Micro processor capable of being connected with a coprocessor
BACKGROUND OF THE INVENTION 
     The present invention relates to a central processing unit and, more particularly, to a micro processor capable of being connected with a coprocessor. 
     In a micro processor fabricated on a single semiconductor chip, the number of circuit elements formed within the single chip is restricted, and hence it is difficult to fabricate a unit in the same chip that executes high level instructions such as a floating-point arithmetic operation, a function arithmetic operation, etc., at a high speed. In order to solve this problem, therefore, a coprocessor is employed, which executes the high level instructions in place of the micro processor. The coprocessor operates under the control of the micro processor and is thus called a &#34;slave processor&#34;. The micro processor operates by itself as a central processing unit (CPU) to control the coprocessor along with a memory and peripheral units and is thus called a &#34;master processor&#34;. 
     When the master processor decodes the high level instruction, it accesses the slave processor. If the master processor is not connected with the slave processor, the master processor should execute the high level instruction by use of its arithmetic unit. Therefore, the master processor requires means for detecting whether or not the slave processor is connected thereto. 
     For this purpose, a prior art system provides in the master processor a flag representative of the presence or absence of the slave processor. When the master processor decodes the high level instruction, it reads out and checks the content of the flag. When the flag is stored with information representative of the presence of the slave processor, the master processor accesses the slave processor to supply command information thereto. When the content of the flag represents that the slave processor is not provided, the master processor executes the instruction by itself. According to this prior art, however, the master processor should check the content of the flag irrespective of the presence of the slave processor. For this reason, the executing time of the high level instruction is prolonged which lowers the arithmetic speed. Moreover, in a case where a plurality of slave processors are required, a plurality of flags corresponding to the respective slave processors are necessitated which undesirably increases the hardware of the master processor. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide an improved micro processor capable of being connected with a coprocessor. 
     Another object of the present invention is to provide a master processor which can detect the presence or absence of a slave processor without employing a flag. 
     A further object of the present invention is to provide a new control system between a master processor and a slave processor. 
     A micro processor according to one aspect of the present invention comprises an execution unit executing a program, a bus control unit coupled to the execution unit and controlling address and data buses to supply to a coprocessor a command code responsive to an instruction to be executed by the coprocessor, and a terminal connected to the execution unit and supplied with a busy signal which takes a first level when the coprocessor is in an arithmetic operating state and a second level when the coprocessor is not in the arithmetic operating state, whereby the execution unit detects the level of the busy signal after the command code is supplied to the coprocessor and controls the bus control unit to read status information of the coprocessor when the level of the busy signal is the second level. 
     A processor according to the present invention reads out the status information of the coprocessor. If the coprocessor is absent, the busy signal is held at the second level and the contents of the read-out status information take an abnormal value. Therefore, the absence of the coprocessor is detected without employing a flag representative of the presence or absence of the coprocessor. In a case where the coprocessor is provided, the coprocessor is brought into the arithmetic operating state in response to the supplied command code, so that the busy signal takes the first level. When the arithmetic operation by the coprocessor is completed, i.e. when the busy signal changes to the second level, the status information of the coprocessor is read out. If the status information an arithmetic exception, an error representation is carried out, for example. In a case where the status information does not contain the arithmetic exception, the arithmetic result is read out. 
     According to another aspect of the present invention, there is provided a micro processor comprising an execution unit executing a program, a bus control unit coupled to the execution unit and supplying to a coprocessor a command code representative of an instruction to be executed by the coprocessor, a first terminal coupled to the execution unit and supplied with a busy signal representing that the coprocessor is in an executing state of the instruction, and a second terminal coupled to the execution unit and supplied with a status read request signal representing that the status information of the coprocessor is to be outputted, whereby when both the busy signal and the status read request signal are absent, the execution unit controls the bus control unit to read out the arithmetic result of the coprocessor without checking the status information of the coprocessor. Thus, the execution time of the instruction is further shortened. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which 
     FIG. 1 is a block diagram representative of an embodiment of the present invention; 
     FIG. 2 is a flow chart representative of an operation of a master processor shown in FIG. 1; 
     FIG. 3 is a timing chart representative of the operation shown in FIG. 2; 
     FIG. 4 is a block diagram representative of a modification of the embodiment shown in FIG. 1; 
     FIG. 5 is block diagram representative of another embodiment of the present invention; 
     FIG. 6 is a flow chart representative of an operation of a master processor shown in FIG. 5; 
     FIGS. 7A and 7B are timing charts representative of the operation shown in FIG. 6; and 
     FIG. 8 is a block diagram representative of a modification of the embodiment shown in FIG. 5. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to FIG. 1, there are shown a master processor 10 as a CPU, a slave processor 30 as a coprocessor, and a memory 20. The slave processor 30 operates under the control of the master processor 10 and executes at a high speed high level instructions such as a floating-point arithmetic operation, a function arithmetic operation, etc., in place of the master processor 10. The memory 20 is accessed by the master processor 10 and stores data and a program, including low level instructions and high level instructions. The master processor 10, the memory 20 and the slave processor 30 are interconnected via an address bus 40 and a data bus 50. 
