to the ECU  1 , data of the rewrite control program RP arranged at the first part of the file F 3 . 
     In the boot loader BL, the affirmative determination is executed by the CPU  11  in step S 170 . Next, the transfer determining program is executed by the CPU  11  in step S 180  illustrated in  FIG. 5 . 
     In the transfer determining program, when the rewrite control program RP is sent from the B type external tool  5   b , because the ROM identification code CD has not been sent therefrom, the negative determination is executed by the CPU  11  in step S 210 . Next, the rewrite control program download flag DF is set in the register by the CPU  11  in step S 220 , and a unit quantity of the rewrite control program RP is written in the RAM  15  by the CPU  11  in step S 230 . 
     In the boot loader BL, every time a unit quantity of the rewrite control program RP is sent from the B type control tool  5   b , the affirmative determination is executed by the CPU  11  and the operations in step S 220  and S 230  are executed thereby. 
     After completion of the send of the rewrite control program RP, the B type external tool  5   b  sends, to the ECU  1 , the ROM identification code CD located at the boundary between the rewrite control program RP and the new application program AP 2  as the rewrite target data, which is illustrated by the reference numeral ( 6 ) in  FIG. 7 . 
     In the boot loader BL, the affirmative determination is executed by the CPU  11  in step S 170 . Next, the transfer determining program is executed by the CPU  111  in step S 180  (see  FIG. 5 ). 
     In the transfer determining program, because the ROM identification code CD has been received by the CPU  11 , the affirmative determination is executed by the CPU  11  in step S 210 . In addition, because the rewrite control program download flag DF has been set in the register by the CPU  11  in step S 220  so that the rewrite control program RP has been stored in the RAM  11 , the affirmative determination is executed by the CPU  11  in step S 260 . As a result, the execution location of the CPU  11  jumps to the leading address of the rewrite control program RP in the RAM  15  by the CPU  11  in step S 270 . 
     Thereafter, a unit quantity of the new application program AP 2  is sequentially sent from the B type external tool  5   b  to the ECU  1 , which is illustrated by the reference numeral ( 7 ) in  FIG. 7 . As a result, in the microcomputer  7 , the rewrite control program RP downloaded to be stored in the RAM  15  from the B type external tool  5   b  is executed by the CPU  11 . The execution of the rewrite control program RP by the CPU  11  allows the earlier application program AP 1  stored in the flash ROM  13  to be sequentially updated into the new application program AP 2  sent from the B type external tool  5   b  every sent unit of the program AP 2 , which is illustrated by the reference numeral  150  in  FIG. 7 . 
     After all units of the new application program AP 2  has been stored in the storage area of the flash ROM  13 , the response to the write data transfer is sent from the CPU  1  to the B type external tool  5   b  based on the rewrite control program RP, which is illustrated by the reference numeral ( 8 ) in  FIG. 7 . The procedure indicated by the reference numeral ( 8 ) in  FIG. 7  is similar to that indicated by the reference numeral ( 11 ) in  FIG. 6 . The response to the write data transfer indicates, to the external tool  5 , that the download-data size of the new application program AP 2  has been entirely stored in the RAM  15  so that the application program rewrite operations have been completed 
     Thirdly,  FIG. 8  illustrates a sequence chart representing communication procedures between the C type external tool  5   c  and the ECU  1 . 
     In  FIG. 8 , the communication procedures up to the reference numeral ( 4 ) are substantially identical those up to the reference numeral ( 4 ) in  FIG. 6  except for the procedure indicated by the reference numeral ( 3 ). In  FIG. 8 , as the procedure indicated by the reference numeral ( 3 ), the C type external tool  5   c  sends, to the ECU  1 , the data download request. The data download request includes the download-data size information indicating the total volume of the new application program AP 2 , that is, the total volume obtained by subtracting the volume of the ROM identification code CD from the total volume of the file F 2 . The descriptions of the communication procedures up to the reference numeral ( 4 ) are therefore omitted. 
