Abstract:
An instruction fetch control system prefetches a branch instruction in a pipeline system and fetches a branch target instruction of the branch instruction. The control system comprises a first branch judgement circuit for conducting a branch condition judgement in a stage prior to the branch judgement stage in which a second and original branch judgement of the branch instruction is conducted, and a circuit for starting a prefetch of instructions following said branch target instruction without waiting for the branch judgement stage where the first branch judgement circuit judges that the branch is successful.

Description:
This application is a continuation of application Ser. No. 07/457,561, filed Dec. 27, 1989, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a control system for fetching an instruction group following a branch target instruction when a branch instruction is executed in a computer system using a one-cycle pipeline system and more particularly, to an instruction fetching control system for performing a prior branch judgement by judging whether a condition judgement can be performed in a cycle preceding a branch judgement cycle in which the branch instruction is executed. 
     FIG. 1 shows a view for explaining an execution of a branch instruction in a prior-art one-cycle pipeline system. The numbers ( 1 ) to ( 12 ) at the top of the drawing represent respective timings of machine cycles and process part symbols D, A, T, B, E, and W of respective instructions represent respective processing states (which are also called “cycles” or stages hereinafter) in the pipeline control. 
     D indicates a decoding cycle for decoding an instruction to be executed. A is an address calculating cycle for performing an address calculation to fetch the branch target instruction, T is an address translating cycle for translating a logical address into a real address, B is a cycle for fetching a branch target instruction, E is a branch judging cycle for performing a judgement as to whether the branch condition is successful or not and W is a cycle for storing an arithmetic operation result. These cycles are well known. 
     In the pipeline system, an instruction to be executed is prefetched. The prefetch of the executed instruction is also conducted in a pipeline manner. The periods for period part symbols IA, IT and IB in the instruction prefetch pipeline in the lower part of FIG. 1 represent process cycles for prefetching an instruction following the branch instruction and IA*, IT* and IB* represent process cycles for prefetching the instruction following the branch target instruction. For example, address calculating cycle IA, address translating cycle IT and cycle IB for fetching data from a buffer in the instruction prefetch pipeline shown in process IPP, is for fetching fetching instruction {circle around (N)}+3 where the branch of branch instruction B is not successful. 
     In FIG. 1, symbols “Next”, shown in the lower part of FIG. 1, represent the kind of the instruction to be prefetched. These symbols show that the instruction (also called “Next side” of the part) following the branch instruction is prefetched. “Target” indicates that an instruction (called “Target side” of the part) following the branch target instruction is prefetched and “(Next)” shows the case in which the instruction following the branch instruction is prefetched and the case in which the instruction is not fetched are selected. 
     Branch instruction {circle around (B)} is first executed in the example shown in FIG.  1  and instruction sets {circle around (N)}, {circle around (N)}+1, {circle around (N)}+2 and {circle around (N)}+3 following the branch instruction {circle around (B)} are then executed sequentially. The branch condition is successful in the branch judging cycle (E state) of branch instruction {circle around (B)} in timing step ( 5 ) and branch target instruction {circle around (T)} and instruction sets {circle around (T)} {circle around (T)}+1, {circle around (T)}+2, . . . following the branch target instruction are executed. FIG. 1 shows the case where the number of branch target instructions {circle around (T)}, {circle around (T)}+1, {circle around (T)}+2 obtained by one time fetch of the branch instruction carried out during B state of branch instruction {circle around (B)} is three and the branch is carried out depending on the result of the branch judgement during E state of branch instruction {circle around (B)} and the execution of branch target instructions {circle around (T)}, {circle around (T)}+1, {circle around (T)}+2 is started smoothly. 
     In a pipeline system, as recited above, an instruction prefetch is processed as a pipeline. Thus, it is necessary to determine the fetch address at which an address calculation IA should be conducted in a stage which is two cycle prior to stage IB in which a prefetch of an instruction is actually carried out, as shown in FIG.  2 . FIG. 2 shows a view for explaining an instruction prefetch pipeline. Period part symbols IA, IT, IB show respective process stages. IA designates an address calculation cycle (ADRS), IT represents an address translation cycle (TLB/TAG) and IB represents an instruction fetching cycle (FETCH). 
     Generally, i.e., without being limited to a branch instruction, it is necessary to determine the addresses of instructions “Next 1” and “Next 2” to be prefetched in the IA cycle two cycles prior to the IB cycle in which an instruction is actually fetched. 
     Where the pipeline process is carried out as shown in FIG.  1  and an instruction prefetch is executed in the form shown in FIG. 2, then, there is the problem that an instruction prefetch, which should be carried out in the same cycle at timing ( 5 ) in FIG. 1 as branch judging cycle (E state) of branch instruction {circle around (B)} in FIG. 1 or in the next cycle (timing ( 6 ) and ( 7 ) in FIG.  1 ), is conducted as to a part (“target side”) following branch target instruction {circle around (T)} or a part (“next side”) following branch instruction {circle around (B)}. 
     As shown in FIG. 3, in the example (No. 1) where a target side instruction fetch does not exist when the branch target instruction starts, the branch condition of branch judging cycle (E state) of branch instruction {circle around (B)} is successful and only T of one machine instruction of the branch target occurring during B state (timing ( 4 ) in FIG. 3) is fetched, branch target instruction {circle around (T)} can then be executed during timing ( 5 ) but, at this time instruction {circle around (T)}+1 of Target Side is not yet fetched, and thus, execution of instruction {circle around (T)}+1 cannot start during timing ( 6 ). 
