Patent Publication Number: US-6990570-B2

Title: Processor with a computer repeat instruction

Description:
This application claims priority to S.N. 98402454.7, filed in Europe on Oct. 6, 1998 (TI-27686EU) and S.N. 98402455.4, filed in Europe on Oct. 6, 1998 (TI-28433EU). 
   FIELD OF THE INVENTION 
   The present invention relates to processing engines configurable to repeat program flow. 
   BACKGROUND OF THE INVENTION 
   It is known to provide for parallel execution of instructions in microprocessors using multiple instruction execution units. Many different architectures are known to provide for such parallel execution. Providing parallel execution increases the overall processing speed. Typically, multiple instructions are provided in parallel in an instruction buffer and these are then decoded in parallel and are dispatched to the execution units. Microprocessors are general purpose processor engines which require high instruction throughputs in order to execute software running thereon, which can have a wide range of processing requirements depending on the particular software applications involved. Moreover, in order to support parallelism, complex operating systems have been necessary to control the scheduling of the instructions for parallel execution. 
   Many different types of processing engines are known, of which microprocessors are but one example. For example, Digital Signal Processors (DSPs) are widely used, in particular for specific applications. DSPs are typically configured to optimise the performance of the applications concerned and to achieve this they employ more specialised execution units and instruction sets. 
   In a DSP or microprocessor, machine-readable instructions stored in a program memory are sequentially executed by the processor in order for the processor to perform operations or functions. The sequence of machine-readable instructions is termed a “program”. Although the program instructions are typically performed sequentially, certain instructions permit the program sequence to be broken, and for the program flow to repeat a block of instructions. Such repetition of a block of instructions is known as “looping”, and the block of instructions are known as a “loop”. For certain processor applications, in particular signal processing, the processing algorithms require so-called “nested loop” computations. Nested loops are loops of program code which are contained within the body of an outer loop of a program code. Often, the inner loop is a single instruction which needs to be iterated a varying number of times dependent on the current step of the outerloop. 
   When performing a loop, memory access, for example to program memory, has to be performed in order to fetch the instructions to be repeated. Typically, the memory, such as the program memory, resides off chip and the memory access represents a considerable processor cycle overhead. This mitigates against power saving and fast processing at low power consumption, in particular for applications where program loops are likely to be utilized frequently. 
   The present invention is directed to improving the performance of processing engines such as, for example but not exclusively, digital signal processors. 
   SUMMARY OF THE INVENTION 
   In accordance with a first aspect of the invention there is provided a processing engine, comprising an execution mechanism. The processing engine also comprises a repeat count register and a repeat count index register. The execution mechanism is responsive to a repeat instruction to initialize the repeat count index register with the content of said repeat count register and to modify the content of said repeat count register. 
   In accordance with a second aspect of the invention there is provided a method for operating a processing engine comprising executing a repeat instruction. The repeat instruction is executed by
         i) initializing a repeat count index register with the content of a repeat count register, and   ii) modifying said repeat count register.       

