Branch instruction execution device and computer program product

Abstract: A microcomputer MCU adopting the general purpose register method is enabled to have a small program capacity or a high program memory using efficiency and low system cost, while enjoying the advantage of simplification of the instruction de-coding as in the RISC machine having a fixed length instruction format of the prior art, by adopting an instruction format of a fixed length of 2 n bits which is smaller than the length of the maximum data word fed to instruction execution means. The control of the coded division is executed by noting the code bits.

Description: BACKGROUND OF THE INVENTION The present invention relates to a CPU and an architecture of a microcomputer, particularly a RISC (Reduced Instruction Set Computer) type microcomputer and, more particularly, to a technology effective if applied to a microcomputer to be packaged in a device for controlling it. Moreover, the present invention relates to a circuit for coded division such as a dividing circuit for a coded binary number of arbitrary length and multi-precision and, more particularly, to a technology effective if applied to a step division of the RISC type microcomputer. The most serious bottleneck for reducing the number of machine cycles necessary for executing one instruction is known to be the decoding of the instruction. In order to speed up this decoding, it is known as effective to adopt an instruction format of fixed length so that where the boundary of the instruction resides may be informed before a preceding instruction has been interpreted. In the so-called "RISC type computer", most instructions are executed for one cycle by adopting the instruction format of fixed length and a pipe line of multiple steps. The conventional RISC computer has used a 32-bit instruction format without exception. This 32-bit fixed length instruction format is advantageous in that what register is to be read can be determined without decoding the operation code by fixing fields in the instruction formats of a source register and a destination register, and in that no alignment is required when an immediate value is decoded. On the contrary, the 32-bit fixed length instruction format requires 32 bits even no matter simple content of an instruction might be described. As a result, the number of bytes occupied by the instruction code is increased to raise a problem that the ratio of the memory area to be occupied by a program is accordingly increased. If the memory area occupied by the program is increased, a memory having a larger capacity has to be packaged to raise the cost of the microcomputer system, thus making it difficult to construct a system having an excellent performance ratio to the cost. Since the RISC processor is given an architecture for speeding up the executions of instructions by reducing the number of instructions, there arises a tendency that the undefined operation codes grow more for the instruction set. The multiplicity of the undefined operation codes deteriorates the code efficiency of the object program and degrades the memory using efficiency the worse. An example of a RISC CPU is disclosed in Furber: "VLSI RISC Architecture and Organisation", 1989, Marcel Dekker, New York, U.S., XP-002183651, page 106 to 109. The instruction set processed by this CPU contains 16-bit jump instructions having a range of +/- 254 bytes, and 32-bit branch instructions having a range of +/- 1 Mbytes. EP-A-0 440 908 (prior art under Article 54(3) and (4) EPC) discloses an instruction set also including 16-bit and 32-bit branch instructions. The 32-bit branch instructions include conditional and unconditional branch instructions each having an address area of 20 bits, and a further unconditional branch instruction having an address area of 24 bits including an absolute branch destination address. SUMMARY OF THE INVENTION It is an object of the invention to provide a central processing unit and a computer program product comprising instruction code for execution by said central processing unit, the invention making more efficient use of a fixed bit length of branch instructions. This object is solved by a CPU according to claim 1 and a computer program product according to claim 9. The dependent claims relate to preferred embodiments of the invention. Embodiments of the present invention solve the various problems accompanying the adoption of a fixed length instruction format having a smaller bit number than that of a data word length. They provide a CPU which can achieve one or plural items selected from: that neither the use of immediate data nor the assignment of an absolute address is restricted even if the bit number of the fixed length instruction format is less than that of the data word length; that a description such as a necessary displacement can be executed in the fixed length instruction format having a limited bit number; that a contribution is made to the prevention of a misalignment of the program arrangement on a memory; and that the code efficiency or memory using efficiency is improved better from the standpoint of the content of a supporting instruction. Embodiments of the invention to be disclosed herein will be briefly summarized in the following. (1) In a microcomputer adopting the general purpose register method, there is adopted a fixed length instruction format which has a smaller bit number than that of the maximum data word length fed to instruction execution means. (2) In order that the bit number set in the fixed length instruction format may prevent a misalignment of a program on a memory, the fixed length instruction format and the maximum data word length may be set to a bit number of a power of 2. If the maximum data word length is 32 bits, for example, the instruction format is fixed to 16 bits. (3) In case the aforementioned relation holds between the maximum word length of data and the bit number of the instruction format, a plurality of instruction may be prefetched in a common cycle so as to fetch the instructions efficiently by making use of an internal bus of a bit number equal to that of the maximum data word length or to reduce the bus access number for the instruction fetch. (4) In case the internal bus is shared between the data transfer and the instruction fetch, the pipe control may be executed to prefer the data fetch thereby to delay the' whole instruction execution schedule including an instruction fetch conflicting with that data fetch, so as to simplify either a processing when the data fetch and the instruction fetch conflict or a post-processing caused by the former. (5) In order to simply cope with the state, in which the uses of the general purpose registers in response to the instructions before and after the pipe-line execution, the pipe-line control may be executed, because the general purpose register method is adopted, by detecting the state, in which the uses of the general purpose registers in response to the plurality of instructions to be executed in the pipe-line manner conflicts, on the basis of the information of a register assigned area contained in the instruction format, thereby to delay the execution of an instruction after the register conflicting state on the basis of the register conflicting state detected and the execution cycle number of the instruction to be preferentially executed. (6) In order that the restriction on the bit number of the fixed length instruction format may not limit the use of immediate data, it is advisable to support the instruction containing a description for assigning the immediate data in a data relation for offsetting the value of a displacement relative to the value of a predetermined register. (7) Even in the fixed length instruction format having a restricted bit number, the displacement necessary for the data processing or the bit number of the immediate data may be maximized to support an instruction for implicitly assigning a predetermined general purpose register which is fixed as an operand despite of no register assigning field in the instruction. (8) Even in the fixed length instruction format having a restricted bit number, likewise, the displacement necessary for the processing or the bit number of the immediate data may be maximized to support an instruction containing a description for reflecting the truth or false of the arithmetic result for a specified condition upon a predetermined status flag. (9) A proper branch destination assigning displacement length is fixedly assigned in accordance with the kinds of branching instructions. For a 16 bit fixed length instruction format, the displacement of a condition branching instruction is fixed at 8 bits, and the displacements of a subroutine branching instruction and an unconditional branching instruction are fixed to 12 bits. (1) According to the means described above, the adoption of a 16 bit fixed length instruction format for a 32 bit data word length makes it possible to grasp the decision of where an instruction boundary resides, before a preceding instruction is completed, like the RISC machine of the prior art having the 32 bit fixed length instruction format in the point that the instruction format has the fixed length, thereby to warrant an advantage such as a simplification of the instruction decoding. (2) The program capacity is smaller than that of the case, in which the 32 bit fixed length instruction format is adopted. Specifically, in the RISC architecture for speeding up the executions of instructions by reducing the kinds of them, there is a tendency that many undefined operation codes are in the instruction set. If the instruction length is halved at this time from that of the prior art, the using efficiency of the program memory is improved. (3) The various problems intrinsic to the adoption of a fixed length instruction format having a smaller bit number than that of a data word length are solved by the facts: that neither the use of immediate data nor the assignment of an absolute address is restricted even if the bit number of the fixed length instruction format is less than that of the data word length; that a description such as a necessary displacement can be executed in the fixed length instruction format having a limited bit number; that a contribution is made to the prevention of a misalignment of the program arrangement on a memory; and that the code efficiency or memory using efficiency is improved better from the standpoint of the content of a supporting instruction. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing one example of a system making use of a microcomputer MCU according to one embodiment of the present invention; Fig. 2 is a block diagram showing a microcomputer according to one embodiment of the present invention; Fig. 3 is a diagram for conceptionally explaining a pin arrangement for a package of a microcomputer according to the present embodiment; Fig. 4 is an explanatory diagram showing one example of the relation between the data word length and the instruction word length to the bus width in the microcomputer according to the present embodiment; Fig. 5 is a block diagram showing one example an internal block of a central processing unit, especially its instruction control system included in the microcomputer of the present embodiment; Fig. 6 is a block diagram showing the structure of one half of an execution unit of the central processing unit; Fig. 7 is a block diagram showing the structure of the remaining half of the execution unit of the central processing unit; Fig. 8 is a diagram for explaining one example of a stage of a pipe-line processing by the central processing unit; Fig. 9 is a diagram for explaining one example of a pipe-line sequence in a register conflicting state; Fig. 10 is a diagram for explaining another example of a pipe-line sequence in a register conflicting state; Fig. 11 is a diagram for explaining one example of a pipe control sequence in case a data fetch and an instruction fetch conflict; Fig. 12 is an operation timing chart showing one example when a plurality of cycle instructions are executed; Fig. 13 is an explanatory diagram showing a table of one half of data transfer instructions to be executed in the microcomputer of the present embodiment; Fig. 14 is an explanatory diagram showing a table of the remaining half of data transfer instructions to be executed in the microcomputer of the present embodiment; Fig. 15 is an explanatory diagram showing a table of logical operation instructions