Multi-Processing System for Controlling Microcomputers and Memories

This invention relates to a microcomputer system and more particularly to a microcomputer system wherein the transfer of data between a plurality of microcomputer systems is effected through a common memory.

The microcomputer which performs an arithmetic operation at a far lower speed than the conventional mini-computer has presented difficulties in providing a numerical control system demanded to carry out a quick arithmetic operation. It has therefore been considered necessary to use a plurality of central processing units (hereinafter referred to as "CPU") and let the "CPU" share in arithmetic operation in order to provide a numerical control system capable of a high speed arithmetic operation. In this case, however, control between the respective CPU's and processing of data will unavoidably become complicated with respect to hard ware.

It is accordingly the object of this invention to provide such type of microcomputer system as makes it easy to provide a system demanded to carry out a quick arithmetic operation.

Accordingly to an aspect of the invention, there is provided a microcomputer system which comprises a plurality of microcomputers, a program area memory and data area memory both exclusively used with the respective microcomputers and a common data memory through which the plural microcomputers are connected, wherein, when the transfer of data is not carried out between the selected ones of said plural microcomputers, data is transferred between the microcomputers and the corresponding exclusive memories; each of the microcomputers performs an arithmetic operation independently of the arithmetic operation cycle of any other microcomputer, thus enabling an arithmetic operation, however complicated, to be effected as quickly as when a single microcomputer is used; a given one of the microcomputers which tries to have an access to the common data memory is placed on a waiting position only until any other microcomputer finishes an acess to said common data memory; and when said common data memory remains disengaged, any of the microcomputers can have an acess to said memory without waiting, thereby attaining smooth transfer of data between the respective microcomputers.

A microcomputer system embodying this invention comprises a plurality of microcomputers; memories exclusively used with the respective microcomputers; a data memory used in common to the microcomputers; and means for controlling the access of any of the microcomputers to the common data memory. This access-controlling means includes a flip-flop circuit which is set when the memory access cycle of one of the microcomputers is brought to an end, and is reset when the memory access cycle of the other microcomputer is finished; an address decoder for detecting the access of any of the microcomputers to the common data memory; and a logic circuit coupled to all the above-mentioned elements, thereby determining by simple arrangement the access priority of the respective microcomputers for control of their access to the common data memory.

A microcomputer system according to another embodiment of this invention comprises three or more microcomputers. In such case, a proper combination of these microcomputers can eliminate the necessity of separately providing exclusive memories, common data memories and logic circuits, thus simplifying the arrangement of the entire microcomputer system.

Other advantages and features of this invention will be understood from the description which follows.

FIG. 1 is a block circuit diagram of a microcomputer system according to an embodiment of this invention which comprises two microcomputers;

FIG. 2 is a timing chart illustrating the operation of the circuit of the microcomputer system of FIG. 1; and

FIG. 3 is a block circuit diagram of a microcomputer system according to another embodiment of the invention which comprises three microcomputers.

A microcomputer system of FIG. 1 comprises two central processing units CPUa 11 and CPUb 12. The CPUa 11 can have an access to its exclusive read only memory ROMa 14 and randam access memory RAMa 15 through eight bidirectional data bus lines A13. The ROMa 14 is stored with an instruction program for deciding the processing sequence of the CPUa 11. Data required for the arithmetic operation of the CPUa 11 is written in or read out from RAMa 15. Similarly, the CPUb 12 can have an access to its exclusive ROMb 17 and RAMb 18 through eight bidirectional data bus lines B16. The ROMb 17 is stored with a program for the operation of the CPUb 12, and the RAMb 18 is stored with data required for the arithmetic operation of the CPUb 12.

RAMc 19 is a common RAM accessible from both CPUa 11 and CPUb 12. Data required for the arithmetic operation of the CPUa 11 and CPUb 12 is written in or read out from the RAMc 19.

