Abstract:
A multi-issue processor includes a register file and a plurality of issue slots, each one of the plurality of issue slots having a plurality of functional units and a plurality of holdable registers. The plurality of issue slots include a first set of issue slots and a second set of issue slots, and the register file is accessible by the plurality of issue slots. A location of at least a part of the plurality of holdable registers in the first set of issue slots is different from a location of at least a corresponding part of the plurality of holdable registers in the second set of issue slots.

Description:
TECHNICAL FIELD 
     The present invention relates to a multi-issue processor comprising: a plurality of issue slots, each one of the plurality of issue slots comprising a plurality of functional units and a plurality of holdable registers, the plurality of issue slots comprising a first set of issue slots and a second set of issue slots; and a register file accessible by the plurality of issue slots. 
     BACKGROUND ART 
     Multi-issue processors exhibit a lot of parallel hardware to enable the concurrent execution of multiple operations in a single processor cycle and thus exploiting instruction-level parallelism in programs. Examples of multi-issue processors are VLIW (Very Large Instruction Word) processors and superscalar processors. In case of a VLIW processor, the software program contains full information regarding which operations should be executed in parallel and these operations are packed into one very long instruction. The compiler ensures that all dependencies between operations are respected and that no resource conflicts can occur. Apart from this program information the hardware does not require any additional information to correctly execute the program, which results in relatively simple hardware. In case of a superscalar processor the software to be executed is presented as a program composed of a sequential series of operations. The processor hardware itself determines at runtime which operation dependencies exist and decides which operations to execute in parallel based on these dependencies, while ensuring that no resource conflicts will occur. A relatively simple compiler suffices for translating a high-level programming language to sequential code, but the processor hardware is very complex. 
     In multi-issue processors, the parallel hardware responsible for executing these operations is organized in issue slots. Each issue slot contains one or more functional units that perform the actual operations. Commonly, in every processor cycle a single operation is started on one functional unit in every issue slot. In some processors more than one functional unit is put in an issue slot as a trade-off between maximum available parallelism and instruction width cost, in case of a VLIW processor, or hardware complexity, in case of a superscalar processor. 
     Since in each clock cycle at most one operation can be started on one functional unit in each issue slot, power may be wasted by functional units in that issue slot that are not being used in a given processor cycle. If the input of these functional units changes during the time that they are not used they will still consume comparable power to when they are being used, even though their output is irrelevant. 
     This waste of power can be eliminated by putting holdable registers, i.e. a register, the state of which remains unchanged in case of a different input, at the inputs of all functional units within an issue slot. These holdable registers will leave the inputs of the functional units unchanged, when these functional units are not being used. Since the inputs of these functional units remain unchanged, no combinatorial gates are switched and no dynamic power dissipation occurs. These holdable registers can be implemented, for example, by means of clock gating. Another advantage of these registers is that the additional pipeline stage they are forming allows running the processor at a higher clock frequency. A disadvantage of adding registers to all inputs of functional unit inputs is that it increases the amount of state that must be saved during interrupts. An interrupt allows a processor to quickly respond to external events and it causes the processor to temporarily postpone the further execution of the current program trace and instead perform another trace. The state of the postponed trace must be saved such that, when the interrupt has been serviced, the processor can restore its original state and can correctly proceed with the original trace. In order to obtain a predictable and short interrupt latency, it must always be possible to interrupt the processor whenever desired. This is especially important in real-time applications. Interrupting a processor at an arbitrary point in the program can imply that a significant amount of state must be saved. 
     The non-prepublished European patent application 00203591.3 [attorneys&#39; docket PHNL000576], filed on 18 Oct. 2000, provides a solution for decreasing the amount of state that must be saved during interrupts. A second compact instruction set is applied, that is used in an interrupt service routine and only uses a limited set of processor resources. In case of an interrupt, it is sufficient to save the state of only the limited set of processor resources used by the second compact instruction set, while simply freezing the state in all other resources. However, the resources used by the second compact instruction set still have a considerable amount of state that must be saved and restored during interrupts, when registers are put at all the inputs of each functional unit in this limited set of resources. 
     DISCLOSURE OF INVENTION 
     An object of this invention is to provide a solution to further reduce the amount of state that must be saved during interrupt handling for multi-issue processors, while maintaining a significant reduction in power consumption and improved performance. 
     This object is achieved with a multi-issue processor of the kind set forth characterized in that a location of at least a part of the plurality of holdable registers in the first set of issue slots is different from a location of at least a corresponding part of the plurality of holdable registers in the second set of issue slots. 
     Ideally, the holdable registers are put at all inputs of each functional unit within an issue slot. In that case it is guaranteed that each input of a functional unit, that is not being used, will remain unchanged and no unnecessary power dissipation will occur. However, this increases the amount of state that has to be saved during interrupt handling. By varying the position of the holdable registers for different issue slots, and not putting a holdable register in front of all inputs of every functional unit, less state saving is required during interrupt handling. This may result in a lower reduction of the power consumption or a reduced increase in performance. Depending on the type of application an optimal choice between these demands can be made. 