   Preferred embodiments in accordance with the first and second aspects of the invention provide improved processing throughput by initializing the repeat count index register and modifying the repeat count register in parallel, by combining them in a single instruction. Furthermore, since the repeat count index register is initialized from a register (repeat count register) memory access delays are reduced, and the repeat may be initiated more quickly. 
   Preferably, the processing engine is responsive to the said repeat instruction comprising first and second parts, and responsive to a first part to initialize said repeat count index register, and to said second part to modify the content of said repeat count register. 
   Preferably, the second part comprises a data instruction having an operand for modifying the content of said repeat count register. 
   More preferably, the data instruction comprises an arithmetic operation including said operand and said repeat count register content, suitably implemented by way of an Arithmetic Logic Unit. 
   The operand may be a constant value, or a register value which would provide flexibility in updating the repeat count register since the register could itself be updated with the result of another operation. 
   Suitably, the repeat count index register is initializable from program memory and/or the program and/or a processing register. 
   The execution mechanism repeats the execution of a subsequent instruction whilst the repeat count index register content satisfies a predetermined condition, which is generally that the register contents are not equal to zero. Typically, the content of the repeat count index register is decremented by one for each execution of the second instruction. 
   Generally, the processing engine comprises repeat count circuitry including said repeat count index register coupled to a decrement unit for decrementing the content of said repeat count index register, a comparator for comparing the content of said repeat count index with a predetermined value, and a control unit coupled to the output of said comparator to inhibit further execution of said second instruction for said content of repeat count index register corresponding to said predetermined value. 
   Typically, the execution mechanism comprises an instruction pipeline including a plurality of pipeline stages. The execution mechanism is adapted to be responsive at respective pipeline stages to initialize the repeat count index register and modify the content of the repeat count register. Preferably, the execution mechanism is responsive at an early stage, for example a decode stage of the pipeline, of the instruction pipeline to modify the repeat count index register for execution of the subsequent instruction. 
   In accordance with the preferred embodiments of the invention, fewer processing cycles are necessary in order to execute repeat cycles having the number of iterations for the next loop updated as part of the current repeat instruction. Consequently, there is a corresponding reduction in power consumption by the data processing apparatus. Therefore, embodiments of the invention are particularly suitable for use in portable apparatus, such as wireless communication devices. Typically, such a wireless communication device comprise a user interface including a display such as liquid crystal display or a TFT display, and a keypad or keyboard for inputting data to the communications device. Additionally, a wireless communication device will also comprise an antenna for wireless communication with a radio telephone network or the like. 
   Further aspects and advantages of the invention will become apparent from the following description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Particular embodiments in accordance with the invention will now be described, by way of example only, and with reference to the accompanying drawings in which like reference signs are used to denote like parts, unless otherwise stated, and in which: 
       FIG. 1  is a schematic block diagram of a processor in accordance with an embodiment of the invention; 
       FIG. 2  is a schematic diagram of a core of the processor of  FIG. 1 ; 
       FIG. 3  is a more detailed schematic block diagram of various execution units of the core of the processor of  FIG. 1 ; 
       FIG. 4  is schematic diagram of an instruction buffer queue and an instruction decoder controller of the processor of  FIG. 1 ; 
       FIG. 5  is a representation of pipeline phases of the processor of  FIG. 1 ; 
       FIG. 6  is a diagrammatic illustration of an example of operation of a pipeline in the processor of  FIG. 1 ; 
       FIG. 7  is a schematic representation of the core of the processor for explaining the operation of the pipeline of the processor of  FIG. 1 ; 
       FIGS. 8A and 8B  show the structure of various repeat instructions; 
       FIG. 9  shows two known algorithms for stepping through a data structure; 
       FIG. 10  is a schematic diagram of the circuitry used for an instruction in accordance with a preferred embodiment of the invention; 
       FIG. 11  illustrates the pipeline stages of an instruction in accordance with a preferred embodiment of the invention; and 
       FIG. 12  is a schematic illustration of a wireless communication device suitable for incorporating in an embodiment of the invention. 
   