There will now be outlined an operation carried out across the respective CPU's and the various memories. Now let it be assumed that the common memory RAMc 19 has a capacity of storing an N number of words. For the access of the CPUa 11 to the common memory RAMc 19, a read-write instruction is sent forth to any desired address included in the address A to A+N-1 of the RAMc 19 according to a program prepared for the CPUa 11. Data is written in or read out from the selected address upon receipt of the read-write instruction. Where the CPUa 11 issues a read-write instruction to any other address than those of the RAMc 19 which are designated as A to A+N-1, then the exclusive memory ROMa 14 or RAMa 15 of the CPUa 11 is set for access. Similarly, for the access of the CPUb 12 to the RAMc 19, it is advised to issue a read-write instruction from the CPUb 12 to any desired address included in the address A to A+N-1 of the RAMc 19.

A program instruction to write data in or read out data from any other address than those designated as A to A+N-1 sets the exclusive ROMb 17 or RAMb 18 of the CPUb 12 for access.

Where the CPUa 11 issues a read-write instruction to any address of the exclusive ROMa 14, RAMa 15 other than the A to A+N-1 address of the common RAMc 19, then address information is set forth from the CPUa 11 to the bus line A13. Since, at this time, an address signal line ADa 20 is supplied with an address-designating signal having a logic level of "1", the address information which has been carried through the bus line A13 is stored in an address register A21. As mentioned above, the CPUa 11 is going to have an access to an address other than those designed as A to A+N-1. Therefore, the address register A21 is stored with information denoting any specified address of the exclusive ROMa 14 or RAMa 15. The address information stored in the address register A21 is transferred through an address bus line A22 to indicate the specified address of the exclusive ROMa 14 or RAMa 15. If an instruction produced is of the "readout" type, then data corresponding to the designated address is read out to the CPUa 11. An address decoder A23 is originally designed to produce a signal having a logic level of "1" at the output terminal only when information stored in the address register A21 represents any of the addresses A to A+N-1 of the RAMc 19. Therefore, in the above-mentioned case where the CPUa 11 is going to have an access to any address other than those of the RAMc 19 designated as A to A+N-1, the address decoder A23 generates an output having a logic level of "0". Accordingly, an output signal WAITa 25 from an AND circuit 24 which can not produce an AND output under the above-mentioned condition has a logic level of "0". At this time, the CPUa 11 issues a read-write signal R/Wa 26 having a logic level of "1" immediately after sending forth a "0" signal to the address signal line 20. Since, as mentioned-above, the address decoder A23 issues a "0" output, an AND circuit 27 can not generate an AND output, but sends forth a "0" output. As the result, a bidirectional bus driver DRVa 28 is turned off causing the CPUa 11 and RAMc 19 to be disconnected from each other. Further at this time, the address bus line A22 does not make an access to the RAMc 19, preventing any data from being written in or read out from said RAMc 19. Where "1" read-write signal R/Wa 26 given forth by the CPUa 11 is of the "read" type, then data of any address of the ROMa 14 or RAMa 15 specified by the address register A21 through the address bus line A22 is load in the CPUa 11 through the bus line A13. Where the above-mentioned read-write signal R/Wa 26 is of the "write" type, then the CPUa 11 also sends forth a write control signal (not shown) together with said read-write signal R/Wa 26. As the result, "write" data is supplied to the RAMa 15 through the bus line A13. At this time, the "write" data delivered from the CPUa 11 is written in that of the address of the RAMa 15 which was previously specified by the address register A21 upon receipt of said write control signal.

For briefness, the operation of the "write" control signal is omitted from the following description.

With the microcomputer system of this invention arranged as described above, it will be understood that so long as the CPUa 11 has an access to any address other than the address A to A+N-1 of the RAMc 19, the CPUa 11 carries out an arithmetic operation independently of that of the CPUb 12 at a speed originally designed for the CPUa 11 itself. So long as the CPUb 12 has an access to any address other than the addresses A to A+N-1, an arithmetic operation is effected between the CPUb 12 and the exclusive ROMb 17 or RAMb 18. This arithmetic operation is performed quickly at such a speed as originally designed for the CPUb 12 itself.