     In order to solve this problem, the means for fetching the branch target instruction {circle around (T)} together with instructions {circle around (T)}+1, {circle around (T)}+2 . . . , following the branch target instruction in B state of branch instruction {circle around (B)} is considered. In this case, the amount of instruction which can be fetched by one fetch operation, is about 1 to 2 instruction lengths, although the amount of instruction depends on the method of forming a central processor unit (CPU) and an instruction length. As shown in the example (No. 2) where the target side instruction is not fetched upon a start of the branch target instruction shown in FIG.  4  and where two instructions comprising branch target instruction {circle around (T)} and {circle around (T)}+1 are fetched, instruction {circle around (T)}+2 of the target side is not fetched at timings ( 5 ) and ( 6 ). Therefore, instruction {circle around (T)}+1 cannot be executed in timing ( 7 ) and the central processor unit (CPU) falls into a state of waiting for an instruction to be executed to be fetched. 
     In order to solve the problem, as shown in FIG. 4, for example, it is necessary to determine the fetch address, at least upon the timing ( 4 ) when an instruction {circle around (T)}+2 of a target side is fetched in timing ( 6 ). The determination of the fetch address uses a result of an address calculation at an A state of branch instruction {circle around (B)}. Thus, it is not necessary to wait for an E cycle to determine whether the branch condition is successful or not. However, when the target side instruction fetch is driven, it is necessary thereafter to add an extra hardware to switch the process to an instruction fetch of a part following the branch instruction. 
     Namely, when the branch target instruction address is maintained and the instruction fetch following the branch target instruction is conducted, the means for updating this instruction fetch, namely, the means for calculating an address of an instruction following the branch target instruction, is necessitated independently of the address calculating means of the instruction following the branch instruction of the next side. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an instruction fetching control system for effectively prefetching an instruction set following the branch target instruction, where the branch instruction is a non-condition branch instruction or an instruction in which the branch can be predicted. 
     A condition branch instruction for judging the value of the flag set (or condition code) which is updated in accordance with an execution of the operation instruction, represents the branch instruction. 
     Where an operation instruction is provided immediately before the condition branch instruction and the flag sets is updated in E state of the operation instruction, the branch judgement cannot be carried out at least until the E state of the branch instruction is executed. This is because the updating of the flag sets in the operation instruction is usually executed after an operation cycle. 
     However, where an operation instruction is not executed immediately before it, the judgement can be carried out without waiting for the original judgement cycle (i.e. E cycle) of the branch instruction. 
     On the other hand, some branch instructions are branched without any conditions and the resulting non-condition branches are determined based on an instruction code of the branch instruction. For example, in commercially available systems, BAL (branch and link) instructions and BAS (branch and safe) instructions are non-condition instructions and they can be judged based on the instruction code. The BC (branch condition) instruction is a condition branch instruction, but where bits  8  to  11  of the instruction code are X′F′ (hexa decimal number) and bits  12  to  15  are not X′O′, the process performs non-condition branch. Where bits  8  to  11  or bits  12  to  15  are X′O′, the process is not branched. 
     In this case, the process is branched without any condition, whether or not the branch condition is successful or unsuccessful. Thus, a control can be effectively moved to fetch an instruction set following the branch instruction. 
     This is conducted to drive a head address in which instruction {circle around ( 1 )} of buffer memory  15  is stored. 
     The present invention provides an instruction fetching control system for executing and prefetching an instruction and for prefetching the branch target instruction of the branch instruction prior to the branch judgement cycle. The system comprises means for performing a prior branch judgement based on an instruction code within the branch instruction prior to the branch judgement cycle of the pipeline when the above instruction is executed, means for detecting whether the condition judgement can be carried out in the preceding cycle and for performing the branch judgement, and means for starting a prefetch of instruction sets following the above branch target instruction without waiting for the branch judgement cycle in the pipeline of the branch instruction when the branch judgement means determines that the branch is successful. 
     When the branch instruction is executed, it is judged whether the branch judgement can be carried out in the cycle preceding the branch condition judgement cycle of the pipeline. This is done in order to perform an instruction prefetching when the branch instruction is executed, thereby performing the branch judgement. If the branch is successful, the instruction of a part following the branch target instruction upon the next cycle of the decode start of the branch target instruction is fetched to the instruction register and the instruction decode is carried out in a pipeline manner. Thus, the central processor unit (CPU) is prevented from falling into a state in which it must wait for an instruciton fetch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a view for explaining an execution of a branch instruction in a one cycle pipeline, 
     FIG. 2 shows a view for explaining an instruction prefetch pipeline, 
     FIG. 3 is a view representing an example (No. 1) where an instruction fetch on a target side is not carried out when a branch target instruction starts, 
     FIG. 4 is a view for explaining an example (No.2) where the instruction fetch on the target side does not occur when the branch target instruction starts, 
     FIG. 5 is a view for explaining an execution of the non-condition branch instruction in a one cycle pipeline, 
     FIG. 6 is a block diagram representing an embodiment of the present invention, 
     FIG. 7 is a view for explaining the case where a general instruction is executed in the embodiment of the present invention, 
     FIG. 8 is a view for explaining the case where the condition branch instruction is executed in the embodiment of the present invention, 
     FIG. 9 is a view for explaining the case where the non-condition branch instruction is executed in the embodiment of the present invention, 
     FIGS. 10A,  10 B and  10 C show concrete circuits of branch judgement circuit  5 , 
     FIG. 11 is a view representing an example of a structure of the branch condition selection circuit, 
     FIG. 12 is a truth-value table of the branch condition selection circuit, and 
     FIG. 13 is view for explaining a branch prediction cycle. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 5 shows a view for explaining an execution of the non-condition branch instruction in one pipeline cycle according to the present invention. In this example, branch target instructions {circle around (T)} {circle around (T)}+1, {circle around (T)}+2 are fetched for every instruction. 