     When receiving the response to the data download request, the C type external tool  5   c  sends (downloads), to the ECU  1 , the data stored in the file F 2  as the write target data, which is illustrated by the reference numeral ( 5 ) in  FIG. 8 . In more detail, the C type external tool  5   c  downloads, to the ECU  1 , the ROM identification code CD located at the top of the whole data stored in the file F 2  as the rewrite target data, which is illustrated by the reference numeral ( 5 ) in  FIG. 8 . 
     In the boot loader BL, the affirmative determination is executed by the CPU  11  in step S 170 . Next, the transfer determining program is executed by the CPU  11  in step S 180  illustrated in  FIG. 5 . 
     In the transfer determining program, because the ROM identification code CD has been sent from the C type external tool  5   c , the affirmative determination is executed by the CPU  11  in step S 210 . Next, because no rewrite control program RP is downloaded from the C type external tool  5   c  and the rewrite control program download flag DF has not been set in the register by the CPU  11 , the negative determination is executed by the CPU  11  in step S 260 . 
     As a result, the rewrite control program RP previously stored in the flash ROM  13  at the first address area is copied into the RAM  15  from the leading address thereof. Thereafter, the execution location of the CPU  11  jumps to the leading address of the rewrite control program RP in the RAM  15  by the CPU  11  in step S 270 . 
     After the procedure in step S 270 , a unit quantity of the new application program AP 2  is sequentially sent from the C type external tool  5   c  to the ECU  1 , which is illustrated by the reference numeral ( 6 ) in  FIG. 8 . As a result, in the microcomputer  7 , the rewrite control program RP copied to be stored in the RAM  15  from the flash ROM  13  is executed by the CPU  11 . The execution of the rewrite control program RP by the CPU  11  allows the earlier application program AP 1  stored in the flash ROM  13  to be sequentially updated into the new application program AP 2  sent from the C type external tool  5   c  every sent unit of the program AP 2 , which is illustrated by the reference numeral  150  in  FIG. 8 . 
     After all units of the new application program AP 2  has been stored in the storage area of the flash ROM  13 , the response to the write data transfer is sent from the CPU  1  to the C type external tool  5   c  based on the rewrite control program RP, which is illustrated by the reference numeral ( 7 ) in  FIG. 8 . The procedure indicated by the reference numeral ( 7 ) in  FIG. 8  is similar to that indicated by the reference numeral ( 11 ) in  FIG. 6 . The response to the write data transfer indicates, to the external tool  5 , that the download-data size of the new application program AP 2  has been entirely stored in the RAM  15  so that the application program rewrite operations have been completed 
     As described above, in the program rewriting system RS according to the first embodiment, any type of the external tool  5  is configured to send, to the ECU  1 , the ROM identification code CD before sending the new application program AP 2 ; this ROM identification code CD indicates that the following data from the identification code CD is the new application program AP 2 . 
     In the boot loader BL installed in the microcomputer  7  used for the ECU  1 , when the rewrite control program RP is sent from the external tool  5 , it is determined whether the ROM identification code CD has been sent therefrom in step S 210 . It is determined that the write target data sequentially sent from the external tool  5  is a part of (a module of) the rewrite control program RP until it is determined that the ROM identification code CD has been sent therefrom in step S 210 . As a result, the write target data is stored in the RAM  15  as part of the rewrite control program RP in step S 230 . When it is determined that the ROM identification code CD has been sent from the external tool  5  in step S 210 , it is determined that the write target data sent from the external tool  5  after the determination in step S 210  is part of the new application program AP 2 . Thereafter, it is determined whether the rewrite control program download flag DF has been set in the register of the CPU  11  in order to judge whether the rewrite control program RP sent from the external tool  5  has been stored in the RAM  15 . 