     The master processor 10 includes an instruction decoder unit 11 for decoding instructions which are executed by the master processor 10 and the slave processor 30, an execution unit 12 for controlling the entire operation of the master processor 10 and for executing instructions under a micro program control, an effective address calculation unit 13 for calculating an effective address of each operand of respective instructions, and a bus control unit 14 for controlling the address bus 40 and the data bus 50 to access the memory 20 and the slave processor 30, these units 11 to 14 being coupled as shown via internal bus lines. The bus control unit 14 supplies a bus cycle signal BC of a high level to the execution unit 12 via a line 16 when it transfers command information of the instruction to be executed by the slave processor 30 to the data bus 50. The master processor 10 further includes a terminal 15 connected to the execution unit 12 and supplied with a busy signal BS which will be described later. 
     The slave processor 30 includes an arithmetic unit 31 for performing an arithmetic operation of high level instructions, a command port 32 for receiving, via the data bus 50, command codes of the instructions executed by the slave processor 30, a status port 33 for receiving the status of the arithmetic unit 31 and for outputting them to the data bus 50, an operand port 34 for receiving via, the data bus 50, data necessary to the arithmetic operation of the unit 31 and for outputting the arithmetic result to the data bus 50, and an address decoder 15 for receiving address information via the address bus 40 and for selecting one of the ports 32, 33 and 34 in response thereto, these units 31 to 35 being coupled as shown via internal bus lines. The slave processor 30 further includes an output terminal 37 and an N-channel MOS transistor 36 of an open-drain type. The source-drain path of the transistor 36 is connected between the terminal 37 and a ground and the gate thereof is connected to the arithmetic unit 31. 
     The terminal 37 of the slave processor 30 is connected via a line 61 to the terminal 35 of the master processor 10. The line 61 is pulled up to a power voltage Vcc via a resistor 60. The arithmetic unit 31 of the slave processor 30 supplies a high level to the MOS transistor 36 during a time period when the unit 31 is in the arithmetic operating state to turn the transistor 36 ON. Thus, the slave processor 30 produces the busy signal BS of a low level which is in turn supplied to the execution unit 12 of the master processor 10 via the terminal 15. The transistor 36 is in a non-conductive state except during an arithmetic operating period of the unit 31, so that the busy signal BS takes the high level. In other words, the slave processor 30 does not produce the busy signal BS. 
     Referring to FIG. 2, there is shown a flow chart representative of an operation of the execution unit 12 in a case where the instruction decoder unit 11 decodes the instruction to be executed by the slave processor 30. This flow chart indicates a dyadic operation of &#34;A+B→B&#34;. When the data of first and second operands A and B are stored in the memory 20, the execution unit 12 prompts the bus control unit 14 to read out the data of the first and second operands A and B from the memory 20 and to store them into the operand port 34 of the slave processor 30 (steps 201 and 202). Thus, the bus control unit 14 controls the address and data buses 40 and 50 and writes the data of the first and second operands A and B into the operand port 34. If the operands A and B are in registers (not shown) in the slave processor 30, the steps 201 and 202 are not carried out. Thereafter, the execution unit 12 supplies to the bus control unit 14 a command code inputted from the instruction decoder unit 11 and prompts the unit 14 to supply it to the command port 32 (step 203). In response to this prompt, the bus control unit 14 transfers the command code on the data bus 50 and the address information of the command port 32 on the address bus 40, as shown in FIG. 3. The command code on the data bus 50 is thus written into the command port 32. During this bus cycle period, the bus control unit 14 supplies the bus cycle signal BC of the high level shown in FIG. 3 to the execution unit 12 via the signal line 16. As a result, the execution unit 12 is brought into a standby condition until the bus cycle signal BC changes to the low level, i.e. until the write operation of the command code into the command port 32 is completed (step 204). 