   DESCRIPTION OF PARTICULAR EMBODIMENTS 
   Although the invention finds particular application to Digital Signal Processors (DSPs), implemented for example in an Application Specific Integrated Circuit (ASIC), it also finds application to other forms of processing engines. 
     FIG. 1  is a block diagram of a microprocessor  10  which has an embodiment of the present invention. Microprocessor  10  is a digital signal processor (“DSP”). In the interest of clarity,  FIG. 1  only shows those portions of microprocessor  10  that are relevant to an understanding of an embodiment of the present invention. Details of general construction for DSPs are well known, and may be found readily elsewhere. For example, U.S. Pat. No. 5,072,418 issued to Frederick Boutaud, et al, describes a DSP in detail and is incorporated herein by reference. U.S. Pat. No. 5,329,471 issued to Gary Swoboda, et al, describes in detail how to test and emulate a DSP and is incorporated herein by reference. Details of portions of microprocessor  10  relevant to an embodiment of the present invention are explained in sufficient detail hereinbelow, so as to enable one of ordinary skill in the microprocessor art to make and use the invention. 
   Several example systems which can benefit from aspects of the present invention are described in U.S. Pat. No. 5,072,418, which was incorporated by reference herein, particularly with reference to FIGS. 2–18 of U.S. Pat. No. 5,072,418. A microprocessor incorporating an aspect of the present invention to improve performance or reduce cost can be used to further improve the systems described in U.S. Pat. No. 5,072,418. Such systems include, but are not limited to, industrial process controls, automotive vehicle systems, motor controls, robotic control systems, satellite telecommunication systems, echo canceling systems, modems, video imaging systems, speech recognition systems, vocoder-modem systems with encryption, and such. 
   A description of various architectural features and a description of a complete set of instructions of the microprocessor of  FIG. 1  is provided in co-assigned application Ser. No. 09/410,977 (TI-28433), which is incorporated herein by reference. 
   The basic architecture of an example of a processor according to the invention will now be described. 
     FIG. 1  is a schematic overview of a processor  10  forming an exemplary embodiment of the present invention. The processor  10  includes a processing engine  100  and a processor backplane  20 . In the present embodiment, the processor is a Digital Signal Processor  10  implemented in an Application Specific Integrated Circuit (ASIC). 
   As shown in  FIG. 1 , the processing engine  100  forms a central processing unit (CPU) with a processing core  102  and a memory interface, or management, unit  104  for interfacing the processing core  102  with memory units external to the processor core  102 . 
   The processor backplane  20  comprises a backplane bus  22 , to which the memory management unit  104  of the processing engine is connected. Also connected to the backplane bus  22  is an instruction cache memory  24 , peripheral devices  26  and an external interface  28 . 
   It will be appreciated that in other embodiments, the invention could be implemented using different configurations and/or different technologies. For example, the processing engine  100  could form the processor  10 , with the processor backplane  20  being separate therefrom. The processing engine  100  could, for example be a DSP separate from and mounted on a backplane  20  supporting a backplane bus  22 , peripheral and external interfaces. The processing engine  100  could, for example, be a microprocessor rather than a DSP and could be implemented in technologies other than ASIC technology. The processing engine, or a processor including the processing engine, could be implemented in one or more integrated circuits. 
     FIG. 2  illustrates the basic structure of an embodiment of the processing core  102 . As illustrated, the processing core  102  includes four elements, namely an Instruction Buffer Unit (I Unit)  106  and three execution units. The execution units are a Program Flow Unit (P Unit)  108 , Address Data Flow Unit (A Unit)  110  and a Data Computation Unit (D Unit)  112  for executing instructions decoded from the Instruction Buffer Unit (I Unit)  106  and for controlling and monitoring program flow. 
     FIG. 3  illustrates the P Unit  108 , A Unit  110  and D Unit  112  of the processing core  102  in more detail and shows the bus structure connecting the various elements of the processing core  102 . The P Unit  108  includes, for example, loop control circuitry, GoTo/Branch control circuitry and various registers for controlling and monitoring program flow such as repeat counter registers and interrupt mask, flag or vector registers. The P Unit  108  is coupled to general purpose Data Write busses (EB, FB)  130 ,  132 , Data Read busses (CB, DB)  134 ,  136  and an address constant bus (KAB)  142 . Additionally, the P Unit  108  is coupled to sub-units within the A Unit  110  and D Unit  112  via various busses labeled CSR, ACB and RGD. 
   As illustrated in  FIG. 3 , in the present embodiment the A Unit  110  includes a register file  30 , a data address generation sub-unit (DAGEN)  32  and an Arithmetic and Logic Unit (ALU)  34 . The A Unit register file  30  includes various registers, among which are 16 bit pointer registers (AR 0 –AR 7 ) and data registers (DR 0 –DR 3 ) which may also be used for data flow as well as address generation. Additionally, the register file includes 16 bit circular buffer registers and 7 bit data page registers. As well as the general purpose busses (EB, FB, CB, DB)  130 ,  132 ,  134 ,  136 , a data constant bus  140  and address constant bus  142  are coupled to the A Unit register file  30 . The A Unit register file  30  is coupled to the A Unit DAGEN unit  32  by unidirectional busses  144  and  146  respectively operating in opposite directions. The DAGEN unit  32  includes 16 bit X/Y registers and coefficient and stack pointer registers, for example for controlling and monitoring address generation within the processing engine  100 . 