     In FIG. 5, a stage of timing ( 2 ), an address calculation (shown by symbol a in FIG. 5) of branch target instruction {circle around (T)} is carried out in A state of branch instruction {circle around (B)}. Then a branch judgement (shown by a symbol b in FIG. 5) is made based on the instruction code of branch instruction {circle around (B)}, as to whether or not the branch is a non-condition branch or whether or not the branch is predictable. When the branch is judged to be successful, an address calculation in process state IA*( 1 ) in an instruction prefetch pipeline in a stage of timing ( 3 ) is performed for branch target instruction {circle around (T)}+1, as shown by symbol c in FIG. 5, and an address calculation is performed in a process state IA*( 2 ) in a stage of timing ( 4 ) for branch target instruction {circle around (T)}+2(shown by symbol d in FIG.  5 ). 
     In timing stage ( 5 ), a fetch of first instruction {circle around (T)}+1 of the target side can be conducted. Thus, instruction {circle around (T)}+1 following head instruction {circle around (T)} of the branch target, and also instruction {circle around (T)}+2 following the instruction {circle around (T)}+1 of the branch target, can be carried out smoothly. 
     FIG. 6 shows an embodiment of the present invention. FIG. 7 shows a view for explaining an execution of a general instruction in the embodiment shown in FIG.  6 . FIG. 8 is a view for explaining the execution of a branch instruction when the branch cannot be predicted. FIG. 9 is a view for explaining the execution of a branch instruction when the branch prediction can be performed. 
       1  of the embodiment shown in FIG. 6 is an instruction prefetch buffer used for instruction fetch,  2  is an instruction register,  4  is a selector for switching an ordinary instruction and branch target instruction using branch judgement circuit  23 ,  5  is a branch judgement circuit for an instruction code according to the present invention,  6  is a general register set (GR),  7  is a register set (XR, BR, D) for calculating an address,  8  is an address adder,  9  is a register (LAR#T) for receiving the result of an address adder,  10  and  11  are flags (F 1  and F 2 ) for maintaining the result of branch judgement circuit  5  to be matched with a progress of a pipeline,  12  is an address translating circuit for performing an address translation based on the value of register (LAR#T)  9 ,  13  is a register for maintaining a result of an address translation circuit,  14  is the register (LAR#B) for maintaining the value of register (LAR#T)  9  without being translated and shifting it by one timing,  15  is a buffer memory,  16  is a register (PAR#E) for maintaining the value of register (PAR#B)  13  to be matched with a progress of a pipeline and to be shifted by one timing,  18  is a register (PAR#W) for maintaining the value of register (PAR#E)  16  to be matched with a progress of a pipeline by being shifted by one timing,  19  is a register for receiving an operand of an arithmetic operation from buffer memory  15 ,  20  is a register for receiving an operand of an arithmetic operation from general register set  6 ,  21  is an arithmetic logical unit circuit,  22  is a register for maintaining an operation result,  23  is a conventional branch judgement circuit,  30  is a register (LIA #A) for maintaining an instruction prefetch address, and  31  is a selector for selecting an instruction length of the instruction which is prefetched. The input signals {circle around ( 1 )}, {circle around ( 2 )} and {circle around ( 3 )} to the selector are explained below. 
     Signal {circle around ( 1 )} represents an amount of fetch of an ordinary instruction prefetch. Signal {circle around ( 2 )} represents an instruction length of the branch target instruction to be fetched in instruction prefetch buffer  1  from buffer  15  by a branch instruction for determining the branch in branch judgement circuit  5 . Signal {circle around ( 3 )} represents an instruction length of a branch target instruction to be fetched to the instruction prefetch buffer  1  from buffer  15  by the branch instruction for determining the branch in branch judgement circuit  23 . 
       32  represents a selector of an instruction fetch address, which operates in the same manner as selector  31 . With regard to signals {circle around ( 1 )}, {circle around ( 2 )} and {circle around ( 3 )} of selector  31 , signal â selects a content of register  30  in accordance with an ordinary instruction prefetch, signal {circle around (b)} selects a content of register  9  where the branch is determined by branch judgement circuit  5 , and signal ĉ selects a content of register  14  where the branch is determined by branch judgement circuit  23 , respectively. 
       33  is a circuit for forming a selection control signal based on the result of branch judgement circuit  5  or branch judgement circuit  23 . Branch judgement circuit  23  is controlled with a priority whereby the result of branch judgement circuit  5  and branch judgement circuit  23  simultaneously show the branch. 