to be executed by the microcomputer of the present embodiment; Fig. 16 is an explanatory diagram showing a table of one half of arithmetic operation instructions to be executed by the microcomputer of the present embodiment; Fig. 17 is an explanatory diagram showing a table of the remaining half of arithmetic operation instructions to be executed by the microcomputer of the present embodiment; Fig. 18 is an explanatory diagram showing a table of instructions to be executed by the microcomputer of the present embodiment; Fig. 19 is an explanatory diagram showing a table of branching instructions to be executed by the microcomputer of the present embodiment; Fig. 20 is an explanatory diagram showing a table of one half of system control instructions to be executed by the microcomputer of the present embodiment; Fig. 21 is an explanatory diagram showing a table of the remaining half of system control instructions to be executed by the microcomputer of the present embodiment; Fig. 22 is a diagram for explaining the description types of Figs. 13 to 21; Fig. 23 is an explanatory diagram showing a table of addressing modes in the mnemonic designations shown in Figs. 13 to 21; Fig. 24 is an explanatory diagram showing one example of the relations between the displacement lengths of branch instructions and the appearance frequency of instructions having the displacement lengths; Fig. 25 is an explanatory diagram showing one example of the relations between the displacement lengths of branch always instructions and the appearance frequency of instructions having the displacement lengths; Fig. 26 is an explanatory diagram showing one example of the relations between the displacement lengths of subroutine call instructions and the appearance frequency of instructions having the displacement lengths; Fig. 27 is an explanatory diagram showing one example of the relations between the displacement lengths of jump instructions or jump subroutine instructions and the appearance frequency of instructions having the displacement lengths; and Fig. 28 is a diagram for explaining.the structure of a register of an example as a programmer' s model. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in the following in connection with the embodiments thereof in the order of items which are broadly itemized into: a microcomputer adopting an instruction forma of fixed length having a smaller bit number than that of the maximum data word length; and coded divisions to be executed by such microcomputer. The contents of the former will be described under Items [1] to [17] whereas the contents of the latter will be described under Items [18] to [24]. [1] Application System of Microcomputer Fig. 1 shows one example of a system which makes use of a microcomputer MCU according to one embodiment of the present invention. This microcomputer MCU is coupled through an external control bus ECB, an external data bus EDB and an external address bus EAB to an external memory EMRY and an external input/output block EI/0 defined by the user and is further connected with a port input/output bus PIOB to an external equipment EEQ. This external equipment EEQ is a predetermined device to package the microcomputer system. [2] Block Structure of Microcomputer Fig. 2 shows the microcomputer MCU according to one embodiment of the present invention. The microcomputer MCU, as shown, is packaged in a single semiconductor substrate such as a silicon substrate by the well-known semiconductor integrated circuit manufacture technology. Reference letters CPU appearing in the Figure designates a central processing unit for controlling the microcomputer MCU in its entirety. Letters Port/I/O designate an input/output port to be used for the central processing unit CPU to input/output a signal, to drive an external display device and to inspect the state of an external switch. The central processing unit CPU inputs/outputs by reading/writing a register assigned to a certain address. Letters Port/Cont designate a port for an input/output of the data bus. Letters Port/Address designate a port,for an input/output of the address bus. Letters I/O designate such a peripheral device on the chip of the microcomputer MCU as includes a serial communication interface, a timer and so on. Letters DMAC designate a direct memory access (DMA) controller. A ROM (Read Only Memory) is an instruction memory packaged on-chip and stores the instructions (i.e., operation programs) of the central processing unit CPU and a constant table. A RAM (Random Access Memory) is a memory packaged on-chip and is used as the working area of the central processing unit CPU or a primary storage area for data. Letters BSC designate a bus state controller for controlling the bus access to the inside and outside of the microcomputer MCU. Letters CACHE designate a cache memory, i.e., a storage device for latching an instruction, which is anticipated to be most frequently used by recognizing the access pattern of an instruction of the central processing unit CPU, to reduce the frequency for accessing to an application program or the like from an external memory having a low access rate, thereby to accelerate the processing. Letters BURST/REFRESH designate a burst/refresh device for burst control of a continuous data transfer and for refresh control of a dynamic random access memory (DRAM) and is applied to a high-speed page mode access, when the DRAM is used as an external memory, and to a refreshing of the DRAM. Letters edb designate an internal data bus coupled through a port Port/Data to the external data EDB, and letters eab designate an internal address bus coupled through a port Port/Add to the external address bus EAB. These two internal buses are not coupled to the central processing unit CPU. On the other hand, characters IDB31-0 designate internal data buses of 32 bits, and IAB23-0 designate internal address buses of 24 bits. These two internal buses are coupled to the central processing unit CPU. This central processing unit CPU is given an instruction from the instruction memory ROM or the cache memory CACHE so that the data issued from the data memory RAM or the external memory EMRY are subjected to a predetermined processing in accordance with the given instruction. [3] Pin Arrangement of Package of Microcomputer Fig. 3 conceptionally shows a pin arrangement of the package of the aforementioned microcomputer MCU. In the same Figure, free running timers FRT0 and FRT1, an analog/digital converter A/D, a digital/analog converter D/A, and serial communication interfaces SCI0 and SCI1 exemplify the peripheral circuit I/O of Fig. 2, and corresponding ports PORT1 to PORT4 shown in Fig. 3 are input/output ports corresponding to individual peripheral devices and accordingly to the port Port/I/O of Fig. 2. The microcomputer MCU is filled in a QFP type package of 112 pins. Address buses (AO-23) of 24 bits are connected with the aforementioned address bus EAB, and data buses (D0-31) of 32 bits are connected with the aforementioned data bus EDB. These buses A0-23 and D0-31 are used when the central processing unit CPU, the direct memory access controller DMAC or the burst/refresh control block BURST/REFRESH accesses the external memory EMRY. Clock signals are those for specifying the basic timing of the instant when the inside of the microcomputer MCU and its external systems operate synchronously. If a not-shown quartz oscillator is coupled to terminals EXTAL and XTAL, for example, it resonates to oscillate with an electric circuit in the chip of the microcomputer MCU. This chip detects the oscillating voltage to generate internally synchronizing clocks  1 and  2, as will be described hereinafter. These clocks  1 and  2 are non-overlap clock signals which have neither of their high periods overlapped, although not especially limitative thereto. At the same time, the microcomputer MCU outputs a clock signal having a waveform and a phase substantially identical to those of the signal  1 from its terminal CLK so as to synchronize its external system and the inside of the LSI. The control signal includes: an operation mode setting signal and an interruption inputting signal such as a reset signal (RES) or a standby signal (STBY); a data bus control signal such as a read strobe (RD) or a write strobe (WRHH); or a DRAM refresh control or bus arbitration signal. Letters Vss and Vcc designate a ground terminal and a power supply terminal. The port PORT1 inputs/outputs two channels of the control signal DMAC concerning the direct memory access controller DMAC. The port PORT2 inputs/outputs the FRT signal for controlling and reading the free running timers FRT0 and FRT1 from the outside of the microcomputer MCU. The port PORT3 inputs/outputs the analog signals which are fed to the analog/digital converter A/D and the digital/analog converter D/A. Reference potentials of the analog/digital converter A/D and the digital/analog converter D/A are fed from terminals AVcc and AVss. The port PORT4 inputs/outputs the serial communication signals which belong to three clock, data transmission and data reception systems for two channels. [4] Instruction Word Length and Data Word Length Fig. 4 shows one example of the relation of the data word length and the instruction word length to a bus width (i.e., the number of signal lines composing a bus) in the microcomputer MCU of the present embodiment. This microcomputer MCU has an architecture of RISC type and an instruction forma of fixed length. The buses such as internal data buses IDB31-0, through which data and instructions are transferred, are given 32 bits (corresponding to 32 signal lines). At this time, the bit number of later-described various registers is 32 bits. The data on a memory are arranged at the units of byte (of 8 bits), word (of 16 bits) and long word (of 32 bits) in a memory area having a width of 32 bits. Addresses are assigned at the byte unit to the memory. As a result: the byte data are accessed at the unit of n addresses (n: an integer); the word data are accessed at the unit of 2n addresses; and the long word data are accessed at the unit of 4n addresses. Other memory accesses are deemed to belong to an address error. On the other hand, instructions are formatted into an instruction format having a fixed length of 16 bits (corresponding to 16 signal lines). In short, an instruction of fixed length is composed of bits of the n-th power of 2, and the relation of m ≧ n holds, if the number of signal lines composing the bus width is the m-th power of 2. In Fig. 4 showing several examples of the aforementioned instruction format of fixed length of 16 bits: four bits of "rrrr" designate an assigned field of a source register; four bits of "RRRR" designate an assigned field of a destination register; bits of "d---dd" designate a displacement: and bits "ii-i" designate an immediate data. In an instruction format of 16 bits, the destination register assigned field RRRR falls at the eighth bit from the fourth bit with reference to the lefthand side of the format, and the source register assigned field rrrr falls at the twelfth bit from the ninth bit. The lefthand four bits of the instruction format are assigned to at least an operation code assigned field. The instruction system of the microcomputer MCU allows the destination register assigned field RRRR to be used as a portion of the operation code assigned field and the source register assigned field rrrr to be used as a portion of the displacement or immediate data. Thanks to adoption of the instruction format of fixed length of 16 bits for the data word length of 32 bits, where the instruction boundary resides can be decided before the end of a preceding instruction like the RISC machine of the prior art having the instruction format of fixed length of 32 bits, thereby to warrant the advantage such as simplification of the instruction decoding process. In addition, the program capacity can be made smaller than that of the case in which the instruction format of fixed length of 32 bits is adopted. In the RISC architecture intended to accelerate the execution of instructions by reducing the number of kinds of instructions, there is a tendency that undefined operation codes increase in an instruction set. If the instruction length is reduced at this time to one half of the prior art, the efficiency of using the program memory can be improved. Thanks to the reduced instruction word length, as compared with the data word length, the substantially useless bit array can be made less