     An embodiment of the invention is characterized in that the multi-issue processor further comprises a first instruction set means having access to the first set of issue slots and a second instruction set means having access to the second set of issue slots. An advantage of this embodiment is that the location of the holdable registers in an issue slot can be made dependent of the instruction set means that controls this issue slot. If the second instruction set means is used in an interrupt service routine, the holdable registers in the second set of issue slots can be positioned to optimally reduce the amount of state that must be saved during interrupt handling. However, this solution is not optimal for reduction of the power consumption. The positioning of the holdable registers still creates an additional pipeline stage enabling an increase in the clock frequency of the processor. Many interrupts require very simple interrupt service routines and therefore a compact second instruction set using a limited set of issue slots is sufficient. Therefore the non-optimal reduction in power consumption only holds for a small set of issue slots within the multi-issue processor. The first set of issue slots is not used during interrupt handling and as a result their state does not have to be saved. The holdable registers can be placed to optimally reduce the power consumption and increasing the clock frequency by creating an additional pipeline stage. For the overall processor this results in a well-balanced consideration between increasing performance, decreasing power consumption and reducing state saving overhead. 
     An embodiment of the invention is characterized in that in the first set of issue slots the location of the plurality of holdable data registers is at individual data inputs of the functional units, while in the second set of issue slots the location of the plurality of holdable data registers is at common data inputs of the functional units. An advantage of this embodiment is that the amount of state that has to be saved during interrupt handling is strongly reduced, since the holdable registers are not positioned at all individual inputs of the functional units of the second set of issue slots, but only at their common inputs. However, the use of one functional unit of an issue slot of the second set of issue slots results in changing inputs at the other functional units of that issue slot and therefore causes unnecessary power dissipation. In case that entire issue slot is not being used, the functional units will consume no power. In the first set of issue slots the holdable registers are positioned at all inputs of the functional units to optimally reduce power consumption, resulting in a significant overall reduction in the power consumption. Furthermore, the holdable registers in the first and second set of issue slots form an additional pipeline stage in the architecture, allowing the processor to run at a higher clock frequency. As a result, a good compromise is obtained between reduction in power consumption, increase in performance and reduction in the amount of state that has to be saved during interrupt handling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features of the described embodiments will be further elucidated and described with reference to the drawings: 
         FIG. 1  is a schematic diagram of a VLIW processor. 
         FIG. 2  is a schematic diagram of issue slot UC 1 , UC 2  and UC 3  only used by a first instruction set. 
         FIG. 3  is a schematic diagram of issue slot UC 0  used by a second instruction set, during interrupt handling. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a schematic block diagram illustrates a VLIW processor comprising a plurality of issue slots, including issue slots, UC 0 , UC 1 , UC 2  and UC 3 , and a distributed register file including register file segments RF 0  and RF 1 . The processor has a controller SQ and a connection network CN for coupling the register file segments RF 0  and RF 1 , and the issue slots UC 0 , UC 1 , UC 2  and UC 3 . The issue slots UC 0 , UC 1 , UC 2  and UC 3  are used by a first instruction set and this first instruction set includes the normal VLIW instructions. The issue slot UC 0  is the only issue slot that is used by a second instruction set. This second instruction set is used in an interrupt service routine. 
     Referring to  FIG. 2 , a schematic block diagram illustrates issue slots UC 1 , UC 2  and UC 3 . Referring to  FIG. 3 , a schematic block diagram illustrates issue slot UC 0 . Referring now to both  FIG. 2  and  FIG. 3 , each issue slot comprises a decoder DEC, a time shape controller TSC, an input routing network IRN, an output routing network ORN, and a plurality of functional units, including functional units FU 0 , FU 1  and FU 2 . The decoder DEC is coupled to the time shape controller TSC and to the functional units FU 0 , FU 1  and FU 2 . The input routing network IRN is coupled to the functional units FU 0 , FU 1  and FU 2 . The output routing network ORN is also coupled to the functional units FU 0 , FU 1  and FU 2 . The decoder DEC decodes the operation O applied to the issue slot in each clock cycle. Results of the decoding step are operand register indices ORI and the decoder DEC passes these indices to the connection network CN, shown in  FIG. 1 . Further results of the decoding step are result file indices RFI and register indices RI. The decoder DEC passes these indices to the time shape controller TSC. The time shape controller TSC delays the result file indices RFI and the register indices RI by the proper amount, according to the input/output behavior of the functional unit on which the operation must be executed. Subsequently, the time shape controller TSC passes the result file indices RFI and the register indices RI to the connection network CN, shown in  FIG. 1 . The decoder DEC also selects one of the functional units FU 0 , FU 1  and FU 2  to perform an operation, using the coupling SEL. Furthermore, the decoder DEC passes information on the type of operation that has to be performed to the functional units FU 0 , FU 1  and FU 2 , using the coupling OPT. The input routing network IRN passes the operand data OD for the issue slot UC 1 , UC 2  and UC 3  to the inputs of functional units FU 0 , FU 1  and FU 2 . The functional units FU 0 , FU 1  and FU 2  pass their output data to the output routing network ORN and subsequently the output routing network ORN passes the result data RD to the communication network CN, see  FIG. 1 . 