       34  is an address adder,  35  is a register (LIA #T) for receiving a result of an address adder,  36  is an address translation circuit,  37  is a register (PIA=B) for receiving the result of an address translation. 
       50  is a decoder circuit for determining in D cycle that the instruction sets a condition code,  51  is a flag for maintaining an output of decoder circuit  50 ,  52  is a flag for maintaining the output from flag  51 ,  53  is a flag for maintaining an output from flag  52 ,  54  is a flag for maintaining the output of flag  53 ,  60  is a timing judgement circuit for detecting a timing of a production of the condition code and a branch judgement using the condition code,  70  is a condition code generating circuit and  71  is a condition code flag. 
     For convenience of explanation, three general registers  6  and three buffer memories  15  are shown in FIG. 6, but the general registers  6  are the same device and the buffer memories are the same device. 
     A state of an instruction execution in the present invention is explained as being divided into a case of a general instruction, a case of the branch instruction where the branch prediction is impossible and a case of an execution of the branch instruction where the branch instruction can be predicted, in accordance with three separate sections. 
     Further, examples of the branch prediction will be explained following the above explanations. 
     (1) The case of executing a general instruction. 
     FIG. 7 shows a view for explaining the case where a general instruction is carried out in the embodiment of the present invention shown in FIG.  2 . By referring to FIG. 7, execution of a general instruction, namely, execution of an instruction which is not the branch instruction, will be explained. 
     In this computer&#39;s initial state, an address of first executed instruction {circle around ( 1 )} is set in register (LIA #A)  30  for maintaining the instruction prefetch address. This address value is “IA” designated by symbol a in FIG.  7 . The signal {circle around ( 1 )} representing a fetch amount of the instruction prefetch which is input to selector  31  is “0” and corresponds to an address of the head region of buffer memory  15  in which instruction {circle around ( 1 )} is stored. At this time, both branch judgement circuits  5  and  23  show a non-branch condition. 
     In timing ( 1 ), the value “0” of signal {circle around ( 1 )} selected by selector  31  is added by added  34  to the value “IA” in register (LIA #A)  30  and is set to register (LIA#T)  35  and register (LIA #A)  30 , as indicated by symbol b in FIG.  7 . Namely, this is conducted to fetch a head address of buffer memory  15  in which instruction {circle around ( 1 )} is stored. 
     In timing ( 2 ), an address (IA) stored in register (LIA #T)  35  is translated to a real address by address translation circuit  36  and the result is set in a register (PIA #B, shown by symbol “PIA” in FIG.  7 ), which is shown by symbol “c” in FIG.  7 . 
     Simultaneously the amount (or length) of instruction fetched by this address appears in signal {circle around ( 1 )} (the value designated by a symbol “e”) and a calculation of the next instruction fetch address is carried out in adder  34 . The result (the value shown by “IA+l” in FIG. 7) is set in register (LIA#T)  35  and register (LIA #A)  30 , shown by symbol “d” in FIG.  7 . This operation is carried out to fetch an address of a region of the buffer memory  15  in which instruction {circle around ( 2 )} is stored. 
     Access to the addresses IA, IA+l, and IA+2l in which instructions {circle around ( 1 )}, {circle around ( 2 )} and {circle around ( 3 )} are stored is shown below. Until instruction fetch buffer  1  is filled by a prefetch instruction train, operation of this instruction fetch starts at every cycle (or with an interval). The values stored in register (LIA #A)  30  and register (LIA #T)  35  are incremented from IA to IA+2l, to IA+3l . . . to IA+nl. The value stored in register (PIA#B)  37  is incremented in this same manner, from PIA+1l to PIA+2l . . . to PIA+nl. Therefore, by sequentially, in every l address, reading an instruction from an address of buffer  15  in which instruction {circle around ( 1 )} is stored, a pipeline operation is executed. 
     Next, in timing ( 3 ), buffer memory  15  is accessed by the value “PIA” of register (PIA#B)  37  and the result (instruction {circle around ( 1 )} in FIG. 7) is set in instruction prefetch buffer  1  as shown by symbol “e” in FIG.  7 . At this time, if instruction prefetch buffer  1  is empty, because of an initial state, or if the instruction prefetch buffer  1  is not empty, the instruction is stored in instruction prefetch buffer  1  in the order of the instructions in sequence, and when the next cycle starts, the instruction is set in instruction register  2 . 
     In timing ( 4 ), decoding of instruction {circle around ( 1 )} within instruction register  2  starts (D state). Namely, in accordance with the value of instruction register  2 , general register set (GR) 6  is accessed and the value of the register necessary for an address calculation is set in register set (XR, BR, D) 7 . 
     In this embodiment, the value of the general register of the register number designated by the X 2  portion and the P 2  portion in the instruction code is read into registers (XR and BR) within a register set (XR, BR, D) 7  and the D 2  portion in the instruction code is set in register (D) in the register set (XR, BR, D) 7  without suffering any change. Generally, the address in instruction register  2  is a logic address. As a preprocess of translating the logical address to a real address, the base address is obtained from general register  6  by X 2  and B 2  of the address portion and the displacement D 2  of the address portion is added to them by address adder  8 . 
     When decoding of an instruction is completed, the next instruction (instruction {circle around ( 2 )} in FIG. 7) is set in instruction register  2  from instruction prefetch buffer  1 . 
