Patent Publication Number: US-7917735-B2

Title: Data processing apparatus and method for pre-decoding instructions

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a data processing apparatus and method for pre-decoding instructions, whereafter the pre-decoded instructions are placed in a cache for access by processing circuitry within the data processing apparatus. 
     2. Description of the Prior Art 
     In a typical data processing apparatus, significant power is consumed in decoding instructions prior to execution within the execution pipelines of the processing circuitry. This issue can become particularly problematic in processing circuitry that supports multiple instruction sets, since often multiple separate decoders will need to be provided for decoding instructions from the various instruction sets. By way of example, in some implementations approximately 15% of the processor power may be consumed by the instruction decoders. 
     It is typically the case that one or more caches are provided within the data processing apparatus for caching the instructions and data required by the processing circuitry. At any particular level in a cache hierarchy, separate instruction and data caches may be provided (often referred to as a Harvard architecture), or alternatively a unified cache may be provided for storing the instructions and data (often referred to as a Von Neumann architecture). When instructions are fetched from memory for storing in a cache, some known systems have employed pre-decoding mechanisms for performance orientated reasons. In accordance with such mechanisms, instructions are pre-decoded prior to storing in the cache, and in such cases the cache often then stores instructions in a wider format than the instructions stored in main memory, to accommodate the additional information produced by the pre-decoding process. To assist in improving performance when the instructions are later decoded and executed, the extra information provided in the pre-decoded instructions as stored in the cache has been used to identify branch instructions, identify classes of instructions (e.g. load/store instructions, coprocessor instructions, etc) to later assist multi-issue circuitry in dispatching particular instructions to particular execution pipelines, and to identify instruction boundaries in variable length instruction sets. 
     For example, the article “Performance Evaluation Of A Decoded Instruction Cache For Variable Instruction Length Computers”, IEEE Transactions on Computers, Volume 43, number 10, pages 1140 to 1150, October 1994, by G Intrater et al., discusses the storing of pre-decoded instructions in a cache. The article “The S-1 Project: Developing High-Performance Digital Computers” by L. Curtis Widdoes, Jr., Lawrence Livermore National Laboratory, 11 Dec. 1979, describes the S1 Mark IIA computer, where a decoded instruction cache expanded the 36-bit instruction word to a 56-bit instruction cache format to reduce instruction decoding time (see also the paper “Livermore S-1 Supercomputer—A Short History” appearing on the website http://www.cs.clemson.edu/˜mark/s1.html). Further, the idea of using pre-decoding mechanisms to pre-identify branches and pre-identify instruction boundaries is discussed in the AMD K5 Processor Data sheet, Publication no. 18522E-0, September 1996, Section 4.5, Innovative x86 Instruction Predecoding, page 6, which discusses adding 4 bits per instruction byte to identify start, end, opcode position, and number of Rops (RISC operations) the individual x86 instruction requires for later translation. 
     Whilst the above-mentioned pre-decoding mechanisms can improve the performance of the processing circuitry, they do not typically significantly alleviate the earlier mentioned power cost associated with the later decoder circuits used to decode the instructions once they are output from the instruction cache. 
     When instructions are fetched from memory, a plurality of instructions are typically fetched at a time, sufficient to fill a cache line within the cache, and when employing pre-decoding techniques, a cache line&#39;s worth of instructions will hence be passed through the pre-decoding circuitry prior to storing the pre-decoded instructions in the cache. Some instruction sets do not require the instructions to be aligned with cache line boundaries (for example the Thumb 32 instruction set developed by ARM Limited, United Kingdom), and accordingly situations can arise where a particular instruction crosses a cache-line boundary. In such instances, the instruction can be considered to comprise of a plurality of instruction portions, and it will be appreciated that the pre-decoding circuitry may not have access to all of the plurality of instruction portions of a particular instruction when it performs the pre-decoding operation, if that instruction crosses a cache-line boundary. In such situations, the pre-decoding operation will be performed on only part of that instruction, and accordingly will be incomplete. This can lead to the pre-decoded instruction being corrupted, which could lead to incorrect operation of the processing circuitry if the instruction is subsequently read from the instruction cache. 
     SUMMARY OF THE INVENTION 
     Viewed from a first aspect, the present invention provides a data processing apparatus comprising: processing circuitry for executing a sequence of instructions fetched from memory; pre-decoding circuitry for receiving the instructions fetched from memory and for performing a pre-decoding operation to generate corresponding pre-decoded instructions; a cache for storing the pre-decoded instructions for access by the processing circuitry; for a first set of instructions, each instruction comprising a plurality of instruction portions, and the pre-decoding circuitry generating a corresponding pre-decoded instruction comprising a plurality of pre-decoded instruction portions; if when applying the pre-decoding operation to an instruction in the first set, the pre-decoding circuitry does not have access to all of the plurality of instruction portions of that instruction, the pre-decoding operation performed being incomplete and the pre-decoding circuitry being arranged to provide in association with at least one pre-decoded instruction portion generated an indication that that pre-decoded instruction portion relates to an incomplete pre-decoding operation. 
     In accordance with the present invention, pre-decoding circuitry receives the instructions fetched from memory and performs a pre-decoding operation to generate corresponding pre-decoded instructions, which are then stored in a cache for access by the processing circuitry. For a first set of instructions which can be executed by the processing circuitry, each instruction comprises a plurality of instruction portions and the pre-decoding circuitry generates a corresponding pre-decoding instruction comprising a plurality of pre-decoded instruction portions. If at the time one of those instructions is subjected to the pre-decoding operation, the pre-decoding circuitry does not have access to all of the plurality of instruction portions of that instruction, the pre-decoding circuitry provides in association with at least one pre-decoded instruction portion that it does generate, an indication that that pre-decoded instruction portion relates to an incomplete pre-decoding operation. 
     By providing such an indication in association with one or more pre-decoded instruction portions, if a pre-decoded instruction including such a pre-decoded instruction portion is later read from the cache by the processing circuitry, that indication will flag to the processing circuitry that the pre-decoded instruction is potentially corrupted, allowing the processing circuitry to take appropriate action. In one embodiment, this may involve invalidating any pre-decoded instruction portion for which the indication is provided, and causing the corresponding instruction portion(s) to be re-fetched from memory and re-passed through the pre-decoding circuitry. However, in an alternative embodiment, the manner in which the additional information created by the pre-decoding operation is encoded in the pre-decoded instruction enables the original instruction to be recreated, such that if the indication of an incomplete pre-decoding operation is detected, the original instruction can be recreated and re-passed through the pre-decoding circuitry, without the need to re-fetch that instruction from memory. This will typically be more power efficient than embodiments that require the earlier-mentioned invalidate and re-fetch approach, and will also provide a significantly faster mechanism for fixing a pre-decode error, hence improving performance. 
     Hence, through use of the present invention, incorrect operation resulting from execution of instructions that have been incompletely pre-decoded can be avoided without significant additional cost and complexity in the design. 
     There may be a number of reasons why the pre-decoding circuitry does not have access to all of the plurality of instruction portions of an instruction at the time the pre-decoding operation is applied. However, in one embodiment, the cache has a plurality of cache lines for storing the pre-decoded instructions and the pre-decoding circuitry does not have access to all of the plurality of instruction portions of an instruction in the first set if that instruction crosses a cache line boundary between two cache lines, and only the instructions for one of the two cache lines have been fetched from memory at the time the pre-decoding operation is performed. 
     Whilst the use of the indication mechanism of the present invention enables situations to be detected where an incomplete pre-decoding operation has been applied, it would be desirable where possible to avoid the use of such indications, as this would give power saving benefits by avoiding unnecessary re-passing of an instruction through the pre-decoding circuitry. In one embodiment, a reduction in the number of indications produced by the pre-decoding circuitry is facilitated by providing a buffer within the pre-decoding circuitry for temporarily buffering one or more instruction portions. In particular, in the event that fetch circuitry used to fetch the sequence of instructions from memory is fetching a stream of instructions from memory that are associated with multiple consecutive cache lines, the buffer is employed to buffer one or more instruction portions of an instruction crossing a cache line boundary and associated with a first cache line. As a result, when the remaining one or more instruction portions of that instruction are retrieved by the fetch circuitry associated with a second cache line, the pre-decoding circuitry then has access to all of the plurality of instruction portions of that instruction when applying the pre-decoding operation. Hence, in such a situation, the requirement to provide an indication of an incomplete pre-decoding operation can be avoided, thereby reducing the number of situations where an instruction needs to be re-passed through the pre-decoding circuitry. 
     The pre-decoding operation may take a variety of forms. In one embodiment, the pre-decoded instructions include sideband information for reference by the processing circuitry, when performing the pre-decoding operation for an instruction in the first set, the sideband information being incorporated in at least one pre-decoded instruction portion of the pre-decoded instruction. When the pre-decoding operation is incomplete, and the sideband information is included in a generated pre-decoded instruction portion, said indication provided by the pre-decoding circuitry identifies that that sideband information is incomplete. Hence, in such embodiments, the sideband information created by the pre-decoding operation is incorporated in at least one pre-decoded instruction portion, and in such instances if the pre-decoding operation is incomplete, the indication provided by the pre-decoding circuitry identifies that the sideband information is incomplete, and hence cannot be relied upon as being accurate. 
     In one particular embodiment, the instructions in the first set of instructions comprise first and second instruction portions, and the sideband information is included in the pre-decoded instruction portion corresponding to the first instruction portion. In such instances, it can be seen that if the first instruction portion is at the end of a first cache line, and the second instruction portion is at the beginning of a second cache line, and the pre-decoding circuitry does not have access to the second cache line at the time the pre-decoding operation is applied to that instruction, then the sideband information included in the pre-decoded instruction portion corresponding to the first instruction portion will be incomplete, and accordingly cannot be assumed to be correct. Through use of the indication mechanism of the present invention, such a situation can later be detected when the pre-decoded instruction is read from the cache. 