     In step S 260 , when it is determined that the rewrite control program RP sent from the external tool  5  has been stored in the RAM  15 , the execution location of the CPU  11  jumps to the leading address of the rewrite control program RP in the RAM  15  so that the rewrite control program RP runs in step S 270 . In contrast, when it is determined that the rewrite control program RP has not been stored in the RAM  15  in step S 260 , the rewrite control program RP is copied to the RAM  15  from the first address area of the flash ROM  13  in step S 280 . Thereafter, the execution location of the CPU  11  jumps to the leading address of the rewrite control program RP in the RAM  15 , allowing the rewrite control program RP to run in step S 270   
     When the external tool  5  is the A type external tool  5   a  or the B type external tool  5   b , the rewrite control program RP sent from the A type external tool  5   a  or the B type external tool  5   b  is stored in the RAM  15 . Thereafter, when the ROM identification code CD is sent from the A type external tool  5   a  or the B type external tool  5   b , it is determined that the rewrite control program RP sent from the A type external tool  5   a  or the B type external tool  5   b  has been stored in the RAM  15  in step S 260 . This allows the rewrite control program RP to run. Execution of the rewrite control program RP permits the earlier application program AP 1  stored in the flash ROM  13  to be updated to the new application program AP 2  therein; this new application program AP 2  is sent from the external tool  5   a  or  5   b  after the rewrite control program execution. 
     In contrast, when the external tool  5  is the C type external tool  5   c , because no rewrite control program is sent therefrom, when the ROM identification code CD is sent from the C type external tool  5   c , it is determined that the rewrite control program RP has not been stored in the RAM  15  in step S 260 . This allows the rewrite control program RP previously stored in the first address area of the flash ROM  13  is copied to the RAM  15  so that the rewrite control program RP is run on the RAM  15 . Thereafter, execution of the rewrite control program RP permits the earlier application program AP 1  stored in the flash ROM  13  to be updated to the new application program AP 2  therein; this new application program AP 2  is sent from the external tool  5   c  after the rewrite control program execution. 
     As set forth above, even if using the external tool  5  with any one of the A to C types, the boot loader BL according to the first embodiment allows the earlier application program AP 1  to be accurately updated (rewritten) into the new application program AP 2 . 
     In addition, the boot loader BL according to the first embodiment eliminates the need to develop dedicated boot loaders for the types A to C of external tools, making it possible to reduce the number of processes for developing the ECU  1  and/or to simplify management of the ECU  1  and the boot loader  7 . 
     Note that, in step S 280  of  FIG. 5 , the execution location of the CPU  11  can jump to the leading address of the rewrite control program RP stored in the flash ROM  13  without copying it to the RAM  15 . In this modification, because the first address area in which the rewrite control program RP has been stored is different from the second address area in which the earlier application program AP 1  is stored in the flash ROM  13 , it is possible to run the rewrite control program RP on the flash ROM  13 . 
     Second Embodiment 
     A program rewriting system according to a second embodiment of the present invention will be described hereinafter. Note that elements of the program rewriting system RS 1  according to the second embodiment, which are substantially identical to those of the program rewriting system RS according to the first embodiment, are represented by the same reference characters as in  FIGS. 1 to 3 . Descriptions of the elements of the program rewriting system according to the second embodiment are therefore omitted or simplified. 
     As compared with the program rewriting system RS according to the first embodiment, the program rewriting system according to the second embodiment is operative to send a destination address, which for example corresponds to destination information, before sending the write target data, such as the rewrite control program RS or the new application program AP 2 . 
     Specifically, the destination address preferably indicates a leading address of a predetermined memory space at which the write target data to be sent should be stored in the target memory, such as the flash ROM  13  or the RAM  15 . Any type of the external tool  5  is configured to send, to the ECU  1 , the data download request including the destination address of the write target data and the download-data size information. 
     In addition, in the program rewriting system RS 1  according to the second embodiment, no ROM identification codes have been stored in the files F 1 , F 2 , and F 3  stored in at least one of the A type external tool  5   a , B type external tool  5   b , and the C type external tool  5   c.    
     Next, the specific instructions (procedures) of the boot loader BL according to the second embodiment will be described hereinafter with reference to  FIGS. 9 and 10 . Similar to the first embodiment, the boot loader BL according to the second embodiment is able to meet any types of external tools  5   a  to  5   c , and to realize the rewrite operations in cases of using any one of the external tools  5   a  to  5   c.    
       FIG. 9  schematically illustrates the overall procedure of the boot loader BL. 