     If the instruction which has started is a branch instruction, a branch judgement is performed by branch judgement circuit  5  based on the instruction code or the immediately prior state of the pipeline, and the result is set in flag (F 1 )  10 . If the instruction is not the branch instruction, the output of branch judgement circuit  5  is “0” as set by flag (F 1 )  10 . (Execution of the branch instruction will be explained later.) 
     In the next timing ( 5 ), operand address calculation is carried out (A state) in accordance with the value of register set (XR, BR, D)  7 . Namely, the value of register (XR,BR,D) is added by address adder  8  and the addition result (the value shown by symbol “OA {circle around (1)}” in FIG. 7) is set in register (LIR #T) 9 . The content of flag (F 1 )  10  is then moved to flag (F 2 )  11  and in timing ( 5 ) the decode of the next instruction {circle around ( 2 )} starts (D state). 
     In the next timing ( 6 ), the value (OA {circle around ( 1 )}) of register (LA#T) 9  is translated by address translating circuit  12 , and the result (the value shown by symbol “POA {circle around (1)}” in FIG. 7) is set in register (PAR#B)  13  (T state). The value of register (LAR#T) 9  is also set in register  14  (LAR#B). In the next timing ( 7 ), buffer memory  15  is accessed, in accordance with the value of register (PAR#D)  13  (B state). The fetch data is set in register  19 . General register set  6  is accessed simultaneously by a signal (not shown) obtained by keeping the content of part of the address portion of instruction register  2  until the B cycle is begun and the result is set in register  20 . The value (POA {circle around ( 1 )}) in register (PAR#B), are set in the (PAR#E)  16 . 
     The next timing ( 8 ) is the operation cycle (E state) and the result obtained by arithmetic operation circuit  21 &#39;s calculation of the values of registers  19  and  20  is set in register  22 . Namely, the data of buffer memory  15  is, for example, added to the data of general register set  6 . The value of register (PAR#E)  16  is set in register (PAR#W)  18 . 
     In the next timing ( 9 ), a storing cycle (W state) operates to store the value of register  22  in buffer memory  15  and general register set  6 . At this time, the value (POA {circle around ( 1 )}) of register (PAR#W)  18  is used for accessing the buffer memory. 
     Instruction {circle around ( 2 )} is executed in accordance with a similar sequence. 
     The general instruction is executed as recited above. It is a matter of course that the access to the buffer memory and the access to the general register set cannot be conducted depending on a definiton of the instruction. 
     (2) The case of executing a branch instruction where the branch cannot be predicted. 
     Execution of a condition branch instruction (also referred to as a “branch instruction”) will first be explained by referring to FIG.  8 . 
     In the case of the branch instruction, the instruction fetch can be conducted in the same manner as in the general instruction case, as explained in FIG. 7, and the decode (D state) of branch instruction {circle around (B)} starts in timing ( 4 ) in FIG.  8 . 
     In this case, branch judgement circuit  5  performs a branch judgement based on an instruction code and an immediately prior pipeline state, and “0” (which designates that the branch prediction is impossible), is outputted (as shown by symbol “a” in FIG.  8 ). Address calculation and address translation by general register set (XR, BR, D)  7 , address adder  8  and address translation circuit  12  are conducted in the same sequence as shown in FIG.  7 . Namely, in timing ( 6 ) the value of register (LAR#T)  9  (shown by a symbol “b” and “TA {circle around (B)}”) is subjected to an address translation and the result (PTA {circle around (B)}) is set in register (LAR#B)  13  (shown by symbol “c” in FIG.  7 ). 
     In the next timing ( 7 ), namely in B state, the fetch data from buffer memory  15  serves as an instruction for the branch target and is set in instruction register  2 . Branch judgement circuit  23  carries out a branch judgement of branch instruction {circle around (B)} in the B state (timing  7 ). 
     If the judgement result is non-branch, then selectors  31  and  32 , instruction registers  2  and selector  4  in an instruction prefetch circuit, selects an ordinary process using the same sequence as in the case of a general instruction other than the branch instruction. 
     On the other hand, when the branch is successful, selector  4  selects a branch target instruction which is set in instruction register  2  to start an execution of branch target instruction {circle around (T)}, as shown by symbol “d” in FIG.  8 . Where the branch is successful as a result of the branch judgement of this timing, selector  4  selects a fetch data from buffer memory  15  to be set in instruction register  2 . Where a branch is not successful, selector  4  selects an instruction from instruction prefetch buffer  1  to be set in instruction register  2 . 
     Execution of the instructions {circle around (N)}, {circle around (N)}+1, {circle around (N)}+2, and {circle around (N)}+3 which start following branch instruction {circle around (B)} is interrupted and similarly, the instruction prefetch sequence which is being executed is also interrupted (as shown by portion expressed as a pipeline process sequence and as shown by a broken line in FIG.  8 ). 
     When an output “1” of branch judgement circuit  23  (shown by the portion designated by symbol “h” in FIG.  8 ), selector  31  selects signal {circle around ( 3 )} and selector  32  selects signal ĉ, namely, register (LAR#B)  14 . Signal {circle around ( 3 )} shows the length of branch target instruction {circle around (T)} fetched in timing ( 7 ) and therefore an address of instruction {circle around (T)}+1) following the branch target instruction is outputted at the output of adder  34 . 