     In response to the change of the bus cycle signal BC from the high level to the low level, the execution unit 12 checks the level of the busy signal BS supplied via the terminal 15 to detect whether or not the arithmetic operation by the unit 31 is completed (step 205). If the slave processor 30 is absent, the terminal 15 is pulled up to the high level by the resistor 60. That is, the busy signal BS takes the high level. Therefore, the execution unit 12 regards the arithmetic operation by the unit 31 as being completed and prompts the bus control unit 14 to read status information of the status port 33 (step 206). The bus control unit 14 transfers the address information of the status port 33 to the slave processor 30 via the address bus 40 and reads the status information stored therein which is in turn supplied to the execution unit 12 via the data bus. The execution unit 12 judges the content of the read-out status information (step 207). Since the slave processor 30 is absent and since each bit line of the data bus is pulled up or pulled down, all of the bits of the read-out status information are &#34;1&#34; or &#34;0&#34;. Namely, the content of the status information takes an abnormal value. As a result, the execution unit 12 detects the absence of the slave processor 30 and executes the instruction to be executed by the slave processor 30 by itself, as represented as a slave processor absence operation (step 208). The execution unit 12 is not equipped with a function similar to the arithmetic unit 31, and therefore various low level instructions would be combined to execute the high level instruction, so that a processing time would be prolonged. 
     When the slave processor 30 is coupled to the master processor 10, as shown in FIG. 1, the unit 31 executes the arithmetic operation of the data between the first and second operands A and B stored in the operand port 34 in response to the command code supplied from the command port 32 and further supplies the high level to the transistor 36, which is thereby turned ON to change the busy signal BS to the low level, as shown in FIG. 3. Accordingly, the execution unit 12 receives the low level busy signal BS in response to the change of the bus cycle signal BC from the high level to the low level and thus regards the arithmetic unit 31 as being in an arithmetic operating state to inhibit the slave processor 30 from being accessed. The address and data buses 40 and 50 are thus brought into a free state. Although the execution unit 12 takes a standby condition in the flow chart shown in FIG. 2, since the buses 40 and 50 are free, the execution unit 12 may prefetch instructions from the memory by use of buses 40 and 50 or peripheral units (not shown) may use the buses 40 and 50. The execution unit 12 cyclically checks the level of the busy signal BS. 
     When the arithmetic operation by the unit 31 is completed, the unit 31 stores the status information and the arithmetic result into the status port 33 and operand port 34, respectively. The arithmetic unit 31 further supplies the low level to the transistor 36 to turn it OFF. The busy signal BS is thus changed to the high level by the pull-up resistor 60, as shown in FIG. 3. As a result of detecting the high level busy signal BS, the execution unit 12 regards the arithmetic unit 31 as having completed its arithmetic operation and prompts the bus control unit 14 to read the status information (step 206). Thus, the bus control unit 14 supplies the address information of the status port 33 to the slave processor 30 via the address bus 40 and reads out the status information from the status port 33 via the data bus 50, as shown in FIG. 3. The read-out status information is judged by the execution unit 12 (step 207). If the status information includes arithmetic exception data representative of overflow or zero-divisor, the execution unit 12 performs an error representation as a slave processor arithmetic exception operation 210. When the status information does not include the abnormal value and the arithmetic exception, the execution unit 12 prompts the bus control unit 14 to read out the arithmetic result (step 209). As a result, the bus control unit 14 supplies the address information of the operand port 34 to the slave processor 30 via the address bus 40 and the arithmetic result stored in the operand port 34 is read out and then transferred to the data bus 50. 
     Since the absence of the slave processor 30 is detected by reading and judging the content of the status information of the status port, the master processor 10 does not require a flag representative of presence or absence of the slave processor 30. Moreover, the reading of the status information is carried out only when the busy signal takes information representative of completion of an arithmetic operation of the unit 31, and therefore the buses 40 and 50 are not occupied with reading the status information. 
     Referring to FIG. 4, a plurality of slave processors (three processors in the drawing) 30-1, 30-2 and 30-3 are coupled to the master processor 10 via the address and data buses 40 and 50. Each of the slave processors 30-1 to 30-3 has the same construction as that shown in FIG. 1, and busy signal output terminals 37-1 to 37-3 thereof are connected in common to the terminal 15 of the master processor 10 via the line 61. Since each port of the respective slave processors 30-1 to 30-3 is assigned to an individual address space, only one of the ports among the slave processors 30-1 to 30-3 is accessed by the address information produced by the master processor 10. Since each of the slave processors 30-1 to 30-3 includes a transistor of an open-drain type (transistor 36 in FIG. 1), the busy signal BS is controlled by one of the slave processors 30-1 to 30-3 which is in an arithmetic operating state. Thus, the master processor 10 can control a plurality of slave processors 30-1 to 30-3 in the same manner as described hereinbefore. 