FIG. 2 is a timing chart, based on which the microcomputer system of FIG. 1 carries out program instructions. A first program instruction A1 performed by the CPUa 11 is an order for an access to the common RAMc 19. A second program instruction A2 is also a command for an access to the common RAMc 19. A third program instruction A3 is given for an access to the ROMa 14 or RAMa 15 other than the common RAMc 19. A first one B1 of the program instructions pusued by the CPUb 12 which are processed at the same time as those of the CPUa 11 is an order for an access to the ROMb 17 or RAMb 18 other than the common RAMc 19. A second program instruction B2 carried out by the CPUb 12 is a command for an access to the common RAMc 19. A third program instruction B3 followed by the CPUb 12 is also given for an access to the common RAMc 19. FIG. 2 also sets forth the timing in which these program instructions B1 and B3 are carried out.

There will now be described by reference to FIG. 2 the timing in which the CPUa 11 and CPUb 12 of FIG. 1 has an access to the common RAMc 19. The flip-flop circuit 29 of FIG. 1 selectively controls the access of the CPUa 11 to the corresponding exclusive memory ROMa 14 or RAMa 15 and the access of the CPUb 12 to the corresponding exclusive memory ROMb 17 or RAMb 18. The flip-flop circuit 29 is reset when the CPUb 12 finishes each program instruction. While being reset, the flip-flop circuit 29 produces an output having a logic level of "0" at a Q terminal 30 and an output having a logic level of "1" at a Q terminal 31. When initially reset, the flip-flop circuit 29 presents a condition shown in FIG. 2(a). Where, under this condition, the CPUa 11 of the subject microcomputer system carries out the first program instruction A1, then the CPUa 11 sends forth, as shown in FIG. 2(b) a memory access cycle signal MACa 32 having a logic level of "1". Then the CPUa 11 produces, as shown in FIG. 2(c), an address signal ADa 20 having a logic level of "1". Upon receipt of the address signal ADa 20, address information obtained according to the first program instruction A1 carried out by the CPUa 11 passes through the bus line A13 to be stored in the address register A21. Since the first program instruction A1 orders the CPUa 11 to have an access to the common memory RAMc 19, the address decoder A23 produces, as previously mentioned, an output having a logic level of "1", as illustrated in FIG. 2(d).

At this time, an output from the Q terminal 30 of the flip-flop circuit 29 has a logic level of "0", causing the AND circuit 24 to generate an output WAITa 25 also having a logic level of "0". So long as the signal WAITa 25 continues to have a logic level of "0", the CPUa 11 can send forth the read-write signal R/Wa 26 after the address signal ADa 20 has its logic level changed from "1" to "0". Where the signal WAITa 25 has a logic level of "1", the CPUa 11 is prevented from producing a read-write signal R/Wa 26 having a logic level of "1", even after the address signal ADa 20 has its logic level changed from "1" to "0". The CPUa 11 can generate a read-write signal R/Wa 26 having a logic level of " 1", only after the signal WAITa 25 has its logic level changed from "1" to "0".

Since the signal WAITa 25 now has a logic level of "0", the CPUa 11 gives forth, as shown in FIG. 2(e), a read-write signal R/Wa 26 having a logic level of "1", after the address signal ADa 20 has its logic level changed from "1" to "0". When the read-write signal R/Wa 26 has a logic level of "1", the AND gate circuit 27 generates an AND output, because the address decoder A23 issues an output having a logic level of "1". As the result, said AND gate circuit 27 delivers an output of "1" to the bidirectional bus driver DRVa 28 for actuation. At this time, an output from the Q terminal of the flip-flop circuit 29 has a logic level of "1", causing a read-write signal R/Wa 26 having a logic level of "1" to be conducted through an AND gate circuit 33 and OR gate circuit 34 to the common memory RAMc 19. Since on terminal of an AND gate circuit 35 is supplied with a "1" output from the Q terminal of the flip-flop circuit 29, address information stored in the address register A21 is transmitted to the common memory RAMc 19 through the address bus line S22, AND gate circuit 35 and OR gate circuit 36 in turn. As the result, the CPUa 11 takes a read-write action with respect to that of the RAMc 19 addresses which was previously designated by the address register A21 according to a read-write signal R/Wa 26 given forth from the CPUa 11. Since, at this time, the address bus line S22 does not specify any address of the ROMa 14 and RAMa 15, data stored in those memories is not read out.