     There are a number of ways in which the indication can be provided in association with a generated pre-decoded instruction portion. In one embodiment, the pre-decoding circuitry is arranged to provide said indication in association with the generated at least one pre-decoded instruction portion by setting an incomplete field provided in said at least one pre-decoded instruction portion, thereby causing an error to be detected when the pre-decoded instruction including that pre-decoded instruction portion is later retrieved from the cache. Hence, in such an embodiment, the pre-decoded instruction portion is directly marked by setting of an incomplete field provided in that pre-decoded instruction portion, and this incomplete field is analysed when the instruction is read from the cache, in order to detect situations where the pre-decoding operation applied to a particular instruction has been incomplete, and accordingly corrective action should be taken. 
     In many embodiments, the processing circuitry will be able to execute instructions not only from the first set of instructions, but also from one or more additional sets of instructions. For example, the processing circuitry may also be able to execute instructions from a second set of instructions, where the instructions of the second set are such that when the pre-decoding operation is applied to such instructions, the pre-decoding operation will always be complete. As an example, the instructions of the second set may be constrained so as to always be aligned with a cache line boundary. For example, the ARM instruction set developed by ARM Limited constrains the addresses of the 32-bit instructions so that they are always aligned to a 32-bit boundary, and hence are aligned with a cache line boundary. As another example, the Thumb 16 instruction set developed by ARM Limited constrains the addresses of the 16-bit instructions so that they are always aligned to a 16-bit boundary, and hence are aligned with a cache line boundary. However, as mentioned earlier, Thumb 32 instructions will not necessarily be aligned with cache line boundaries, as 16 bit alignment is maintained for such 32-bit instructions. 
     As another way of providing the indication of an incomplete pre-decoding operation, in accordance with an alternative embodiment, when an instruction in the first set is encountered by the pre-decoding circuitry at a time where it does not have access to all of the plurality of instruction portions of that instruction, the pre-decoding circuitry is arranged to set an identifier field provided in at least one generated pre-decoded instruction portion to identify that at least one pre-decoded instruction portion as relating to an instruction in the second set of instructions. By such an approach, this forces an error when the instruction is later read from the cache, since it will be detected that the relevant instruction portion is in fact part of an instruction of the first instruction set, rather than from the second instruction set as indicated by the identifier field, and this can be used to instigate the same corrective action as discussed earlier. Hence, in accordance with the above embodiment, even though the pre-decoding circuitry knows that the instruction is from the first instruction set, in situations where the pre-decoding operation is incomplete, it marks the instruction as being from the second instruction set so as to provoke detection of an error later. In one particular embodiment, the first instruction set is the Thumb 32 instruction set, and the second instruction set is the Thumb 16 instruction set, and hence a Thumb 32 instruction which is incompletely pre-decoded is marked as a Thumb 16 instruction so as to force an error when that instruction is later read from the cache. 
     The manner in which such errors are detected can take a variety of forms. However, in one embodiment the data processing apparatus further comprises error detection circuitry responsive to a pre-decoded instruction being accessed from the cache, to detect if at least one pre-decoded instruction portion of that pre-decoded instruction has said indication associated therewith, and on such detection to recreate the original instruction from the pre-decoded instruction and to cause the original instruction to be re-passed through the pre-decoding circuitry. This provides a power efficient mechanism for later producing the correct pre-decoded instruction. In an alternative embodiment, if the original instruction cannot be recreated from the pre-decoded instruction, then the error detection circuitry invalidates the at least one pre-decoded instruction portion in the cache, and causes the corresponding at least one instruction portion to be re-fetched from the memory and re-passed through the pre-decoding circuitry. 
     Typically the cache will comprise a data portion having a plurality of cache lines for storing the pre-decoded instructions, and a tag portion having an entry for each cache line, each entry storing an address identifier associated with the pre-decoded instructions stored in the corresponding cache line. As an alternative to providing the indication of an incomplete pre-decoding operation in association with the relevant pre-decoded instruction portion(s), in an alternative embodiment the indication is stored in the entry of the tag portion associated with the cache line containing the at least one pre-decoded instruction portion relating to an incomplete pre-decoding operation. When set, this indication will flag that the one or more pre-decoded instruction portions appearing at the end of the cache line associated with a particular instruction crossing a cache line boundary cannot be relied upon to have been pre-decoded correctly. 
     If there is a “spare” bit available in the pre-decoded instruction portion that needs to be identified as relating to an incomplete pre-decoding operation, it is more area efficient to provide the earlier-mentioned incomplete field in that spare bit location, since by such an approach the required indication can be provided without any increase in size of the pre-decoded instruction. However, in situations where there is no spare space in the pre-decoded instruction, and accordingly it would be necessary to increase the size of the pre-decoded instruction to enable the provision of such an incomplete field, then the alternative approach described above of providing the indication in association with the relevant entry in the tag portion may be more area efficient, since the indication will at most need to be provided in association with only one of the instructions in a particular cache line, namely the instruction at the end of the cache line if that instruction crosses a cache line boundary. 
     Viewed from a second aspect, the present invention provides a method of pre-decoding instructions in a data processing apparatus, the data processing apparatus including processing circuitry for executing a sequence of instructions, the method comprising the steps of: retrieving instructions from memory and performing a pre-decoding operation on those instructions to generate corresponding pre-decoded instructions; storing each pre-decoded instruction in a cache; for a first set of instructions, each instruction comprising a plurality of instruction portions, and the pre-decoding operation generating a corresponding pre-decoded instruction comprising a plurality of pre-decoded instruction portions; when applying the pre-decoding operation to an instruction in the first set, if the pre-decoding operation does not have access to all of the plurality of instruction portions of that instruction, the pre-decoding operation performed being incomplete and the pre-decoding operation providing in association with at least one pre-decoded instruction portion generated an indication that that pre-decoded instruction portion relates to an incomplete pre-decoding operation. 
     View from a third aspect, the present invention provides a data processing apparatus comprising: processing means for executing a sequence of instructions fetched from memory; pre-decoding means for receiving the instructions fetched from memory and for performing a pre-decoding operation to generate corresponding pre-decoded instructions; cache means for storing the pre-decoded instructions for access by the processing means; for a first set of instructions, each instruction comprising a plurality of instruction portions, and the pre-decoding means for generating a corresponding pre-decoded instruction comprising a plurality of pre-decoded instruction portions; if when applying the pre-decoding operation to an instruction in the first set, the pre-decoding means does not have access to all of the plurality of instruction portions of that instruction, the pre-decoding operation performed being incomplete and the pre-decoding means providing in association with at least one pre-decoded instruction portion generated an indication that that pre-decoded instruction portion relates to an incomplete pre-decoding operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described further, by way of example only, with reference to embodiments as illustrated in the accompanying drawings, in which: 
         FIG. 1  schematically illustrates a data processing apparatus according to one embodiment; 
         FIG. 2  is a flow diagram illustrating a series of steps carried out by a data processing apparatus according to one embodiment; 
         FIG. 3  is a flow diagram illustrating in more detail a series of steps carried out by pre-decoding circuitry when providing an abnormal indicator; 
         FIG. 4  is a flow diagram illustrating a series of steps carried out by processing circuitry when fetching and executing instructions; 
         FIG. 5  illustrates one particular example embodiment of the data processing apparatus; 
         FIG. 6  illustrates two instruction formats prior to pre-decoding taking place; 
         FIG. 7  illustrates how data representing program instructions can be stored in memory space; 
         FIGS. 8A to 8D  illustrate several possible situations in which pre-decoding errors could arise; 
         FIG. 9  illustrates the format of two example instructions after pre-decoding in accordance with one embodiment; 
         FIG. 10  is a table listing situations where pre-decoding errors may or may not occur; 
         FIG. 11  is a flow diagram illustrating the operation of the data processing system in some embodiments; 
         FIG. 12  illustrates the use of an incomplete field within a pre-decoded Thumb 32 instruction in accordance with one embodiment; 
         FIG. 13  illustrates two possible routes through the data processing apparatus for a Thumb 32 instruction that crosses a cache line boundary, in accordance with one embodiment; 
         FIG. 14  is a flow diagram illustrating a pre-decoding operation sequence in accordance with one embodiment; 
         FIG. 15  illustrates the format of two example instructions after pre-decoding in accordance with one embodiment; 
         FIG. 16  is a flow diagram illustrating in more detail the steps performed to implement step  202  of  FIG. 11  in accordance with one embodiment; 
         FIG. 17  illustrates components provided within the instruction cache in accordance with one embodiment; 
         FIG. 18  illustrates how each entry of the tag RAM of the cache is extended to store speculative processor state in accordance with one embodiment; 
         FIG. 19  schematically illustrates the construction of a two-way set associative instruction cache as used in one embodiment; 
         FIG. 20  is a flow diagram illustrating the pre-decoding operation according to one embodiment; 
         FIG. 21  illustrates a cache lookup procedure in accordance with one embodiment; 
         FIG. 22  illustrates one embodiment where the pre-decoder circuitry is associated with a combined level two cache within the data processing apparatus; 
         FIG. 23  illustrates the use of an additional instruction/data identifier value in association with each cache line in the combined level two cache of  FIG. 22 ; 
         FIG. 24  schematically illustrates a portion of a processing pipeline having decoding circuitry including a shared portion used to decode pre-decoded instructions corresponding to instructions in different instruction sets; 
         FIG. 25  illustrates how different portions of an ARM instruction may be pre-decoded into a pre-decoded instruction having a shared format with Thumb32 instructions; 
         FIG. 26  illustrates how a Thumb32 and an ARM instruction can respectively be pre-decoded into pre-decoded instructions having a shared format to represent shared functionality; 
         FIG. 27  illustrates how an ARM instruction may be pre-decoded into a pre-decoded instruction corresponding to an unused portion within the Thumb32 instruction bitspace; 
         FIG. 28  illustrates how two adjacent Thumb16 instructions may be concatenated and pre-decoded into a pre-decoded instruction corresponding to a single Thumb32 instruction; 
         FIG. 29  is a flow diagram schematically illustrating the re-mapping of ARM instructions to Thumb32 instructions; 
         FIG. 30  illustrates how two Thumb16 instructions can be re-mapped during pre-decoding to a pre-decoding instruction corresponding to a single Thumb32 instruction; and 
         FIG. 31  is a flow diagram schematically illustrating how branch instructions can be identified during pre-decoding and used to trigger early termination of pre-decoding. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  schematically illustrates a data processing system  1  according to one embodiment. The data processing system  1  includes a processor system interface  5  which contains instruction fetching circuitry for fetching data representing program instructions from memory (not shown). The processor system interface  5  passes instructions fetched from memory to pre-decoding circuitry  10 . The pre-decoding circuitry  10  performs pre-decoding operations on the instructions to generate pre-decoded instructions, which are passed to a line-fill buffer  15  and stored in an instruction cache  20 . Instructions stored in the instruction cache  20  are executed by processing circuitry  25 . The processing circuitry  25  includes decoding circuitry  30  for generating control signals from the pre-decoded instructions which control the processing circuitry so as to perform processing operations. The error detection circuitry  35  is employed to detect certain errors occurring in the pre-decoded instructions and to cause the relevant instructions, or the cache lines containing those instructions, to be re-passed through the pre-decoding circuitry  10  (either directly or via initiation of a linefill operation) before being supplied to the processing circuitry  25 . In the event such errors are detected, an error signal is sent from the error detection circuitry  35  to the processing circuitry  25  to cause the decoding circuitry  30  to cease any decoding that may have begun in respect of such pre-decoded instructions for which an error has been detected, the processing circuitry  25  instead awaiting the pre-decoded instruction obtained by the re-pass through the pre-decoding circuitry  10 . 
