Abstract:
A decode unit ( 20 ) decodes instructions in a processor. These, instructions include instructions of a first length in a first instruction mode and instructions of a second, shorter length in a second instruction mode. The decode unit has decoding circuitry ( 50-60 ) which decode the instructions. A register holds the instruction mode and generates an instruction mode signal. Switching circuitry (MUX 6 ,MUX 7 ) is responsive to the instruction mode signal to output decoded instructions from the decode unit depending on the instruction mode. A detector ( 70 ) is provided for detecting a length change instruction of the second, shorter length while in the second instruction mode which indicates that the subsequent instruction is of the first length. The detector also temporarily alters the state of the instruction mode signal to allow the first length instructions to be decoded without changing the instruction mode held in the register.

Description:
FIELD OF THE INVENTION 
     The present invention relates to decoding instructions in a computer system such as a processor. 
     BACKGROUND OF THE INVENTION 
     In a computer system, instructions are typically fetched from a program memory, decoded and supplied to an execution unit where they are executed to run the program stored in the program memory. It is advantageous for such a computer system to be able to support more than one instruction mode. A novel computer system described herein can support three instruction modes. 
     According to a first instruction mode, during each machine cycle two 32 bit instructions are decoded, referred to herein as GP 32  mode. 
     According to a second instruction mode, during each machine cycle a pair of 16 bit instructions are decoded, referred to herein as GP 16  mode. 
     According to a third instruction mode, four 32 bit instructions are decoded during each machine cycle, referred to herein as VLIW mode. 
     In practice, a prefetch unit fetches a word from memory having a length of 128 bits. This word can contain eight 16 bit instructions (GP 16  mode), four independent 32 bit instructions (GP 32 ) or four interrelated 32 bit instructions (VLIW mode). The four 32 bit instructions in VLIW mode are interrelated in the sense that they have to conform to a certain grammar such that they can be fetched and supplied to the decoder together. The prefetch unit supplies an 128 bit sequence to the decode unit on each machine cycle. However, the decode units should supply to the execution unit decoded outputs only for the instructions to be decoded in that machine cycle, which depends on the instruction mode. 
     In the second instruction mode, the nature of the 16 bit instructions can sometimes limit the manipulations to be carried out by the execution unit. It is therefore advantageous to allow a 32 bit instruction to be included in a sequence of instructions in GP 16  mode. If this is done however then it would be necessary to include an instruction to alter the instruction mode of the computer system each time prior to execution of a 32 bit instruction and then to have an additional instruction to change the mode back again to continue to decode and execute 16 bit instructions. 
     SUMMARY OF THE INVENTION 
     According to the present invention in one aspect there is provided a decode unit for decoding instructions in a processor including instructions of a first length in a first instruction mode and instructions of a second, shorter length in a second instruction mode, the decode unit including: decoding circuitry for decoding instructions; a register for holding the instruction mode and generating an instruction mode signal; switching circuitry responsive to the instruction mode signal to output decoded instructions from the decode unit depending on the instruction mode; and means for detecting a length change instruction of the second, shorter length while in the second instruction mode which indicates that the subsequent instruction is of the first length and for temporarily altering the state of the instruction mode signal to allow the first length instructions to be decoded without changing the instruction mode held in the register. 
     In another aspect the present invention provides a processor comprising: at least one execution unit for executing instructions; an instruction mode indicator which indicates one of a plurality of instruction modes for the processor; a decode unit for decoding instructions prior to dispatch to the at least one execution unit; and an instruction supply mechanism for supplying instructions to the decode unit, wherein said instructions are represented by bit sequences the length of which depends on an instruction mode of the processor, and wherein the decode unit comprises: decoding circuitry for decoding the instruction; switching circuitry responsive to an instruction mode signal generated by the instruction mode indicator to output decoded instructions from the decode unit depending on the instruction mode; means for detecting a length change instruction of a second length while in a second instruction mode which indicates that the subsequent instruction is of a first length and for temporarily altering the state of the instruction mode signal to allow the first length instruction to be decoded without changing the instruction mode held at the instruction mode indicator. 
     In a further aspect, the invention provides a method of decoding instructions in a processor, the instructions each being of a predetermined length and including a length change instruction which indicates that a subsequent instruction is of a different length, the method comprising: detecting the length change instruction in a decode unit; temporarily altering the outputs of the decode unit to permit the different length instruction to be decoded; and after the different length instruction has been decoded, defaulting to decoding of the instructions of the predetermined length. 