     The address is set in register (LIA#T)  35  in address “TA+l” designated by symbol “e” in FIG.  8 . The instruction fetch, in accordance with this address, is conducted in timing ( 9 ) (shown by a portion designated by symbol “f” in FIG.  8 ). The decoding of instruction ({circle around (T)}+1) starts in timing ( 10 ) (as shown by a portion designated by symbol “g” in FIG.  8 ). Namely, one cycle in which an instruction cannot start is caused between timing ( 8 ) of decode start cycle of branch target instruction {circle around (T)}, and timing ( 10 ) of the decode start cycle of the following instruction ({circle around (T)}+1), thereby preventing a pipeline operation. 
     In the case of executing a branch instruction where the branch cannot be predicted, the operation of the present embodiment is the same as that of the prior art. If this is intended to be improved, the selection of an instruction between the branch side and the ordinary side is required to be changed so that it does not occur before address adder  34 , but immediately before address translation circuit  36  or immediately before an access to buffer memory  15 . However, in order to achieve this change, a pair of address adders or a pair of address translation cicuits are necessary, thus requiring a great increase in the amount of hardware. This would result in an uneconomical device. 
     (3) Execution of a branch instruction where the branch is predicted. 
     The case of an execution of a branch instruction where the branch can be predicted will be explained by referring to FIG.  9 . In FIG. 9, as in FIG. 7, an instruction fetch of branch instruction {circle around (B)} is carried out and the branch instruction is decoded in timing ( 4 ) (as shown by the portion designated as symbol “a”). 
     In this instance, branch judging circuit  5  performs a branch judgement based on an instruction code or an immediately prior pipeline state and when the branch is predicted to be successful, “1” is outputted, otherwise “0” is outputted (as shown by the portion designated by symbol “b” in FIG.  9 ). Therefore, as in the case of the condition branch instruction in FIG. 8, an address calculation (A state), an address translation (T state) and a branch target instruction fetch (B state) are carried out in respective timings ( 5 ), ( 6 ) and ( 7 ). 
     In timing ( 8 ), namely in E state, selector  4  selects a branch target instruction by an output (shown by the portion designated by symbol “h” in FIG. 8) of branch judgement circuit  23 . Therefore, the decode of branch target instruction {circle around (T)} starts (shown by the portion designated by symbol “c” in FIG.  8 ). 
     A branch judgement is conducted in D cycle of branch instruction {circle around (B)}. Branch judgement circuit  5  outputs “1” when branch instruction {circle around (B)} is in D cycle. And then, the output is transmitted through flags  10  and  11 , and in timing ( 6 ) selector  31  selects signal {circle around ( 2 )} and selector  32  selects signal {circle around (b)}, namely reigster (LAR#P)  9 . Signal {circle around ( 2 )} is a signal designating the length of a branch target instruction which is expected to be fetched in timing ( 7 ) (i.e., a value represented by signal “l” in FIG.  9 ), and signal {circle around ( 2 )} is added to the content of register (LAR#T)  9  (the value designated by symbol “TA {circle around (B)}” in FIG. 9) in address adder  34 . Thus, an address (TA+l) of the instruction following the branch instruction appears in the output of adder  34 . 
     The address is set in register (LIA#T)  35  as shown by address (TA+l) designated by symbol “e” in FIG.  9 . The instruction fetch using this address is conducted in timing ( 8 ) as shown by the portion designated by symbol “f” in FIG.  9 . Then, the decode of instruction {circle around (T)}+l starts in timing ( 9 ) (as shown by a portion designated by symbol “g” in FIG.  9 ). 
     Therefore, when executing the branch instruction where the branch can be predicted, useless time is not created between the branch target instruction and the following instruction. 
     In timing ( 7 ), branch judgement circuit  23  transmits a signal for designating the success of the branch to selector circuit  33 . If as a result of this judgement, selectors  31  and  32  are switched in the same manners in the condition branch, then the instruction fetch of the instruction following the branch target instruction starts again, and thus, the instruction fetch of the instruction following the branch target instruction must be prevented from again starting. In order to prevent this, if a branch instruction enabling branch judgement circuit  5  to output “1” activates an instruction fetch of a portion following the branch target instruction, it is necessary for selector  33  to prevent an instruction fetch from being further activated by the following branch judgement. However, where the branch is simultaneously successful in branch judgement circuit  23  and branch judgement circuit  5  because of the reasons other than stated above, the condition of branch judgement circuit  23  has priority. This is because the instruction judged by branch judgement circuit  23  is executed prior to an instruction to be judged by branch judgement circuit  5 , and in case of a successful branch of the present branch instruction, the following branch instruction cannot be executed. 
     In FIG. 6, decoder  50  decodes a content of instruction register  2  to detect whether an instruction is for updating the condition code, thereby providing a flag designating whether or not the updating of the condition code exists, so that the flag is sequentially stored in flags  51  to  54 . Decoder  50 ′ decodes a content of instruction register  2  to detect whether an instruction is for updating the content of general register  6 , thereby providing a flag designating whether or not the updating of general register  6  exists, so that the flag is sequentially stored in flags  51 ′ to  55 ′. 