     Pre-decoding instructions before storing them in an instruction cache is a technique that has previously been implemented to enhance the performance of a data processing apparatus. However the performance benefit of pre-decoding instructions in this manner generally comes at the cost of greater power consumption when later fetching those instructions from the instruction cache for execution, due to the greater length of the pre-decoded instruction resulting from the pre-decoding operation. However, since fetching cache lines from main memory is usually a rare event compared with fetching from the instruction cache, it has been realised that a pre-decoder would normally operate far less on a given instruction than the later decoder and thus the potential existed for a power saving in the data processing apparatus to be achieved. Yet it was also realised that it would only be possible to reduce overall power consumption if one could identify a feature of an instruction for which the power consumed by accessing a wider instruction cache is outweighed by the power saving made by not having to decode that feature after the instruction cache. 
     As stated above instruction pre-decoding is generally implemented as a performance-enhancing technique and the added power consumption is viewed as a price worth paying for the performance benefit. However, it has been found that the decoding circuitry required for the identification of abnormal instructions in the decode stage after the instruction cache could contribute significantly to the total gate count of the decoder. This is even more significant in a secure data processing apparatus since abnormal instructions must be decoded in a consistent, deterministic way so that no matter what condition the processor is in and no matter what instructions it has recently executed it will always behave in the same way with a specific abnormal instruction. Because abnormal instructions must be decoded in a consistent, deterministic way, more gates must be put into the decoder to deal with corner cases in the instruction set architecture. By shifting this identification to the pre-decode stage and providing an abnormal instruction identifier in association with the pre-decoded instruction, it has been found that the added power consumption of this arrangement was outweighed by the power saving of not having to identify abnormal instructions at the decode stage. 
       FIG. 2  schematically illustrates a series of steps carried out by data processing apparatus  1  when performing a pre-decode operation in accordance with one embodiment. The flow begins at step  300  within pre-decoding circuitry  10 , an instruction having been retrieved from memory (via the processor system interface  5 ). At step  302  it is established if the instruction is abnormal or not. In one embodiment an abnormal instruction is defined to be one that is either undefined or unpredictable. An undefined instruction is one that is within the instruction space, yet not part of the instruction set. There can clearly be a great number of such undefined instructions, since in a 32-bit (for example) instruction space there are 2 32  (over 4 billion) possible encodings. On the other hand, an unpredictable instruction is one which is a defined instruction, yet is being used in a manner such that it will not result in a well-defined outcome in the data processing apparatus, for example a load or store instruction which attempts to perform a base register writeback with the base set to the program counter register. 
     If the instruction is identified as abnormal, then at step  304  pre-decoding circuitry  10  provides an identifier identifying that instruction as abnormal. This identifier is associated with the pre-decoded instruction that the pre-decoding circuitry  10  generates, in one embodiment side band bits in the pre-decoded instruction serving as the identifier. 
     If however at step  302  the instruction is identified as not being abnormal, then step  304  is omitted. The flow proceeds to step  306  where pre-decoding circuitry  10  performs any other pre-decode steps that are required (examples of other pre-decode steps are given later in the description). Finally, at step  308  the pre-decoded instruction is stored in instruction cache  20  (via linefill buffer  15 ). 
       FIG. 3  schematically illustrates in more detail a series of steps carried out in one embodiment by pre-decoding circuitry  10  in step  304  of  FIG. 2 . Firstly, at step  320  it is checked if the pre-decoding circuitry  10  is currently configured to convert all abnormal instructions into a predetermined undefined instruction. If so, then the flow proceeds to step  324 , where this conversion takes place. In embodiments of the present invention in which the pre-decoding circuitry is configured to do this, this allows the mechanism for handling abnormal instructions in the processing circuitry to be particularly simple, since only one particular undefined instruction will ever be received by the processing circuitry, and its response (for example to call a dedicated exception routine) is then well-defined and predictable for all abnormal instructions. From step  324 , the flow continues to step  306  in  FIG. 2 . Hence in this embodiment, the use of the pre-determined undefined instruction itself provides the “indicator” of the abnormal instruction. 
     Otherwise the flow proceeds to step  326  where it is checked if the pre-decoding circuitry  10  is currently configured to label all abnormal instructions as undefined. This configuration of pre-decoding circuitry  10  also simplifies the downstream mechanisms for handling abnormal instructions, since all abnormal instructions are then handled as if they are undefined instructions and the processing circuitry&#39;s response is then well-defined and predictable for all abnormal instructions (for example, such instructions could trigger an exception, or alternatively could be NOP (no-operation) handled, passing through the processing circuitry without having any effect on the internal state of the processing circuitry as a result). If all abnormal instructions should be labelled as undefined, then the flow proceeds to step  330 , where the corresponding indicator is generated, in this case adding side band bits to the pre-decoded instruction indicating that the instruction is undefined. 
     If the pre-decoding circuitry  10  is not currently configured to label all abnormal instructions as undefined, then the flow proceeds to step  328 , where it is checked if the identified abnormal instruction is an undefined instruction. If so, the flow proceeds to step  330  and side band bits are added to the pre-decoded instruction indicating that the instruction is undefined. If the abnormal instruction is not undefined, then in this embodiment it must be unpredictable, and at step  332  side band bits are added to the pre-decoded instruction indicating that the instruction is unpredictable. All paths through  FIG. 3  finally merge to proceed to step  306  in  FIG. 2 . 
       FIG. 4  illustrates a series of steps carried out by processing circuitry  25  when fetching and executing instructions that may have an associated indicator labelling the pre-decoded instruction as abnormal. At step  350  a pre-decoded instruction is fetched from the instruction cache  20  into the processing circuitry  25 . 
     Next, at step  352  the pre-decoded instruction is examined to establish if it has been labelled as abnormal, by means of an associated indicator. If the pre-decoded instruction is not so labelled as abnormal, then at step  354  normal execution (preceded by further decoding by decoding circuitry  30  as necessary) of the instruction is carried out. 
     On the other hand, if the instruction is abnormal then the processing circuitry  25  executes the instruction in a predetermined manner (step  356 ). In this embodiment the processing circuitry is configured to do this in one of two ways. Firstly, at step  358 , if the processing circuitry is configured to NOP the instruction, then at step  360  the instruction is executed as NOP, passing through the processing circuitry without any state of the processing circuitry changing as a result. Alternatively, if the processing circuitry is configured to trigger an exception in response to an abnormal instruction, then at step  362  a suitable exception is called. 
     Hence, according to the above described embodiment, a data processing apparatus is provided in which power savings may be made by identifying abnormal instructions at a pre-decode stage. By generating an identifier showing that an instruction is abnormal, when the pre-decoded instruction is later retrieved from an instruction cache, the processing circuitry may more swiftly handle the abnormal instruction. Furthermore, the early identification of abnormal instructions enables the provision of a data processing apparatus which responds in a predictable and well-defined manner to instructions which could otherwise jeopardize the operation and security of the device. 
     The data processing system  1  processes instructions from one or more instruction sets. For example, some processors produced by ARM Limited of Cambridge may execute instructions from the ARM instruction set, as well as from the mixed-length Thumb 2 instruction set. Thumb 2 instructions may be 16 bits or 32 bits long. Thus, the decoding circuitry  30  can include decoders for each set of instructions. 
       FIG. 5  shows an example of the data processing apparatus  1  in which the decoding circuitry  30  includes an ARM decoder  40 , a Thumb-32 decoder  45  and a Thumb-16 decoder  50 . The decoders  40 ,  45 ,  50  are used to decode pre-decoded instructions stored in the instruction cache  20 . The decoders  40 ,  45 ,  50  generate control signals from the pre-decoded instructions which control the processing circuitry so as to perform processing operations. A multiplexer  65  selects which of the decoders  40 ,  45 ,  50  is used depending on the processor state. 
       FIG. 6  shows a Thumb-16 (T16) instruction  100  and a Thumb-32 (T32) instruction  110  before they are pre-decoded. A T16 instruction  100  consists of a 16-bit opcode (including all fields). A T32 instruction  110  consists of two portions. The first portion T32a consists of 16-bits. Bits [15:13] in the first portion T32a of a T32 instruction  110  are set to 0b111. Bits [12:0] of the first portion T32a store the first 13 bits of the T32 opcode. The second portion T32b of a T32 instruction  110  contains 16 bits which comprise the second half of the T32 opcode. For T16 and T32 instructions, the two portions of the T32 are the same length as each other, and the same length as a T16. However, this is not necessarily the case for all mixed-length instruction sets. 