     Like the first embodiment, the CPU  11  of the microcomputer  7  of the ECU  1  shifts its execution location from the earlier application program AP 1  to the boot loader BL when the write request is sent from the external tool  5  to the ECU  1 . When the CPU  11  launches the procedures of the boot loader BL, in step S 310 , the CPU  11  receives a unit quantity of the data sent from the external tool  5  and received by the communication driver  9 . Next, in step S 320 , the CPU  11  analyzes the received unit quantity of data to determine whether the received unit quantity of data is the write request based on the analyzed result. 
     If it is determined that the write request has been received (the determination in step S 320  is YES), the CPU  11  goes to step S 330  to execute an operation required to send, to the external tool  5 , a response to the write request, and thereafter returning to the step S 310 . The response to the write request is a response message indicating, to the external tool  5 , that the write request is normally received. 
     If it is not determined in step S 320  (the determination in step S 320  is NO), the CPU  11  goes to step S 340 . In step S 340 , the CPU  11  analyzes the received unit quantity of data in step S 310  to determine whether the received unit quantity of data is the data download request based on the analyzed result. 
     The data download request is a command to indicate, to the ECU  1 , that the external tool  5  starts to send the write target data; this write target data specifically is the rewrite control program RP or the new application program AP 2 . The data download request includes the download-data size information indicating the total volume of the write target data, and the destination address thereof. 
     If it is determined that the data download request has been received (the determination in step S 340  is YES), the CPU  11  goes to step S 350  to store the download-data size information in the RAM  15 . Subsequently, in the next step S 360 , the CPU  11  executes a transfer determining program, such as a subroutine, according to the second embodiment, which is illustrated in  FIG. 10  hereinafter, and thereafter returns to step S 310 . The transfer determining program is part of the boot loader BL. 
     When launching the transfer determining program, the CPU  11  determines whether the destination address included in the data download request is an address in the flash ROM  13  in step S 410  of  FIG. 10 . 
     When it is determined that the destination address included in the data download request is not an address in the flash ROM  13  (the determination in step S 410  is NO), the CPU  11  goes to step S 420 . In step S 420 , the CPU  11  sets the rewrite control program download flag DF in its register; this flag DF indicates that the download of the rewrite control program RP has been completed. Next, the CPU  11  executes an operation required to send, to the external tool  5 , a response to the data download request, and thereafter exits from the transfer determining program, returning to step S 110 . The response to the data download request is a response message indicating, to the external tool  5 , that the data download request is normally received. In this case, after sending the response to the data download request, the rewrite control program RP is sent from the external tool  5  to the ECU  1 , and the rewrite control program RP is stored in the RAM  15  by the operation of the CPU  11  in step S 380  described hereinafter. 
     When it is determined that the destination address included in the data download request is an address in the flash ROM  13  (the determination in step S 410  is YES), the CPU  11  goes to step S 440 . In step S 440 , the CPU  11  determines whether the rewrite control program download flag DF is set to its register. 
     When it is determined that the rewrite control program download flag DF is set to its register (the determination in step S 440  is YES), the CPU  11  determines that the rewrite control program RP sent from the external tool  5  has been stored in the RAM  15 , going to step S 450 . In step S 450 , the CPU  11  causes its execution location to jump to the leading address of the rewrite control program RP in the RAM  15 . 
     Thereafter, the CPU  11  runs the rewrite control program RP from the leading address thereof. Specifically, the CPU  11  executes the rewrite operations to update the new application program AP 2  sent from the external tool  5  in the flash ROM  13 . After completion of the new application program update, the CPU  11  sends, to the external tool  5 , the response to the write data transfer. 
     In step S 440 , when it is not determined (the determination in step S 440  is NO), the CPU  11  determines that the rewrite control program RP sent from the external tool  5  has not been stored in the RAM  15 , going to step S 460 . In step S 460 , the CPU  11  copies the rewrite control program RP previously stored in the flash ROM  13  at the first address area into the RAM  15  from the leading address thereof. After completion of the rewrite control program copy, the CPU  11  shifts to the step S 450 . 
     In this case, the CPU  11  executes the rewrite control program RP copied from the flash ROM  13  into the RAM  15  in step S 460 . 
     Thereafter, the CPU  11  runs the rewrite control program RP from the leading address thereof. Specifically, the CPU  11  executes the rewrite operations to update the new application program AP 2  sent from the external tool  5  in the flash ROM  13 . After completion of the new application program update, the CPU  11  sends, to the external tool  5 , the response to the write data transfer. 