     The length change instruction is named herein as Gp 32 nxt. It thus allows a 32 bit instruction to be decoded and executed while the machine remains in GP 16  mode. This gives GP 16  mode a greater flexibility than it would have if all instructions were restricted to the 16 bit length. Moreover, it overcomes the need to change the mode of the computer system just to execute a single 32 bit instruction in an instruction sequence of 16 bit instructions. 
     It will be appreciated that the length change instruction and the following 32 bit instruction needs to be atomic, i.e. executable without any interrupts. This is because once the decoder has detected the length change instruction, the next output from the decoder will be as though a 32 bit instruction has been decoded. The decode circuitry comprises in the preferred embodiment a first decoder having an input connected to receive a bit sequence of the first length and operating on receipt of said bit sequence to generate a first decoded output; a second decoder having an input connected to receive a bit sequence of the second length and operating on receipt of said bit sequence to generate a second decoded output; and a communication path for supplying a bit sequence simultaneously to said first and second decoders. 
     Thus, as described in the following, in order to manage different instruction modes, the decode unit has a plurality of dedicated decoders each of which receives and decodes the bit sequence during each machine cycle. Depending on the instruction mode of the machine, the outputs of selected one of the decoders are supplied to the execution unit for execution. The outputs of the other decoder are not required and thus are not selected. 
    
    
     For a better understanding of the present invention and to show how the same may be carried into effect reference will now be made by way of example to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a prefetch unit; 
     FIG. 2 illustrates the different instruction modes of the processor; 
     FIG. 3 illustrates the organisation of a prefetch buffer; 
     FIG. 4 is a circuit diagram illustrating the key components of the prefetch buffer; and 
     FIG. 5 is a block diagram of a decode unit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a block diagram of a prefetch unit  2  for a processor, the prefetch unit  2  comprising a prefetch buffer  4  with associated control bits  6  and control circuitry comprising a prefetcher  8  and an aligner  10 . The prefetcher  8  is connected to a program memory  12  and is responsible for initiating memory accesses to the program memory  12  using memory access control signals  14   a ,  4   b . The address in memory to which a fetch is initiated is held in a prefetch program counter  16  in the prefetcher  8 . Control of the prefetch program counter is not discussed herein, but it can be assumed that fetches are initiated from memory in accordance with a sequence of instructions to be executed by the processor. That is, the prefetch program counter may be incremented each time as a sequence of adjacent instructions is fetched, or it may change according to branches, traps, interrupts etc. Responsive to a memory fetch initiated by the prefetcher, instruction words are supplied from the program memory  12  to the prefetch buffer  4  as represented by data-in path  18 . 
     The aligner  10  controls reading of instructions from the prefetch buffer to a decode unit  20  along data-out path  22 . To do this, the aligner issues and is responsive to prefetcher align (PFAL)/decoder control signals  24   a ,  24   b . The aligner  10  has an align program counter  26  which keeps track of how many instructions have been dispatched to the decoder  20  in each machine cycle, and a state machine  28  which generates a read pointer RP for controlling the prefetch buffer in a manner which is described in more detail hereinafter. 
     Instructions in the program memory  12  can have a length of 16 bits or 32 bits. The prefetch buffer supports three different instruction modes as described with reference to FIG. 2 as follows. The instruction mode is held in a process status register (PSR)  3  at the decode unit  20  and can be changed. The change of instruction mode can be caused for example by specific change mode instructions which set the PSR accordingly. Change mode signals chmd 1 ,chmd 2  are issued by the decoder  20  responsive to a change in instruction mode. 
     According to a first instruction mode, two instructions each having a length of 32 bits are supplied to the decoder from the prefetch buffer in each machine cycle, for example w 0 ,w 1  in CYCLE  0 . This mode is referred to herein as GP 32  mode. 
     According to a second instruction mode, a pair of 16 bit instructions are supplied during each machine cycle to the decoder  20  from the prefetch buffer  4 . This pair is denoted slot 0 , slot 1  in bit sequences w 0 ,w 1  etc. This is referred to herein as GP 16  mode. 
     According to a third instruction mode, four instructions w 0 ,w 1 ,w 2 ,w 3  each of 32 bits in length are supplied to the decoder in each machine cycle. This is referred to herein as VLIW. In VLIW mode the instructions in each instruction word (128 bits) conform to certain grammatical rules. 