When execution of the first program instruction A1 is brought to an end, the CPUa 11 generates a memory access cycle signal MACa 32 having a logic level of "0", as shown in FIG. 2(b).

At this time, the first program instruction B1 for the CPUb 12 is not delivered to the common memory RAMc 19. As the result, the address decoder B37 belonging to the CPUb 12 produces an output of "0", as shown in FIG. 2(i). An AND gate circuit 38 is not opened, but supplies a "0" output to a bidirectional driver DRVa 39 to render it nonconducting, thereby disconnecting the CPUb 12 from the common memory RAMc 19. Accordingly, the CPUb 12 makes an access to the corresponding exclusive ROMb 17 or RAMb 18. Since, as mentioned above, the address decoder b37 issues a "0" output, an AND gate circuit 40 produces an output WAITb 41 having a logic level of "0". Consequently, as described with respect to the first program instruction A1 for the CPUa 11, the CPUb 12 specifies any address of the corresponding exclusive ROMb 17 or RAMb 18 through an address bus line B43 according to the prescribed address information stored in an address register B42. Thus, data is written in or read out from that of the addresses of the ROMb 17 or RAMb 18 which was previously designated, according to a read-write signal R/Wb 44 delivered from the CPUb 12. Namely, the CPUb 12 carries out an arithmetic operation according to access information supplied from the ROMb 17 or RAMb 18. This means that a memory access cycle signal MACb 45 (FIG. 2g) issued upon execution of the first program instruction B1 by the CPUb 12 is obtained at the same time as the memory access cycle signal MACa 32 produced upon execution of the first program instruction A1 by the CPUa 11, thereby enabling both CPUa 11 and CPUb 12 to carry out the respective allotted arithmetic operations at the same time.

Thereafter, the CPUa 11 and CPUb 12 get ready to execute the corresponding program instructions A2, B2. Since these second program instructions A2, B2 order both CPUa 11 and CPUb 12 to have an access to the common memory RAMc 19, it is necessary to keep either of said CPU elements in a waiting position. With the microcomputer system of this invention shown in FIG. 1, the CPUa 11 is given a priority under the above-mentioned condition. Therefore, the CPUa 11 first has an access to the common memory RAMc 19 and the CPUb 12 has an access thereto. When the CPUa 11 has carried out the second program instruction A2, the CPUa 11 sends forth an address signal Ada 20 having a logic level of "1" (FIG. 2(c). As the result, the address register A21 is stored with an address information allowing the CPUa 11 to have an access to the common memory RAMc 19. Accordingly, the address decoder A23 generates an output of "1" as shown in FIG. 2d. Since, at this time, an output from the Q terminal 30 of the flip-flop circuit 29 has a logic level of "0", the AND circuit 24 issues an output WAITa 25 also having a logic level of "0" as shown in FIG. 2(f). Thereafter, a read-write signal R/Wa 26 (FIG. 2(e)) is delivered from the CPUa 11. Where the second program instruction A2 given to the CPUa 11 is of the "read" type, then data stored in that of the addresses of the common memory RAMc 19 which was previously designated according to address information stored in the address register A21 is read out to the CPUa 11 through the bidirectional driver A28. Where said second program information A2 is of the "write" type, then data stored in the CPUa 11 is written through the bidirectional driver A28 in that of the addresses of the common memory RAMc 19 which was previously specified. Thus, transfer of data is effected between the CPUa 11 and common memory RAMc 19.

