Abstract:
An exemplary embodiment of the present invention is a method and system for reducing the number of branch instructions required to test combinations of millicode branch points. The method is implemented via a pipe-lined computer processor executing a millicode routine. The processor interrogates a millicode condition code; interrogates a first field of the TMBP instruction, the results of which determine a logical function to be performed on the millicode condition code; interrogates a second field of the TMBP instruction which specifies a first millicode branch point; interrogates a third field of the TMBP instruction, which specifies a second millicode branch point; and sets a millicode condition code based upon the results of the interrogating and used for executing subsequent TMBP instructions or conditional branch instructions.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to improvements in pipelined computer processors that execute relatively simple instructions in hardware controlled execution units and execute relatively complex instructions in a milli-mode architected state with vertical microcode (i.e. millicode) routines executing in the same hardware controlled execution units. More particularly, this invention relates to a specialized millicode instruction which reduces the number of branch instructions required to test combinations of millicode branch points. 
     A new generation of S/390 ESA CMOS machines were introduced in 1997 by International Business Machines Corporation known as the G4 generation. Included was a pipelined computer processor which provides for the use of millicode. A milli-mode operation enables implementation of complex functions in a large, hardware controlled, pipelined, general purpose digital computer without a microprocessor. Milli-mode implements these complex functions with the flexibility provided by firmware and avoids a packaging problem introduced by the inclusion of microprocessor hardware. Rather than a microprocessor, milli-mode uses the pre-existing dataflow and hardware controlled execution units of a pipe-lined processor to accomplish complex functions. Additional hardware controlled instructions (private milli-mode only instructions) are added to provide control functions or to improve performance. These private milli-mode instructions augment the architected instruction set. Milli-mode routines can intermingle the milli-mode only instructions with architected instructions to implement complex functions as illustrated by U.S. Pat. No. 5,694,587, issued Dec. 2, 1997. U.S. Pat. No. 5,694,587 describes specialized millicoded instructions for a PSW Validity Test, Load With Access Test, and Character Translation Assist, which were employed in the IBM machine known as the G4 S/390 machine introduced in 1997. Related to U.S. Pat. No. 5,694,587 were additional applications related to millicode which are implemented in the same G4 S/390 machine introduced in 1997 by International Business Machines Corporation. 
     However, there are still improvements which may be made in the millicode environment. For example, while executing a millicode routine, it is important to have that routine executed as quickly as possible to improve the overall performance of the system. One of the operations that is done most frequently within a millicode routine is branching. Whenever branches are encountered within a millicode routine, the Instruction Processing Unit of the processor sometimes fetches and decodes instructions located on the incorrect path of the branch. For millicode execution there are many hardware conditions that the millicode can interrogate, which assist in controlling the actual execution of a specific millicode routine. Each of these conditions can be interrogated with a millicode instruction, (e.g. Branch Relative Special (BRS)) but, as before, the hardware may initially predict the wrong path of the branch and start fetching and decoding instructions that will not be executed. In addition, due to the format of the instruction, the branch target can only be 64K bytes away from the BRS, which may not give as much branch capability as the millicode routine would desire. It would be advantageous, therefore, to provide a solution that would reduce the number of branch instructions required to test combinations of millicode branch points, thereby resulting in fewer branches being decoded by the Instruction Processing Unit, and ultimately, fewer fetches down wrong branch paths. 
     SUMMARY OF THE INVENTION 
     An exemplary embodiment of the present invention is a method and system for reducing the number of branch instructions required to test combinations of millicode branch points. The method is implemented via a pipe-lined computer processor executing a millicode routine. The processor activates a millicode condition code; interrogates a first field of the Test Millicode Branch Points (TMBP) instruction, the results of which determine a logical function to be performed on the millicode condition code; interrogates a second field of the TMBP instruction which specifies a first millicode branch point; interrogates a third field of the TMBP instruction, which specifies a second millicode branch point; and modifies the milli-code condition code based on the results of the interrogating. A conditional branch instruction can then be executed based on the resultant milli-code condition code of the TMBP instruction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating the components of a millicode system in accordance with a preferred embodiment of the present invention; and 
     FIG. 2 is a pictorial representation of the millicode instruction “TMBP”. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following exemplary embodiment of the invention is applicable in the overall context of a system generally conforming to IBM 390 system architecture. While the System 390 environment will be used as an example, it should be understood that the present invention is also readily applicable to other IBM and non-IBM architectures as well. 