   The A Unit  110  also comprises the ALU  34  which includes a shifter function as well as the functions typically associated with an ALU such as addition, subtraction, and AND, OR and XOR logical operators. The ALU  34  is also coupled to the general-purpose busses (EB, DB)  130 ,  136  and an instruction constant data bus (KDB)  140 . The A Unit ALU is coupled to the P Unit  108  by a PDA bus for receiving register content from the P Unit  108  register file. The ALU  34  is also coupled to the A Unit register file  30  by busses RGA and RGB for receiving address and data register contents and by a bus RGD for forwarding address and data registers in the register file  30 . 
   As illustrated, the D Unit  112  includes a D Unit register file  36 , a D Unit ALU  38 , a D Unit shifter  40  and two multiply and accumulate units (MAC1, MAC2)  42  and  44 . The D Unit register file  36 , D Unit ALU  38  and D Unit shifter  40  are coupled to busses (EB, FB, CB, DB and KDB)  130 ,  132 ,  134 ,  136  and  140 , and the MAC units  42  and  44  are coupled to the busses (CB, DB, KDB)  134 ,  136 ,  140  and data read bus (BB)  144 . The D Unit register file  36  includes 40-bit accumulators (AC 0 –AC 3 ) and a 16-bit transition register. The D Unit  112  can also utilize the 16 bit pointer and data registers in the A Unit  110  as source or destination registers in addition to the 40-bit accumulators. The D Unit register file  36  receives data from the D Unit ALU  38  and MACs 1&amp;2  42 ,  44  over accumulator write busses (ACW 0 , ACW 1 )  146 ,  148 , and from the D Unit shifter  40  over accumulator write bus (ACW 1 )  148 . Data is read from the D Unit register file accumulators to the D Unit ALU  38 , D Unit shifter  40  and MACs 1&amp;2  42 ,  44  over accumulator read busses (ACR 0 , ACR 1 )  150 ,  152 . The D Unit ALU  38  and D Unit shifter  40  are also coupled to sub-units of the A Unit  108  via various busses labeled EFC, DRB, DR 2  and ACB. 
   Referring now to  FIG. 4 , there is illustrated an instruction buffer unit  106  comprising a  32  word instruction buffer queue (IBQ)  502 . The IBQ  502  comprises 32×16 bit registers  504 , logically divided into 8 bit bytes  506 . Instructions arrive at the IBQ  502  via the 32-bit program bus (PB)  122 . The instructions are fetched in a 32-bit cycle into the location pointed to by the Local Write Program Counter (LWPC)  532 . The LWPC  532  is contained in a register located in the P Unit  108 . The P Unit  108  also includes the Local Read Program Counter (LRPC)  536  register, and the Write Program Counter (WPC)  530  and Read Program Counter (RPC)  534  registers. LRPC  536  points to the location in the IBQ  502  of the next instruction or instructions to be loaded into the instruction decoder(s)  512  and  514 . That is to say, the LRPC  534  points to the location in the IBQ  502  of the instruction currently being dispatched to the decoders  512 ,  514 . The WPC points to the address in program memory of the start of the next 4 bytes of instruction code for the pipeline. For each fetch into the IBQ, the next 4 bytes from the program memory are fetched regardless of instruction boundaries. The RPC  534  points to the address in program memory of the instruction currently being dispatched to the decoder(s)  512  and  514 . 
   The instructions are formed into a 48-bit word and are loaded into the instruction decoders  512 ,  514  over a 48-bit bus  516  via multiplexors  520  and  521 . It will be apparent to a person of ordinary skill in the art that the instructions may be formed into words comprising other than 48-bits, and that the present invention is not limited to the specific embodiment described above. 
   The bus  516  can load a maximum of two instructions, one per decoder, during any one instruction cycle. The combination of instructions may be in any combination of formats, 8, 16, 24, 32, 40 and 48 bits, which will fit across the 48-bit bus. Decoder  1 ,  512 , is loaded in preference to decoder  2 ,  514 , if only one instruction can be loaded during a cycle. The respective instructions are then forwarded on to the respective function units in order to execute them and to access the data for which the instruction or operation is to be performed. Prior to being passed to the instruction decoders, the instructions are aligned on byte boundaries. The alignment is done based on the format derived for the previous instruction during decoding thereof. The multiplexing associated with the alignment of instructions with byte boundaries is performed in multiplexors  520  and  521 . 
   The processor core  102  executes instructions through a 7 stage pipeline, the respective stages of which will now be described with reference to  FIG. 5 . 
   The first stage of the pipeline is a PRE-FETCH (P 0 ) stage  202 , during a which stage a next program memory location is addressed by asserting an address on the address bus (PAB)  118  of a memory interface, or memory management unit  104 . 
   In the next stage, FETCH (P 1 ) stage  204 , the program memory is read and the I Unit  106  is filled via the PB bus  122  from the memory management unit  104 . 
   The PRE-FETCH and FETCH stages are separate from the rest of the pipeline stages in that the pipeline can be interrupted during the PRE-FETCH and FETCH stages to break the sequential program flow and point to other instructions in the program memory, for example for a Branch instruction. 