Upon receipt of the second program instruction B2, the CPUb 12 delivers an address signal ADb 46 to the address register B42, which in turn is stored with that address information supplied from the CPUb 12 through the bus line B16 which indicates that said CPUb 12 is going to have an access to the common memory RAMc 19. At this time, the address decoder B37 issues an output having a logic level of "1" (FIG. 2(i)). Since an output from the Q terminal 31 of the flip-flop circuit 29 has a logic level of "1", the AND gate circuit 40 which received a "1" output from the decoder B37 sends forth a "1" output WAITb 41 (FIG. 2(k)). So long as said signal WAITb 41 continues to have a logic level of "1", the CPUb 12 does not issue a read-write signal R/Wb 44, and consequently is prevented from executing the second program instruction B2. Namely, the CPUb 12 is kept in a waiting position, until the signal WAITb 41 has its logic level changed from "1" to "0".

When the CPUa 11 finishes the execution of the second program instruction A2 and in consequence the memory access cycle signal MACa 32 has a logic level of "0", then an inverter 47 issues an output having a logic level of "1". Since, at this time, the address decoder B37 generates an output of "1", an AND gate circuit 48 sends forth an output of "1". This "1" output from the AND gate circuit 48 sets the flip-flop circuit 29 which has been reset, as shown in FIG. 2a, up to this point. When the flip-flop circuit 29 is thus set, an output from the Q terminal 30 has a logic level of "1", and an output from the Q terminal 31 has a logic level of "0". When the signal WAITb 41 has a logic level of "0", the CPUb 12 issues a read-write signal R/Wb 44 having a logic level " 1". Since, at this time, one input terminal of an AND gate circuit 49 is supplied with a "1" output from the Q terminal 30 of the flip-flop circuit 29, address information stored in the address register B42 designates any of the addresses of the common memory RAMc 19 through the address bus line B43, AND gate circuit 49 and OR gate circuit 36 in turn. Upon receipt of a "1" read-write signal R/Wb 44 from the CPUb 12, the AND gate circuit 38 which is already supplied with a "1" output from the address decoder B37 sends forth a "1" output to a bidirectional driver DRVb 39 to render it conducting. Since, an AND gate circuit 50 is supplied with a "1" output from the Q terminal 30 of the flip-flop circuit 29, a read-write signal R/Wb 44 delivered from the CPUb 12 is conducted through the AND gate circuit 50 and OR gate circuit 34 to the common memory RAMc 19, thereby allowing the CPUb 12 to have an access to said common memory RAMc 19. As the result, transfer of data is carried out between the CPUb 12 and common memory RAMc 19. When the execution of the second program instruction B2 by the CPUb 12 is brought to an end, then the memory access cycle signal MACb 45 has a logic level of "0" as shown in FIG. 2b. This memory access cycle signal MACb 45 is inverted by an inverter 51 and carried to the reset input terminal of the flip-flop circuit 29. As the result, the flip-flop circuit 29 which has been set up to this point is reset, as shown in FIG. 2a, in the timing in which the memory access cycle signal MACb 45 has its logic level changed to "0".

When the CPUa 11 finishes the execution of the second program instruction A2 (at this time the memory access cycle signal MACa 32 has a logic level of "0") and the flip-flop circuit 29 has been changed from a reset state to a set state through the inverter 47 and AND gate 48, then the AND gate circuit 24 gives forth an output WAITa 25 having a logic level of "1" (FIG. 2(f)). This "1" signal WAITa 25 obstructs the issue of a read-write signal R/Wa 26 from the CPUa 11 but not inhibit the generation of the memory access cycle signal MACa 32 and address signal ADa 20. After execution of the second program instruction A2, therefore, the CPUa 11 is made ready to carry out the succeeding third program instruction A3. When an address signal ADa 20 associated with the third program instruction A3 has its logic level changed from "0" to "1", as shown in FIG. 2(c), by the CPUa 11, then the address decoder A23 produces a "0" output (because, as previous described, the third program instruction A3 does not order the CPUa 11 to have an access to the common memory RAMc 19). As the result, the AND gate circuit 24 generates an output WAITa 25 having a logic level of "0" (FIG. 2(f)). Thereafter, the CPUa 11 can issue a read-write signal R/Wa 26. When the CPUa 11 fully carries out the third program instruction A3, the memory access cycle signal MACa 32 has a logic level of "0".