     Referring now to FIG. 1, elements of a system relevant to this invention include a system storage  11 , and a cache memory unit  12 . The system storage  11  contains the instructions that the processor is executing as well as the data those instructions are manipulating. The cache memory unit  12 , which includes a copy of the data and instructions the processor is presently executing, in this preferred embodiment is a cache providing interleaved double word addressing. 
     An instruction unit subsystem  16  includes an instruction buffer (not shown in FIG.  1 ), instruction registers  18  and an instruction decoder  20 . The instruction unit subsystem receives macro instructions, millicode instructions, and data from the cache memory unit  12 . Instructions are parsed and placed into the instruction registers  18 . The decoder  20  reads the contents of the instruction registers  18 , decodes the instruction (or causes an operation exception), and passes the instruction to an instruction queue for sequential execution by a hardware execution unit  24 . Each hardware execution unit  24  has access to a set of general purpose registers and access registers  23  for millicode instruction execution. Control logic controls the exchange of data between the two sets of registers when beginning or terminating a millicode routine. 
     Milli-mode detection logic  26  is coupled to the instruction registers and detects when a macro-instruction, which is being decoded, is of a type that is to be interpreted in a milli-mode operation. When this occurs, the milli-mode detection logic  26  generates an entry point address and passes this address along to the instruction fetch control logic  25  and places the decoder  20  in to a milli-mode operating state. In this state the decoder is enabled to decode macro-instructions which are not interpreted in millicode, as well as milli-mode instructions, all of which can be executed in execution unit  24 . The special milli-mode instructions provide control functions needed by the milli-code routines. The set of millicode routines reside outside of the program addressable storage. 
     The system effects of an executed instruction are architecturally visible in the completion logic  30 . Signal lines between the completion logic  30  and the decoder  20  allow the decoder  20  to keep track of instruction completion. A program status word (PSW) in register  31  controls execution of the macro-program. Similarly, the system also includes a milli-PSW register  33 , which controls execution of the milli-routine. Both the execution unit  24  and completion logic  30  are connected to read from/write to the PSW and the milli-PSW registers. Thus, at any given point the execution units or the completion logic can read or update the appropriate one of the PSW and/or milli-PSW registers. A processor state unit  40  maintains the entire updated status of the architected system both in regular mode and milli-mode operation. In the event of a detected error, the processor state unit  40  provides a resource to recreate the status of the system for a check point state in order to allow a retry of the error causing operation. 
     Milli-mode is enabled when the milli-mode detection logic  26  recognizes that the macro-instruction being decoded is to be implemented with millicode. In response to this recognition, the detection logic  26  signals the decoder  20 , the instruction fetch controls  25  and register controls in the execution unit  24 . In response to the milli-mode recognition signal from the detection logic  26 , the decoder  20  suspends macro-mode decoding and causes the system to subsequently use the milli-registers  23 , instead of normal macro-mode registers  21 . The milli-mode detection logic  26  generates a millicode entry point address. 
     The entry point address (generated by the milii-mode detection logic  26 ) is used by the control logic  25  to address the cache  12 . Milli-instructions from the cache are sent to the instruction registers  18  where the decoder  20  decodes them and schedules them for execution. 
     When the processor enters milli-mode, it executes and completes the macro-instructions already in the pipeline conceptually prior to the instruction that caused entry into milli-mode. As the processor completes the macro-instructions, it updates the appropriate GPRs. At the same time, the processor decodes and executes the milli-instructions that implement the macro instruction that caused entry into milli-mode. 
     At some point the macro-instruction immediately prior to the instruction that caused entry to milli-mode will be indicated completed in the completion logic  30 . Only then does the processor begin to complete the milli-instructions. The processor then continues decoding, executing and completing the milli-instructions. 
     Eventually, the detection logic  26  recognizes a millicode END (MEND) milli-instruction. When the detection logic  26  detects a MEND milli-instruction, it causes the processor to cease fetching milli-instructions. Further, when MEND is detected, the detection logic puts the decoder in macro-mode and causes the processor to begin fetching macro-instructions. Millicode explicitly updates all registers, so there is no transfer of register content when going from milli-mode operation to regular operation. 