   The next instruction in the instruction buffer is then dispatched to the decoder/s  512 / 514  in the third stage, DECODE (P 2 )  206 , where the instruction is decoded and dispatched to the execution unit for executing that instruction, for example to the P Unit  108 , the A Unit  110  or the D Unit  112 . The decode stage  206  includes decoding at least part of an instruction including a first part indicating the class of the instruction, a second part indicating the format of the instruction and a third part indicating an addressing mode for the instruction. 
   The next stage is an ADDRESS (P 3 ) stage  208 , in which the address of the data to be used in the instruction is computed, or a new program address is computed should the instruction require a program branch or jump. 
   Respective computations take place in the A Unit  110  or the P Unit  108  respectively. 
   In an ACCESS (P 4 ) stage  210  the address of a read operand is output and the memory operand, the address of which has been generated in a DAGEN X operator with an Xmem indirect addressing mode, is then READ from indirectly addressed X memory (Xmem). 
   The next stage of the pipeline is the READ (P 5 ) stage  212  in which a memory operand, the address of which has been generated in a DAGEN Y operator with an Ymem indirect addressing mode or in a DAGEN C operator with coefficient address mode, is READ. The address of the memory location to which the result of the instruction is to be written is output. 
   In the case of dual access, read operands can also be generated in the Y path, and write operands in the X path. 
   Finally, there is an execution EXEC (P 6 ) stage  214  in which the instruction is executed in either the A Unit  110  or the D Unit  112 . The result is then stored in a data register or accumulator, or written to memory for Read/Modify/Write or store instructions. Additionally, shift operations are performed on data in accumulators during the EXEC stage. 
   The basic principle of operation for a pipeline processor will now be described with reference to  FIG. 6 . As can be seen from  FIG. 6 , for a first instruction  302 , the successive pipeline stages take place over time periods T 1 –T 7 . Each time period is a clock cycle for the processor machine clock. A second instruction  304 , can enter the pipeline in period T 2 , since the previous instruction has now moved on to the next pipeline stage. For instruction  3 ,  306 , the PRE-FETCH stage  202  occurs in time period T 3 . As can be seen from  FIG. 6  for a seven stage pipeline a total of 7 instructions may be processed simultaneously. For all 7 instructions  302 – 314 ,  FIG. 6  shows them all under process in time period T 7 . Such a structure adds a form of parallelism to the processing of instructions. 
   As shown in  FIG. 7 , the present embodiment of the invention includes a memory management unit  104  which is coupled to external memory units (not shown) via a 24 bit address bus  114  and a bi-directional 16 bit data bus  116 . Additionally, the memory management unit  104  is coupled to program storage memory (not shown) via a 24 bit address bus  118  and a 32 bit bi-directional data bus  120 . The memory management unit  104  is also coupled to the I Unit  106  of the machine processor core  102  via a 32 bit program read bus (PB)  122 . The P Unit  108 , A Unit  110  and D Unit  112  are coupled to the memory management unit  104  via data read and data write busses and corresponding address busses. The P Unit  108  is further coupled to a program address bus  128 . 
   More particularly, the P Unit  108  is coupled to the memory management unit  104  by a 24 bit program address bus  128 , the two 16 bit data write busses (EB, FB)  130 ,  132 , and the two 16 bit data read busses (CB, DB)  134 ,  136 . The A Unit  110  is coupled to the memory management unit  104  via two 24 bit data write address busses (EAB, FAB)  160 ,  162 , the two 16 bit data write busses (EB, FB)  130 ,  132 , the three data read address busses (BAB, CAB, DAB)  164 ,  166 ,  168  and the two 16 bit data read busses (CB, DB)  134 ,  136 . The D Unit  112  is coupled to the memory management unit  104  via the two data write busses (EB, FB)  130 ,  132  and three data read busses (BB, CB, DB)  144 ,  134 ,  136 . 
     FIG. 7  represents the passing of instructions from the I Unit  106  to the P Unit  108  at  124 , for forwarding branch instructions for example. Additionally,  FIG. 7  represents the passing of data from the I Unit  106  to the A Unit  110  and the D Unit  112  at  126  and  128  respectively. 
   In general, an instruction for initiating a loop sets up the number of iterations of the loop. This may be done by setting a variable for the maximum number of iterations, that variable being decreased for each iteration and the iteration ceasing when a lower cut-off is reached, or vice versa. Optionally, a parameter such as “step” may be initialized before the loop in order to determine the step-size through which the iteration variable is stepped down from or up to its minimum or maximum value. 
   As mentioned in the introductory portion of the description, for certain processor applications, in particular signal processing, the processing algorithms require so-called “nested loop” computations. Often, the inner loop is a single instruction which needs to be iterated a varying number of times dependent on the current step of the outer loop. An example of such an algorithms is given below.
         Algorithm (1):   for (j=1 to NUMBER — OF — ITERATION)   for (i=1 to initial — value+j*iteration — step)   single — cycle — function (x(i), y(j);       