     In all modes, each fetch operation initiated to the program memory  12  retrieves an instruction word of 128 bits in length. Thus, in GP 16  mode, the instruction word comprises eight 16 bit instructions, paired as slot 0 ,slot 1  for each machine cycle. In GP 32  and VLIW mode, the instruction word comprises four 32 bit instructions. 
     The organisation of the prefetch buffer  4  is illustrated in FIG.  3 . In diagrammatic terms, the prefetch buffer can be considered to have four successive lines L 0  to L 3 , each having a capacity of 128 bits. There is a single write port WPO having a width of 128 bits which receives data from the program memory via the data-in path  18  and an input latch FF-in and writes it into the selected line under the control of a write pointer WP [3:0]. Each line comprises four storage locations each having a capacity of 32 bits and each of which is shown diagrammatically divided into two 16 bit sections for the purposes of explanation. The storage locations are denoted F 0  to F 15 . Each line in FIG. 3 is referred to herein as a group of storage locations and has the capacity for one 128 bit line from memory. This allows up to four successive memory accesses to be made, even if the first instruction word has not been received or executed by the processor. While the instruction word in storage locations F 0  to F 3  is being decoded and subsequently executed, memory fetches can continue to be implemented into the storage locations F 4  to F 7 , F 8  to F 11  and F 12  to F 15  until the buffer is full. By the time that a memory fetch has been made into the last group F 12  to F 15 , it is most likely that the first group F 0  to F 3  will have been completely read out into the decode unit and will thus be ready to receive a subsequent instruction word from memory. The number of cycles required to decode an instruction word in each group varies depending on the instruction mode of the machine in a manner which will be described in more detail in the following. Nevertheless, a minimum of one cycle is required for reading and decoding, and therefore the use of the prefetch buffer hides memory latency. 
     In order to save a cycle when the prefetch buffer is empty or flushed after a branch, data can bypass the prefetch buffer through a bypass circuitry BS. As described in more detail later, the bypass circuitry is implemented as a plurality of multiplexors (MUX 0  to MUX 3  in FIG.  4 ). 
     FIG. 4 is a more detailed diagram of the prefetch buffer and its associated read circuitry. The storage locations F 0  to F 15  are illustrated aligned vertically for the purposes of explanation. 
     The control bits  6  described above in FIG. 1 include empty flags EF 1  to EF 4  which indicate when a complete 128 bit line of storage locations is empty such that a subsequent memory fetch can be initiated. When a fetch is instituted from memory, and data has been received by the prefetch buffer, the empty flag is cleared to indicate that those storage locations are now full. 
     Reading from the prefetch buffer will now be described with reference to the schematic diagram of FIG.  4 . The prefetch buffer includes four read ports RP 1 ,RP 2 ,RP 3  and RP 4 . These read ports each take the form of multiplexors each capable of connecting selected ones of the storage locations F 0  to F 15  to a 32 bit output, pf-buf-out 1 ,  2 ,  3  or  4 . However, the read ports are not identical. The first read port RP 1  has sixteen inputs each of which is connected to a respective storage location F 0  to F 15  and each of which can be connected to the output pf-buf-out 1 . The second read port RP 2  has eight inputs which are respectively connected to storage locations F 1 ,F 3 ,F 5 ,F 7 ,F 9 ,F 11 ,F 13 ,F 15  to selectively connect the contents of those storage locations to the output pf-buf-out 2 . 
     The third read port RP 3  has four inputs connected to storage locations F 2 ,F 6 ,F 10  and F 14  for selectively connecting the contents of those storage locations to the output pf-buf-out 3 . The fourth read port RP 4  also has four inputs which are connected to storage locations F 3 ,F 7 ,F 11  and F 15  for selectively connecting the contents of those storage locations to the output pf-buf-out 4 . 
     The read ports RP 1  to RP 4  are controlled by the read pointer RP from the aligner  10  in dependence on the instruction mode of the machine and the consequential number of machine cycles required for decoding each instruction word. 
     Alternatively, for instructions supplied directly from memory along data-in path  18 , the control of instructions supplied to the decoder in dependence on the instruction mode and machine cycles is additionally controllable by multiplexors MUX 0 ,MUX 1 ,MUX 2  and MUX 3 . These receive at their input respective bits of the 128 bit data-in path  18  to supply a 32 bit sequence to each multiplexor in each machine cycle as described in the following. 