     FIG. 10A shows the concrete circuitry of branch judgement circuit  5 . The output of decoder instruction register  2  is decoded by decoder  80 . Decoder  80  decodes an instruction to detect whether the instruction is for a condition branch or for a non-condition branch. Further, gate  82  determines that a plurality of instructions which are transmitted in a continuous manner from an immediately preceding stage of the branch instruction, do not update the condition code, and gate  82 ′ determines that a plurality of instructions which are transmitted in a continuous manner from an immediately preceding stage of the branch instruction, do not update the content of the general register. The output of AND gate  82  is connected to AND gate  84  through AND gate  83 . As a result, in the case of a non-condition branch, or in the case of a predictable instruction even in the case of a condition branch, the output of branch circuit  5  is turned to “1”. AND gate  82  in timing judgement circuit  60  determines that all the flags  51 ,  52 ,  53  and  54  are in a timing for “0”. Namely, AND gate  82  detects that a plurality of, for example, four, continuous instructions immediately preceding the branch instruction do not update the condition code. AND gate  82 ′ of the timing circuit  60 ′ receives the signal from flags  51 ′ to  55 ′, which correspond to respectives timings of A, T, B, E, and W cycles in the pipeline. The output of AND circuit  82 ′ is connected to the input of AND circuit  83 ′. AND gate  82 ′ of timing circuit  60 ′ determines that all the flags  51 ′,  52 ′,  53 ′,  54 ′ and  55 ′ are in a timing for “0”. Namely, AND gate  82 ′ detects that a plurality of, for example, four, continuous instructions immediately preceding the branch instruction do not update the content of the general register for the timings of A, T, B, E and W cycles. 
     Circuit  83  next judges that the condition branch is successful and the output of the timing judgement circuit  60  is “1”. Then judgement circuit  81  determines that a specific relationship between condition code flag  71  and instruction register  2  has been established. In this case, the output of AND circuit  84  becomes “1” and then the instruction is determined as the predictable branch instruction. Alternatively, where decoder  80  determines that the judgement of the branch instruction should be made based on the value stored in general register  6 , for example, determines that the instruction is a counter branch instruction, further AND gate  82 ′ determines that a plurality of, for example, four, immediately preceding continuous instructions do not update the content of general register  6  as recited above, and checking circuit  81 ′ detects that the value of general register  6  is not “0000 0001” of the hexa decimal number, namely, that the branch is successful because the value of general register  6  is other than “1”. Then the output of AND circuit  84 ′ is turned to “1” and then the instruction is determined as the predictable branch instruction. Therefore, when the non-condition branch is successful or when the output of AND circuits  84  or  84 ′ is “1”, OR circuit  85  produces the output “1” and the output of branch judgement circuit  5  is determined as “1”. Therefore, it becomes possible for a branch judgement to be conducted prior to an original branch judgement stage. 
     Judgement circuit  81  performs a branch judgement based on information within the branch instruction (which appears as the output of instruction register  2 ) and the condition code (which is obtained when the operation result of arithmetic operation circuit  21  is outputted from condition code flag  71  through condition code generation circuit  70 ), when the output of instruction register  2  and the condition code are in a specific relation and the logic of judgement circuit  81  are usually subjected to an instruction definition by respective CPUs. 
     Therefore, judgement circuit  81  outputs a judgement flag (bit) when the branch is successful as a result of a branch instruction, based on the combination of an instruction code, condition code, and data circuit of judgement circuit  81 , as shown in FIG.  10 B. The condition becomes successful by the combination of 4 bits, M 1 , M 2 , M 3  and M 4 , for example, of the mask field of instruction register  2  for the branch instruction and the pattern transmitted from condition codes C 1  and C 2  from condition code flag  71 . For example, when M 4  is 1 and both C 1  and C 2  are (1, 1), the outputs of AND circuits  90  and  91  become “1”, thereby producing “1” output A through OR circuit  92 . Similarly, when C 1 =0 and C 2 =1 and M 2 =1; or C 1 =0, C 2 =0 and M 1 =1, the output A of OR circuit  92  becomes “1”. When the output of timing judgement circuit  60  is 1 simultaneously with the “1” output A, the output of branch judgement circuit  5  becomes “1”. Therefore, even in case of the condition branch, the process can be branched to an address designated by the address portion of the condition branch instruction and the branch can therefore be predicted. 
     FIG. 10C shows a detail circuit of judgement circuit  81 ′ which is used when the decoder  80  determines that the instruction is the counter branch instruction and which will be explained later. 
     Next, an example of selector  33 , as shown in FIG. 11, is described. In FIG. 11, numerals  40  and  41  show flags (F 2   a  and F 2   b ), numeral  42  shows an AND gate and numeral  43  shows an OR gate. In selector  33  of the branch judgement circuit shown in FIG. 11, the flag  11  in FIG. 6 is further divided into a flag (F 2   a )  40  and a flag (F 2   b )  41 , and is controlled to be set in flag  40  when the branch instruction is in the “B state” and to be set in flag  41  when the branch instruction is in the “E state” in accordance with the progress of the pipeline operation. The selction signal of selectors  31  and  32  shown in FIG. 6, is produced in accordance with the truth value table shown in FIG.  12 . 
     (4) Examples of the method for predicting a branch. 
     (a) by an instruction code. 
     The instruction capable of forming a branch judgement based only on an instruction code exists among the branch instructions. This judgement can detect the non-condition branch instruction as stated above and can detect the non-condition non-branch instruction where the non-condition non-branch instruction is included in the instruction set. 