       FIG. 7  shows how data representing program instructions can be stored in the memory space  150 . Data is arranged in blocks  170 , each block representing a portion of a program instruction. For example, each block may represent a T16 instruction, the first portion T32a of a T32 instruction or the second portion T32b of a T32 instruction. Blocks are arranged in cache lines  160 . For simplicity, in  FIG. 7  a cache line  160  is depicted as containing four blocks, but it will be appreciated that a cache line  160  can store other (typically large) numbers of blocks  170 . A T32 instruction may span the boundary between two cache lines  160 , so that the last block of one cache line represents T32a and the first block of the following cache line represents T32b. 
     When a program is executed, the processing circuitry executes instructions stored in the instruction cache  20  if possible. If an instruction is not present in the instruction cache  20 , then a cache miss occurs and blocks of data representing the instruction are fetched from memory. The fetching circuitry within the processor system interface  5  fetches a cache line  160  including the block or blocks representing the desired instruction from memory. The fetched blocks are pre-decoded by the pre-decoding circuitry  10  before being placed in the instruction cache  20  ready for further decoding. However, when blocks are fetched, the pre-decoding circuitry  10  cannot determine with certainty which portion of an instruction a block represents. Thus, the pre-decoding circuitry  10  makes a speculative identification of which portion of an instruction a fetched block represents. 
     The instruction blocks may have some bits which give an indication of which portion a block is likely to represent. For example, with the mixed-length Thumb2 instructions a block representing the first portion T32a of a T32 instruction will have bits [15:13] set to 0b111. A pre-decoding operation is performed on a block, depending on the identification made by the pre-decoding circuitry  10 . For example, an abnormal instruction identifier may be added to the block indicating whether or not the block is an abnormal instruction. However the same bit pattern could potentially also arise for the second portion T32b of a T32 instruction, depending on the value of the opcode for that instruction. Thus, the speculative identification may be incorrect. If an incorrect identification of a block is made, then the pre-decoding circuitry  10  may perform the wrong pre-decoding operation on that block, resulting in a pre-decoding error. 
       FIGS. 8A to 8D  illustrate several possible situations in which errors could arise.  FIG. 8A  shows an error which can occur when the processor branches into the middle of a cache line. If a cache miss occurs, the processor system interface fetches the cache line from memory. The pre-decoding circuitry  10  starts pre-decoding from the point at which the program branches to so that the processor may unstall as soon as possible. The pre-decoding circuitry  10  then continues pre-decoding blocks up to the end of the cache line. The pre-decoding circuitry  10  then returns to the beginning of the cache line, makes a speculative identification of which portion the first block in the cache line represents, and pre-decodes the remaining blocks in the cache line. The speculative identification of the first block in the cache line may be incorrect, causing a pre-decoding error. For example, in  FIG. 8A , the first block of in the cache line, which actually represents a T32b portion, could be mistakenly identified as a T32a portion or a T16 portion, depending on the value of bits [15:13] of the block. 
     Another type of error can occur when a branch misprediction occurs, causing the program flow to branch into the middle of a T32 instruction by mistake, as illustrated in  FIG. 8B . In this case the pre-decoding circuitry  10  may incorrectly identify the following T32b portion as a T16 instruction or as a T32a. Subsequent blocks may also be misidentified. For example, if the block at the branch point, which actually represents a T32b portion, is mistakenly identified as a T32a, then the following block will be identified as a T32b. This could cause further errors. 
     If a T32 instruction spans a cache line boundary, as illustrated in  FIG. 8C , an error may occur when the pre-decoding circuitry  10  decodes the second cache line. The T32b portion may be identified as a T16 instruction or as a T32a. 
       FIG. 8D  illustrates an error which may occur if the cache line contains data which does not represent a program instruction (a literal). In this case the pre-decoding circuitry  10  may interpret the literal as an instruction portion. This may cause a knock on effect in the identification of subsequent blocks. For example, if the pre-decoding circuitry  10  identifies the literal as a T32a portion, it will pre-decode the following block as a T32b portion of the T32 instruction. 
     Thus, there are several situations in which pre-decoding errors may arise. Therefore, in embodiments the data processing system  1  is provided with error detection circuitry  35  which can detect if the speculative identification made by the pre-decoding circuitry  10  was or was not correct and if not, signal a pre-decoding error and correct the error instructions in the cache  20 . Pre-decoding errors may be corrected by sending at least part of pre-decoded instructions for which errors have been detected back to the pre-decoding circuitry  10  to be pre-decoded again. In some situations the pre-decoding circuitry  10  may be able to regenerate the original instruction blocks from the pre-decoded blocks. An alternative method of correcting errors is to invalidate a cache line in the cache  20  which contains an error and cause the cache line to be re-fetched from memory and pre-decoded again. 
     In order for the error detecting circuitry to be able to detect as which portion the pre-decoding circuitry  10  speculatively identified a block, the pre-decoding circuitry marks each block with an instruction portion identifier that indicates as which portion the block has been identified. The instruction portion identifier is added to a block as one or more additional bits. 
     In one embodiment, the pre-decoding operation is performed in order to add an abnormal instruction identifier to a T32 instruction indicating whether or not the instruction is abnormal or not. This abnormal instruction identifier is added to the first portion T32a of the T32 instruction in the form of sideband information. In this embodiment, no sideband information is added to the second portion T32b of the T32 instruction or the T16 instruction. 
     Thus, the T16 opcode and the second half of the T32 opcode are unmodified by the pre-decoding circuitry  10 . This means that it does not matter if the pre-decoding circuitry  10  mistakenly identifies a block representing a T16 instruction as the second portion T32b of a T32 instruction, because the opcode will be unmodified and so the block stored in the instruction cache  20  will be the same regardless of the identification made. Similarly, it does not matter if the pre-decoding circuitry  10  identifies a block which actually represents a T32b as a T16. Again, no pre-decoding error will arise from an incorrect identification. This means that the number of pre-decoding errors which occur can be reduced. If fewer pre-decoding errors occur, then the error detection circuitry  35  needs to correct fewer errors and the pre-decoding circuitry  10  does not need to pre-decode as many instructions more than once. Thus, the power consumed by the data processing system  1  will be reduced. 
     As it does not matter whether a block is identified as a T16 or a T32b, the pre-decoding circuitry  10  adds the same instruction portion identifier to a block for each case. Thus, the instruction portion identifier needs only to identify whether or not a block represents the first portion T32a of a T32 instruction. This means only one bit is required for the instruction portion identifier, and so the instruction does not need to be unduly extended. This means less power is consumed in storing pre-decoded instructions in the cache  20 . In one embodiment, the instruction portion identifier is set to “1” when a block is identified as representing a T32a, and set to “0” when a block represents either a T32b or a T16 instruction. 
       FIG. 9  shows the format of the T16 and T32 instructions after pre-decoding in one example embodiment. If a block is identified as representing the first portion T32a of a T32 instruction, the pre-decoding circuitry  10  adds an instruction portion identifier (ID bit) at bit [16] and sets the instruction portion identifier to “1”. Sideband information is inserted indicating whether or not the instruction is an abnormal instruction. Since the instruction portion identifier at bit [16] now indicates that the block represents a T32a portion of a T32 instruction, bits [15:13] are redundant for identifying the block. Thus, sideband information can be inserted into these bits without loss of information. One way of doing this is do insert sideband bits in bits [15:14] of the T32a portion, one bit indicating whether or not the instruction is undefined and one bit indicating whether or not the instruction is unpredictable. Thus, sideband information may be added to the block without extending the length of the block, reducing the power consumed in storing the instructions in the cache  20 . 
     As sideband information for a T32 instruction is only added to the first portion T32a of the instruction, this allows the pre-decoding operation performed on the second portion T32b to be the same as for a T16 instruction, so that the number of errors which occur can be reduced. If a block is identified as representing a T16 instruction, the pre-decoding circuitry  10  adds an instruction portion identifier (ID bit) at bit [16] and sets the instruction portion identifier to 0. The T16 opcode in bits [15:0]is not modified. If a block is identified as representing the second portion of a T32 instruction, then the same pre-decoding operation is performed on the block as for a T16 instruction. Bit [33] of the T32 instruction (corresponding to bit [16] of the block representing the second portion T32b of the T32 instruction) is set to 0. The remainder of the second half of the T32 opcode is not modified. 
     Thus, the pre-decoding circuitry does not need to detect errors when a T16 has been identified as a T32b or vice versa, and so has fewer errors to detect. The only errors which need to be detected are when a T32b portion is incorrectly speculatively identified as a T32a portion, when two consecutive blocks are marked as a T32a portion (this may occur when a T32 instruction spans a cache line boundary), or when a T32a portion is mistakenly identified as a T32b portion (caused by a knock on effect from a mis-identification of a previous block).  FIG. 10  shows a table listing the situations in which errors may or may not occur. The column Native Instruction Stream shows which portion a sequence of blocks in a cache line actually represent. The column Pre-decode Speculation shows as which portion the pre-decoding circuitry has speculatively identified the blocks. Some pre-decoding errors may occur when a branch mispredict occurs causing the program flow to branch into the middle of a T32 instruction. Other errors may occur when the flow continues from a previous cache line. 
     It will be appreciated that the present technique is not restricted to instructions within the Thumb2 instruction set, but is applicable to instructions from any variable-length instruction set, or to instructions from instruction sets of differing lengths. The present technique can be implemented by ensuring that for at least one portion of an instruction from a first set of instructions and at least one portion of an instruction from a second set of instructions the pre-decoding operation performed on a block is the same regardless of as which of the portions the pre-decoding circuitry  10  identifies the block. Thus, the power consumed by the system can be reduced. 
       FIG. 11  shows a flow chart illustrating the operation of the data processing system  1  in some embodiments. Firstly, in step  200 , the instruction fetch circuitry within the processor system interface  5  fetches a cache line from memory comprising a plurality of blocks of data representing program instructions. Next, in step  202  the blocks of data are passed to the pre-decoding circuitry  10 , which pre-decodes the blocks by performing pre-decoding operations on the blocks and generates pre-decoded blocks. The pre-decoding operations will be described in more detail with reference to  FIG. 15 . In step  204  pre-decoded instructions made up of pre-decoded blocks are stored in the instruction cache  20 . Subsequently, in step  206  the error detection circuitry  35  checks for errors in the pre-decoded blocks stored in the cache  20 . In step  208  the error detection circuitry  35  identifies whether or not any pre-decoding errors have been detected. 