     On the other hand, in step S 340  in  FIG. 9 , when it is determined that the data download request has not been received (the determination in step S 340  is NO), the CPU  11  goes to step  370 . In step S 370 , the CPU  11  analyzes the received unit quantity of data in step S 110  to determine whether the received unit quantity of data is part of the write target data based on the analyzed result. 
     When it is determined that the write target data has not been received (the determination in step S 370  is NO), the CPU  11  returns to step S 110 . When it is determined that the write target data has been received (the determination in step S 370  is YES), the CPU  11  goes to step S 380 . In step S 380 , the CPU  11  writes the current received data, that is, the write target data, into the RAM  15  at a predetermined address thereof as data constituting the rewrite control program RP. 
     In the next step S 390 , the CPU  11  determines whether received data volume is equivalent to the total volume indicated by the download-data size information stored in the RAM  15  in step S 350 . In other words, the CPU  11  determines whether the download-data size of the write target data has been entirely written in the RAM  15  in step S 390 . 
     When it is determined that the download-data size of the write target data has not been entirely written in the RAM  15  (the determination in step S 390  is NO), the CPU  11  returns to step S 310 . 
     In step S 390 , when it is determined that the download-data size of the write target data has been entirely written in the RAM  15  (the determination in step S 390  is YES), the CPU  11  goes to step S 395 . In step S 395 , the CPU  11  executes an operation required to send, to the external tool  5 , a response to the write data transfer, and thereafter exits from the transfer determining program, returning to step S 310 . The response to the write data transfer is a response message indicating, to the external tool  5 , that the download-data size of the write target data has been entirely written in the RAM  15 . 
     Next, individual operations of each of the external tools  5   a  to  5   c  and of the ECU  1  will be described hereinafter with reference to  FIGS. 1 and 12 . 
     First,  FIG. 11  illustrates a sequence chart representing communication procedures between the external tool  5   a  or and the ECU As illustrated by the reference numeral ( 1 ) in  FIG. 11 , when the predetermined operation is executed in the external tool  5   a  or  5   b  so that the write request RS is sent therefrom to the ECU  1 , the execution location of the CPU  11  is shifted from the earlier application program AP 1  to the boot loader BL. Thereafter, the boot loader BL is executed by the CPU  11 . These operations of the CPU  11  are represented by the reference character  100  in  FIG. 11 . 
     In the boot loader BL, the affirmative determination is executed by the CPU  11  in step S 320 , and the response to the write request is sent therefrom to the A type external tool  5   a  or B type external tool  5   b  in step S 330 , which is illustrated by the reference numeral ( 2 ) in  FIG. 11 . 
     When receiving the response to the write request, the A type external tool  5   a  or B type external tool  5   b  sends, to the ECU  1 , the data download request, as a preparation for sending the rewrite control program RP, which is illustrated by the reference numeral ( 3 ) in  FIG. 11 . The data download request includes the destination address of the rewrite control program RP in the RAM  15  and the download-data size information indicating the total volume of the rewrite control program RP. 
     In the boot loader BL, the affirmative determination is executed by the CPU  11  in step S 340 . Next, the download-data size information is stored in the RAM  15  by the CPU  11  in step S 350 , and the transfer determining program is executed by the CPU  11  in step S 360  illustrated in  FIG. 10 . 
     In the transfer determining program, because the destination address included in the data down load request sent from the external tool  5   a  or  5   b , which is illustrated by the reference numeral ( 3 ) in  FIG. 11 , is not an address of the flash ROM  13  but the RAM  15 , the negative determination is executed by the CPU  11  in step S 410 . In this case, the rewrite control program download flag DF is set in the register by the CPU  11  in step S 420 , and the response to the data download request is sent therefrom to the A type external tool  5   a  or B type external tool  5   b  in step S 430 , which is illustrated by the reference numeral ( 4 ) in  FIG. 11 . 