than that of the case, in which the two word lengths are equalized, so that the efficiency of using the program memory can be enhanced. As a result, the efficiency of using the memory for storing the program can be improved to keep an application field, which uses a memory having a limited capacity on the board or a program memory packaged on-chip in the processor, away from problems such as shortage in the storage capacity of the program memory at the time of constructing the system or unavoidably large scale of the memory. Thus, the system cost can be reduced. Since, moreover, the instruction format of fixed length of 16 bits has one half of the data word length and bus width of 32 bits, an instruction misalignment to the program memory such that an instruction of single format is arranged across the boundary of the memory (or word) occurs less frequently than the case, in which the instruction word length is shortened by setting the instruction word length to a fragmentary bit number (e.g., bit number other than a power of 2) with respect to the bus width or data word length. [5] Instruction Control System of CPU Fig. 5 shows one example of the internal block, i.e., its instruction control system of the aforementioned central processing unit CPU. Letters IRH and IRL designate individual instruction buffers (i.e., instruction queues) of 16 bits for latching instructions of fixed length of 16 bits one by one. These registers IRH and IRL are individually loaded with the instructions in one instruction fetch cycle. This is because the instructions are transferred at the unit of 32 bits through the internal data buses IDB31-0. These instructions are loaded in the instruction register IR1 through a multiplexer MPX. The instruction register IR1 has 16 bits. This instruction register IR1 latches the instruction which is present at the instruction decode stage. In the instruction register IR1. with reference to the lefthand end indicated as "RRRR" in accordance with the instruction format of fixed length of 16 bits, the fifth to eighth bits are caused to belong to the destination register field, and the ninth to twelfth bits indicated as "rrrr" are caused to belong to the source register field. These fields are fixed in the instruction field. At this time, as has been described hereinbefore, the source register field never fails to be used for selecting the source operand, and the destination register field never fails to be used for selecting the second source operand or destination operand. Each register field may mean a portion of the operation code or the number of a register. Whether or not the contents of the register assigned fields RRRR and rrrr are pieces of information for selecting a register is determined at the left end of the aforementioned instruction format of fixed length by the contents of the 1st to 4th bit operations codes. Nevertheless, the values of the register fields RRRR and rrrr are inputted to and decoded by a source register decoder SRD and a destination register decoder DRD through the instruction register IR2. The decoded results by the aforementioned source register decoder SRD and destination register decoder DRD are used to decide what of general purpose registers R0H and R0L, - - -, and R15H and R15L contained in an execution unit for arithmetic operations is to be selected. On the other hand, the values of the register fields RRRR and rrrr outputted from the aforementioned instruction register IR2 are fed through temporary latches WBR0 and WBR1 to a write back register decoder WBRD so that the arithmetic results obtained by the instruction executions are written for selecting the general purpose registers in accordance with the decoded results. Moreover, the values of the register fields RRRR and rrrr, i.e., the register numbers latched in the aforementioned temporary latch WBR1 and being used are fed to a register content check block RCCB so that they are compared with the outputs of the instruction registers IR1 and IR2 to find out an even (or register conflict), in which each instruction, e.g., a subsequent instruction accesses a common register when the instructions sequentially fetched are executed in the pipe-line manner. The register conflict is fed to a flag operation & pipe control decoder FO&PCD in response to the signal S1. If the signal S1 thus asserted is fed to the flag operation & pipe control decoder FO&PCD, skip controls are executed in accordance with the number of instruction execution cycles being done, to cancel or delay the execution cycles of the instructions using the conflicting registers. In short, when the flag operation & pipe control decoder FO&PCD outputs the control signal Skip, the decoding of the instruction to be subsequently executed is delayed in the source register decoder SRD and the destination register decoder DRD. As a result, the execution cycle of a succeeding instruction is started at a timing after the writing of the conflicting registers is ended by executing the preceding instruction. The instruction latched by the aforementioned instruction register IR1 is fed to a mapping control block MCB to index the addresses of a high-speed control read only memory (i.e., Hardware Sequence ROM) HSC-ROM. This mapping control block MCB has a role to calculate the entry address of a proper high-speed control read only memory MHSC-ROM in accordance with the content. The high-speed control read only memory HSC-ROM has its output composed of two portions: a micro code field MCF and a pipe control field PCF, which are latched by a hardware sequence control instruction register HIR such as a micro instruction register. The former in the micro instruction field feeds a control signal to the execution unit EXEC through a shallow decode logic, i.e.. an instruction decoder ID. The latter establishes the sequence of an instruction having two or more cycles through the flag operation & pipe control decoder FO&PCD or controls the pipe line. The flag operation & pipe control decoder FO&PCD has eight flags C, W, B, L, M, I, S and S for controlling the pipe lines. Moreover, this flag operation & pipe control decoder FO&PCD has a status register SR which has a true bit T (as will be shortly referred to as "T bit") used for condition branching. This T bit is set to the truth or false of the arithmetic result of selected conditions described in the instruction, as will be described hereinafter. The content of the aforementioned instruction register IR1 is transferred before an arithmetic execution phase (EX) to the instruction register IR2 so that whether or not a conflict is caused between instructions in the pipe line state of later memory access phase (MA) and write back phase (WB) is checked through the aforementioned register content check block RCCB, and this result is outputted as the signal S1. What is stored at this time in the instruction register IR2 is the value of the register field. The values of the register fields RRRR and rrrr latched in the instruction registers IR1 and IR2, as has been described hereinbefore, are fed to the aforementioned source register decoder SRD, destination register decoder DRD and write back register decoder WBRD. These source register decoder SRD, destination register decoder DRD and write back register decoder WBRD generates a selection signal for selecting one pair of the sixteen general purpose registers R0H and R0L, - - - , and R15H and R15L and feeds it to the execution unit EXEC. The memory interface MIF detects whether or not the central processing unit CPU has to access a memory and whether the memory is of the read or write type, and feeds a signal necessary for accessing the memory. On the other hand, an instruction fetch & instruction register control block IF&IRC has a function to determine whether or not an instruction fetch from a memory is necessary and when the contents of the instruction registers IRH and IRL are to be updated, and to output a necessary signal. The function of this instruction fetch & instruction register control block IF&IRC is to output a predetermined control signal with reference to the state of the pipe line, the state of the memory and the state of the instruction queues (IRH and IRL) thereby to control the instruction queue or the instruction fetch. What features the present embodiment is that the instruction fetch is carried out at the unit of 32 bits so that it contains two instructions having an instruction length of 16 bits. This makes it unnecessary to fetch again the instruction, which has been fetched simultaneously as the preceding instruction is fetched, in another phase. These events are totally judged to control when the instruction fetch is to be executed. The instruction fetch & instruction register control block IF&IRC is constructed as an finite state machine, and a detailed description of the structure of this machine will be omitted because the structure per se is well known in the art. Incidentally, letters IMB appearing in Fig. 5 designate a buffer for sending immediate data contained in an instruction to the execution unit EXEC. Moreover, the instruction queues IRH and IRL and the latch timing of the instruction register IR1 are synchronized with the aforementioned clock signal  1. The latch timings of the instruction register IR2, the micro instruction register MIR and the registers WBR0 and WBR1 and the output timing of the signal S1 by the aforementioned register content check block RCCB are synchronized with the aforementioned clock signal  2. Since the data bus width is 32 bits whereas the fixed length instruction is 16 bits, there are provided two instruction buffers IRH and IRL. Despite of this provision, however, the number of instruction buffers is determined depending upon how many fixed length instructions can be transferred within the data bus width, for example: four instruction buffers in case of a fixed length instruction of 8 bits; and eight instruction buffers in case of a fixed length instruction of 4 bits. [6] Execution Unit of CPU Figs. 6 and 7 show one example of the execution unit EXEC of the central processing unit CPU. In the two Figures, buses indicated at A, B, C and D are commonly connected. The execution unit EXEC includes: an instruction fetch block IFB and a general purpose block GRB for fetching an instruction and updating a program counter; an operation block OPB for addition/subtraction and shift operations; and memory access block MAB and a multiplication block MULT for accessing a memory and aligning data. These individual blocks are coupled to one another through the four data buses A. B, C and D having a width of 32 bits. The aforementioned instruction buffers (or instruction queues) IRH and IRL, multiplexer MPX and instruction register IR1 forming part of the aforementioned instruction fetch block IFB are shown in the block structure of Fig. 5 separately of the execution unit EXEC but may be contained in the execution unit EXEC, as shown in Fig. 6. An immediate buffer IMB is a logic for cutting and bit-shift, if necessary, immediate data. Letters PCH and PCL designate program counters for latching addresses for fetching an instruction. An arithmetic unit high AUH and an arithmetic unit low AUL are adders capable of performing an addition of 32 bits for updating the program counters. A procedure address register high PRH and a procedure address register low PRL are procedure address registers for latching return addresses for a function call. A vector base register high VBRH and a vector base register low VBRL are used as storage areas of an interrupt vector area for latching the base addresses. A global base register high GBRH and a global base register low GBRL are used as storage registers for the base address of the I/O. A break register high BRH and a break register low BRL are used as storage registers for return destination addresses from the break routine. The aforementioned general purpose register block GRB includes sixteen general purpose registers of 32 bit length, as indicated at R0H and R0L to R15H and R15L. In the aforementioned operation block OPB, a shifter high SFTH and a shifter low SFTL are hardware for bit shifts and rotations. An arithmetic logic unit high and an arithmetic logic unit low are operators for arithmetic logical operations. Letters SWP&EXT designate a hardware for executing a swap instruction, a code (or sign) extension or a zero extension. An aligner ALN is a hardware for aligning the data which are accessed in byte or word from a memory or I/O. A memory