Patent Publication Number: US-11379237-B2

Title: Variable-length-instruction processing modes

Description:
This application is the U.S. national phase of International Application No. PCT/GB2016/050978 filed 7 Apr. 2016, which designated the U.S. and claims priority to GB Patent Application No. 1509746.2 filed 5 Jun. 2015, the entire contents of each of which are hereby incorporated by reference. 
     This disclosure relates to data processing systems. More particularly, this disclosure relates to modal control of hardware to support the execution of program instructions. 
     It is known to provide data processing systems supporting multiple instruction sets. For example, processing systems as designed by ARM Limited of Cambridge, England may support both the ARM and Thumb instruction sets. 
     Viewed from one aspect the present disclosure provides apparatus for processing data comprising: 
     a processor to perform processing operations upon operands stored within a register file of said processor and as specified by program instructions of an instruction set, said processor operating in at least a first mode of operation having a first set of active processing circuitry ready to perform processing operations and a second mode of operation having a second set of active processing circuitry ready to perform processing operations, said second set of active processing circuitry different from said first set of active processing circuitry; wherein 
     a first proper subset of program instructions within said instruction set are processed by said processor operating in a selectable one of said first mode and said second mode; and 
     a second proper subset of program instructions within said instruction set are required to be processed by said processor operating in said second mode. 
     Viewed from another aspect the present disclosure provides apparatus for processing data comprising: 
     processing means for performing processing operations upon operands stored within a register file of said processor means and as specified by program instructions of an instruction set, said processing means operating in at least a first mode of operation having a first set of active processing means for performing processing operations and a second mode of operation having a second set of active processing means for performing processing operations, said second set of active processing means different from said first set of active processing means; wherein 
     a first proper subset of program instructions within said instruction set are processed by said processing means operating in a selectable one of said first mode and said second mode; and 
     a second proper subset of program instructions within said instruction set require to be processed by said processing means operating in said second mode. 
     Viewed from a further aspect the present disclosure provides a method of processing data comprising: 
     performing processing operations upon operands stored within a register file of a processor and as specified by program instructions of an instruction set using at least a first mode of operation having a first set of active processing circuitry for performing processing operations and a second mode of operation having a second set of active processing circuitry for performing processing operations, said second set of active processing circuitry different from said first set of active processing circuitry; wherein 
     a first proper subset of program instructions within said instruction set are processed using a selectable one of said first mode and said second mode; and 
     a second proper subset of program instructions within said instruction set require to be processed using said second mode. 
    
    
     
       Embodiments may now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  schematically illustrates a data processing apparatus having a first mode of operation and a second mode of operation; 
         FIG. 2  schematically illustrates an instruction set including a first proper subset of program instructions and a second proper subset of program instructions; 
         FIG. 3  schematically illustrates different forms of mode switching instruction; 
         FIG. 4  is a flow diagram schematically illustrating switching between modes triggered by an interrupt; 
         FIG. 5  is a flow diagram schematically illustrating switching between modes triggered by a switch instruction; 
         FIG. 6  is a flow diagram schematically illustrating switching between modes triggered by a preferred metric value; and 
         FIG. 7  is a flow diagram schematically illustrating switching between modes triggered by an unsupported instruction. 
     
    
    
       FIG. 1  schematically illustrates a data processing apparatus  2  comprising a processor  4  coupled to a memory  6 . The processor  4  includes fetch circuitry  8  for fetching program instructions for execution from the memory  6 . The fetch circuitry  8  is controlled by a mode signal to operate in either a first mode of operation or a second mode of operation. In the first mode of operation, the fetch circuitry  8  serves as first fetch circuitry to fetch program instructions from the memory  6  at a first fetch rate. In the second mode, the fetch circuitry  8  serves as second fetch circuitry to fetch program instructions from a memory  6  at a second fetch rate. The fetch circuitry  8  includes an instruction fetch buffer  10  into which program instructions fetched by the fetch circuitry are stored. In the first mode, only a portion of the instruction fetch buffer  10  is used (clocked and/or powered) and this serves as a first instruction fetch buffer. In the second mode, the whole of the instruction fetch buffer  10  is used and this serves as a second instruction fetch buffer. The first instruction fetch buffer has a lower storage capacity than the second instruction fetch buffer. 