   where j is the current step variable of the outer loop, and i is the current step level of the inner loop. The variable NUMBER — OF — ITERATION is the total number of repeats of the outer loop. Iteration step is the step-size for the inner loop. Single — cycle — function is a generic description for any suitable single cycle processor instruction or parallel executed instructions. As can be seen, the number of iterations of the inner loop is dependent on the current step (j) of the outer loop. The effects of this are diagrammatically illustrated in  FIGS. 8A and 8B . 
     FIG. 8A  shows the number of iterations  802  of the inner loop, increasing as the step position  804  of the outer loop increases, as would be the case for algorithm (1) above. Alternatively, algorithm (1) could be modified such that,
 j=NUMBER — OF — ITERATION to 1; 
   In which case the number of iterations  806  of the inner loop would decrease as the step position  808  of the outer loop decreased, as shown in  FIG. 8B . 
   An example of applications where such algorithms may be applied are: the initialization phases of FIR/IIR filters; symmetrical matrix computations; and Levinson &amp; Schurr-like recursions. 
   Hitherto, such algorithms have been coded “in-line” with the loop argument (NUMBER — OF — ITERATION) being part of the program code as a parameter or immediate constant value. This requires duplication of code and increases the code size overhead for the processor. 
   Optionally, a known loop structure is used where the loop iteration size parameter is a memory operand. Although such an algorithm uses a looped structure and therefore saves on code size, there is a processor cycle overhead since memory accesses are required. The processor cycle overhead occurs since the instruction pipeline stage where memory is read and the pipeline stage where the repeat counter register is initialized are different. This causes a latency in instruction execution which causes a redundant slot in the pipeline. 
   There is an increasing requirement for electronic apparatus, in particular portable electronic apparatus, to reduce power consumption. Furthermore, for portable electronic apparatus it is desirable to have as an efficient and small a program code size as possible in order to reduce the program memory size and/or increase the functionality of the portable electronic apparatus. The foregoing requirements are not satisfied by known loop methods. 
   By way of background description, known methods of implementing a single loop repeat shall now be described. 
     FIG. 9  of the drawings shows two known algorithms for stepping through a data structure or the like for repeating an operation on separate parts of the data structure. Referring now to the algorithm labelled “State of the Art-1”, line ii) thereof represents the multiplication of two data elements located in data memory at positions pointed to by address registers AR 1  and AR 2 . The “+” indicates that the address AR 1  and AR 2  are post-incremented after the results of the operation, and the “%” indicates that address AR 1  is post-incremented in a circular addressing mode. The “*” indicates an indirect addressing mode. The result of the operation is stored in accumulator  1 , AC 1 . Such an operation is known as a Multiply and Accumulate (MAC) operation. In line (ii) the data elements pointed to by addresses AR 1  and AR 2  are multiplied together and the result added to the contents of accumulator  1 , AC 1 . Moving on to line (iii), the first part indicates that a high part (bits  31  to  16 ) of accumulator  2 , AC 2 , are stored in the memory location pointed to by address AR 3 . The address AR 3  is post incremented. The second part of line (iii) shows a parallel operation, indicated as parallel by “∥”, and in which the contents of AC 2  are the results of a previous step. The data element pointed to by AR 2  is added to the high contents (bits  31  to  16 ) of accumulator  1 , AC 1 . Address pointer AR 2  is post-decremented by the contents of data register DRØ. The data register contents, DRØ, are decremented by an iteration step value ready for the next operation on the data structure. 
   Operations (i) through (iv) are repeated for lines (v) through (ix). As can be seen from “State of the Art-1” instructions (ii) &amp; (iii) are repeated twice explicitly in a first part, and three times in a second part. In this manner of in line coding, it is possible to step through a data structure in memory in steps corresponding to the value “iteration-step”, and perform the same function at each step. 
   An algorithm such as “state of the Art-1” requires a low level of processor cycles, but requires a large amount of code. For example, 85 words of the 85 cycles for 10 repeats. 
   Referring now to the algorithm labelled “State of the Art-2” in  FIG. 9 , line (i) thereof initializes a memory variable directly accessible on the current memory page at the address represented by @inner-counter. Line (ii) is the initialization of a block — repeat — counter with the number of repeats for the block given by NB-ITERATION-1. At lines (iii) all the code within the braces at lines (iv) and (xi) will be repeated until the block — repeat — counter is zero. Line (v) performs the same operation as line (i) in “State of the Art-1”, and line (vi) is a single instruction repeat statement responsive to which the repeat counter is fetched from memory address @inner-counter, and which will repeat line (vii). Lines (vii) and (viii) corresponds to lines (ii) and (iii) in “State of the Art-1”. At line (ix) the variable stored at address @inner-counter is incremented by an amount given by the iteration step. Line (x) corresponds to line (iv) of “State of the Art-1”. By utilizing nested repeat loops, the same effect as achieved by “State of the Art-1” may be achieved but with only 13 words of code for 10 repeats, for example. However, the number of processor cycles is relatively high at  141 . 
   In accordance with a preferred embodiment of the invention, the processing engine is responsive to instructions relating to instruction repeat loop management. In a preferred embodiment the execution mechanism is responsive to a Computed Single Repeat (CSR) instruction comprising two parts. The format of the CSR instruction is given below:
 