read buffer high MRBH and a memory read buffer low MRBL are temporary registers for latching the data which are read from a memory. A memory write buffer high MWBH and a memory write buffer low MWBL are temporary registers for latching data to be written in a memory. A memory address buffer high MABH and a memory address buffer low MABL are temporary registers for latching addresses at the time of a memory access. A MULT buffer MLTB is a temporary register for transferring a multiplier and a multiplicant to the multiplication block MULT. The connection relations of the inside and outside of the central processing unit CPU through the buses are as follows. Specifically, letters MTBL and MTBH are bilateral special purpose buses for connecting the multiplication block MULT. In Figs. 6 and 7, letters IDBH and IDBL correspond to the data buses IDB31-0 of Fig. 2, and letters IABH and IABL correspond to the address buses IAB23-0 of Fig. 2. The values of the aforementioned program counters PCH and PCL are outputted to the address buses IABH and IABL, and the instruction buffers IRH and IRL fetch the data from the data buses IDBH and IDBL so that the outputs of the temporary registers MWBH and MWBL are fed to the data buses IDBH and IDBL. The temporary registers MRBH and MRBL input the data from the data buses IDBH and IDBL and the special purpose buses MTBH and MTBL. The address informations latched by the temporary registers MABH and MABL are outputted to address buses IABH and IABL. The multiplying temporary register MLTB has its output fed to the special purpose buses MTBH and MTBL. [7] Pipe Line Stage by CPU Fig. 8 shows one example of a stage of a pipe line processing by the central processing unit CPU. This central processing unit CPU has a basic pipe line structure of five stages having the following basic phases: IF: Instruction Fetch; ID: Instruction Decode; Ex: Execute; MA: Memory Access; and WB: Write-Back. In Fig. 8 showing one example of the execution content of each pipe stage, the Address Bus correspond to the address buses IAB23-0 of Fig. 2, and the Data Bus correspond to the IDB31-0 of the same. Letters IR of Fig. 8 correspond to the instruction buffers IRH and IRL of Figs. 6 and 5. In Fig. 8, letters A-Bus, B-Bus, C-Bus and D-Bus are the A bus, B bus, C bus and D bus of Fig. 7, respectively. Likewise, letters MAB and MRB of Fig. 8 are the MABH, MABL, MRBH and MRBL of Fig. 7. [8] Pipe Line Sequence in Register Conflicting State The pipe line sequence in the aforementioned register conflicting state will be described with reference to Fig. 9. First of all, the meanings of signals shown in the same Figure will be described in the following. The waveforms of the aforementioned non-overlap two-phase clock signals  1 and  2 acting as operation reference clock signals are shown at the top of Fig. 9. One cycle is defined as a period starting from the rise of the clock signal  1 and ending the next rise of the signal  1. Subsequently, the states of an address bus and a data bus are shown. Next letters IRLatch designate a latch signal of an instruction buffer (IR(32 bits) or IRH and IRL). The IRLatch presents an input latch signal waveform of the IR1 register. The IR1 (16 bits) latches the instruction which is present at an instruction decode stage. The aforementioned hardware sequence control instruction register HIR is a register for latching a partially decoded micro code, a sequence information or a pipe line control information. Letters Reg. Content Flag appearing in Fig. 9 designate a flag indicating it necessary to check the conflict between an LOAD instruction and an instruction using the execution unit EXEC. This conflict is checked in a 4th cycle to set a Content Flag (or C flag). At the same time, there is set a LOAD Flag (or L flag) indicating it necessary to load an operand. Likewise, there is set in the 4th cycle a Bus Cycle Flag (or B flag) indicating a bus operation necessary. This flag indicates whether or not the bus cycle is being executed. An instruction fetch inhibit flag (i.e., IF Inhibit Flag: I Flag) is one indicating that an instruction fetch is interrupted and replaced by a data access. A skip signal (i.e., Skip Sig.) is a flag meaning that a processing to be executed in the execution unit EXEC in that cycle is canceled. The Execution indicates a processing to be executed in the execution unit EXEC. The Reg. Write is a signal to be written in a register. In response to the Reg. Write in the ordinary operation, the destination register, as instructed, latches through the C-Bus. At the time of executing the LOAD instruction and the MULT instruction, the destination register, as instructed, latches through the D-Bus. In this meaning, the signal Reg. Write is shown as divided those for the C-Bus and the D-Bus in the timing chart so that the writing operation is executed in preference of the C-Bus if the two signals Reg. Write for the C-Bus and D-Bus conflict in the same register. In short, only the write from the C-Bus is executed. The signal written in the 5th cycle, as indicated dotted lines, indicates the write in the register which is inhibited by the Skip Sig. The MAB means a memory address bus for outputting an address when a data access is executed. The MRB Latch meaning a memory read buffer is a signal for latching data. The PC indicates the value of a program counter. Fig. 9 is a timing chart exemplifying the sequences between LOAD instructions (LOAD @R1, R2) and ADD instructions (ADD R2, R3). The register R2 for the LOAD instruction to latch the data and the register R2 for the ADD instruction to use are so common that the value of the register R2 is used for the operations between its value determined if the instruction execution is performed in the ordinary pipe line flow. In this example, what timing the pipe line control is to be executed at is shown over seven cycles when the uses of the registers R2 conflict. The lowermost column indicates the situations of execution of the pipe line. Since the register R2 of the destination of the LOAD instruction and the source register R2 of the ADD instruction conflict, a stall (or delay) occurs at the 5th cycle, as shadowed. For this stall, it becomes necessary at first to detect whether or not the register conflicting state takes place and to recognize how many cycle the execution cycle (EX) is to be delayed for avoiding the register conflict. The former detection is carried by asserting the signal S1 outputted by the aforementioned register content check block RCCB for comparing the register selecting information contained in the preceding instruction outputted by the aforementioned register WBR1 and the register selecting information (e.g., the information contained in the ADD instruction for selecting the register R2 according to this example) contained in the succeeding instruction. The latter recognition can be achieved from the decoded result of the operation code. Since the number of the execution cycle (EX) of the LOAD instruction is one in the shown example, the stall occurs only in the 5th cycle. Fig. 10 shows another example of the pipe line sequence in the register conflicting state. Fig. 10 is a timing chart exemplifying the sequences among the MULT instructions (MUL R1, R2) as multiplying instructions, the ADD instructions (ADD R4, R3) and the SUB instructions (SUB R3, R2). The register R2 for the MUL instruction to latch the data and the register R2 to be used by the SUB instructions are common. Unless the register conflicting state is reflected upon the pipe line sequences, it occurs that the register R2 is used for another operation before its value is determined if the instruction is executed. In the present example, what timing the control of the pipe line is executed at in case of such conflict of register uses is shown over seven cycles. The format of expression of the present Figure is similar to that of Fig. 9 and illustrates the executions of multiplications in four cycles, although not detailed. The MULT instructions are executed in four stages EX, ML, ML and ML. The multiplier can execute the multiplications for latching the result of 16b*16b in 32b in the four cycles. These calculations can be executed by determining the partial product of 16b*4b and their cumulative sum for each cycle. In case of this example, the SUB instruction is fetched in the register R1 with a delay of 2 cycles from the MULT instruction, and the execution cycle (EX) of the SUB instruction is delayed by two cycles because the MULT instruction are multiplied in the four cycles EX, ML, ML and ML. [9] Pipe Line Sequence at Memory Access Conflicting Time Fig. 11 shows a pipe control sequence exemplifying the case, in which a data fetch from a memory and an instruction fetch conflict. In this case, the data fetch is preferred so that the instruction execution schedule containing a conflicting instruction fetch is shifted in its entirety. For this control, the instruction fetch wait flag (IF Wait Flag) is set to delay the start of the instruction fetch cycle while the load flag (LOAD Flag) and the bus cycle flag (Bus Cycle Flag) are conflicting. [10] Sequence of Instruction Execution of Plural Cycles Fig. 12 is a timing chart showing one example when a plurality of cycle instructions are executed. Here will be explained by way of example an instruction "AND.B #imm, @R1" or a kind of AND instructions (i.e., logical product). This is an instruction for calculating the logical product between the 8 bit data of a memory selected relative to the register R1 and the 8 bit immediate data. This AND.B instruction is a plurality cycle instructions to be executed in response to the macro instruction 1, the micro instruction 1 and the micro instruction 2. The aforementioned macro instruction 1 is an instruction for fetching a byte operand from the area of a memory, which is selected according to the content of the register R1. The aforementioned micro instruction 1 is an instruction for taking an AND of the aforementioned byte operand and the immediate data. The aforementioned micro instruction 2 is an instruction for writing the byte operand in the area of a memory, which is selected according to the content of the register R1. The execution content of the aforementioned AND.B instruction is described in the C language: ANDM(int i)/*AND.B#imm:8, @R1*/ With reference to this description, there are idle cycles between the ID (Instruction Decode) stage and the EX (Execution) stage of the micro instruction 1 and between the µ -IF (Micro Instruction Fetch) stage and the ID stage of the micro instruction 2. This is because the operand fetched at the MA (Memory Access) stage of the macro instruction 1 has to be used at the EX (Execution) stage of the micro instruction 1. [11] Instruction Assignment of CPU The instructions to be assigned to the central processing unit CPU are: data transfer instructions shown in Figs. 13 and 14; logical operation instructions shown in Fig. 15; arithmetic operation instructions shown in Figs. 16 and 17; shift instructions shown in Fig. 18; branch instructions shown in Fig. 19; and system control instructions shown in Figs. 20 and 21. Fig. 22 explains the description formats of Figs. 13 to 21. According to these formats, the items of the instructions in Figs. 13 to 21 are mnemonically indicated. The addressing modes in this mnemonic indications are tabulated in Fig. 23. As apparent from the various instruction codes, all of the integer calculations, branching methods and control instructions of the general purpose register system can be selected even in the 65,536 combinations which can be expressed in the instruction format of fixed length of 16 bits. The decoding can be realized with less logical expressions by devising the bit assignments to group instructions of similar functions. An instruction array having the operation code starting from "1111" is wholly reserved so that calculations of single or double accuracy can be selected in conformity with the IEEE floating point standards. [12] Displacement Length of Branch Instruction Figs. 24, 25, 26 and 27 plot the relations between the displacement lengths of branch instructions and instructions in various programs extracted as samples and the appearance frequencies of the instructions having those displacement lengths. Figs. 24 and 25 relate to conditional branch instructions (i.e., branch instructions) and unconditional branch instructions (i.e., branch always instructions); Fig. 26 relates to subroutine call instructions; and Fig. 27 relates