Abstract:
An integrated circuit has a field programmable circuit region arranged as a generally rectangular array of rows and columns of circuit areas. Some of the circuit areas each provide a respective processing unit for performing operations on data on at least one respective input signal path to provide data on at least one respective output signal path. Others of the circuit areas each provide a respective switching section; and the processing units and the switching sections are arranged alternately in each row and in each column. Each of a substantial proportion of the switching sections provides a programmable connection between at least some of the signal paths of those of the processing units adjacent that switching section in the same column and in the same row. A dense layout can be obtained with efficient local interconnections, especially in the case where one or more of the processing units has a plural-bit input and/or a plural-bit output, and at least some of the signal paths are provided by respective plural-bit busses.

Description:
NOTICE OF RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/341,566, filed Jul. 13, 1999, now U.S. Pat. No. 6,252,792 entitled FIELD PROGRAMMABLE PROCESSOR ARRAYS, and naming Alan Marshall, Anthony Stansfield and Jean Vuillemin as joint inventors, which application is hereby incorporated by reference in its entirety, which application is itself a US counterpart to EP Patent Application 97300563.0, filed Jan. 29, 1997, entitled FIELD PROGRAMMABLE PROCESSOR ARRAYS, and naming Alan Marshall, Anthony Stansfield and Jean Vuillemin as joint inventors, which application is hereby incorporated by reference in its entirety. This application claims the benefit of both above-cited applications. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to field programmable processor arrays, that is, arrays of processing units which are selectively connectable by signal paths one to another, and in particular to such arrays which are provided as integrated circuits. 
     The problems with which the present invention (or at least preferred embodiments of it) is concerned are to provide a dense circuit layout, efficient interconnections between the processing units and flexibility in the manner in which the processing units may be interconnected. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided an integrated circuit having a field programmable circuit region arranged as a generally rectangular array of rows and columns of circuit areas, wherein: some of the circuit areas each provide a respective processing unit (such as an arithmetic logic unit) for performing operations on data on at least one respective input signal path to provide data on at least one respective output signal path; others of the circuit areas each provide a respective switching section; the processing units and the switching sections are arranged alternately in each row and in each column; and each of a substantial proportion (and preferably all) of the switching sections provides a programmable connection between at least some of the signal paths of those of the processing units adjacent that switching section in the same column and in the same row. By arranging the circuit in this way, a dense layout can be obtained with efficient local interconnections, especially in the case where one or more of the processing units has a plural-bit input and/or a plural-bit output, and at least some of the signal paths are provided by respective plural-bit busses. 
     A first type of the connections provided by the switching sections are between such signal paths which may be generally collinear with or parallel to each other, allowing the paths to be selectably concatenated to produce longer connections. In this case and in the case where at least one of the processing units and the processing units adjacent thereto each have a first input, a second input and an output; the output of said one processing unit is preferably connectable: by such a first type of connection to the first input of the next processing unit in one direction in the same row; by such a first type of connection to the first input of the next processing unit in the one direction in the same column; by such a first type of connection to the second input of the next processing unit in the opposite direction in the same row; and by such a first type of connection to the second input of the next processing unit in the opposite direction in the same column. In this way, the output of a first processing unit can be selectably passed to any of the four adjacent processing units in the same row or column (“a second processing unit”), where it can be processed and then passed back to the first processing unit or passed on to any of the other three adjacent processing units adjacent the second processing unit. 
     A second type of the connections provided by the switching sections are between such signal paths which may be generally orthogonal to each other. Thus the signal paths can change direction to enable flexible routing. In this case and in the case where at least one of the processing units and the processing units adjacent thereto each have a first input, a second input and an output, the output of said one processing unit is preferably connectable: by such a second type of connection in the same column to the first input of the diagonally adjacent processing unit in said one row direction and said one column direction; by such a second type of connection in the same row to the first input of the diagonally adjacent processing unit in said opposite row direction and said one column direction; by such a second type of connection in the same column to the second input of the diagonally adjacent processing unit in said opposite row direction and said opposite column direction; and by such a second type of connection in the same row to the second input of the diagonally adjacent processing unit in said one row direction and said opposite column direction. Thus, the output from a first processing unit can be selectably routed to the first inputs of two of the diagonally adjacent processing units, and to the second inputs of the other two diagonally adjacent processing units. 