     Completion of a MEND milli-instruction causes the processor completion logic to being completing macro-instructions. 
     The processor can also enter milli-mode in response to an interrupt. When the completion logic detects an interrupt, the interrupt priority logic  45  determines that an interrupt is to be serviced and it signals the instruction fetch unit, causing the decoder to initiate milli-mode. The recognition of an interruption condition causes the processor to halt macro-mode execution at the next interruptible point. The interrupt priority logic  45  also generates control inputs which are used by the milli-mode detection logic to generate an entry point address with which to address the cache. These milli-instructions are sent to the instruction registers where the decoder decodes them and schedules them for execution at the appropriate hardware execution elements. 
     The processor proceeds to decode, execute and complete the milli-instruction in the milli-routine for interrupts. Eventually, the decoder recognizes a MEND milli-instruction. This causes the decoder to stop decoding in milli-mode. Depending on whether or not there are additional interrupts that require servicing, the decoder hardware will either redo the interrupt process or return to decoding macro-instructions from the cache. 
     As described above, one of the operations frequently performed by a millicode routine is branching. Upon encountering a branch the instruction unit subsystem  16  of the processor sometimes fetches and decodes instructions on the incorrect path of the branch. The Test Millicode Branch Points (TMBP) instruction of the present invention significantly reduces the number of incorrect “fetches” as described further herein. In addition, due to limitations in the design of a pipeline processor, there is a limit to the number of outstanding branches that can be decided, but not yet executed, before the pipeline stalls and no more instructions are able to be decoded. This invention now allows multiple branch points to be tested via the TMBP instruction (that is, not via multiple branch instructions), and then only one branch instruction is executed after all of the branch points have been interrogated, thus avoiding stalls in the pipeline. 
     FIG. 2 illustrates a preferred embodiment of a TMBP instruction. The TMBP instruction comprises three mask fields. The first mask field (MI) is a 4-bit field which controls how the millicode condition code is modified by the TMBP instruction. The millicode condition code (not shown), which is provided by the hardware during the execution of a millicode routine, indicates the results of an operation or comparison and is represented by two bits, which can each be a ‘0’ or a ‘1’ bit. The second and third mask fields (M 2 , M 3  respectively) each specify a millicode branch point to be tested. 
     When the TMBP instruction is executed, the M 1 , M 2 , and M 3  fields of the instruction are interrogated by the hardware. Bits  0 - 1  of the M 1  field control what logical function is to be performed on bit ‘0’ of the condition code, and bits  2 - 3  of the M 1  field control what logical function is performed on bit ‘1’ of the condition code. 
     For purposes of illustration, the logical value of the hardware branch point M 2  is defined as Q 0 . That is, Q 0  would be a ‘1’b if the branch point specified by M 2  were true, and would be a ‘0’b if the branch point were false. Assume also for purposes of illustration, that the logical value of the hardware branch point M 3  is defined as Q 1 . Accordingly, Q 1  would be a ‘1’b if the branch point specified by M 3  were true, and a ‘0’b if the branch point were false. 
     The two bits of the Millicode condition code are specified as CC. 0  and CC. 1 , and in the current implementation are controlled by the M 1  bits as follows: 
     M 1 . 0 - 1  New CC. 0   
     ‘00’b Q 0   
     ‘01’b CC. 0  ‘AND’Q 0  ‘AND’Q 1   
     ‘10’b CC. 0  ‘AND’Q 0   
     ‘11’b CC. 0  ‘XOR’Q 0   
     M 1 . 2 - 3  New CC. 1   
     ‘00’b Q 1   
     ‘01’b CC. 1  ‘OR’Q 0  ‘OR’Q 1   
     ‘10’b CC. 1  ‘AND’Q 1   
     ‘11’b CC. 1  (unchanged) 
     Thus, it can be realized that either the current values of the M 2  and M 3  branch points can be placed directly into the condition code, or that logical functions can be performed on the M 2  and M 3  branch points in conjunction with the existing condition codes, with different functions being performed on the two different bits of the condition code. Then, with multiple executions of the TMBP instruction, many different branch points can be tested, without executing any actual branch instruction. After all of the branch points have been tested via the TMBP instruction, any conditional branch instruction can be executed to branch on the new condition code. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.