     If the error detection circuitry  35  identifies in step  208  that a pre-decoding error has been detected, the error detection circuitry  35  acts to correct the pre-decoding error. In one embodiment, the error detection circuitry  35  in step  210  sends at least part of any incorrectly pre-decoded blocks back to the pre-decoding circuitry  10 . Flow then returns to step  202 , in which the pre-decoding circuitry again pre-decodes the incorrectly pre-decoded blocks. 
     As an alternative to step  210 , the error detection circuitry  35  may instead perform step  212 . In step  212 , the error detection circuitry  35  invalidates the cache line in the instruction cache  20 , which contains the incorrectly pre-decoded instruction. Flow then returns to step  200 , in which the cache line containing the incorrectly pre-decoded instruction is re-fetched from memory. 
     If, on the other hand, the error detection circuitry  35  identifies in step  208  that no errors have been detected, flow passes to step  216 . In step  216 , the decoding circuitry  30  within the processing circuitry  25  decodes the pre-decoded instructions and generates control signals. Next, in step  220  the processing circuitry performs processing operations in response to the control signals. The process then comes to an end. 
     Another pre-decode error that can occur, and hence should be identified, is when a Thumb-32 instruction crosses a cache line boundary, and the second half-word is not available to the pre-decoding circuitry at the time the pre-decoding operation is performed. In normal operation, the pre-decoder can correctly pre-decode a Thumb-32 instruction that crosses a cache line boundary providing it has the first half-word from the previous cache line and the second half-word from the new cache line. However, when a Thumb-32 instruction crosses a cache line boundary and the new cache line has not been fetched at the time the pre-decoding operation is being performed, the pre-decoding operation will be incomplete and accordingly the sideband signals will not be pre-decoded correctly. 
     In order to enable the processing circuitry to later determine, when reading a pre-decoded instruction from the instruction cache, that the pre-decoding operation was incomplete, in one embodiment an incomplete field is added to the pre-decoded instruction to form an incomplete pre-decode identifier, this field being set if the pre-decoding circuitry does not have access to both portions of the Thumb-32 instruction when performing the pre-decoding operation, and in one particular embodiment the pre-decoded Thumb-32 instruction can be represented as shown in  FIG. 12 . In particular, when comparing  FIG. 12  with the earlier-discussed  FIG. 9 , it will be seen that each pre-decoded instruction portion has been extended from 17 to 18 bits, so that bits  0  to  17  represent the first pre-decoded instruction portion, and bits  18  to  35  represent the second pre-decoded instruction portion. Bits  0  to  16  of the first pre-decoded instruction portion are the same as in the earlier discussed  FIG. 9 , with the sideband signals (identifying the presence of an abnormal instruction) being inserted in bit positions  14  and  15 , and with the instruction portion identifier being inserted at bit position  16 . Further, bit position  17  is used to provide the incomplete field, this bit being set if the pre-decoding circuitry did not have access to the second half of the Thumb-32 instruction when performing the pre-decoding operation, thus indicating that the sideband signals in bit position  14  and  15  cannot be relied upon, and may in fact be incorrect. 
     Considering the second half of the pre-decoded instruction, bits  17  to  34  are the same as bits  16  to  33  of the earlier described embodiment shown in  FIG. 9 . Bit position  35  is added so as to make the second pre-decoded instruction portion the same length as the first pre-decoded instruction portion, and in one embodiment bit  35  is set equal to 0. However, as will be discussed in more detail later, in an alternative embodiment bit position  35  may be used for another purpose. 
     In an alternative embodiment, rather than employing the incomplete field in bit position  17 , the pre-decoding circuitry may be arranged in situations where it only has access to the first half-word of the Thumb 32 instruction when performing the pre-decoding operation, to set the instruction portion identifier in bit position  16  to identify that first pre-decoded instruction portion as relating to a Thumb 16 instruction, i.e. by setting bit position  16  to a logic 0 value. If such a pre-decoded instruction portion is later fetched from the instruction cache, the error detection circuitry  35  would observe that the instruction portion is actually part of a Thumb 32 instruction that has been incorrectly marked as a Thumb 16 instruction, and will accordingly signal an error, preventing the potentially corrupt pre-decoded instruction being executed by the processing circuitry. Accordingly, through this mechanism, the processing circuitry can be prevented from making use of an incompletely pre-decoded instruction without the need for a separate incomplete bit as shown in  FIG. 12 . 
     As another alternative embodiment to the embodiment shown in  FIG. 12 , the incomplete pre-decode identifier can be stored within the cache in association with the tag RAM entry for a particular cache line. When such an identifier is set (preferably this being provided as a single bit field), this will flag that the pre-decoded instruction portion appearing at the end of the cache line was subjected to an incomplete pre-decode operation, and hence cannot be relied upon to have been pre-decoded correctly. 
     In some embodiments, it may be more space efficient to provide this single identifier bit in association with each tag RAM entry, rather than seeking to accommodate that extra piece of information in each pre-decoded instruction. 
       FIG. 13  illustrates the two possible paths that a Thumb 32 instruction crossing a cache line boundary may take through the data processing apparatus. As shown in  FIG. 13 , a buffer  380  is provided for storing the last half word of a particular cache line fetched from the memory system via the processor system interface  5 , this buffered information being used if that last half word was the first half word of a Thumb 32 instruction. If the processor is continuously fetching consecutive cache lines from memory, then by buffering this last instruction portion, the pre-decoding circuitry  10  can wait for the next cache line to be routed from memory to the pre-decoding circuitry  10  via the processor system interface  5  and the multiplexer  60 , and at that point can perform the full pre-decode operation and generate the appropriate sideband signals using both half words of the instruction spanning the cache line boundary. As a result, when the pre-decoded instruction then passes through the linefill buffer  15  into the instruction cache  20 , it can subsequently be read from the instruction cache and passed directly through the decoding circuitry  30 , as indicated schematically by the path “A” in  FIG. 13 . 
     However, if the processor is not continuously fetching consecutive cache lines, the pre-decoded instruction portion corresponding to the first half of a Thumb 32 instruction crossing a cache line boundary will be subjected to an incomplete pre-decoding operation, and accordingly the incomplete pre-decode identifier will need to be set, using any of the above-mentioned three possible techniques. In such instances, when a Thumb 32 instruction including such a first pre-decoded instruction portion is read from the instruction cache  20 , the pre-decode error detection circuitry  35  will detect the setting of the incomplete pre-decode identifier, and in one embodiment will cause the entire Thumb 32 instruction to be re-routed back via the multiplexer  60  into the pre-decoding circuitry  10 , as indicated schematically by the path “B” in  FIG. 13 . In particular, a control signal is routed back to the multiplexer  60  from the pre-decode error detection circuitry  35  over path  37  to cause the multiplexer to select the instruction data presented over the loopback path  39 . In embodiments of the present invention, this is possible due to the way the information created by the pre-decoding operation is added into the instruction when creating the pre-decoded instruction. In particular, the sideband signals are added into bit positions  15  and  14  because those bit positions becomes redundant once the instruction portion identifier is inserted in bit position  16 . As a result, it is possible for the original instruction to be recreated readily from the pre-decoded instruction, and accordingly the instruction can be rerouted through the pre-decoding circuitry  10  without needing to re-fetch the instruction from memory. 
     In an alternative embodiment, if the original instruction cannot be recreated from the pre-decoded instruction, then the pre-decode error detection circuitry  35  can be arranged to cause the relevant two adjacent cache lines in the instruction cache to be invalidated, and a linefill operation to be performed to cause those two cache lines to be refetched sequentially from memory and passed through the pre-decoding circuitry. 
     It will be appreciated that following the detection of the error and repassing of the instruction through the pre-decoding circuitry, the instruction will then follow path A and can be retrieved into the decoding circuitry  30  when required by the processing circuitry. 
       FIG. 14  schematically illustrates the operation of the pre-decoding circuitry in accordance with the above-described technique. The pre-decoding operation starts at step  400 , and at step  402  it is determined whether the instruction is a Thumb 32 instruction. If it is, it is then detected at step  404  whether that Thumb 32 instruction crosses a cache line boundary, and if it does it is then determined at step  406  whether the processor is fetching from sequential cache lines. 
     If the processor is fetching from sequential cache lines, then the first instruction portion of the Thumb 32 instruction appearing at the end of the cache line is buffered in the register  380  at step  410 , whereafter at step  412  the pre-decoding circuitry  10  awaits the receipt of the next cache line from the processor system interface  5 . Once the next cache line is available, then the pre-decoding operation is performed on the Thumb 32 instruction at step  414  in the usual manner. The process will also proceed directly to step  414  if it is detected at step  402  that the instruction is not a Thumb 32 instruction, since in one embodiment of the present invention the instructions of any other instruction sets executed by the processor will be aligned with cache line boundaries. Similarly, if the instruction is a Thumb 32 instruction, but at step  404  it is determined that the Thumb 32 instruction does not cross the cache line boundary, then again the processor proceeds directly to step  414 . 
     If at step  406  it is determined that the processor is not fetching from sequential cache lines, then the process proceeds to step  408  where an incomplete pre-decoding operation is performed in respect of the first half word of the Thumb 32 instruction, and the incomplete field is set to identify that the sideband signals added into the pre-decoded instruction portion have been produced using an incomplete pre-decoding operation. 
     From the above description, it will be seen that by providing such an incomplete pre-decode indication in association with one or more pre-decoded instruction portions, if a pre-decoded instruction including such a pre-decoded instruction portion is later read from the cache by the processing circuitry, that indication will flag to the processing circuitry that the pre-decoded instruction is potentially corrupted, allowing the processing circuitry to take appropriate action. Hence, incorrect operation resulting from execution of instructions that have been incompletely pre-decoded can be avoided without significant additional cost and complexity in the design. 
     In the embodiments discussed above, an abnormal instruction identifier is added only to T32 instructions. For example, this can be done by adding sideband information to the first portion T32a of a T32 instruction. An abnormal instruction identifier is not added to a T16 instruction. To some extent, this is acceptable as the T16 instruction set is small enough that abnormal instructions can be decoded more easily than for T32 instructions, and so less power is consumed by the decoding circuitry in decoding these abnormal instructions. 