     Preferably substantially all of the input and output signal paths are oriented in directions substantially parallel to the rows or the columns, thus enabling a dense layout to be achieved. 
     The integrated circuit preferably further comprises a plurality of inter switching section signal paths, each of which extends from a respective first one of the switching sections to a respective second one of the switching sections in the same row in a direction primarily generally parallel to that row, or in the same column in a direction primarily generally parallel to that column, each of the inter switching section signal paths being programmably connectable by the respective first switching section to others of the signal paths at that first switching section, and being programmably connectable by the respective second switching section to others of the signal paths at that second switching section. Accordingly, medium and long range connections can be selectably provided. 
     For one type of the inter switching section signal paths, there may be no such switching sections in the respective row or column between the respective first and second switching sections. 
     For another type of the inter switching section signal paths, the respective first and second switching sections may have a number (preferably one less than a power of two) of other such switching sections therebetween in the respective row or column. Thus, long range connections are provided reducing the number of intermediate switches which may be required, and accordingly reducing the propagation delay which would be caused thereby. 
     For a further type of the inter switching section signal paths, each signal path may have a spine portion extending in a direction generally parallel to the respective row or column and first and second end portions each extending in a direction generally orthogonal to the respective row or column and interconnecting the spine portion and the respective first and second switching sections, respectively. Since the spine portions do not connect to the switches directly, the choice of the physical position of the conductors in the spine portion is flexible, and this flexibility enables denser layouts to be achieved. For at least some of the inter switching section signal paths of said further type, the respective first and second switching sections may have a number (preferably one less than a power of two) of other such switching sections therebetween in the respective row or column. Thus, long range connections are provided reducing the number of intermediate switches which may be required, and accordingly reducing the propagation delay which would be caused thereby. At least some of the inter switching section signal paths of said further type preferably each have at least one tap portion extending in a direction generally orthogonal to the respective row or column and interconnecting the spine portion and a respective such other switching section. Accordingly, even greater flexibility is provided. 
     At least some of the switching sections preferably each include a respective register and/or buffer having an input and an output each switchably connectable to at least some of the signal paths at that switching section. This allows signals to be retimed or buffered without using one of the processing units for that purpose. Retiming and buffering of time-critical signals allows configurations to run at higher clock speeds, increasing the rate of operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A specific embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 shows part of a processor array, illustrating six switching sections and the locations of six arithmetic logic units; 
     FIG. 2 is a diagram of part of the arrangement shown in FIG. 1 on a larger scale, illustrating one of the switching sections and one of the locations of the arithmetic logic units; 
     FIG. 3 shows part of the processor array shown in FIG. 1 on a smaller scale, illustrating the locations of the arithmetic logic units and “vertical” busses extending across them; 
     FIG. 4 is similar to FIG. 3, but illustrating “horizontal” busses extending across the locations of the arithmetic logic units; 
     FIG. 5 shows the interconnections between the busses of FIGS. 2,  3  and  4  at the location of one of the arithmetic logic units; 
     FIG. 6A shows in detail the circuitry of one type of programmable switch in the switching sections, for connecting a pair of 4-bit busses which cross each other; 
     FIG. 6B shows in detail the circuitry of another type of programmable switch in the switching sections, for connecting a pair of 4-bit busses which meet each other end to end; 
     FIG. 6C shows in detail the circuitry of another type of programmable switch in the switching sections, for connecting carry-bit busses; 
     FIG. 7 shows the circuitry of a series of NOR gates which may be used in the programmable switches of FIGS. 5 and 6; 
     FIG. 8 shows a modification to the circuitry of FIG. 7; 
     FIG. 9 shows a buffer and register which may be used in each switching section; 
     FIG. 10 is a schematic drawing illustrating how enable signals may be distributed to the programmable switches in the switching sections; and 
     FIG. 11 shows in more detail the circuitry of the arrangement shown in FIG.  10 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, the terms “horizontal”, “vertical”, “North”, “South”, “East” and “West” have been used to assist in an understanding of relative directions, but their use is not intended to imply any restriction on the absolute orientation of the embodiment of the invention. 