     (b) by a relation between a set cycle of the flag set and a branch prediction cycle. 
     This is conducted by detecting that the value of the flag set set by the result of the arithmetic operation, which constitutes information used for branch judgement, cannot be changed from the branch prediction cycle of the branch instruction to the original branch cycle. The branch judgement can then be conducted based on the value of the flag set. 
     It is necessary to detect that the value of the flag set is not changed. 
     In FIG. 13, the newest value of the flag set in the timing ( 6 ) cycle in which the D state of the branch prediction cycle of branch instruction {circle around (B)} is carried out is the value updated by the E state of operation instruction  1 . 
     Then it is checked whether instructions  2  to  5  are the instructions which do not update the flag set. If they are, the value of the flag set in timing ( 6 ) is the same value as in timing ( 10 ), which is the original branch judgement cycle of branch instruction {circle around (B)}. 
     Therefore, the branch judgement can be carried out in timing ( 6 ). 
     If any of instructions  2  to  5  are instructions for updating the flag sets, the branch judgement cannot be carried out in timing ( 6 ) and is conducted in timing ( 10 ) which is the original branch judgement cycle. 
     The branch judgement uses the timing judgement circuit  60  (shown in FIG. 6) for detecting a timing between the production of the condition code. 
     In FIG. 13, instruction  1  is for updating the condition code (flag) while instructions  2 ,  3 ,  4  and  5  are not. 
     In the D cycle of instruction  1 , the setting of the condition code is decoded by decoder  50  to be set in flag  51 . The value is set sequentially in flags  52 ,  53 ,  54  and  55  in accordance with the execution of instruction  1 . 
     On the other hand, instructions  2 ,  3 ,  4  and  5  are respectively decoded in the D cycle as instructions for preventing the condition codes from being set and the resultant state is set in flag  51 . 
     As a result, in timing ( 6 ), which is the first branch judgement cycle of branch instruction {circle around (B)}, the setting of the condition code for instruction  1 , i.e., the existance of the updating is set in flag  55 . However, flag  54  is set so that instruction  2  does not update the condition code, flag  53  is set so that instruction  3  does not update the condition code, flag  52  is set so that instruction  2  does not update the condition code, and flag  51  is set so that instruction  5  does not update the condition code. 
     Timing judgement condition  60  judges whether the branch judgement can be conducted based on the condition code in the D cycle by branch instruction B, taking into consideration the values of the above flags. Namely, where flags  51 ,  52 ,  53  and  54  all show that the condition code is not updated, the branch instruction B can perform the branch judgement based on the condition code in the D cycle using branch judgement circuit  5  as shown in FIGS. 10A and 10B. Conversely, updating of the condition code occurs after the timing ( 6 ), namely, after the D cycle of the branch instruction B. For example, when flag  52  indicates updating, instruction  4  performs a updating in timing ( 8 ) in E cycle. The branch judgement cannot be conducted by using the condition code in timing ( 6 ). 
     (c) based on the branch prediction by a counter branch instruction. 
     The counter branch instruction performs subtraction of the value of the designated general register and determines that the branch is successful, when the result is other than 0, and is used for forming a do-loop for a high class instruction. Namely, the do-loop is repeated untill a content of the counter formed by the general register  6  and the arithmetic logical unit becomes “0”, thereby repeatedly performing the branch to a head address of the do-loop. When the value of the counter becomes “0”, the do-loop is completed and the process proceeds to an address following the last address of the do-loop. Therefore, in case of the counter branch instruction, the present invention makes it possible to prefetch the instruction following the branch target instruction of the counter branch instruction, thereby enabling the do-loop to be performed at a high speed. 
     In this type of branch instruction, the value to be subtracted is usually “1”. Thus, the branch is successful where the value to be read out from the general register is other than “1”. 
     FIG. 10C checks whether or not the branch judgement should be conducted based on the content of general register  6 . In the branch cycle, the value stored in general register  6  is read out through line  5 ′ (shown in FIG. 6) to branch judgement circuit  5  and then it is checked whether or not the value is “1”. Namely, in the counter branch instruction, the content of general register  6  is subjected to a subtraction. When it is detected by AND circuit  93  in judging circuit  81 ′ (shown in FIG. 10A) that the result of the subtraction is not “0000 0001” of hexa decimal number in case of 32 bit buffer register  6 , NAND  94  of the judging circuit  81 ′ output “1” to be supplied to AND circuit  84 ′. Therefore, in this case, branch judgement cricuit  5  can perform a predictable branch judgement based on the content of general register  6 . However, in this case, it is necessary to detect that the value of the general register is not changed from the branch prediction stage to the original judgement stage. As recited above, this judgement can be conducted by decoder  50 ′; flags  51 ′,  52 ′,  53 ′,  54 ′ and  55 ′; and timing cirucit  60 ′, shown as a dotted line in FIG. 6 in the same manner as the decoder  50 ; flags  51  to  54 ; and timing circuit  60 . 
     In accordance with the present invention, when executing a branch instruction where the branch can be predicted, there is the possibility that the execution of the instruction following the branch target instruction is kept waiting. Though depending on the ratio of the number of non-condition instructions  2  to the number of instructions to be executed, the possibility of the branch instruction being high, greatly increases the total capability.