to jump instructions or jump subroutine instructions. Here, the "branch" is to select one of numerous instruction sets which can be selected in the execution of a computer program. The "jump" means a departure from the implicit or specific execution order of instructions, which is actually done in the execution of the computer program. The "displacement" is used to select a jumped address. For the more bit number of the displacement length, therefore, the farther address can be jumped to. The frequency distributions of the displacement in response to the branch instructions, as shown in Figs. 24 to 26, are the data which were obtained by analyzing the various programs of Microcomputer H8/500 of Hitachi, Ltd. These Figures illustrate the distributions of the displacement values of the individual kinds of the branch instructions used. The abscissa indicates the log 2 values of the used displacement values. At the righthand of the origin 0, the log 2 {i.e., displacement} is expressed by a positive integer (1, 3, 5, 7, - - -) in case the displacement value is positive. At the lefthand, the -log 2 {i.e., -displacement} is expressed by a negative number for a displacement having a negative value. The ordinate indicates the appearance frequency at the unit of %. The data were sampled for nine different programs. As could be apparent from Figs. 24 and 25, the branch instructions and the branch always instructions having the higher appearance frequencies are distributed the closer to the center so that the distribution can be substantially covered with a displacement of 8 bits. It could also be found that the distribution of the subroutine call instructions of Fig.. 26 can be wholly covered with a displacement field of 12 to 13 bits although it is considerably wide. In case of the jump instructions or jump subroutine instructions shown in Fig. 27, moreover, the value of the abscissa takes such a displacement as is defined to mean the difference between the address, at which the jump instruction is present, and the address of the jump destination. It could be found that the jump destination is also far. In the microcomputer MCU of the present embodiment, the displacement of the conditional branch instructions is fixed at 8 bits, and the displacement of the subroutine branch instructions and the unconditional branch instructions is fixed at 12 bits so that those instructions are confined in the instruction format of fixed length of 16 bits. In the various branch instructions shown in Fig. 19, for example: the individual instructions BC, BT and BF are made to belong to the conditional branch instructions; the instruction BSR in the same Figure is made to belong to the subroutine branch instructions; and the instruction BRA is made to belong to the unconditional branch instructions. The detailed contents of the individual instructions should be referred to the description items of instructions, as follows. In the modular programming method for preparing a program as a set of relatively small subroutines (or functions), the conditional branch instructions will jump within the functions. Since most functions have a size as large as several hundreds bytes, the distribution can be substantially covered with the displacement of 8 bits. On the other hand, the subroutine branch has a tendency to jump to the outside of a function itself, i.e., to a far place so that it requires a displacement of a larger bit number than that of the condition branch instruction. The unconditional branch may be used for calling another function at the last of functions so as to accelerate the program. Since it seems advantageous that the unconditional branch be handled similarly to the subroutine branch condition, the bit number of the displacement is equalized to that of the subroutine branch. Thus, the fixed assignment of the proper displacement length according to the kind of the branch instruction contributes to realization of the instruction format of fixed length of 16 bits without any substantial trouble. [13] Processing of Immediate Data In case the instruction format of 16 bit fixed length is adopted, it is not practical to limit all the immediate values to 16 bits or less in view of the fact that the data word length is 32 bits. In the present embodiment, a method of using the value of a register such as the program counter PC and the relative address is adopted so as to select the immediate values of 16 bits or more within one instruction format. The instructions for the immediate processing are exemplified by the load instructions shown in Fig. 13, such as MOV.W@(disp, PC)Rn or MOV.L@(disp, PC)Rn. These instructions are those for storing the immediate data in the general purpose register Rn. If the data are words/long words, there are referred to the data in a table stored in the address which is specified by adding the displacement to the program counter PC. If the data are words, the displacement is shifted leftward by 1 bit to 9 bits so that the relative distance from the table is changed from -256 to +254 bytes. The program counter PC is the head address which is behind the present instruction by two instructions. This makes it necessary to arrange the word data at the boundary of 2 bytes. If the data are the long words, the displacement is shifted leftward by 2 bits to 10 bits so that the relative distance from the operand is changed from -512 to +508 bytes. The program counter PC is the head address which is behind the present instruction by two instructions, but its less significant 2 bits are corrected to B and 00. This makes it necessary to arrange the long word data at the boundary of 4 bytes. [14] Implicit Register Selection The implicit register selection is said to select a general purpose register fixed as an operand notwithstanding that no register selecting field is present in an instruction. The general purpose register, as specified herein, is used for determining a memory address, for example, or for storing the data fetched from a memory. The instruction for this implicit register selection can be exemplified by the MOV @(disp, R1)R0 or MOV R0, @(disp, R1), as shown in Fig. 14. As is apparent from the codes corresponding to the instruction in the same Figure, the instruction contains only the operation code and the displacement dddddddd of 8 bits but not the register selecting field. This displacement is used for determining a memory address together with the value of the implicitly selected register R1. Thanks to this implicit register selection, even in an instruction requiring the value of the register and the displacement, the instruction word length can be restricted within 16 bits without any reduction in the bit number required as the displacement. [15] Functionally Composite Instruction The functionally composite instruction can be exemplified by a bit operation instruction such as AND.B #imm, @R1 shown in Fig. 15. This instruction is one composed of three instructions for taking a logical product (i.e., AND operation) between the 8 bit data of a memory selected relative to the register R1 selected implicitly like before and the immediate data of 8 bits to execute the reading of the memory, the AND operation and the write return of the result of the AND operation in said memory. The operation of this kind appears highly frequently in controlling the packaged devices, and the adoption of such functionally composite instruction in the instruction format of 16 bit fixed length contributes to an improvement in the code efficienty. [16] Truth/False Setting Instruction for Selected Condition An instruction for setting the truth/false of the arithmetic result for a selected condition can be exemplified by eight kinds of CMP instructions shown in Fig. 16, for example. These are instructions for comparing operands to set the comparison result to the T (True) bit of the aforementioned status register SR. For example, the instructions as designated at COMP/EQ. Rm and RnFF compare whether or not the values of the registers Rm and Rn are equal, and set the T bit to 1, if YES, but clear the same to 0. By assigning the T bit to the status register so that the operation of setting the truth/false for the compared result to the T bit may be executed in response to one instruction of the instruction format of 16 bit fixed length, the next instruction such as the aforementioned conditional branch instruction BT for the operation based on the resultant truth/false may refer directly to the T bit. Thus, the description of the condition necessary for the arithmetic result according to the preceding instruction need not be made in said BT instruction itself so that the area of the displacement necessary for the BT instruction can be accordingly enlarged in the limited instruction format of fixed length. As a result, this structure contributes to realization of the instruction format of 16 bit fixed length. [17] List of Instructions The featuring ones of the instructions of having the formats of 16 bit fixed length have been representatively described hereinbefore. In order to clarify the whole aspect of the instruction format of 16 bit fixed length, all the instructions of the microcomputer of the present embodiment will be further described sequentially in the alphabetical order. The descriptions of the individual instructions include the names of instructions, the formats (wherein "imm" and "disp" designate numerical values or symbols) expressed by the input formats of the assembler, the notes for using the instructions, the descriptions of the operations expressed by the C language, the operation examples (indicating the states before and after the instruction executions) exemplified assembler-mnemonically, and the codes. Before the descriptions of the individual instructions, here will be described the register structures as the programmer's models to be noted when a program is to be executed, with reference to Fig. 28. The registers as the programmer's models are exemplified not only by the aforementioned general purpose registers R0 (i.e., R0H, R0L) to R15 (i.e., R15H, R15L) but also by control registers such as a status register SR, a procedure register PR (i.e., PRH, PRL), a global base register GBR (BGRH, GBRL), a program counter PC (PCH, PCL), a vector base register VBR (VHRH, VBRL) or a break register BR (BRH, BRL). In the example of Fig. 28: the register R0 is an accumulator; the register R1 is an index register; and the register R15 is a stack pointer. In the status register SR of Fig. 28: the M/Q bit is referred to in the DIV instruction; the I bit is a mask bit; the sign "-" is a reserved bit; the D bit is referred to in the PASS instruction; the C bit is one reflecting on the carry/borrow/overflow/underflow/shift-out; and the T bit is one expressing the truth (1) and the false (0), as has been described hereinbefore. The descriptions of the operations, whose contents are indicated in C, assume the use of the following resources, although not especially limitative thereto: unsigned char Read_Byte (unsigned long Addr); unsigned short Read_Word (unsigned long Addr); unsigned long Read_Long (unsigned long Addr); The contents of the individual sizes of the address Addr are returned. The reads of the words other than the address (2n) and the long words other than the address (4n) are detected as address errors. unsigned char Write_Byte (unsigned long Addr, unsigned long Data); unsigned short Write_Word (unsigned long Addr, unsigned long Data); unsigned long Write_Long (unsigned long Addr, unsigned long Data); The data Data are written with their individual sizes in the address Addr. The writes of the words other than the address (2n) and the long words other than the address (4n) are detected as address errors. Delay_Slot (unsigned long Addr); The slot instruction of the address (Addr-4) is executed. This means that the execution is shifted for Delay_Slot(4) to the instruction of not the 4th address but the 0-th address. If, moreover, the execution is to be shifted from that function to the following instructions, these are detected as the slot invalid instructions immediately before the execution: BC, BF, BT, BRA, BSR, JMP, JSR, RTS, BRK, PASS, RTB, RTE, TRAP unsigned long R[16]; unsigned long CR[3]; unsigned long PC; unsigned long BR; unsigned long VBR; Individual Register Bodies #define SR (CR[0]) #define PR (CR[1]) #define GBR (CR[2]) Correspondences between Proper Names of Registers and Indications of CRn Formats Definitions of SR Structures #define M ((*(struct SR0 *)(&CR[0])).M0) #define