     The processor array which forms the embodiment of the invention is provided in an integrated circuit. At one level, the processor array is formed by a rectangular (and preferably square) array of “tiles”  10 , one of which is shown bounded by a thick line in FIG.  1 . Any appropriate number of tiles may be employed, for example in a 16×16, 32×32 or 64×64 array. Each tile  10  is rectangular (and preferably square) and is divided into four circuit areas. It is preferable for these tiles to be logically square (to provide symmetry in connection), although it is of less significance that they be physically square (this may have some advantage in providing symmetry in timing, but this will generally be less likely to be of significance). Two of the circuit areas  12 , which are diagonally opposed in the tile  10 , provide the locations for two arithmetic logic units (“ALUs”) . The other two circuit areas, which are diagonally opposed in the tile  10 , provide the locations for a pair of switching sections  14 . 
     Referring to FIGS. 1 and 2, each ALU has a first pair of 4-bit inputs a, which are directly connected within the ALU, a second pair of 4-bit inputs b, which are also directly connected within the ALU, and four 4-bit outputs f, which are directly connected within the ALU. Each ALU also has an independent pair of 1-bit carry inputs hci, vci, and a pair of 1-bit carry outputs co, which are directly connected within the ALU. The ALU can perform standard operations on the input signals a, b, hci, vci to produce the output signals f, co, such as add, subtract, AND, NAND, OR, NOR, XOR, NXOR and multiplexing and optionally can register the result of the operation. The instructions to the ALUs may be provided from respective 4-bit memory cells whose values can be set via the “H-tree” structure described below, or may be provided on the bus system which will be described below. 
     At the level shown in FIGS. 1 and 2, each switching section  14  has eight busses extending across it horizontally, and eight busses extending across it vertically, thus forming an 8×8 rectangular array of 64 crossing points, which have been numbered in FIG. 2 with Cartesian co-ordinates. All of the busses have a width of four bits, with the exception of the carry bus vc at X=4 and the carry bus hc at Y=3, which have a width of one bit. At many of the crossing points, a 4-gang programmable switch  16  is provided which can selectively connect the two busses at that crossing point. At some of the crossing points, a 4-gang programmable switch  18  is provided which can selectively connect two busses which meet end to end at that crossing point, without any connection to the bus at right angles thereto. At the crossing point at ( 4 ,  3 ), a programmable switch  20  (for example as shown in FIG. 6C) is provided which can selectively connect the carry busses vc, he which cross at right angles at that point. 
     The horizontal busses in the switching section  14  will now be described. 
     At Y=0, busses h 2 s are connectable by programmable switches  16  to the vertical busses at X=0, 1, 2, 5, 6. The busses h 2 s have a length of two tiles and are connectable end to end in every other switching section  14  by a programmable switch  18  at ( 4 ,  0 ). 
     At Y=1, a bus be extending from an input b of the ALU to the West is connectable by switches  16  to the vertical busses at X=0, 1, 2, 3. Also, a bus fw extending from an output f of the ALU to the East is connectable by switches  16  to the vertical busses at X=5, 6, 7. The ends of the busses be, fw are connectable by a programmable switch  18  at ( 4 ,  1 ). 
     At Y=2, a bus hregs is connectable by programmable switches  16  to the vertical busses at X=1,2, 3, 5, 6, 7. 