     However, in one embodiment, the first portion T32a of a T32 instruction is extended when an incomplete pre-decode identifier is added. In order to make best use of the memory space available, two T16 instructions should be the same length as one T32 instruction (or an ARM instruction). Thus, as the first portion of the T32 instruction has been extended to accommodate the incomplete pre-decode identifier, an additional bit can also be added to the T16 instruction during pre-decoding. This bit can be used to store an abnormal instruction identifier. For example, the bit could contain sideband information which indicates whether or not the instruction is an undefined instruction. In this way, the power consumed in decoding undefined instructions can be reduced. 
     The second portion T32b of a T32 instruction does not need an abnormal instruction identifier because this information will be contained in the first portion T32a. However, it is important that the pre-decoding operation performed on the T32b portion is the same as the operation performed on a T16 instruction by the pre-decoding circuitry  10 . Thus, if a block is speculatively identified as representing a T32b portion, then the pre-decoding circuitry  10  adds an abnormal instruction identifier which indicates, provisionally assuming (against the speculation) that the block represents a T16 instruction, whether or not that T16 instruction is abnormal. Thus, the operation performed on a block is the same regardless of whether it is identified as a T16 instruction or a T32b portion. If the pre-decoding circuitry  10  mistakenly identifies a T16 instruction as a T32b portion, then this does not matter because the abnormal instruction identifier is added to the block anyway, and so a pre-decoding error does not occur. If the speculation was correct, then the abnormal instruction identifier in the T32b portion can be ignored. 
       FIG. 15  shows the format of a T16 instruction and a T32 instruction once pre-decoding operations have been performed on them according to at least some embodiments. For a block speculatively identified as a T16 instruction, an instruction portion identifier is added in bit [16] and set to “0”, and a sideband bit is added in bit [17] indicating whether the instruction is abnormal. The T16 opcode in bits [15:0] is unmodified. The same operation is performed on a block speculatively identified as representing the second portion T32b of a T32 instruction. An instruction portion identifier is added in bit [34] and set to “0”, and a sideband bit is added in bit [35] indicating, while provisionally identifying the block as a T16 instruction, whether the T16 instruction is abnormal. The second half of the T32 opcode in bits [33:18] is unmodified. 
     For a block speculatively identified as the first portion T32a of a T32 instruction, the pre-decoding circuitry  10  adds sideband information to bits [15:14] of the block. An instruction portion identifier is added in bit [16] and set to “1”, and an incomplete pre-decode identifier is added in bit [17] indicating whether the T32 instruction spans a cache line boundary. 
     Thus, the same pre-decoding operation is performed on a block regardless of whether it is identified as a T16 instruction or as a T32b portion. The error-resistant pre-decode format used for instructions also consistently utilizes extra bits which are available to convey an abnormal instruction identifier. 
     The process of step  202  in  FIG. 11 , in which blocks are pre-decoded by the pre-decoding circuitry  10 , is described in more detail with reference to  FIG. 16 . In step  250 , the pre-decoding circuitry  10  speculatively identifies which instruction portion a block represents. In step  254 , the pre-decoding circuitry  10  detects whether or not the block has been identified as the first portion T32a of a T32 instruction. 
     If the pre-decoding circuitry  10  identifies the block as the first portion T32a of a T32 instruction, then flow proceeds to step  260 , in which the pre-decoding circuitry  10  detects whether the T32 instruction is an abnormal instruction. In step  262 , the pre-decoding circuitry  10  adds sideband information to the block depending on whether or not the T32 instruction is an abnormal instruction. In step  264 , the pre-decoding circuitry  10  adds an instruction portion identifier in the form of an ID bit which is added at bit [16] of the block and set to “1”. 
     On the other hand, if at step  254  the pre-decoding circuitry  10  did not identify the block as a T32a portion, then flow proceeds to step  270 . The pre-decoding circuitry  10  provisionally assumes that the block represents a T16 instruction, and detects whether or not that T16 instruction is an abnormal instruction. Then, in step  272 , independent of whether the block is a T16 instruction or the second portion T32b of a T32 instruction, the pre-decoding circuitry  10  adds sideband information to the block indicating whether or not the T32 is an abnormal instruction. At step  274 , the pre-decoding circuitry  10  then adds an instruction portion identifier in the form of an ID bit which is added at bit [16] of the block and set to 0. 
     For both paths through the flow diagram in  FIG. 16 , flow then proceeds to step  280 , in which any further pre-decoding operations are performed by the pre-decoding circuitry  10 . This may include, for example, adding the operation discussed in relation to  FIG. 14  for adding an incomplete instruction identifier. Once any further pre-decoding operations are complete, the pre-decoded blocks are then passed to the cache  20  in step  284 . Flow then returns to step  204  in  FIG. 11 . 
     Another problem that can occur within the data processing apparatus when using the pre-decoding circuitry is when there is a mix of instruction states within a particular cache line. As a particular example, an ARM processor may be able to execute instructions from either the ARM, Thumb, ThumbEE or Jazelle instruction sets developed by ARM Limited, United Kingdom, and it is possible for one cache line to include instructions from more than one of the instruction sets. 
     At any particular point in time, the processing circuitry will be in a particular processor state, dependent on which instruction set the instructions currently being executed belong to. Hence, at one point in time, the processing circuitry may be in the ARM processor state, whilst at a subsequent point it may branch into a sequence of Thumb instructions, and hence enter the Thumb processor state. 
     When pre-decoding instructions, the pre-decoding circuitry needs to assume a speculative processor state when performing the pre-decoding, and this chosen speculative processor state would then typically be used for pre-decoding a whole cache line&#39;s worth of instructions received via the processor system interface  5 . In one particular embodiment, the speculative processor state is chosen to be the state of the processor at the time the pre-decoding operation is performed by the pre-decoding circuitry. It will be appreciated that the instruction cache  20  could store pre-decoded instructions which have become corrupted if, for example, the chosen speculative processor state is not an appropriate processor state for all of the instructions in the cache line. For example, if the processor is in Thumb state when the pre-decoding operation is performed for a particular cache line, but the cache line mixes up ARM and Thumb instructions, then it will be appreciated that the ARM instructions may be pre-decoded incorrectly, resulting in corrupted pre-decoded instructions for those ARM instructions. Indeed, it is also possible for an entire cache line to be pre-decoded in the wrong state if the last instruction in one cache line is a state changing branch instruction, and the next cache line is already being subjected to the pre-decoding operation by the pre-decoding circuitry. 
     Whilst it would be possible to mark every half word that is pre-decoded with an indication of the speculative processor state used for the pre-decoding, this would consume a significant amount of area in the instruction data RAMs of the instruction cache. Since it is rare that cache lines mix state, in one embodiment the cache line is marked with the state the processor was in when the cache line was pre-decoded, this proving to be more area efficient. Accordingly, in one embodiment each entry in the instruction tag RAMs of the cache can be extended to store the speculative processor state as shown schematically in  FIG. 18 . As will be understood by those skilled in the art, each entry in an instruction tag RAM will typically store an address portion  480 , and will also often store one or more other control bits, for example a valid bit to indicate whether the associated cache line is valid. As shown in  FIG. 18 , an additional two bits can be provided for each entry to identify the speculative processor state  485  that was used when performing the pre-decoding operation on the corresponding cache line in the instruction data RAM. 
       FIG. 17  illustrates in more detail components provided within the instruction cache  20  when such an extended tag RAM entry is used within the instruction tag RAMs. As shown in  FIG. 17 , as the pre-decoded instructions for a particular cache line are stored in that cache line within an instruction data RAM  450  of the instruction cache, the corresponding entry in the relevant instruction tag RAM  460  is populated with the address portion  480  and the speculative processor state information  485 , which will have been output by the pre-decoding circuitry  10 . 
     When the processor subsequently issues an access request seeking to read an instruction from the instruction cache  20 , the cache performs a lookup operation using the address comparison circuitry  464 , the state comparison circuitry  468  and the hit/miss circuitry  470 . In particular, the address comparison circuitry  464  determines whether the address specified by the access request matches any of the address portions stored in selected entries within the instruction tag RAMs  460 . In one embodiment, the instruction cache  20  is arranged as an n-way set associative cache, and in one particular embodiment takes the form of the two-way set associative cache shown in  FIG. 19 . Accordingly, in such an embodiment there will be two tag RAMs  460 - 0  and  460 - 1 , and an associated two data RAMs  450 - 0  and  450 - 1 . For each cache line  492 , there will be a corresponding entry  490  in the associated tag RAM for storing the information shown schematically in  FIG. 18 . 
     It will be appreciated by those skilled in the art that when an access request is issued by the processing circuitry, an index portion of the address is used to identify a particular set in the instruction cache, each set comprising one cache line in each way, and accordingly for the example of  FIG. 19  each set comprising two cache lines. A tag portion of the address specified in the access request is then compared with the address portion  480  stored in the corresponding entry in each tag RAM, and accordingly for the example of  FIG. 19  the address comparison logic  464  will perform two address comparisons, namely comparing the tag portion of the address with the address portion  480  from the entry in each tag RAM that is associated with the set specified by the index portion of the address. 
     Furthermore, in addition to performing the address comparison using the address comparison circuitry  464 , state comparison circuitry  468  also performs a comparison of the current processor state against the speculative processor state  485  stored in those accessed entries in the tag RAM. The hit/miss circuitry  470  is arranged to only detect a hit condition if both the address comparison circuitry  464  identifies a match, and the state comparison circuitry  468  also identifies a match, i.e. a hit condition is only detected if the instruction seeking to be accessed is present in the instruction cache and the speculative processor state used to pre-decode that instruction matches the current processor state. 
     In the event of a hit condition, the multiplexer  472  is caused to output the instruction read from the data RAM  450  of the appropriate way in the instruction cache, from where that instruction is then provided to the decoding circuitry  30 . 
     If a hit condition is not detected, then a miss signal is issued over path  474 , causing a linefill request to be issued from the processor system interface  5 . As a result, the relevant cache line of instructions will be fetched from memory and passed through the pre-decoding circuitry  10 . 