repeat ( CSR ),  CSR+= register — content;
 
where the op-code “repeat(CSR)” initiates a repeat loop which repeats a following instruction or pair of instructions executable in parallel for example, a number of times corresponding to the value of the content of a Computed Single Repeat (CSR) register. The second part of the instruction modifies the content of the CSR register by adding a value to the content. In the preferred embodiment the value is the content of an A unit  110  register, (DR 0 –DR 3 , AR 0 –AR 7 ).
 
   Table 1 shows the instruction format for four types of CSR instruction. The seven left most alphas “O” represent the repeat (CSR) instruction op-code. These are then followed by a parallel enable bit “E”, which explicitly enables the repeat (CSR) instruction to be executed in parallel with another instruction. The third group of four alphas from the left represents the post-modification, if any, of the CSR register which may be a constant value or the contents of a register in the A unit  110  register file. The fourth group of four alphas is an op-code extension which is unused for the repeat (CSR) instruction. 
   
     
       
         
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Instruction Format 
               Alpha Code 
               Description 
             
             
                 
             
           
          
             
               OOOO OOOE xxxx oooo 
               repeat (CSR) 
               computed 
             
             
                 
                 
               repeat with 
             
             
                 
                 
               no modify 
             
             
               OOOO OOOE kkkk oooo 
               repeat (CSR), CSR+=k4 
               computed 
             
             
                 
                 
               repeat with 
             
             
                 
                 
               post modify 
             
             
                 
                 
               (positive 
             
             
                 
                 
               step k4) 
             
             
               OOOO OOOE kkkk oooo 
               repeat (CSR), CSR−=k4 
               computed 
             
             
                 
                 
               repeat with 
             
             
                 
                 
               post modify 
             
             
                 
                 
               (negative 
             
             
                 
                 
               step k4) 
             
             
               OOOO OOOE FSSS oooo 
               repeat (CSR), CSR+=DAx 
               computed 
             
             
                 
                 
               repeat with 
             
             
                 
                 
               DAx index 
             
             
                 
                 
               post 
             
             
                 
                 
               increment 
             
             
                 
             
          
         
       
     
   
   An example of an algorithm utilizing the repeat (CSR) instruction is given below: 
                                          BRC0 = #n           CSR=#initial — value           •           •           •           block repeat {                         repeat (CSR), CSR += 4                         AC1+=(*AR1+%)*(*AR2+)                         };           •           •           •                        
where a block repeat counter BRC 0  has an initial value n for a block repeat including the repeat (CSR) instruction.
 