     The program instructions fetched from the memory  6  may be 16-bit instructions or 32-bit instructions. More generally, the instructions may be N-bit instructions or M-bit instructions, where M is greater than N. In this example embodiment in the first mode the fetch bit width is 16-bits and in the second mode the fetch bit width is 32-bits. If the processing apparatus  2  consumes on average one instruction per cycle, then operating in the first mode, the fetch circuitry  8  is able to fetch one 16-bit program instruction bit each cycle and so keep pace with program execution if predominantly 16-bit program instructions are being executed. Conversely, in the second mode, the fetch circuitry  8  is able to fetch one 32-bit program instruction each cycle and so keep pace with program execution if executing predominantly 32-bit program instructions. It will be appreciated that in practice a mix of 16-bit and 32-bit program instructions may be executed and furthermore the number of program instructions executed each cycle may not be one. When using a 16-bit fetch bit width, it is still possible to fetch 32-bit instructions, but in this case two fetch cycles will be used to fetch each instruction. 
     The instruction fetch buffer  10  provides smoothing between the instruction fetch rate and the instruction consumption rate. Energy consumption can be reduced if the instruction fetch rate capability is matched to the instruction consumption rate. In the example of  FIG. 1 , if the processor  4  is executing predominately 16-bit instructions, then energy may be saved by only fetching on average 16-bits of instruction per cycle permitting portions of the fetch circuitry  8  to be powered down, or at least be clock gated, during such operation (first mode of operation). Conversely, should the processor  4  need to perform a sustained period of execution of 32-bit program instructions, then the second mode of operation of the fetch circuitry  8  may be adopted in order to support a sustained fetch rate of 32-bits per cycle. Efficiency may be improved in embodiments in which the instruction fetch rate capability of the fetch circuitry is mapped to the instruction consumption rate of the processor  4 . Providing an excess instruction fetch rate capability can unnecessarily consume energy. 
     It will be appreciated that in the above it was assumed that the fetch cycles corresponded to instruction execution cycles. In other embodiments, this may not necessarily by the case. Nevertheless, in such other embodiments it may still be appropriate to operate the fetch circuitry in a plurality of modes with different fetch rate capabilities. In a system supporting a mixed 16-bit and 32-bit instruction set, then the 16-bit instructions may be a first proper subset of program instructions within the instruction set and, with respect to the behaviour of the fetch circuitry  8 , the processor  4  may operate in a selectable one of a first mode supporting a 16-bit instruction fetch width or a 32-bit instruction fetch width. The fetch circuitry  8  will have an excess fetch capability when operating in the second mode and fetching only 16-bit program instructions, but nevertheless may operate correctly. However, when executing a sustained stream of 32-bit program instructions, then the fetch circuitry  8 , (despite the action of the instruction fetch buffer  10 ) may be unable to keep pace with instruction consumption if operating in the first mode (16-bit fetch width) and accordingly processor  4  may stall awaiting program instructions to be fetched. Accordingly, the 32-bit program instructions of the instruction set may be considered to be a second proper subset of program instructions that require to be processed by the processor  4  in the second mode (at least if undesired stalls want to be avoided). 
     The program instructions fetched by the fetch circuitry  8  are supplied to decoder circuitry formed of first decoder circuitry  12  and second decoder circuitry  14 . The first decoder circuitry  12  is active in the first mode. Both the first decoder circuitry  12  and the second decoder circuitry  14  are active in the second mode. When the processor  4  is only executing 16-bit program instructions, then it may operate in the first mode in which only the first decoder circuitry  12  is active. The first decoder circuitry  12  is able to decode 16-bit program instructions, but is unable to decode all 32-bit program instructions. When operating in the second mode, both the first decoder circuitry  12  and the second decoder circuitry  14  are active and all instructions within the instruction set may be decoded. A second proper subset of program instructions (e.g. some 32-bit program instructions, such as floating point program instructions) require the second decoder circuitry  14  to be active as the first decoder circuitry  12  is not able to decode program instructions from this second subset of program instructions. 