When the execution of the second program instruction B2 by the CPUb 12 is brought to an end, the flip-flop circuit 29 which has been set up to this point is reset, causing the AND gate circuit 40 to issue an output WAITb 41 having a logic level of "1" (FIG. 2(k)). For the execution of the third program instruction by the CPUb 12, therefore, an address signal ADb 46 has its logic level changed to "1" to store address information in the address register B42. Since, however, an output from the address decoder B37 still has a logic level of "1" as shown in FIG. 2(i) (because the third program instruction B3 orders the CPUb 12 to have an access to the common memory RAMc 19) and an output WAITb 41 from the AND gate circuit 40 has a logic level of "1", the CPUb 12, cannot immediately give forth a read-write signal R/Wb 44 having a logic level of "1", but is kept in a waiting position. Where, under this condition, the CPUa 11 finishes the execution of the third program instruction A3, then the memory access cycle signal MACa 32 has a logic level of "0" (FIG. 2(b)). This "0" memory access cycle signal MACa 32 is conducted through the inverter 47 and AND gate circuit 48 to set, as shown in FIG. 2a, the flip-flop circuit 29 which has been reset up to this paint. When the flip-flop circuit 29 is set, an output WAITb 41 from the AND gate circuit 40 has a logic level of "0" (FIG. 2(k)). Therefore, the CPUb 12 which has been kept in a waiting position immediately issues a read-write signal R/Wb 44 having a logic level of "1" (FIG. 2(j)). As the result, the CPUb 12 causes, as previously described, data stored in that of the addresses of the common memory RAMc 19 which was previously designated through the address line B43 to be read out to the CPUb 12 through the bidirectonal bus driver DRVb 39 or to be written in that of the addresses of the common memory RAMc 19 which was previously specified, thereby carrying out an arithmetic operation based on the third program instruction B3. When the CPUb 12 fully executes the third program instruction B3, the memory access cycle signal MACb 45 has a logic level of "0" (FIG. 2(g)). Consequently, the flip-flop circuit 29 is brought back to the original reset condition (FIG. 2(a)).

With the microcomputer system of this invention shown in FIG. 1, the flip-flop circuit 29 is reset, each time the CPUb 12 completes a memory access cycle. Where, therefore, the CPUa 11 is going to have an access to the common memory RAMc 19, the CPUa 11 is given a priority to be saved from a waiting position, unless the CPUb 12 has already made an access to the common memory RAMc 19 ahead of the CPUa 11. Thus the CPUa 11 never fails to have an access to said common memory RAMc 19, whenever required. Further, both CPUa 11 and CPUb 12 can be given an equal access priority by inserting an AND gate circuit 55 shown in broken lines in FIG. 1 between the inverter 51 and the reset terminal R of the flip-flop circuit 29, connecting one input terminal of the AND gate circuit 55 to the output terminal of the inverter 51 and the other input terminal of said AND gate circuit 55 to the output terminal of the address decoder A23 and further connecting the output terminal of said AND gate circuit 55 to the reset terminal R of the flip-flop circuit 29.

With the microcomputer system of the invention shown in FIG. 1, the CPUa 11 is provided with an exclusive random access memory RAMa 15 and the CPUb 12 is provided with an exclusive random access memory RAMb 18. However, either or both of said exclusive memory elements can be omitted. In such case, either of the data memory area required for the CPUa 11, CPUb 12 may be concurrently used as a common memory RAMc 19.