   Referring now to  FIG. 10 , operation of the execution mechanism  900  for a repeat (CSR) instruction for a preferred embodiment of the invention will be described. Execution mechanism  900  comprises a CSR register  902 , having an output coupled to an Arithmetic Logic Unit (ALU)  904  and a multiplexor  906 . The CSR register may comprise a part of a Register File  908 , which resides in the A Unit  110 , and is directly coupled to the ALU  904 , preferably the A Unit ALU. The Register File  908  is coupled to a gate  910  controllable by register select line  912 , for selectively transferring the content of one of the registers in the Register File  908  to a multiplexor  914 . Multiplexor  914  also receives an immediate constant value via data flow  916 , and is controllable by control line  918  to selectively forward an immediate constant value or the content of one of the registers in the Register File  908  to the ALU  904 . Control line  918  also controls operation of ALU  904 . 
   In response to a repeat (CSR) instruction, the loop is initialized by passing the current contents of the CSR register  902  to the loop counter (repeat count index register)  922 , via multiplexor  906 . 
   Loop counter register  922  is coupled to a decrement unit  924  and a comparator unit  926 . The content of the loop counter register  922 , which represents the index count of the loop, is decreased by 1 in the decrement unit  924 , and the modified counter value transferred via data flow  928  to an input of multiplexor  906 . It will be clear to a person of ordinary skill in the art that an increment unit may be substituted for the decrement unit  924 . Optionally, the decrement/increment unit may comprise a register whose content defines a step size for the loop counter. 
   Referring now to  FIG. 11  the instruction flow for a repeat (CSR) instruction will be described. The same nomenclature for the pipeline stages is used as was used in  FIG. 5 . A block repeat counter, BRCØ is initialized, n, prior to the repeat (CSR) instruction entering the instruction pipeline. The CSR register  902  is initialized before entering the repeat block with the repeat count for the first single repeat, for example k. On the decode stage P 2 ,  1102 , of the repeat (CSR) instruction pipeline, the loop counter register  922 , RPTC, is updated from the CSR register  902  which gives value k for the first single repeat. During the Exec stage P 6 ,  1104 , of the repeat (CSR) instruction pipeline ALU  904  operation takes place, and the current contents of the CSR register  902  are added to either a constant value or other register contents (for example 4). The result is stored in the CSR register  902  at the end of the Exec stage  1104 , thereby updating the register by post-modification. Modification of the CSR register  902 , is by way of register (ALU  904 ) to register (CSR  902 ) operation, and is a generic register type operation. This is because the update of the CSR register is not timing critical as the repeat (CSR) instruction is typically nested within a block repeat, and the size of the block is sufficient for the update to take place during execution of the block instructions. In the present embodiment, the repeat latency is four cycles. 
   The loop counter ( 922  in  FIG. 10 ) RPTC is initialized from the CSR register during the decode stage P 2 ,  1102 , of the repeat (CSR) instruction, and the value read into the loop counter register  922  at the end of the decode stage, shown  1106 . 
   A single instruction represented by AC 0  at  1108   a  is then repeated,  1108 , in accordance with value k in the loop counter  922 , with the loop counter value being tested, decremented and updated during the address stage P 3  of the pipeline. When the single repeat has terminated, i.e. loop counter  922  value=0 at  1110 , the rest of the repeat block is executed. 
   For the next entry into the repeat block  1112 , the CSR register  902  value K+4 is loaded into the loop counter  922  at  1114 , and then updated in the Exec stage of the pipeline ready for the next block repeat. 
   A preferred embodiment of a processing engine configured to operate in accordance with the foregoing provides an advantageous method and apparatus for providing increasing levels of iteration for repeat single instructions dependent on the iteration level of an outer loop. 
     FIG. 12  illustrates an exemplary implementation of a digital system embodying aspects of the present invention in a mobile telecommunications device, such as a mobile telephone with integrated keyboard  12  and display  14 . Digital system  10  with a digital signal processor embodying aspects of the present invention packaged in an integrated circuit  40  is connected to the keyboard  12 , where appropriate via a keyboard adapter (not shown), to the display  14 , where appropriate via a display adapter (not shown) and to radio frequency (RF) circuitry  16 . The RF circuitry  16  is connected to an aerial  18 . Integrated circuit  40  includes a plurality of contacts for surface mounting. However, the integrated circuit could include other configurations, for example a plurality of pins on a lower surface of the circuit for mounting in a zero insertion force socket, or indeed any other suitable configuration. 
   In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention. 
   The scope of the present disclosure includes any novel feature or combination of features disclosed therein either explicitly or implicitly or any generalisation thereof irrespective of whether or not it relates to the claimed invention or mitigates any or all of the problems addressed by the present invention. 
   As used herein, the terms “applied,” “connected,” and “connection” mean electrically connected, including where additional elements may be in the electrical connection path. 
   While the invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various other embodiments of the invention will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications of the embodiments as fall within the true scope and spirit of the invention.