     The selection of which instructions within the instruction word are supplied to the decode unit  20  is made on dependence on the instruction mode as described in the following. In FIG. 3, the symbols w 0  to w 3  are used on different input lines of the multiplexors MUX 0  to MUX 3  to represent different 32 bit sequences, as in FIG.  4 . The definition of each 32 bit sequence depends on the instruction mode, but bits of the data-in path are always allocated as w 0  [0:31], w 1  [32:63], w 2  [64:95], w 3  [96:127]. The inputs to the multiplexors are individually labelled so as to distinguish between them. That is, in GP 16  mode, on the first decode cycle, cycle  0 , the first sequence w 0  is supplied to the decoder  20 . This presents a pair of 16 bit instructions, slot 0 ,slot 1  (w 0 ) for simultaneous decoding by the decode unit  20 . On the next cycle, cycle  1 , the sequence w 1  is supplied, presenting the next pair of 16 bit instructions slot 0 ,slot 1  (w 1 ) for decoding. In GP 16  mode, the read port RP 1  and the multiplexor MUX 0  are the only read devices which are used and the control of the word which is supplied to the decode unit is made by the multiplexor MUX 0  under the control of signal mux-ctrl 0 , and the read pointer RP. If the signal mux-ctrl 0  selects the read port output pf-buf-out 1 , the read pointer selects inputs F 0  to F 3  over four successive cycles CYCLE 0  to CYCLE 3  to read out successively w 0  to w 4 . Once storage location F 3  has been read out, the read port counter will reset the read port RP 1  so that it reads out from storage locations F 4  to F 7  over the next four cycles. If the buffer is not in use, the first instruction pair w 0  is read out by the multiplexor MUX 0 . That is in cycle  0 , input M 00  of the multiplexor MUX 0  is selected. Meanwhile, the 128 bit line is loaded into the first location of the prefetch buffer and the read pointer points to the next location to be read out by the decode unit. Therefore on cycle  1 , the next instruction pair w 1  is read out by the multiplexor MUX 0  by selecting pf-buf-out 1 . 
     In GP 32  mode, in the first machine cycle the first two instructions w 0 ,w 1  are presented to the decode unit  20 . In the subsequent cycle, cycle  1 , the next two instructions w 2 ,w 3  are presented to the decode unit. This utilises read ports RP 1  and RP 2  and the multiplexors MUX 0  and MUX 1 . If the signal mux-ctrl 0  is set to pf-buf-out 1 , and mux-ctrl 1  to pf-buf-out 2 , then the read pointer RP is set to F 0  for RP 1  and F 1  for RP 2  in cycle  0 . In cycle  1 , it is changed to F 2  and F 3  respectively. Instructions are then read over the next two cycles from the next group of storage locations F 4  to F 7  by altering the setting of the read ports RP 1  and RP 2  responsive to the read pointer RP. Alternatively, when read from the data-in path  18 , in the first cycle, the first input M 10  of the multiplexor MUX 1  is set to read w 1  (bits  31  to  63 ) and the first input M 00  of the multiplexor MUX 0  is set to read w 0  (bits  0  to  31 ). Thus, instructions w 0  and w 1  are presented to the decode unit  20  in CYCLE  0 . Meanwhile, the 128 bit line is loaded into the prefetch buffer so that in the subsequent cycle, CYCLE  1 , w 2  and w 3  are read from the buffer by selecting pf-buf-out 1  and pf-buf-out 2 . 
     In VLIW mode, four 32 bit instructions W 0  to W 3  (slot 0  to slot 3 ) are supplied simultaneously to the decode unit  20  in each machine cycle, e.g. CYCLE  0 . The multiplexors MUX 2  and MUX 3  are set according to the control signals mux-ctrl 2  and mux-ctrl 3  respectively to allow the instruction words w 2  and w 3  to be read either from the buffer or from the data-in path  18 . In other respects, the settings of RP 1  and RP 2 , MUX 0  and MUX 1  are as in GP 32  mode. However, in the subsequent cycle, e.g. CYCLE  1  in VLIW mode, it will be noticed that the instruction words w 2  and w 3  which would have been remaining in GP 32  mode have now been read out. Therefore, the read pointer RP can immediately move on to the next set of storage locations F 4  to F 7  to read out the subsequent VLIW instruction word containing the next four instructions. 