     When receiving the response to the data download request, the A type external tool  5   a  or the B type external tool  5   b  sends (downloads), to the ECU  1 , the rewrite control program RP as the write target data, which is illustrated by the reference numeral (S) in  FIG. 11 . In more detail, the A type external tool  5   a  or the B type external tool  5   b  downloads, to the ECU  1 , data constituting the rewrite control program RP as the write target data. In  FIGS. 11 and 12 , the write target data is described as “write data”. 
     In the boot loader BL, every time a unit quantity of the rewrite control program RP is sent from the A type control tool  5   a , the affirmative determination is executed by the CPU  11  in S 370  and the operations in step S 380  are executed thereby. As a result, when the total volume of the rewrite control program RP has been entirely written in the RAM  15 , the affirmative determination is executed by the CPU  11  in step S 390 . Next, the response to the write data transfer is sent from the CPU  11  to the A type external tool  5   a  or the B type external tool  5   b  in step S 395 , which is illustrated by the reference numeral ( 6 ) in  FIG. 11 . 
     When receiving the response to the response to the rewrite data, the A type external tool  5   a  or the B type external tool  5   b  sends, to the ECU  1 , the data download request as a preparation for sending the rewrite control program RP, which is illustrated by the reference numeral ( 7 ) in  FIG. 11 . The data download request includes the download-data size information indicating the total volume of the new application program Ap 2  and the destination address thereof in the flash ROM  13 . 
     In the boot loader BL, the affirmative determination is executed by the CPU  11  in step S 340 . Next, the download-data size information is stored in the RAM  15  by the CPU  11  in step S 350 , and the transfer determining program is executed by the CPU  11  in step S 360  illustrated in  FIG. 10 . 
     In the transfer determining program, because the destination address included in the data down load request sent from the external tool  5   a  or  5   b , which is illustrated by the reference numeral ( 7 ) in  FIG. 11 , is an address of the flash ROM  13 , the affirmative determination is executed by the CPU  11  in step S 410 . In this case, because the rewrite control program download flag DF has been set in the register of the CPU  11  so that the rewrite control program RP sent from the external tool  5   a  or  5   b  before the new application program AP 2  being sent therefrom, the affirmative determination is executed by the CPU  11  in step S 440 . As a result, the execution location of the CPU  11  jumps to the leading address of the rewrite control program RP in the RAM  15  by the CPU  11  in step S 450 . 
     Thereafter, the rewrite control program RP sent form the external tool  5   a  or  5   b  and stored in the RAM  15  is executed by the CPU  11 . 
     Specifically, the response to the data download request is sent therefrom to the A type external tool  5   a  or B type external tool  5   b , which is illustrated by the reference numeral ( 8 ) in  FIG. 11 . When receiving the response to the data download request, the external tool  5   a  or  5   b  sequentially sends (downloads), to the ECU  1 , a unit quantity of the new application program AP 2 , which is illustrated by the reference numeral ( 9 ) in  FIG. 11 . 
     As a result, in the microcomputer  7 , the execution of the rewrite control program RP by the CPU  11  allows the earlier application program AP 1  stored in the flash ROM  13  to be sequentially updated into the new application program AP 2  sent from the external tool  5   a  or  5   b  every sent unit of the program AP 2 , which is illustrated by the reference numeral  150  in  FIG. 11 . 
     After all units of the new application program AP 2  has been stored in the storage area of the flash ROM  13 , the response to the write data transfer is sent from the CPU  1  to the external tool  5   a  or  5   b  based on the rewrite control program RP, which is illustrated by the reference numeral ( 10 ) in  FIG. 11 . Specifically, the procedure indicated by the reference numeral ( 8 ) in  FIG. 11  and that indicated by the reference numeral ( 10 ) in  FIG. 11  are executed by the CPU  11  based on the rewrite control program RP. 
     Secondary,  FIG. 12  illustrates a sequence chart representing communication procedures between the C type external tool  5   c  and the ECU  1 . 
     In  FIG. 12 , the communication procedures up to the reference numeral ( 2 ) are substantially identical those up to the reference numeral ( 2 ) in  FIG. 11 , and therefore the descriptions of the communication procedures up to the reference numeral ( 2 ) are therefore omitted. 