     In respect of the fetch circuitry  8 , the first and second decoder circuitry  12 ,  14  and other elements of the processor  4  illustrated in  FIG. 1 , when portions of the circuitry are inactive, they may be placed into a low power state. This low power state may take various forms, such as clock signal gating, powering down a relevant power domain containing in the inactive circuit elements or other power control techniques. 
     Mode control circuitry  16  is coupled to the first decoder  12  and the second decoder  14  and serves to snoop program instructions that are decoded for execution. The mode control circuitry  16  uses this snooped data to heuristically control switching between the first mode and the second mode in dependence upon detected characteristics of the stream of program instructions executed. If a program instruction is decoded which requires the processor to operate in a second mode, then the mode control circuitry  16  switches operation to this second mode. If greater than a threshold rate of program instructions which are preferentially, but not essentially, executed in the second mode are detected, then a switch may be made to the second mode. Similarly, if greater than a threshold rate of program instruction execution of program instructions of the first proper subset which are preferentially (for example for at least reduced energy consumption reasons) executed within the first mode are detected, then a switch to that first mode may be made. 
     Decoded control signals generated by the first decoder  12  and the second decoder  14  are passed to issue circuitry  18  where corresponding processing operations are issued in to an appropriate one of a number of processing pipelines, such as an integer pipeline  20 , a load store unit  22  and a floating point pipeline  24 . In this example, the floating point pipeline  24  is responsible for executing floating point program instructions. The floating point program instructions are 32-bit program instructions which are part of the second proper subset of program instructions and require the processor  4  to be operating in the second mode. Accordingly, the floating point pipeline  24  is switched between an inactive state in the first mode and an active state in the second mode. The inactive state is a low power state. In practice, it may be that large portions of program code are executed that do not require floating point program instruction execution and accordingly the floating point pipeline  24  may be powered down in accordance with the first mode of operation during such periods of program execution. Program instructions within the first subset of program instructions (which do not include any floating point instructions) may be executed irrespective of whether the floating point pipeline  24  is or is not active. A lower power consumption will be achieved if the floating point pipeline  24  is inactive. 
     A register file  26  is provided within the processor  4  to store operand values to be manipulated. The registers within the register file  26  include high order register file circuitry and low order register file circuitry. When operating in the second mode, both the high order register file circuitry and the low order register file circuitry are active and available for use. When operating in the first mode, the high order register file circuitry is placed into a low power state in which it is unavailable for use while the low order register file circuitry remains active and available for use. First proper subset of program instructions requires access to the low order register file circuitry, but not to the high order register file circuitry. Conversely, the second proper subset of program instructions requires access to the high order register file circuitry and the low order register file circuitry. 
     The mode control circuitry  16  as discussed above is able to switch between the first mode of operation and the second mode of operation heuristically dependent upon detected characteristics of the stream of program instructions executed. Other switch mechanisms are also possible. For example, explicit mode switching program instructions may be provided within the instruction set. A specific 16-bit program instruction for switching from the first mode to the second mode may be included within the first proper subset of program instructions. A 32-bit mode switch instruction for switching from the second mode to the first mode may be included within the second proper subset of program instructions. The first decoder  12  may respond to the 16-bit mode switching program instruction to trigger the mode control circuitry  16  to switch the mode of operation from the first mode to the second mode. The second decoder  14  may respond to the 32-bit mode switching instruction to trigger the mode control circuitry  16  to switch from the second mode to the first mode. 
     Another form of mode switching control which may be performed is associated with interrupt signals received by an interrupt controller  28 . When an interrupt is received, then a redirection of program instruction execution flow is triggered, as will be familiar to those in this technical field. The interrupt controller  28  is configured with data associated with each received interrupt signal indicating which interrupt handler program routine should be initiated when that interrupt is executed. This interrupt configuration data may also include mode data indicating which one of the first mode of operation or the second mode of operation is to be adopted for execution of the associated interrupt handler program. Thus, when an interrupt signal is received by the interrupt controller  28 , execution of an associated interrupt handler program is initiated and a switch of mode of operation is made if the mode flag associated with that interrupt handler program indicates a different mode of processing to the currently operative mode of processing of the processor  4 . 