     Referring to  FIG. 2 , holdable registers  1 - 27  are provided directly at the data and control inputs of the functional units FU 0 , FU 1  and FU 2 . Holdable registers  1 - 5 ,  11 - 15 ,  21  and  23  are referred to as holdable data registers, since they are positioned at the data inputs of the functional units FU 0 , FU 1  and FU 2 . The holdable registers  1 - 27  will leave the inputs of the functional units FU 0 , FU 1  and FU 2  unchanged when a functional unit is not being used. As a result, no combinatorial gates are switched and no power dissipation occurs. Furthermore, to prevent result file indices RFI and register indices RI from changing unnecessarily, and thereby causing unnecessary power dissipation, holdable registers  29 ,  31  and  33  are placed directly after the time shape controller TSC. An advantage of this embodiment is that it reduces the power consumption. In each clock cycle at most one operation can be started on one of the functional units FU 0 , FU 1  and FU 2 , and most functional units finish their operation in a single processor cycle. If the inputs of the functional units, that are not being used, change due to data passed via the input routing network IRN or the decoder DEC, these functional units will consume comparable power to when they are not being used, even though their output is irrelevant. Adding the holdable registers  1 - 33  creates additional state, but that is irrelevant for the issue slots UC 1 , UC 2  and UC 3 . During interrupts, their state only has to be frozen. The holdable registers  1 - 33  do only incur additional area. These registers do not waste additional power due to using clock gating to hold the registers in their inactive state in case the corresponding functional unit is not being used. 
     Referring to  FIG. 3 , issue slot UC 0  is the only issue slot that is used by the second instruction set, used in an interrupt service routine. In order to guarantee a fast interrupt response, it is crucial to minimize the amount of state that has to be saved during interrupt handling. This can be achieved by positioning the holdable registers at common inputs of the functional units FU 0 , FU 1  and FU 2 . Therefore, holdable registers  101 ,  103  and  105  are put directly at the input of the issue slot UC 0  instead of at the data inputs of each functional unit FU 0 , FU 1  and FU 2  in issue slot UC 0 . Furthermore, a holdable register  117  is put at the output of the decoder DEC for passing information of the type of operation OPT that has to be performed, instead of at the input of each functional unit FU 0 , FU 1  and FU 2  in issue slot UC 0 . At the result file index input and register index input terminals of the time shape controller TSC holdable registers  113  and  115  are positioned as well, instead of at their outputs, saving one holdable register. The positioning of the holdable registers  107 ,  109  and  111  at the input of each functional unit FU 0 , FU 1  and FU 2  remains unchanged, since these functional unit inputs are not coupled to a common output of the decoder DEC. 
     An advantage of the positioning of the holdable registers in issue slot UC 0 , is that the amount of state that has to be saved during an interrupt is strongly reduced, when compared to the amount of state present due to the holdable registers in the issue slots UC 1 , UC 2  and UC 3 . The use of one functional unit FU 0 , FU 1  and FU 2  in the issue slot UC 0 , results in changing inputs at the other functional units of issue slot UC 0  and therefore causes unnecessary power dissipation in this issue slot. In case the entire issue slot is not being used, the holdable registers  101 - 111  and  117  will prevent power consumption by the functional units FU 0 , FU 1  and FU 2  of issue slot UC 0 . 
     For the issue slots UC 0 , UC 1 , UC 2  and UC 3 , the location of the holdable registers results in a well balanced consideration between increasing performance, decreasing power consumption and reducing state overhead. Many interrupts require very simple interrupt service routines and therefore only require a compact second instruction set that uses a limited second set of issue slots. In a large subset of the issue slots the holdable registers can be positioned as indicated in  FIG. 2  to optimally reduce the power consumption, resulting in a significant overall reduction of the power consumption. The amount of state that has to be saved during interrupt handling is strongly reduced by positioning the holdable registers in the issue slots, used by the second instruction set, as indicated in  FIG. 3 . Furthermore, the holdable registers added to the issue slots UC 0 , UC 1 , UC 2  and UC 3  form an additional pipeline stage in the architecture, allowing the processor to run at a higher clock frequency. Referring again to  FIG. 1 , the holdable registers positioned in issue slots UC 0 , UC 1 , UC 2  and UC 3  divide the existing data path into two parts, decreasing the time needed to execute one part of the data path and allowing to increase the clock frequency of the processor. 
     A superscalar processor also comprises multiple issue slots that can perform multiple operations in parallel, as in case of a VLIW processor. The principles of the embodiments for a VLIW processor, described in this section, therefore also apply for a superscalar processor. In general, a VLIW processor may have more issue slots when compared to a superscalar processor. The hardware of a VLIW processor is less complicated when compared to a superscalar processor, which results in a better scalable architecture. The number of issue slots and the number of functional units in each issue slot, among other things, will determine the relative decrease in power consumption due to the present invention. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually dependant claims does not indicate that a combination of these measures cannot be used to advantage.