Q ((*(struct SR0 *)(&CR[0])).Q0) #define D ((*(struct SR0 *)(&CR[0])).D0) #define C ((*(struct SR0 *)(&CR[0])).C0) #define T ((*(struct SR0 *)(&CR[0])).T0) Definitions of Bits in SR #define BRKVEC 0x0000001C #define PASSVEC 0x00000014 Definitions of Vector Addresses #define SHAL() SHLL() Definitions of Identical Instructions of Different Names Error (char *er); Error Indicating Function In addition, it is assumed that the PC indicates the address of 4 bytes (i.e., 2 instructions) before of the instruction being executed. This means that (PC=4;) indicates a shift of execution from the instruction of not the 4th address but the 0th address. In the example of use described in the assembler mnemonic format, the codes indicate the binary codes. In the mnemonic: the following expansions are made in codes: r: rrrr R: RRRR imm: iiiiiiii disp: dddd or dddddddd or dddddddddddd. The SS in the MOV instruction code is expanded in the following manner in accordance with the operand size: 01 = Byte 10 = Word 11 = Long Word. ADD (Addition) Instruction Format: ADDRm, Rn ADD #imm, Rn Description: The contents of the general purpose registers Rn and Rm are added, and the result is stored in the Rn. The general purpose register Rn and 8 bit immediate data can be added. The 8 bit immediate data are code-expanded to 32 bits so that they are used with subtractions. Operation: Example of Use: ADDC (Addition with Carry) Instruction Format: ADDC Rm, Rn Description: The contents of the general purpose registers Rn and Rm and the C bit are added, and the result is stored in the Rn. According to the arithmetic result, the carry reflects upon the C bit. Operation: Example of Use: Format: ADDS Rm, Rn Description: The contents of the general purpose registers Rn and Rm are added, and the result is stored in the Rn. Even if an overflow occurs, the result is limited to a range of H'7FFFFFFF to H'80000000. At this time, the C bit is set. Operation: Example of Use: Format: ADDV Rm, Rn Description: The contents of the general purpose registers Rn and Rm are added, and the result is stored in the Rn. If an overflow occurs, the C bit is set. Operation: Example of Use: Format: AND Rm, Rn AND #imm, R0 AND.B #imm, @R1 Description: The logical product of the general purpose registers Rn and Rm are added, and the result is stored in the Rn. A special type can be taken from either the logical product between the general purpose register R0 and the zero-expanded 8 bit immediate data or the 8 bit memory relative to the R1 and the 8 bit immediate data. Note: In the AND #imm, R0, the more significant 24 bits of the R0 are always cleared as a result of the operation. Operation: Example of Use: Format: BC disp BF disp BT disp Description: This instruction is a conditional branch one referring to a T/C bit. For T = 1, the BT is branched, but the BF executes a next instruction. For T = 0, on the contrary, the BF is branched, but the BT executes a next instruction. The BC is branched for C = 1 but executes a next instruction for C = 0. The destination of branch is an address, in which the displacement is added to the PC. This PC is the leading address of the instruction which is behind the present instruction by two instructions. Since the displacement is shifted leftward by 1 bit to have 9 bits, the relative distance from the branch destination ranges from -256 to +254. If the branch destination fails to be reached, this is coped with by the combination with the BRA instruction or the JMP instruction. Operation: Example of Use: Format: BRA disp Description: This is an unconditional delay branch instruction. The branch destination is located at the address which is the addition of the displacement to the PC. This PC is the leading address which is behind the present instruction by two instructions. Since the displacement is shifted leftward by 1 bit to 13 bits, its relative distance from the branch destination ranges from -4096 to +4094. If the branch destination fails to be reached, a change has to be made to the JMP instruction. At this time, the branch destination address has to be transferred to the register in response to the MOV instruction. Note: Because of the delay branch instruction, the instruction immediately after the present instruction is executed prior to the branch. No interruption is accepted between the present instruction and the instruction immediately after. This instruction immediately after is recognized as an invalid one if it is a branch instruction. Operation: Example of Use: Format: BRK Description: A break exceptional process is started. Specifically, after an acknowledge signal is returned to an external device, the PC is relieved to the BR to make a branch to a break interrupt routine in accordance with a predetermined vector. The PC is the leading address of the present instruction. The content of the VBR is independent of the calculation of a vector address. The VBR is combined with the RTB and is used in a break routine call. Note: The present instruction will not accept an interruption. The BR is used together with a hardware break caused by a break terminal. Hence, any break redemand during the breaking should be avoided. The PC but not the SR is protected by the BR. The SR has to be relieved, if necessary, to a register or memory in response to the STC instruction. Operation: Example of Use: Format: BSR disp Description: This makes a branch to a selected address procedure. The content of the PC is relieved to the PR and is branched to the address which is the additions of the displacement to the PC. This PC is the leading address which is behind the present instruction by two instructions. Since the displacement is shifted leftward by 1 bit to 13 bits, its relative distance from the branch destination ranges from -4096 to +4094. If the branch destination fails to be reached, a change has to be made to the JSR instruction. At this time, the branch destination address has to be transferred to the register in response to the MOV instruction. The JSR instruction is combined with the RTS and is used for a procedure call. Note: Because of the delay branch instruction, the instruction immediately after the present instruction is executed prior to the branch. No interruption is accepted between the present instruction and the instruction immediately after. This instruction immediately after is recognized as an invalid one if it is a branch instruction. Operation: Example of Use: Format: CLRC Description: The C bit of the SR is cleared. Operation: Example of Use: Format: CLRT Description: The T bit of the SR is cleared. Operation: Example of Use: Code: Format: CMP/cond Rm,Rn Description: The general purpose registers Rn and Rm are compared. If the result reveals that a selected condition (cond) holds, the T bit of the SR is set. If NO, the T bit is cleared. The content of the Rn is unchanged. Eight conditions can be selected. For the two conditions of the PZ and the PL, the Rn and 0 are compared. Mnemonic Description CMP/EQ Rm,Rn T=1 for Rn=Rm CMP/GE Rm,Rn T=1 for coded values Rn ≧ Rm CMP/GT Rm,Rn T=1 for coded values Rn> Rm CMP/HI Rm,Rn T=1 for uncoded values Rn > Rm CMP/HS Rm,Rn T=1 for uncoded values Rn≧Rm CMP/PL Rn T=1 for Rn> 0 CMP/PZ Rn T=1 for Rn≧ 0 CMP/STR Rm,Rn T=1 if any byte is equal Operation: Example of Use: Format: DIV1 Rm,Rn DIVOS Rm,Rn DIVOU Description: The content of 32 bits of the general purpose register Rn is subjected to one-step division with the content of the Rm. The DIVOS is an intialization instruction for a coded division to store the MSB of the dividend (Rn) in the Q bit, the MSB of the divisor (Rm) in the M bit, and the EOR of the M bit and the Q bit in the C bit. The DIVOU is an initialization instruction for a uncoded division to clear M/Q/C bits to zero. A quotient is obtained by repeating the DIV1 (in combination with the ROTCL, if necessary) by the number of times of the bit number of the divisor. In this repetition, an intermediate result is latched in M/Q/C bits of the assigned register and the SR. The arithmetic result is not warranted if the procedures are unnecessarily rewritten by a software. The sequence of the division can be referred to the following examples of use. The zero division, the detection of the overflow and the arithmetic of the remainder are not prepared. Operation: Example 1 of Use: Example 2 of Use: Example 3 of Use: Example 4 of Use: Format: EOR Rm, Rn EOR #imm, R0 EOR.B #imm, @R1 Description: An exclusive OR is taken between the content of the general purpose register Rn and the Rm, and the result is latched in the Rn. A special form can be exemplified by either an exclusive OR between the general register R0 and the zero-extended 8 bit immediate data or an exclusive OR between an 8 bit memory and an 8 bit immediate data relative to the R1. Operation: Example of Use: Format: EXTS.B Rm,Rn EXTS.W Rm,Rn Description: The content of the general purpose register Rm is code-extended, and the result is latched in the Rn. In case of byte assignment, the content of the bit 7 of the Rm is copied from the bit 8 to the bit 31. In case of word assignment, the content of the bit 15 of the Rm is copied from the bit 16 to the bit 31. Operation: Example of Use: Format: EXTU.B Rm,Rn EXTU.W Rm,Rn Description: The content of the general purpose register Rm is zero-extended, and the result is latched in the Rn. In case of byte assignment, the value 0 is inserted into the bits 8 to 31 of the Rn. In case of word assignment, the value 0 is inserted into the bits 16 to 31. Operation: Example of Use: Format: JMP @Rn Description: A delay branch is unconditionally made to an assigned address. The branch destination is the address which is expressed with the 32 bit data of the content of the general purpose register Rn. Note: An instruction immediately after the present instruciton is executed prior to the branch because of a delay branch instruction. No interruption is accepted between the present instruction and the instruction immediately after. This after instruction is recognized as an invalid one if it is a branch instruction. Operation: Example of Use: Format: JSR @Rn Description: A branch is made to a procedure of an assigned address. The content of the PC is resumed to the PR, and the content of the general purpose register Rn is branched to the address which is expressed with 32 bit data. The PC is a leading address which is behind the present instruction by two instructions. The PC is combined with the RTS and is used for the procedure call. Note: An instruction immediately after the present instruciton is executed prior to the branch because of a delay branch instruction. No interruption is accepted between the present instruction and the instruction immediately after. This after instruction is recognized as an invalid one if it is a branch instruction. Operation: Example of Use: Format: LDC Rm, CRn LDC.L @Rm+, CRn Description: A source operand is latched in the control register CRn. Note: No interruption is accepted by the present instruction. Operation: Example of Use: Format: LDBR Description: The content of the general purpose register R0 is latched in the control register BR. Note: No interruption is accepted by the present instruction. Operation: Example of Use: Format: LDVR Description: The content of the general purpose register R0 is latched in the control register VBR. Note: No interruption is accepted by the present instruction. Operation: Example of Use: Format: MOV #imm, Rn MOV.W @(disp,PC),Rn MOV.L @(disp,PC),Rn. Description: Immediate data are latched in the general purpose register Rn. The data are code-expanded to 32 bits. If the data are a word/long word, reference is made to the data in the table, which are latched by adding the displacement to the PC. If the data are a word, the displacement is shifted leftward by 1bit to 9 bits so that the relative distance from the table is -256 to +254 bytes. The PC is the leading address which is behind the present instruction by two instructions. This makes it necessary to arrange the word data in the boundary of 2 bytes. If the data are a long word, the displacement is shifted leftward by 2 bits to 10 bits, the relative distance from the operand is -512 to +508 bytes. The PC is the leading address, which is behind the present instruction by two instructions, and its less significant two bits are corrected to B, 00. This makes it necessary to arrange the long word data in the boundary of 4 bytes. Note: The table is not automatically skipped so that it is interpreted as an instruction