     Data is passed from the multiplexors MUX 0  to MUX 3  to respective output flip-flops FF 0  to FF 3  via a set of control gates labelled GC 1 , GC 2  and GS 0  to GS 3 . The control gates GC 1 ,GC 2  are responsive to change mode signals chmd 1 ,chmd 2  respectively which indicate to the prefetch unit that there has been a change in the instruction mode in which the machine is operating. The control gates GS 0  to GS 3  are responsive to respective stop signals stop[ 0 ] to stop[ 3 ] to prevent any new data from entering the decode unit from that output flip-flop. These effectively allow the decode unit to be stalled. In a stop condition, the outputs of the flip-flops are recirculated to the input of its associated control switch to prevent unnecessary operation of the subsequent decoder. 
     Operation of the prefetch unit responsive to the change mode signals chmd 1  and chmd 2  will now be described. The output flip-flop FF 0  is connected to a single 32 bit decoder and to two 16 bit decoders. When the machine is in GP 16  mode, the outputs of the two 16 bit decoders are selected for the instruction pair supplied to the flip-flop FF 0 . When the machine is in GP 32  mode, the output of the 32 bit decoder is selected. The remaining flip-flops FF 1  to FF 3  are each connected to respective 32 bit decoders. 
     A first change mode signal chmd 1  signals a change of machine instruction mode from GP 32  to GP 16 . If the machine had been operating in GP 32  mode, consider the situation at the end of cycle  0  which reference to FIG.  2 . Instructions w 0  and w 1  will have been supplied via the flip-flops FF 0  and FF 1  to the respective 32 bit decoders of the decoder  20 . However, the change in instruction mode now implies that the 32 bit sequence which was formerly to be considered as the second instruction W 1  in cycle  0  of GP 32  mode, in fact contains a pair of 16 bit instructions as denoted in cycle  1  of GP 16  mode. Thus, the output of the 32 bit decoder connected to the flip-flop FF 1  needs to be ignored, and the 32 bit sequence w 1  needs to be reapplied to the two 16 bit decoders connected to the output flip-flop F 0 . This is achieved by the recirculation line  42  from the output of the flip-flop FF 1  to the input of the control gate CG 1 . 
     Conversely, control signal chmd 2  denotes a change of instruction mode from GP 16  to GP 32 . Consider again the effect at the end of cycle  0  with reference to FIG.  2 . The instruction pair denoted w 0  has just been decoded in GP 16  mode, and the expectation is that the machine will now wait for the next instruction pair w 1 . However, in GP 32  mode, that word w 1  represents a single instruction and the change mode signal chmd 2  allows it to be applied directly through the control gate GC 2  to the output flip-flop FF 1  so that it can be applied directly to the input of the dedicated 32 bit decoder connected to the output of the flip-flop FF 1 . This allows the instruction w 1  to be decoded as a single 32 bit instruction. In the next cycle, instructions w 2  and w 3  can be transmitted normally as indicated by cycle  1  in GP 32  mode in FIG.  2 . 
     It will be clear from the above that the number of cycles needed co read all four storage locations in a group depends on the instruction mode. That is, in GP 16  mode, four cycles are needed, in GP 32  two cycles are needed and VLIW one cycle is needed. When all the storage locations F 0  to F 3  in the first group have been read, the first empty flag EF 1  is cleared to empty. 
     The aligner controls the setting and clearing of the “empty” flags using information from the read pointer. The aligner detects when the read pointer goes from one line (128 bits) to the next. When this occurs, the “empty” flag corresponding to the page which has just been read is set. 
     The state of an empty flag being cleared is detected by the prefetcher  8  along line  48  and a fetch is initiated to the next prefetch address in the prefetch program counter  16 . Thus, the next instruction line is fetched from memory and the write pointer WP is set to write it into storage locations F 0  to F 3 . In the meantime, the read pointer has moved to the second group F 4  to F 7  to read and decode instructions of that group. When those storage locations are empty, the empty flag EF 2  is cleared, a next memory fetch is initiated by the prefetcher  8  and the read pointer moves onto the group F 8  to F 11 . As can readily be seen, the prefetch buffers masks a latency of memory fetches of at least three cycles in the VLIW mode, and a greater number of cycles in GP 32  and GP 16  mode. Signals are supplied from the decoder along line  24   b  to the aligner  10  indicating what mode the decoder is operating in so that the aligner can adjust the align program counter  26  accordingly and keep track of the next instructions to be decoded so that the read pointer RP can correctly be issued by the state machine  28 . The control signals  24   b  sent from the decode unit  20  to the aligner include an acknowledge bit which controls the aligner, in particular the program counter  26 . 