     It will be appreciated that there are two types of miss condition. The first is the classical one where the instruction seeking to be accessed is not currently in the instruction cache, and in this instance the output from the pre-decoding circuitry  10  can be allocated to the instruction cache in the usual manner. For the two way set associative cache example discussed earlier, this will involve cleaning and invalidating the cache line contents in one of the ways, and allocating the newly pre-decoded instructions to that cache line. However, another miss condition that can now be generated is where the address comparison circuitry  464  detects a match, i.e. the instruction is present in the cache, but the state comparison circuitry  468  does not detect a match, and hence identifies that the relevant instruction has been pre-decoded assuming a wrong processor state. In this instance, the cache miss causes a re-fetch of the cache line from memory, along with a re-pass of the fetched instructions through the pre-decoding circuitry. While in one embodiment the output from the pre-decoding circuitry  10  in such a situation could be allocated in exactly the same manner as discussed above, in an alternative embodiment the output from the pre-decoding circuitry  10  will preferentially be allocated to a different way. This can avoid a potential “thrashing” problem that could otherwise arise in situations where the cache line contains instructions from more than one instruction set. 
     To illustrate this thrashing problem, consider the example where a first part of the cache line contains Thumb instructions and the second part of the cache line contains ARM instructions, with the whole cache line initially being pre-decoded assuming a Thumb processor state. It will be appreciated that if one of the ARM instructions is then accessed while the processing circuitry is in the ARM processor state, it will be detected by the state comparison circuitry  468  that the speculative processor state (i.e. the Thumb state) does not match the current processor state (i.e. the ARM state), and accordingly the re-fetch process described above will be performed. This time, the entire cache line will be pre-decoded assuming the ARM processor state, which now provides the correct pre-decoded ARM instruction that was the subject of the access request. However, if this output from the pre-decoding circuitry was used to overwrite the previous cache line contents, then if at a later stage the processing circuitry whilst in the Thumb processor state seeks to access one of the Thumb instructions in the first part of the cache line, it will be seen that a miss condition will again be detected since the state comparison circuitry  468  will again identify that the speculative processor state and the current processor state do not match. 
     By allocating the output from the pre-decoding circuitry  10  to a different cache line in such situations, this problem can be avoided, since it allows both versions to co-exist in the cache. Accordingly, considering the earlier example, the same set of instructions forming a particular cache line&#39;s worth of pre-decoded instructions will be stored in one cache line as a series of pre-decoded instructions that were pre-decoded assuming the Thumb processor state, and will be stored in another cache line in a different way as a series of pre-decoded instructions that were pre-decoded assuming the ARM processor state. 
     Whilst this avoids the above thrashing problem, when cleaning and invalidating the cache, the clean and invalidate operation will need to operate over multiple cache lines per physical address if the instruction at the physical address has been stored in multiple cache lines due to the above-described procedure. 
       FIG. 20  illustrates the above-described pre-decoding operation of one embodiment. At step  500 , the pre-decoding operation starts, whereafter at step  504  the current processor state of the processing circuitry is determined and set as a speculative processor state. Thereafter, at step  508 , the pre-decoding operation is performed by the pre-decoding circuitry  10  using the speculative processor state. Then each pre-decoded instruction is stored in the cache  20  at step  510  along with an indication of the speculative processor state. 
       FIG. 21  illustrates schematically the process performed by the instruction cache  20  upon receipt of an access request from the processing circuitry. At step  550 , an access request from the processing circuitry is awaited, and on receipt of such an access request a lookup procedure is performed in the cache at step  554  using the address specified by the access request. 
     At step  558 , it is determined whether the address comparison circuitry  464  has detected an address match, and if not the hit/miss circuitry  470  generates a miss condition at step  570 , causing a linefill operation to take place. 
     If at step  558 , an address match is detected, then at step  560  it is determined whether the state comparison circuitry  468  has detected a match between the current processor state and the speculative processor state stored in the relevant instruction tag RAM entry. If so, the hit/miss circuitry  470  generates a hit signal causing the matching pre-decoded instruction to be output from the relevant data instruction RAM  450  via the multiplexer  472  to the decoding circuitry  30  of the processing circuitry  25 . However, if at step  560  it is determined that the current processor state does not match the speculative processor state, then a miss condition is generated at step  568  causing a linefill operation to take place into a different cache way from the cache way in which the address match was detected by the address comparison circuitry  464 . 
     Often a state changing instruction used to trigger a change in processor state will cause a branch to a new address (and hence a different cache line) containing the first instruction to be executed in the new processor state. However, in an alternative embodiment, this may not be the case, and hence following a state changing instruction the next instruction to execute may be the next instruction in the same cache line. In such situations, a problem can arise when a fetch from the instruction cache contains more than one instruction, and one instruction in that fetch (other than the last one) is a state changing instruction. In this case a ‘hit’ will correctly occur for the fetch, but all instructions after the state changing instruction will have been incorrectly pre-decoded in the case where the instruction cache holds a cache line that has been pre-decoded in only one state. To address this, in one embodiment the error detection circuitry  35  is arranged to spot this situation, to trigger a ‘state error’ for the sequential instructions after the state changing instruction, and to initiate either a line fill or a loop back through the pre-decode process. 
     In a transition between the ARM and Thumb instruction sets this potential problem will not occur since the state changing instruction is a BLX type instruction that branches to a new cache line. However in a transition between the Thumb and ThumbEE instruction sets, this problem could arise since the state changing instruction (ENTERX/LEAVEX) does not use a branch and hence a fetch from a new address will not be triggered by the state changing instruction. In this case the error detection mechanism described above can be used to identify the state changing instruction, trigger a ‘state error’ for the sequential instructions after the state changing instruction, and then initiate a new fetch from the memory address immediately after the state changing instruction. 
     Whilst in the above described embodiments the pre-decoder circuitry has been associated with a level one instruction cache used to directly provide instructions to the processing circuitry  25 , in an alternative embodiment as shown in  FIG. 22  the pre-decoder circuitry  10  may be associated with a combined cache storing both instructions and data, for example a level two cache  605 . As shown in this figure, the processor system interface  5  actually communicates with the combined level two cache  605 , rather than directly with memory, and for completeness the level one data cache  615  is also shown in addition to the instruction cache  20  discussed earlier. In this embodiment, instructions from memory would be routed through the pre-decoder circuitry  10  to create pre-decoded instructions for storing in the combined level two cache  605 . However, data retrieved from memory to the level two cache, or written back to memory from the level two cache, will bypass the pre-decoder circuitry  10 . 
     As shown in  FIG. 23 , an additional identifier  630  can be associated with each cache line to identify whether the cache line stores instructions or data. This will be added in addition to the speculative processor state bits  635  and address portion  640  within each tag RAM entry  625 , hence identifying for the corresponding cache line  650  whether the contents of that cache line are pre-decoded instructions, or instead are data values used by the processing circuitry when executing those instructions. By such an approach, the instruction/data identifier  630  enables those cache lines which have been subjected to pre-decoding to be clearly identified. Accordingly, when performing lookup procedures in the cache, it can be determined whether there is any need to check speculative processor state against current processor state during the lookup procedure, in particular such a check only being necessary for those cache lines storing pre-decoded instructions. 
     From the above description, it will be seen that through storing of the speculative processor state in the cache, it is possible to later detect a condition where the assumption of speculative processor state by the pre-decoding circuitry was wrong, and accordingly the pre-decoded instruction read from the cache should not be used by the processing circuitry. This provides a simple and effective, power efficient, mechanism for detecting certain situations where instructions have been corrupted by the pre-decoding process. 
       FIG. 24  illustrates an instruction pipeline having a form similar to those previously described. This instruction pipeline supports pre-decoding and subsequent execution of processing operations corresponding to instructions of three different sets of instructions, namely ARM instructions, Thumb32 instructions, and Thumb16 instructions. It has been recognised that many common ARM instructions share or have functionality closely corresponding to instructions which are also present within the Thumb32 instructions. This recognition may be exploited by using a system in which ARM instructions are pre-decoded by the pre-decoding circuitry  10  to form pre-decoded instructions stored within the instruction cache  20  and using a shared format to represent shared functionality with corresponding Thumb32 instructions. Thus, in the de-coded instructions stored within the instruction cache  20 , the individual pre-decoded instructions corresponding to either ARM instructions or Thumb32 instructions will share the same format to represent the same functionality. The re-mapping performed on ARM instructions by the pre-decoding circuitry  10  seeking to re-map them into a form in which they are represented within the pre-decoded instructions in the same way as Thumb32 instructions represents additional activity at the pre-decoding stage, but this activity will be performed when the instructions are being loaded into the instructions cache  20  whereas the pre-decoded instructions which have been re-mapped in this way will typically be read many times from the instruction cache  20  and executed by the remainder of the system. Thus, the additional activity (energy) consumed in the re-mapping is amortised across many uses of the re-mapped instructions. 
     As will be evident form  FIG. 24 , the re-mapping of the ARM instructions into a form similar to the Thumb32 instructions in their pre-decoded forms enables the Thumb32 decoding circuitry  45  to be re-used for the majority of the de-coding operations which need to be performed in respect of what were originally ARM instructions. This saves gate count within the decoding circuitry and consequently saves energy. 
     There are some ARM instructions which do not share functionality closely enough with any Thumb32 instruction that they can be re-mapped into a shared format and for which decoding circuitry is likely already provided within the Thumb32 decoding circuitry  45 . In respect of these ARM-only instructions additional decoding circuitry  46  is provided and switched into use when such ARM-only instructions are identified. This ARM-only de-coding circuitry  46  is considerably smaller than a full ARM decoding circuitry block which would be required in conventional multiple instruction set systems. 
     As previously described, the pre-decoded instructions are thirty six bits in length. The original ARM instructions are 32 bit instructions. Thus, the additional bits within the pre-decoded instructions can be used to represent the previously discussed side band signals as is shown in  FIG. 25 . The provision of the same side band signals in respect of ARM instructions as well as the other instructions facilitates more efficient decoding within the decoding circuitry  45 ,  50 ,  46 . One bit within the pre-decoded instruction is used to flag ARM-only instructions for which there is no Thumb32 equivalent and for which the ARM-only decoding circuitry  46  is used to perform decoding and generate appropriate control signals therefrom. This ARM-only bit is selected as a bit having a fixed value within all Thumb32 instructions and which is set to the opposite value within pre-decoded instructions corresponding to ARM-only instructions. 