without any countermeasure. In order to avoid this, the table has to be arranged either at the head of a module or after the unconditional branch instruction. If, however, the table is carelessly arranged immediately after the BSR/JSR/TRAP, a simple return will conflicts with the table. When the present instruction is arranged immediately after the delay branch instruction, the PC is the leading address +2 of the branch destination. The table access of the present instruction is a target of the instruction cache. Operation: Example of Use: Format: MOV.L @(disp,Rm), Rn MOV.L Rm, @(disp,Rn) Description: The source operand is transferred to the destination. The memory operand is present in the stack fame so that the data size is limited to the long word. Hence, the displacement is shifted leftward by 2 bits to 6 bits so that bytes of -32 to +28 can be assigned. If the memory operand fails to be reached, the ordinary MOV instruction is used. However, restrictions arise to fix the register to be used. Operation: Example of Use: Format: MOV @ (disp,GBR),R0 MOV R0, @(disp,GBR) Description: The source operand is transferred to the destination. The data size of the memory operand can be assigned within the range of the byte/word/long word. The base address of the I/O is set to the GBR. If the data of the I/O is the byte size, the displacement has 8 bits so that it can be assigned to a range of -128 to +127 bytes. In case of the word size, the displacement is shifted leftward by 1 bit to 9 bits so that it can be assigned to a range of -256 to +254 bytes. In case of the long word size, the displacement is shifted leftward by 2 bits to 10 bits so that it can be assigned within a range of -512 to +508 bytes. If the memory operand failed to be reached, the ordinary MOV instruction is used. If the source operand is a memory, the loaded data are code-extended to 32 bits and are latched in the register. Note: For the loading, the destination register is fixed to R0. Hence, the reference to the R0 is caused, if desired in response to an instruction immediate after, to wait for the end of the execution of the load instruction. The optimization is required, as will correspond in the following items (1) and (2) , by changing the sequence of the orders: Operation: Example of Use: Format: Description: The source operand is transferred to the destination. If the operand is a memory, the data size to be transferred can If the source operand is a memory, the loaded data are code-extended to 32 bits and are latched in a register. If the data of a memory has a byte size in the @(disp,R1) mode, the displacement has 8 bits so that it can be assigned to a range of -128 to +127 bytes. In case of the word size, the displacement is shifted leftward by 1 bit to 9 bits so that it can be assigned to a range of -256 to +254 bytes. In case of the long word size, the displacement is shifted leftward by 2 bits to 10 bits so that it can be assigned within a range of -512 to +508 bytes. Note: In the @(disp,R1) mode, the other operand is fixed in the R0. Hence, in case of the load instruction, like the I/0 data transfer instruction, the optimization can be effected, as will correspond in the following items 1 ○ and 2 ○, by changing the sequence of the orders: Operation: Example of Use: Format: MOVA @(disp,PC),R1 Description: The valid address of the source operand is latched in the general purpose register R1. Since the displacement is shifted leftward by 2 bits to 10 bits, the relative distance from the operand ranges from -512 to +508 bytes. The PC is the leading address behind the present instruction by two instructions and has its less significant 2 bits corrected to B'00. Hence, the source operand has to be arranged in the boundary of 4 bytes. Note: When the present instruction is arranged immediately behind the delay branch instruction, the PC becomes the leading address +2 of the branch destination. Operation: Example of Use: Format: MULS Rm,Rn Description: The content of the general purpose register Rn and the Rm are multiplied in 16 bits, and the result of 32 bits is latched in the Rn. The calculation is accomplished in the coded arithmetic operation. Operation: Example of Use: Format: MULU Rm,Rn Description: The content of the general purpose register Rn and the Rm are multiplied in 16 bits, and the result of 32 bits is latched in the Rn. The calculation is accomplished in the uncoded arithmetic operation. Operation: Example of Use: Format: NEG Rm,Rn Description: A complement of 2 is taken from the content of the general purpose register Rm, and the result is latched in the Rn. The Rm is subtracted from 0, and the result is latched in the Rn. Operation: Example of Use: Format: NEGC Rm,Rn Description: The content of the general purpose register Rm and the C bit are subtracted from 0, and the result is latched in the Rn. The borrow is reflected on the C bit in accordance with the arithmetic result. Operation: Example of Use: Format: NOP Description: Only the increment of the PC is performed, and the execution is shifted to a subsequent instruction. Operation: Example of Use: Format: NOT Rm,Rn Description: A complement of 1 is taken from the content of the general register Rm, and the result is latched in the Rn. Operation: Example of Use: Format: OR Rm , Rn OR #imm, R0 OR.B #imm, @R1 Description: A logical sum is taken between the content of the general purpose register Rn and the Rm, and the result is latched in the Rn. It is also taken as a special form to take a logical sum between the general purpose register R0 and the zero-extended 8 bit immediate data or a logical sum between an 8 bit memory and an 8 bit immediate data in relation to the R1. Operation: Example of Use: Format: PASS #imm Description: This is a conditional software interruption instruction referring to the D bit. A debug interruption is issued for D = 1. For D = 0, on the contrary, only the increment of the PC is performed. If the debug interruption occur, the PC and the SR are released to the stack, and a branch is made to the address which is indicated with the content of a predetermined vector address. The PC is a leading address of an instruction subsequent to the present instruction. At the time of preparing a program, the present instruction is buried in the head of the routine so that the passage can be confirmed, if necessary, with the debug interruption by setting D = 1. The routine to be debugged can be decided by referring to the imm code in a debug interruption routine defined in advance. The imm code is a byte data having an address which is set by decrementing the PC on the stack by (-1). Note: The present instruction is recognized, if arranged just behind the branch instruction, as an invalid instruction irrespective of the value of the D bit. Operation: Example of Use: Format: ROTL Rn ROTR Rn Description: The content of the general purpose register Rn is rotated clockwise or counterclockwise by 1 bit, and the result is latched in the Rn. The bits thus rotated to go outside of the operand are transferred to the C bit. Operation: Example of Use: Format: ROTCL Rn ROTCR Rn Description: The content of the general purpose register Rn is rotated clockwise or counterclockwise by 1 bit with the C bit, and the result is latched in the Rn. The bits thus rotated to go outside of the operand are transferred to the C bit. Operation: Example of Use: Format: RTB Description: A return is made from the brake exception processing routine. Specifically, after the PC has been returned from the BR, an acknowledge signal is returned to an external device, and the processing is continued from the address indicated by the returned PC. Note: The acceptance of an interruption between the present instruction and the branch destination instruction can be controlled with an RTBMSK signal. With this RTBMSK being inputted, an external interruption such as NMI/IRQ is not accepted. (An address error or the like is accepted.) Operation: Example of Use: Format: RTE Description: A return is made from the interruption routine. Specifically, the PC and the SR are returned from the stack. The processing is continued from the address indicated by the returned PC. Note: Because of the delay branch instruction, an instruction immediately after the present instruction is executed before the branch. No interruption between the present instruction and the instruction immediately after. If the latter instruction is a branch one, it is recognized as an invalid instruction. It is necessary that the instruction should not be continuously arranged immediately after a load instruction to the R15. Reference will be erroneously made to the old R15 before the loading. The order of instructions has to be changed in a manner to correspond to the following items 1 ○ and 2 ○: Operation: Example of Use: Format: RTS Description: A return is made from the procedure. Specifically, the PC is returned from the PR. The processing is continued from the address indicated by the returned PC. With this instruction, a return can be made to the call origin from the procedure which was called by the BSR and JSR instruction. Note: Because of the delay branch instruction, an instruction immediately after the present instruction is executed prior to the branch. No interruption is accepted between the present instruction and the instruction immediately after. This instruction is recognized, if a branch one, as an invalid instruction. Operation: Example of Use: Format: SETC Description: The C bit of the SR is set. Operation: Example of Use: Format: SETT Description: The T bit of the SR is set. Operation: Example of Use: Format: SHAL Rn SHAR Rn Description: The content of the general purpose register is arithmetically shifted to the right/left by 1 bit, and the result is latched in the Rn. The bits shifted to the outside of the operand are transferred to the C bit. Operation: Example of Use: Format: SHLL Rn SHLR Rn Description: The content of the general purpose register Rn is logically shifted to the right/left by 1 bit, and the result is latched in the Rn. The bits shifted to the outside of the operand are transferred to the C bit. Operation: Example of Use : Format: SL2 Rn SR2 Rn SL8 Rn SR8 Rn SL16 Rn SR16 Rn Description: The content of the general purpose register Rn is logically shifted to the right/left by 2/8/16 bits, and the results are latched in the Rn. The bits shifted to the outside of the operand are disposed. Operation Example of Use: Format: SLP Description: The CPU is set to a low power consumption mode. In this low power consumption mode, the the CPU has its internal status retained to stop execution of the immediately after instruction and to await a demand for interruption. If the demand is issued, the CPU goes out of the low power consumption mode to start an exceptional processing. Specifically, the SR and the PC are released to the stack, and a branch is made to an interruption routine in accordance with a predetermined vector. The PC is the leading address of the instruction immediately after the present instruction. Operation: Example of Use: Format: STC CRm,Rn STC.L CRm ,@-Rn Description: The control register CRm is assigned to the destination. Note: No interruption is accepted by the present instruction. Operation: Example of Use: Format: STBR Description: The content of the control register BR is latched in the general purpose register R0. Note: No interruption is accepted by the present instruction. Operation: Example of Use: Format: STVR Description: The content of the control register VBR is latched in the general purpose register R0. Note: No interruption is accepted by the present instruction. Operation: Example of Use: Format: SUB Rm,Rn Description: The Rm is subtracted from the content of the general purpose register, and the result is latched in the Rn. The subtraction with the immediate data is replaced by ADD #imm, Rn. Operation: Example of Use: Format: SUBC Rm,Rn Description: The Rm and the C bit are subtracted from the content of the general purpose register Rn, and the result is latched in the Rn. The borrow is reflected upon the C bit in accordance with the arithmetic result. Operation: Example of Use: Format: SUBS Rm,Rn Description: The Rm is subtracted from the content of the general purpose register Rn, and the result is latched in the Rn. Even if an underflow should occur, the result is restricted within a range of H'7FFFFFFF to H'80000000. At this time, the C bit is set. Operation: Example of Use: Format: SUBV Rm,Rn Description: The Rm is subtracted from the