     When receiving the response to the write request, the C type external tool  5   c  sends, to the ECU  1 , the data download request, as a preparation for sending the rewrite control program RP, which is illustrated by the reference numeral ( 3 ) in  FIG. 12 . The data download request includes the download-data size information indicating the total volume of the new application program Ap 2  and the destination address thereof in the flash ROM  13 . 
     In the boot loader BL, the affirmative determination is executed by the CPU  11  in step S 340 . Next, the download-data size information is stored in the RAM  15  by the CPU  11  in step S 350 , and the transfer determining program is executed by the CPU  11  in step S 360  illustrated in  FIG. 10 . 
     In the transfer determining program, because the destination address included in the data down load request sent from the external tool  5   c , which is illustrated by the reference numeral ( 3 ) in  FIG. 12 , is an address of the flash ROM  13 , the affirmative determination is executed by the CPU  11  in step S 410 . In this case, because the rewrite control program download flag DF has not been set in the register of the CPU  11  so that the rewrite control program RP has not been sent from the external tool  5   c , the negative determination is executed by the CPU  11  in step S 440 . As a result, the rewrite control program RP, which has been stored in the first address area of the flash ROM  13 , is copied to the RAM  15  from the first address area of the flash ROM  13 . 
     Thereafter, the execution location of the CPU  11  jumps to the leading address of the rewrite control program RP in the RAM  15  by the CPU  11  in step S 450 . 
     As a result, in the microcomputer  7 , the rewrite control program RP, which is copied from the flash ROM  13  to the RAM  15 , is launched by the CPU  11 . This allows the response to the data download request to be sent to the C type external tool  5   c , which is illustrated by the reference numeral ( 8 ) in  FIG. 11 . 
     When receiving the response to the data download request, the external tool  5   c  sequentially sends (downloads), to the ECU  1 , a unit quantity of the new application program AP 2 , which is illustrated by the reference numeral ( 5 ) in  FIG. 12 . 
     As a result, in the microcomputer  7 , the execution of the rewrite control program RP by the CPU  11  allows the earlier application program AP 1  stored in the flash ROM  13  to be sequentially updated into the new application program AP 2  sent from the C type external tool  5   c  every sent unit of the program AP 2 . This operation is illustrated by the reference numeral  150  in  FIG. 6 . 
     After all units of the new application program AP 2  has been stored in the storage area of the flash ROM  13 , the response to the write data transfer is sent from the CPU  1  to the external tool  5   c  based on the rewrite control program RP, which is illustrated by the reference numeral ( 6 ) in  FIG. 12 . Specifically, the procedure indicated by the reference numeral ( 4 ) in  FIG. 12  and that indicated by the reference numeral ( 6 ) in  FIG. 12  are executed by the CPU  11  based on the rewrite control program RP. 
     As described above, in the program rewriting system according to the second embodiment, any type of the external tool  5  is configured to send, to the ECU  1 , the destination address of the write target data before sending the new application program AP 2 . 
     In the boot loader BL installed in the microcomputer  7  used for the ECU  1 , after the write request is sent from the external tool  5 , when the data download request including the destination address is sent from the external tool  5 , it is determined whether the destination address is an address in the flash ROM  13 . It is determined that the write target data sequentially sent from the external tool  5  is part of the rewrite control program RP until it is determined that the destination address is an address in the flash ROM  13  in step S 410 . As a result, the write target data is stored in the RAM  15  as part of the rewrite control program RP in step S 380 . When it is determined that the destination address is an address in the flash ROM  13  in step S 410 , it is determined that the write target data sent from the external tool  5  after the determination in step S 410  is part of the new application program AP 2 . Thereafter, it is determined whether the rewrite control program download flag DF has been set in the register of the CPU  11  in order to judge whether the rewrite control program RP sent from the external tool  5  has been stored in the RAM  15 . 
     In step S 440 , when it is determined that the rewrite control program RP sent from the external tool  5  has been stored in the RAM  15 , the execution location of the CPU  11  jumps to the leading address of the rewrite control program RP in the RAM  15  so that the rewrite control program RP runs in step S 450 . In contrast, when it is determined that the rewrite control program PP has not been stored in the RAM  15  in step S 440 , the rewrite control program RP is copied to the RAM  15  from the first address area of the flash ROM  13  in step S 460 . Thereafter, the execution location of the CPU  11  jumps to the leading address of the rewrite control program RP in the RAM  15 , allowing the rewrite control program RP to run in step S 450 . 