     One feature of the ARM instruction set is that every instruction includes four bits of condition codes, namely codes N, C, Z, and V. As shown within  FIG. 25 , these condition codes are allocated positions within the pre-decoded instruction corresponding to bit positions  16 ,  17 ,  34 , and  35 . These are bit positions normally used for other purposes within pre-decoded instructions corresponding to Thumb32 instructions, namely identification bits, an incomplete bit, and a side band signal. Given that ARM instructions are stored as 32-bit aligned words in memory, the identification and incomplete bits are not required for ARM instructions within the pre-decoded instructions and only one area of side band bits is necessary thereby freeing bitspace within the pre-decoded instructions for ARM instructions in order to represent the condition codes N, C, Z, and V. The Thumb32 de-coding circuitry  45  can be readily modified to respond to these condition codes within pre-decoded instructions for ARM instructions in a manner which gates the execution of those pre-decoded instructions in dependence upon the associated condition code values. If a pre-decoded instruction corresponding to an ARM instruction meets its condition codes, then it will be executed and appropriate control signals generated therefrom. If a pre-decoded instruction does not meet its condition codes then it will not be executed. 
       FIG. 26  illustrates example of a Thumb32 ADC instruction and an ARM ADC instruction both decoded into the form of a pre-decoded instruction in which shared functionality is represented in a shared format. Corresponding fields within the Thumb32 instruction and the ARM instruction have been given the same names. It will be seen that while the formats of the original Thumb32 instruction and the original ARM instruction are very different, they nevertheless contain nearly all of the same fields and accordingly re-mapping the fields within the ARM instruction into positions within the pre-decoded instruction corresponding to the same position of those fields when a Thumb32 instruction is pre-decoded is relatively simple and efficient. This re-mapping is performed by the pre-decoding circuitry  10  illustrated in  FIG. 24 . 
     The ARM instructions include four condition code bits at bit positions [31:28]. As previously discussed, these condition codes are mapped to bit positions  35 ,  34 ,  17 , and  16  within the pre-decoded instruction corresponding to an ARM instruction. The pre-decoded instruction corresponding to the ARM instructions still contains side band signals at bit positions [15:14], but these will apply to the whole of the 36 bits of pre-decoded instruction since ARM instructions are 32-bit aligned in memory and accordingly the issues associated with instructions spanning cache line boundaries do not arise in relation to ARM instructions since the cache lines are word aligned. Also of note within  FIG. 26  is bit position  33  within the pre-decoded instructions. This is shown as having a value of “should be 0” in respect of both the pre-decoded instructions corresponding to the Thumb32 instruction and the ARM instruction. This “should be 0” value corresponds to bit  31  within the Thumb32 instruction. If bit  31  within the Thumb32 instruction is not a zero, then this is defined as having unpredictable behaviour. 
       FIG. 27  illustrates how the unpredictable behaviour associated with the “should be 0” value within the pre-decoded instruction being set to a “1” can be used to provide instruction bitspace within the pre-decoded instruction to represent functionality of the ARM instructions which is not shared by any Thumb32 instructions. In the example of  FIG. 27 , the ARM instruction for which there is no Thumb32 equivalent is a ADC instruction in it register-shifted register variant. In this variant a shift register Rs specified in bits [11:8] of the ARM instruction stores a value representing a shift amount to be applied to one of the values stored within the two input source registers Rm, Rn. Thumb32 instructions do not have register-shifted register variants of their ADC instruction. The remainder of the fields within the ARM instruction of  FIG. 27  are similar to those illustrated in  FIG. 26 . However, the register-shifted register variant of the ARM instruction does not include an immediate field. This immediate field was mapped to bits [32:30] and [23:22] within the pre-decoded instructions for both the Thumb32 and ARM instructions of  FIG. 26 . This bitspace within the pre-decoded instructions can be re-used to represent the register identifier Rs of the register storing the shift value within the register-shifted register version of the ARM instruction for the pre-decoded instruction form of  FIG. 27 . The decoding circuitry  45  is configured to recognise the pre-decoded instruction supplied to it as having this particular form by the setting of the “should be 0” bit  33  to a value of “1” in the pre-decoded instruction representing an ADC of a register-shifted register variant ARM instruction. 
     This behaviour whereby bitspace corresponding to unpredictable behaviour for the Thumb32 instructions is re-used to represent ARM-only variant instructions can be contrasted with the use of the ARM-only bit at bit position  13 . The ARM-only bit at bit position  13  can be reserved for instructions with much less similarity to any Thumb32 instructions and for which the ARM-only decoding circuitry  46  is provided. Re-using the bitspace corresponding to the unpredictable behaviour in Thumb32 instructions not only preserves instruction bitspace within the pre-decoded instructions it also facilitates re-use of the same gates within the shared portion of the decoding circuitry  45 . 
       FIG. 28  illustrates a further feature which may be provided by the pre-decoding circuitry  10 . It has been recognised that sequences of Thumb16 instructions can include adjacent instructions specifying combined functionality corresponding to a single Thumb32 instruction. The pre-decoding circuitry can recognise this by pre-decoding the adjacent Thumb16 instructions into a single pre-decoded instruction corresponding to the equivalent Thumb32 instruction. The single pre-decoded instruction corresponding to the Thumb32 instruction will be issued as a single entity into the execution pipeline and accordingly executed more rapidly and efficiently then would two separate Thumb16 instructions. The pre-decoding circuitry  10  can concentrate on a relatively small number of common cases where two adjacent Thumb16 instructions may be re-mapped to a corresponding pre-decoded form of a Thumb32 instruction in order to balance the benefit and cost of providing the additional circuitry to identify such opportunities. 
       FIG. 29  is a flow diagram schematically illustrating the re-mapping of ARM instructions to Thumb32 instructions within the pre-decoding circuitry. At step  700 , a cache line is fetched from memory. At step  702 , a determination is made as to whether or not the first instruction to be de-coded is from the ARM instruction set. If the instruction to be pre-decoded is not from the ARM instruction set, then processing proceeds to step  704  at which pre-decoding in accordance with either a Thumb16 or Thumb32 instruction is performed. 
     If the determination at step  702  was that the instruction to be decoded is an ARM instruction, then step  706  determines whether the ARM instruction can be mapped to a T32 format pre-decoded instruction for decoding with a shared portion of the decoder. If the ARM instruction can be mapped in this way then processing proceeds via step  707 . If the ARM instruction cannot be mapped in this way the step  709  pre-decodes the ARM instruction into a pre-decoded instruction with the ARM-only bit set so that a non-shared portion of the decoder will be used. Step  708  then stores the pre-decoded instructions in the cache memory  20 . Step  710  determines whether or not there are more instructions in the fetched cache line fill which pre-decoding is required. If there are more such instructions, then processing returns to step  702 , otherwise the processing halts. 
       FIG. 30  is a flow diagram schematically illustrating how two Thumb16 instructions may be re-mapped into a pre-decoded instruction corresponding to a single Thumb32 instruction. At step  712  a cache line is fetched from memory. At step  714  a determination is made as to whether or not the first instruction to be decoded is a Thumb16 instruction. If the instruction is not a Thumb16 instruction, then processing proceeds to step  716  where the pre-decoding is performed in accordance with the appropriate behaviour for an ARM or a Thumb32 instruction. 
     If the determination at step  714  was that the instruction fetched is a Thumb16 instruction, then step  718  reads the next Thumb16 instruction and step  720  identifies whether the two adjacent Thumb16 instructions correspond to one Thumb32 instruction. 
     If the determination at step  720  was that the two adjacent Thumb16 instructions do not correspond to one Thumb32 instruction, then step  722  generates pre-decoded instructions for two separate Thumb16 instructions. However, if the determination at step  720  was that the two adjacent Thumb16 instructions do correspond to one Thumb32 instruction, then step  724  generates a pre-decoded instruction corresponding to the equivalent Thumb32 instruction as opposed to the two separate Thumb16 instructions that would be generated by step  722 . 
     Step  726  stores the pre-decoded instructions into the instruction cache  20 . Step  728  then determines whether or not there are more instructions within the fetched cache line that require pre-decoding. If there are more such instructions then processing returns to step  714 . 
       FIG. 31  illustrates a flow diagram corresponding to processing which can be performed within the pre-decoding circuitry  10  to identify branch instructions and early terminate pre-decoding operations. If a branch instruction which either will be taken, or is predicated taken, is encountered, then it can be energy efficient to early-terminate pre-decoding as the energy expended upon such pre-decoding is wasted since it is likely that instructions subsequent to the branch (or predicted taken branch) will not be executed. At step  730 , a cache line is fetched from memory. At step  732 , the pre-decoding circuitry  10  identifies whether or not the instruction being pre-decoded is a taken branch. If the instruction being pre-decoded is a taken branch (e.g. an unconditional branch (jump)), then processing terminates. If the determination at step  732  is that the instruction is not a taken branch, then the instruction is pre-decoded at step  734 . Step  736  then stores the pre-decoded instruction within the instruction cache  20  and step  738  determines whether or not there are more instructions within the cache line which required pre-decoding. If there are more such instructions then processing returns to step  732 . 
     It will be appreciated that the processes illustrated by the flow diagrams of  FIGS. 29 ,  30 , and  31  are illustrated as sequential processes and processes which are independent of one another. Those familiar with this technical field will realise that when these processes are implemented using the pre-decoding circuitry  10 , these processes may well occur with a different ordering of steps or with some steps being performed in parallel. Furthermore, the processes which are illustrated separately in  FIGS. 29 ,  30 , and  31  will likely in practise be performed at least partially in parallel. The present techniques encompass all of these various alternatives. 
     Although particular embodiments have been described herein, it will be apparent that the invention is not limited thereto, and that many modifications and additions may be made within the scope of the invention. For example, various combinations of the features of the following dependent claims could be made with the features of the independent claims without departing from the scope of the present invention.