     In the above-mentioned embodiment, the status information of the slave processor 30 is always read out. However, when the status information does not include the abnormal information or the arithmetic exception information, the reading of the status information is not necessary. That is, the arithmetic result can be read from the slave processor without reading the status information. The processing time of the high level instruction is thereby further shortened. 
     For this purpose, as shown in FIG. 5 as another embodiment of the present invention, a master processor 70 is provided with a second input terminal 17 coupled to the execution unit 12 and a slave processor 80 is provided with a second N-channel MOS transistor 38 of an open-drain type and a second output terminal 39. The terminals 17 and 39 are connected to each other via a signal line 63 which is in turn pulled up to the power voltage Vcc via a resistor 62. The drain-source path of the transistor 38 is connected between the terminal 39 and the ground and the gate thereof is connected to the arithmetic unit 31. The unit 31 supplies the high level to the transistor 38 at the end of the arithmetic operation when the necessity of reading the status information does not occur, i.e. when the arithmetic exception and flag updating do not occur as a result of the arithmetic operation. The transistor 38 is thereby turned ON to change the line 63 to the low level. The level of the line 63 is supplied via the terminal 17 to the execution unit 12 as a status read request signal STRD. 
     A detailed description of a circuit operation will be made below with reference to the flow chart shown in FIG. 6 and timing charts shown in FIGS. 7A and 7B. 
     The flow chart of FIG. 6 indicates a dyadic operation of &#34;A+B→B&#34;. Steps 601 to 605 correspond respectively to the steps 201 to 205 of FIG. 2, and therefore the description thereof is not repeated. When the execution unit 12 detects the high level of the busy signal BS, it catches the status read request signal STRD supplied from the terminal 17 to detect the level thereof. Assuming that both the arithmetic exception and flag updating do not occur, the arithmetic unit 31 supplies the high level to the transistor 38 at the end of the arithmetic operation. The transistor 38 is thereby turned ON to change the status read request signal STRD to the low level, as shown in FIG. 7A. Therefore, the execution unit 12 receives the low level signal STRD. As a result, the execution unit 12 prompts the bus control unit 14 to read the arithmetic result without reading the status information of the slave processor 80 (step 612). Thus, the bus control unit 14 supplies the address information of the operand port 34 to the slave processor 80 via the address bus 40 and reads out the arithmetic result stored in the port 34 via the data bus 50, as shown in FIG. 7A. The processing time of the high level instruction is thereby made short. 
     In contrast, when the arithmetic exception or flag updating occurs, the arithmetic unit 31 does not turn the transistor 38 ON. The status read request signal STRD is thereby maintained at the high level, as shown in FIG. 7B. Since the execution unit 12 receives the high level signal STRD, it prompts the bus control unit 14 to read the status information (step 607). Thus, the bus control unit 14 supplies the address information of the status port 33 to the slave processor 80 via the address bus 40 and reads out the status information stored therein via the data bus 50, as shown in FIG. 7B. The execution unit 12 judges the content of the read-out status information (step 608). When the status information contains the arithmetic exception data, the execution unit 12 carries out a slave processor arithmetic exception operation which has been described hereinbefore (step 610). In a case where the status information contains flag updating data, the execution unit 12 updates the content of one of the associated flags 18 (step 609). The updated flag is used for a branch operation or a subroutine procedure, for example. Thereafter, the execution unit 12 prompts the bus control unit 14 to read the arithmetic result (step 612). 
     If the slave processor 80 is absent, both the busy signal BS and the status read request signal STRD are maintained at the high level. All of the bits of the read-out status information take &#34;1&#34; or &#34;0&#34;. As a result, the execution unit 12 executes a slave processor absence operation (step 611) which has been mentioned above. 
     Since the bus cycle for reading the content of the status port 33 can be omitted by use of the status read request signal STRD, the execution time of the high level instruction is further shortened. 
     Referring to FIG. 8, the master processor 70 is connected with a plurality of slave processors (three processors in the drawing) 80-1, 80-2 and 80-3 each having the same construction as that shown in FIG. 5. The busy signal output terminals 37-1, 37-2 and 37-3 of the slave processors 80-1 to 80-3 are connected in common to the terminal 15 of the master processor 70 via the line 61, and the status read request signal output terminals 39-1 to 39-3 are connected in common to the terminal 17 via the line 63. Since the respective ports of the slave processors 80-1 to 80-3 are assigned to respective address spaces, the master processor 70 can individually control these slave processors 80-1 to 80-3. 
     The present invention is not limited to the above-mentioned embodiments, but may be changed and modified without departing from the scope and spirit of the invention.