     When the external tool  5  is the A type external tool  5   a  or the B type external tool  5   b , the rewrite control program RP sent from the A type external tool  5   a  or the B type external tool  5   b  is stored in the RAM  15 . Thereafter, when the data download request including an address in the flash ROM  13  as the destination address is sent from the A type external tool  5   a  or the B type external tool  5   b , it is determined that the rewrite control program RP sent from the A type external tool  5   a  or the B type external tool  5   b  has been stored in the RAM  15  in step S 440 . This allows the rewrite control program RP to run. Execution of the rewrite control program RP permits the earlier application program AP 1  stored in the flash ROM  13  to be updated to the new application program AP 2  therein; this new application program AP 2  is sent from the external tool  5   a  or  5   b  after the rewrite control program execution. 
     In contrast, when the external tool  5  is the C type external tool  5   c , because no rewrite control program is sent therefrom, when the ROM identification code CD is sent from the C type external tool  5   c , it is determined that the rewrite control program RP has not been stored in the RAM  15  in step S 260  This allows the rewrite control program RP previously stored in the first address area of the flash ROM  13  is copied to the RAM  15  so that the rewrite control program RP is run on the RAM  15 . Thereafter, execution of the rewrite control program RP permits the earlier application program AP 1  stored in the flash ROM  13  to be updated to the new application program AP 2  therein; this new application program AP 2  is sent from the external tool  5   c  after the rewrite control program execution. 
     As described above, in the second embodiment, even if using the external tool  5  with any one of the A to C types, the boot loader BL according to the second embodiment allows the earlier application program AP 1  to be accurately updated (rewritten) into the new application program AP 2 . 
     In addition, the boot loader BL according to the second embodiment eliminates the need to develop dedicated boot loaders for the types A to C of external tools, making it possible to reduce the number of processes for developing the ECU  1  and/or to simplify management of the ECU  1  and the boot loader  7 . 
     Note that, in step S 440  of  FIG. 10 , the execution location of the CPU  11  can jump to the leading address of the rewrite control program RP stored in the flash ROM  13  without copying it to the RAM  15 . In this modification, because the first address area in which the rewrite control program RP has been stored is different from the second address area in which the earlier application program AP 1  is stored in the flash ROM  13 , it is possible to run the rewrite control program RP on the flash ROM  13 . 
     In each of the first and second embodiments, the ECU  1  includes the flash ROM  13  as one example of rewritable nonvolatile memories, but can include at least one of other types rewritable nonvolatile memories, such as an EEPROM. 
     In each of the first and second embodiments, note that the write request can be transmitted from the external tool  5  to the ECU  1  through a signal line different from the communication line  3 . 
     In each of the first and second embodiments, the program rewriting system includes an ECU installed in an automobile, but the program rewriting system can include a control unit installed in other types of machines. Moreover, in each of the first and second embodiments, the ECU  1  is operative to control an engine installed in an automobile as a target, but the ECU  1  can control, as a target, other components installed in an automobile, such as a brake, a transmission, and suspensions. 
     In each of the first and second embodiments, it is important to note that, while the present invention has been and will continue to be described in the context of each of fully functional computers, such as the ECU  1  and the external tool  5 . In the present invention, however, the elements provided in each of the ECU  1  and the external tool  5  can be implemented as dedicated hardware devices, such as custom LSI (Large-Scale Integration) circuits. 
     Moreover, those skilled in the art will appreciate that the present invention is capable of being distributed as program products, for example, the boot loader BL in a variety of forms. It is also important to note that the present invention applies equally regardless of the particular type of signal bearing media used to actually carry out the distribution. Examples of suitable signal bearing media include recordable type media such as CD-ROMs and DVD-ROMs, and transmission type media such as digital and analog communications links. 
     While there has been described what is at present considered to be the embodiments and modifications of the present invention, it will be understood that various modifications which are not described yet may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.