FIG. 3 is a block circuit diagram of a microcomputer system according to another embodiment of this invention which is formed of three microcomputers. This embodiment represents an enlargement of the microcomputer system of FIG. 1 comprising two CPU elements. Referring to FIG. 3, CPUa 60 can have an access to the corresponding exclusive read only memory ROMa 61, exclusive random access memory RAMa 62, another random access memory RAM1.64 used in common to the CPUa 60 and CPUb 63, and still another random access memory RAM3.66 used in common to the COUCPU60 and the CPUc 65. Similarly the CPUb 63 can have an access to the corresponding exclusive read only memory ROMb 67, exclusive random access memory RAMb 68, another random access memory Ram1.64 used in common to the CPUb 63 and CPUa 60, and still another random access memory RAM2.69 used in common to the CPUb 63 and CPUc 65. Likewise, CPUc 65 can have an access to the corresponding exclusive read only memory ROMc 70 and exclusive random access memory RAMc 71, another random access memory RAM3.66 used in common to the CPUc 65 and CPUa 60 and still another random access memory RAM2.69 used in common to the CPUc 65 and CPUb 63. Under this arrangement, transfer of data between the CPUa 60 and RAM1.64 is effected through a bidirectional bus driver DRVa1.72, and transfer of data between the CPUa 60 and RAM3.66 is carried out through a bidirectional bus driver DRVa3.73. Transfer of data between the CPUb 63 and RAM1.64 is performed through a bidirectional bus driver DRVb1.74, and transfer of data between the CPUb 63 and RAM2.69 is conducted through a bidirectional bus driver DRVb2.75. Transfer of data between the CPUc 65 and RAM2.69 is achieved through a bidirectional bus driver DRVc2.76, and transfer of data between the CPUc 65 and RAM3.66 is accomplished through a bidirectional bus driver DRVc3.77.

Referring to FIG. 3, UNIT1.78, UNIT2.79, UNIT3.80 are each formed of the same elements as those of the microcomputer system of FIG. 1, that is, the address decoder A23, address decoder B 37, AND circuits 27, 33, 50, 38, 35, 49, 48, 24, 40, OR circuits 34, 36, inverters 47, 51 and flip-flop circuit 29 (the elements included in a region indicated in broken lines in FIG. 1). Therefore, the respective UNITS 1, 2, 3 are operated in the same manner as in the microcomputer system of FIG. 1. Signal lines connecting the UNITS 1, 2, 3 to the external apparatus is denoted by the same designations as used in FIG. 1.

The CPUa 60 can have an access to any of the addresses of the corresponding exclusive ROMa 61 and RAMa 62 the common RAM1.64 and common RAM3.66. With respect to the CPUb 63, CPUc 65, access control is carried out in the same manner as in the CPUa 60.

Where, with the microcomputer system of FIG. 3, the CPUa 60 and CPUb 63, for example, are going by chance to have an access to the common memory RAM1.64 at the same time, then the UNIT1.78 generates a signal WAITa1 to prevent the CPUa 60 from issuing a read-write signal and also a signal WAITb1 to keep the CPUb 63 in a waiting position. Similarly, the UNIT2.79 sends forth signals WAITb2, WAITc2, and the UNIT3.80 produces signals WAITc3, WAITa3. Among these WAIT signals, the one obtained by passing the signals WAITa1, WAITa3 through an OR gate 81 is supplied as a signal WAITa to the CPUa 60. Likewise, the one obtained by conducting the signals WAITb2, WAITb1 through an OR gate 82 is delivered as a signal WAITb to the CPUb 63. Similarly, the one obtained by transmitting the signals WAITc3 and WAITc2 through an OR gate 83 is supplied as a signal WAITc to the CPU c 65. Thus, transfer of data between the three CPU's and memories is effectively carried out according to address information stored in the corresponding address registers 84, 85, 86. The microcomputer system of FIG. 3 is so designed as to determine each time which the three CPU's should be given an access priority.

While the microcomputer system of FIG. 3 comprises three microcomputers, it is also possible to provide any number of CPU's interpose a common random access memory RAM between every adjacent CPU's and carry out transfer of data between the respective CPU's and common random access memories RAM.

Further, with the microcomputer system of FIG. 3, the RAM3.66, UNIT3.80 and bidirectional bus drivers 73, 77 can be omitted. In this case, data can be exchanged between the CPUa 60 and CPUc 65 through the common access memory RAM1.64, CPUb 63 and the common access, memory RAM2.69.

This invention is not restricted to the above-mentioned embodiments, but various changes and modifications can be made without departing from the spirit and scope of this invention.