content of the general purpose register Rn, and the result is latched in the Rn. If an underflow occurs, the C bit is set. Operation: Example of Use: Format: SWAP.B Rm,Rn SWAP.W Rm,Rn Description: The more and less significant bits of the content of the general purpose register Rm are interchanged, and the result is latched in the Rn. If the bytes are assigned, the eight bits 0 to 7 of the Rm and the eight bits 8 to 15 are interchanged. If latched in the Rn, the more significant 16 bits of the Rm are transferred as they are to the more significant 16 bits of the Rn. If the words are assigned, the 16 bits 0 to 15 and the 16 bits of 16 to 31 of the Rm are interchanged. Operation: Example of Use: Format: TAS.B @Rn Description: The content of the general purpose register Rn is used as an address, and T = 1 if the byte data indicated by the address is zero but otherwise T = 0. After this, the bit 7 is set to 1 and written. In this meanwhile, the bus priority is not released. Operation: Example of Use: Format: TEST Rm, Rn TEST #imm, R0 TEST.B #imm, @R1 Description: An AND is taken between the content of the general purpose register Rn and the Rm, and the T bit is set if the result is zero. The T bit is cleared if the result is not zero. The content of the Rn is not changed. As a special form, there can be taken either an AND between the general purpose register R0 and the zero-extended 8 bit immediate data or an AND between the 8 bit memory in relation to the R1 and the 8 bit immediate data. The content of the memory is not changed. Operation: Example of Use: Format: TRAP #imm Description: The trap exceptional processing is started. Specifically, the PC and the SR are released to the stack and branched to the address indicated with the content of the selected vector. This vector is the memory address itself, which is determined by shifting the immediate data leftward by 2 bits for the code extension. The PC is the leading address which is behind the present instruction by two instructions. The PC is combined with the RTE for a system call. Note: The instruction immediately after the present instruction is executed prior to the branching because of a delay branch. No interruption is accepted between the present instruction and the subsequent instruction. This subsequent instruction is recognized as an invalid one if it is a branch instruction. The subsequent instruction has to be arranged not consecutively just after the load instruction from the memory to the R15. The old R15 before the loading is erroneously referred to. The order change of instructions has to be accomplished to correspond to the following 1 ○and 2 ○: Operation: Example of Use: Address Format: XTRCT Rm,Rn Description: The central 32 bits are extracted from the content of 64 bits linking the general purpose registers Rm and Rn, and the result is latched in the Rn. Operation: Example of Use: Although the description thus far made is directed to the case in which our invention is applied to a microcomputer having a program ROM packaged therein and presenting the field of application backgrounding the invention, the invention should not be limited thereto but can also be applied widely to a microcomputer containing neither the program ROM nor its peripheral circuit. The effects to be obtained by the representatives of the invention disclosed herein will be briefly described in the following. (1) In a microcomputer adopting the general purpose register method, there is adopted a fixed length instruction format which has a smaller bit number than that of the maximum data word length fed to instruction execution means. As a result, there is achieved an effect that it is possible to provide a microprocessor which has a small program capacity or a high using efficiency of program memory and a low system cost while enjoying the advantage of simplification of such an instruction decoding as exemplified in the RISC machine of the prior art having a fixed length instruction format of 32 bits or the like. (2) The fixed length instruction format and the maximum data word length are set to a bit number of a power of 2 so that the maximum data word length is 32 bits, for example, the instruction format is fixed to 16 bits. Thus, another effect achieved is that either the instruction or the data can be extended over the ROM/RAM having an identical bit structure while preventing the misalignment of the program on the memory and holding the consistency with the general memory commercially available. (3) When the aforementioned fixed length instruction format and the aforementioned maximum data word length are to be individually set'to the bit numbers of powers of 2, a plurality of instructions efficiently can be efficiently fetched by the use of an internal bus of a bit number equal to that of the maximum data word length, and the bus access number for the instruction fetch can be reduced by prefetching the instructions in a common cycle. (4) In case the internal bus is shared between the data transfer and the instruction fetch, the pipe control is executed to prefer the data fetch thereby to delay the whole instruction execution schedule including an instruction fetch conflicting with that data fetch, if the data fetch and the instruction fetch for the memory conflict. (5) A further effect achieved is that the restriction on the bit number of the fixed length instruction format is enabled not to limit the use of immediate data by supporting the instruction containing a description for assigning the immediate data in a data relation for offsetting the value of a displacement relative to the value of a predetermined register. (6) A further effect achieved is that, even in the fixed length instruction format having a restricted bit number, the displacement necessary for the processing or the bit number of the immediate data can be maximized by supporting an'instruction for implicitly assigning a predetermined general purpose register which is fixed as an operand despite of no register assigning field in the instruction. (7) A further effect achieved is that the displacement necessary for the processing or the bit number of the immediate data can be maximized, even for the fixed length instruction format having a limited bit number, by supporting an instruction containing a description for reflecting the truth or false of the arithmetic result for a specified condition upon a predetermined status flag. (8) Thanks to these effects, it is possible to solve the various problems accompanying the adoption of the fixed length instruction format having a smaller bit number than that of the data word length. For example: neither the use of immediate data nor the assignment of an absolute address is restricted even if the bit number of the fixed length instruction format is less than that of the data word length; a description such as a necessary displacement can be executed in the fixed length instruction format having a limited bit number; a contribution is made to the prevention of a misalignment of the program arrangement on a memory; that the code efficiency or memory using efficiency is improved better from the standpoint of the content of a supporting instruction. (9) The conflicting state between the uses of the general purpose registers in response to the instructions before and after the pipe-line execution making use of the general purpose register method, can be simply coped with by detecting the register conflicting state in the general purpose register on the basis of the information of a register assigned area contained in the instruction format and by adopting the pipe-line control means for relatively delaying execution of the subsequent Instructions on the basis of the detection result and the instruction execution cycle number. (10) For a 16 bit fixed length instruction format, the displacement of a condition branching instruction is fixed at 8 bits, and the displacements of a subroutine branching instruction and an unconditional branching instruction are fixed to 12 bits. As a result, a proper branching destination can be assigned according to the kinds of the branching instruction without exerting substantial affections upon the actual operation while the instruction format length being restricted.

Claims: A CPU (CPU) for processing instructions from an instruction set, comprising an instruction register (IRH, IRL, IR1, IR2) for receiving instructions from a memory (ROM, CACHE), and a program counter (PC) for storing an address of an instruction to be fetched to said instruction register, wherein the instruction set includes at least one conditional branch instruction (BC, BF, BT) and at least one unconditional branch instruction (BRA, BSR) which have the same bit length and each include a displacement area (disp) holding a displacement to be added to the content of the program counter to obtain an address to be jumped to by said conditional or unconditional branch instructions, and wherein the bit width of the displacement area (disp) of the conditional branch instruction is different from the bit width of the displacement area (disp) of the unconditional branch instruction. A CPU according to claim 1, which has a RISC architecture. A CPU according to claim 1 or 2, wherein the bit width of the displacement area (disp) of the conditional branch instruction is smaller than that of the unconditional branch instruction. A CPU according to any of claims 1 to 3, wherein the bit length of said conditional and unconditional branch instructions is 16 bits. A CPU according to claim 4, wherein the bit width of the displacement area (disp) of the conditional branch instruction is 8 bits and that of the unconditional branch instruction is 12 bits. A CPU according to any of claims 1 to 5, which is formed on a semiconductor chip. A CPU according to any of claims 1 to 6, wherein the memory is a Read Only Memory (ROM). A CPU according to any of claims 1 to 6, wherein the memory is a cache memory (CACHE). A computer program product comprising instruction code for execution by a CPU according to any of claims 1 to 8, said instruction code comprising a conditional branch instruction (BC, BF, BT) having a displacement area (disp) holding a displacement to be added to the content of the program counter to calculate the destination address of a conditional branch, and an unconditional branch instruction (BRA, BSR) having the same bit length as said conditional branch instruction and including a displacement area of a different bit length than the displacement area of the conditional branch instruction, to be added to the content of the program counter to calculate the destination address of said unconditional branch.

Inventors: AKAO YASUSHI, ASANO YOICHI, BABA SHIRO, CHAKI HIDEAKI, FUJITA SHUYA, FUKADA KAORU, ISHIDA KATSUHIKO, KAINAGA MASAHIRO, KASHIWAGI YUGO, KAWASAKI SHUMPEI, KIHARA TOSHIMASA, KURAKAZU KEIICHI, MASUMURA SHIGEKI, NOGUCHI KOKI, SAKAKIBARA EIJI, SAWA NORIKO, SUGAWARA TADAHIKO, TAWARA YASUHIRO, TSUKAMOTO TAKASHI, YAMAZAKI TAKANAGA, AKAO, YASUSHI, ASANO, YOICHI, BABA, SHIRO, CHAKI, HIDEAKI, FUJITA, SHUYA, FUKADA, KAORU, ISHIDA, KATSUHIKO, KAINAGA, MASAHIRO, KASHIWAGI, YUGO, KAWASAKI, SHUMPEI, KIHARA, TOSHIMASA, KURAKAZU, KEIICHI, MASUMURA, SHIGEKI, NOGUCHI, KOKI, SAKAKIBARA, EIJI, SAWA, NORIKO, SUGAWARA, TADAHIKO, TAWARA, YASUHIRO, TSUKAMOTO, TAKASHI, YAMAZAKI, TAKANAGA, Akao, Yasushi Hitachi, Ltd., Intel. Prop. Group, Asano, Yoichi Hitachi Ltd., Intel. Prop. Group, Baba, Shiro Hitachi, Ltd., Intel. Prop. Group, Chaki, Hideaki Semiconductor &amp; I.Circuits, Fujita, Shuya Semiconductor &amp; I.Circuits, Fukada, Kaoru Semiconductor &amp; I.Circuits, Ishida, Katsuhiko Semiconductor &amp; I.Circuits, Kainaga, Masahiro Semiconductor &amp; I.Circuits, Kashiwagi, Yugo Semiconductor &amp; I.Circuits, Kawasaki, Shumpei Semiconductor &amp; I.Circuits, Kihara, Toshimasa Semiconductor &amp; I.Circuits, Kurakazu, Keiichi Semiconductor &amp; I.Circuits, Masumura, Shigeki Hitachi Ulsi Systems Co,, Noguchi, Koki Semiconductor &amp; I.Circuits, Sakakibara, Eiji Hitachi Ltd., Intel. Prop.Group, Sawa, Noriko Hitachi Ltd., Intel. Prop. Group, Sugawara, Tadahiko Hitachi,Ltd. Intel. Prop.Group, Tawara, Yasuhiro Semiconductor &amp; I.Circuits, Tsukamoto, Takashi Hitachi Ulsi Systems Co,, Watabe, Mitsuru Hitachi Ltd., Intel. Prop. Group, Yamazaki, Takanaga Semiconductor &amp; I.Circuits
Applicants: HITACHI LTD, HITACHI ULSI SYS CO LTD, HITACHI ULSI SYSTEMS CO.,LTD., HITACHI, LTD.
Application year: 1992
Main classification: G06F9
IPC codes: G06F9, G06F7