       FIG. 2  schematically illustrates an instruction set  30  formed of a mixture of 16-bit program instructions and 32-bit program instructions. A first proper subset of program instructions  32  are instructions which may be processed in a selectable one of either the first mode of operation or the second mode of operation. A second proper subset of program instructions  34  are program instructions, such as floating point program instructions, which require to be processed in the second mode of operation. Program instructions outside of the first proper subset and the second proper subset may be executed in either the first mode of operation or the second mode of operation, but have a preference for execution in the second mode of operation. 
       FIG. 3  schematically illustrates the use of explicit mode switching program instructions within the instruction set  30 . As previously discussed, a 16-bit instruction for execution when operating in the first mode of operation may be provided to switch execution to the second mode of operation. Similarly, an explicit 32-bit mode switching program instruction may be provided for execution in the second mode of operation to switch execution to the first mode of operation. 
     In other example embodiments instead of 16-bit instructions and 32-bit instructions, the system may operate with 32-bit instructions and 64-bit instructions. 
       FIG. 4  schematically illustrates an interrupt triggered switch of mode. At step  36  processing waits until an interrupt signal is received by the interrupt controller  28 . At step  38 , in addition to the other processing operations associated with an interrupt, the interrupt controller  38  serves to read the mode flag associated with the received interrupt. This mode flag is supplied to the mode controller  16 . At step  40  the mode controller  16  determines from the received mode flag whether the processor  4  is operating in the correct mode as indicated by the mode flag for the newly adopted interrupt handling program. If the processor  4  is not currently operating in the correct mode, then step  42  serves to switch the mode of operation. 
       FIG. 5  is a flow diagram schematically illustrating explicit switch instruction triggered mode switching. The mode switching instructions may be either the 16-bit mode switching instruction or the 32-mode switching instruction referred to in relation to  FIG. 3 . At step  44  the instruction decoder circuitry  12 ,  14  waits until a mode switching instruction is received. When a mode switching instruction is received, then a signal is sent to the mode controller  16  to indicate that a mode switch should be performed. Step  46  corresponds to the mode controller  16  switching the mode as indicated by the explicit mode switching program instruction that has been executed. 
       FIG. 6  is a flow diagram schematically illustrating heuristic switching in accordance with a preferred execution metric. At step  48  processing waits until an instruction is decoded by the decoder circuitry  12 ,  14  which is detected by the mode controller  16  as an instruction which is preferred for execution in a different mode to the current mode of operation of the processor  4 . When such a program instruction preferred for execution in a different mode is detected, then processing proceeds to step  50  where an update is made to a preferred metric value. This preferred metric value may, for example, track the mean rate of detection of such program instructions which are executing in their non-preferred mode. Other forms of metric value are also possible. At step  52  a determination is made as to whether or not the updated preferred metric value is above a switching threshold. If the preferred metric value is above the switching threshold, then step  54  initiates a mode switch. The mode switch may be either from the first mode to the second mode when the processor  4  is currently operating in the first mode or from the second mode to the first mode when the processor  4  is currently operating in the second mode. The threshold values for the different switching directions may be different, and indeed, the preferred metric used for heuristically controlling switching in the different directions may also be different. 
       FIG. 7  schematically illustrates mode switching triggered by detection of an unsupported instruction. At step  56  processing waits until an instruction which is not supported in the current mode of operation is detected by the decoder circuitry  12 ,  14 . An example of such an unsupported instruction is a floating point program instruction detected when the processor  4  is operating in the first mode of operation. When such an unsupported instruction is detected, then step  58  serves to switch the mode of operation, e.g. switch from the first mode of operation which the floating point pipeline  24  is inactive to the second mode of operation in which the floating point pipeline  24  is active. 
     Although illustrative embodiments have been described in detail herein with reference to the accompanying drawings, it is to be understood that the claims are not limited to those precise embodiments, and that various changes, additions and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the appended claims. For example, various combinations of the features of the dependent claims could be made with the features of the independent claims.