     FIG. 5 illustrates the details of the decode unit  20 . The decode unit comprises six decoders  50 , 52 , 54 , 56 , 58  and  60 . Four of the decoders  50 , 54 , 58  and  60  are 32 bit decoders which are labelled DEC 32 - 0 , 1 , 2  and  3  respectively because they are each associated with the output devices FF 0  to FF 3  of the prefetch buffer illustrated in FIG.  4 . There are two 16 bit decoders  52 , 56  which are labelled DEC 16 - 0 , 1  respectively. Both of the 16 bit decoders  52 , 56  are associated with the first output device FF 0 . The 32 bit decoders  50 , 54 , 58  and  60  are hard-wired by 32 bit communication paths respectively to the output devices F 0  to F 3 . These communication paths are denoted by the dotted lines CP 0  to CP 3  in FIG.  6 . The first 16 bit decoder  52  is hard-wired to the 16 least significant bits of the output of the output device FF 0 , and the second 16 bit decoder  56  is hard-wired to the 16 most significant bits of the output of the output device FF 0 . Thus, the bits of each instruction are supplied as a parallel sequence to the decoders. 
     The outputs of the top two decoders  50  and  56  are supplied to a first decode multiplexor MUX 6  which has an output connected to an instruction dispatch unit  62 . The outputs of the second two decoders  54 , 56  are connected to a second decode multiplexor MUX 7 , the output of which is also connected to a dispatch unit  62 . The outputs of the lower most decoders  58 , 60  are connected directly to the dispatch unit  62 . The decoders  50  to  60  are always “on”. That is, on each machine cycle they receive the bits on the connection paths CP input to the decoders, perform a decode operation and supply an output. It can readily be seen therefore that of the top four decoders, the outputs of only two of them in each case are of interest. For example, in GP 32  mode, it is the outputs of the decoders  50 , 54  which are required—the outputs of the decoders  52 ,  56  being redundant. A mode signal MODE supplied to the multiplexors MUX 6 ,MUX 7  selects the appropriate outputs in dependence on the instruction mode of the machine. Although the decoders are normally. “on”, they are responsive only to change in state of the bits supplied to them from the output devices FF 0  to FF 3 . If between two machine cycles, there is no change in state in the outputs of the output devices, then the connected decoder will not need to change any of its internal state. Thus, no power will be consumed by that decoder for as long as the outputs of the output device FF 0  to FF 3  connected to is do not change. Therefore, by recirculating The outputs using the control switches GS 0  to GS 3  responsive to the stop signals stop[ 0 ] to stop[ 3 ] when the relevant decoders are not required, a power saving feature is introduced. 
     It is sometimes useful to be able to use a 32 bit instruction in GP 16  mode because the instructions in GP 16  mode are more limited, while those in GP 32  mode allow more complicated operations. Therefore the present system in GP 16  mode allows an instruction sequence: 
     (i) Gp 32 nxt 
     (ii) Instr (32 b) 
     where (i) is the name of a specific instruction and (ii) represents any 32 bit instruction. Instruction (i) has the effect of allowing execution of eight of the 32 bit instructions while remaining in GP 16  mode. The instruction (i) is detected in the decoder and causes the following changes in operation. 
     The decoder recognises a Gp 32 nxt instruction in the detector  70  and causes a MODE signal to switch MUX 6  to supply the output of the 32 bit decoder  50  on the next cycle. The decoder acknowledges the subsequent 32 bit instruction to the aligner as two 16 bit instructions. On the subsequent cycle, the MODE signal to MUX 6  reverts back to select the output of the 16 bit decoder  52  for decoding of subsequent 16 bit instructions. There is no change in the process status register PSR of the machine. The Gp 32 nxt instruction avoids the need for wasteful execution of change mode instructions just to execute a single 32 bit instruction while in GP 16  mode. It will be appreciated that the sequences (i), (ii) above must however be atomic.