     At Y=3, a bus hco extends from the carry output co of the ALU to the West to a programmable switch  20  at ( 4 ,  3 ), which can connect the bus hco (a) to a carry bus hci extending to the carry input hci of the ALU to the East or (b) to a carry bus vci extending to the carry input vci of the ALU to the South. 
     At Y=4, a bus hregn is connectable by programmable switches  16  to the vertical busses at X=0, 1,2,3,5,6. 
     At Y=5, busses h 1  are connectable to the vertical busses at X=0, 1, 2, 3, 5, 6, 7. The busses h 1  have a length of one tile and are connectable end to end in each switching section  14  by a programmable switch  18  at ( 4 ,  5 ). 
     At Y=6, a bus fe extending from an output f of the ALU to the West is connectable by switches  16  to the vertical busses at X=0, 1, 2, 3. Also, a bus aw extending from an input a of the ALU to the East is connectable by switches  16  to the vertical busses at X=5, 6, 7. The ends of the busses fe, aw are connectable by a programmable switch  18  at ( 4 .  6 ). 
     At Y=7, busses h 2   n  are connectable by programmable switches  16  to the vertical busses at X=1, 2, 3, 6, 7. The busses h 2   n  have a length of two tiles and are connectable end to end in every other switching section  14  by a programmable switch  18  at ( 4 ,  7 ), staggered with respect to the programmable switches  18  connecting the busses h 2 s at ( 4 , 0 ). 
     The vertical busses in the switching section  14  will now be described. 
     At X=0, busses v 2   w  are connectable by programmable switches  16  to the horizontal busses at Y=0, 1, 4, 5, 6. The busses v 2   w  have a length of two tiles and are connectable end to end in every other switching section  14  by a programmable switch  18  at ( 0 ,  3 ). 
     At X=1, a bus fin extending from an output f of the ALU to the South is connectable by programmable switches  16  to the horizontal busses at Y=0, 1, 2. Also, a bus bs extending from an input b of the ALU to the North is connectable by switches  16  to the horizontal busses at Y=4, 5, 6, 7. The ends of the busses fn, bs are connectable by a programmable switch  18  at ( 1 ,  3 ). 
     At X=2, busses v 1  are connectable to the horizontal busses at Y=0, 1, 2, 4, 5, 6, 7. The busses v 1  have a length of one tile and are connectable end to end in each switching section  14  by a programmable switch  18  at ( 2 ,  3 ). 
     At X=3, a bus vregw is connectable by programmable switches  16  to the horizontal busses at Y=1,2, 4, 5,6,7. 
     At X=4, a bus vco extends from the carry output co of the ALU to the North to the programmable switch  20  at ( 4 ,  3 ), which can connect the bus vco (a) to the carry bus hci extending to the carry input hci of the ALU to the East or (b) to the carry bus vci extending to the carry input vci of the ALU to the South. 
     At X=5, a bus vrege is connectable by programmable switches  16  to the horizontal busses at Y=0, 1, 2, 4, 5, 6. 
     At X=6, a bus an extending from an input a of the ALU to the South is connectable by switches  16  to the horizontal busses at Y=0, 1, 2. Also, a bus fs extending from an output f of the ALU to the North is connectable by programmable switches  16  to the horizontal busses at Y=4, 5, 6, 7. The ends of the busses an, fs are connectable by a programmable switch  18  at ( 6 ,  3 ). 
     At X=7, busses v 2   e  are connectable by programmable switches  16  to the horizontal busses at Y=1, 2, 5, 6, 7. The busses v 2   e  have a length of two tiles and are connectable end to end in every other switching section  14  by a programmable switch  18  at ( 7 ,  3 ) staggered with respect to the programmable switches  18  connecting the busses v 2   w  at ( 0 .  3 ). 
     As shown in FIG. 2, the busses bs, vco, fs are connected to input b, output co and output f, respectively, of the ALU to the North of the switching section  14 . Also, the busses fe, hco, be are connected to the output f, output co and input b of the ALU, respectively, to the West of the switching section  14 . Furthermore, the busses aw, hci, fw are connected to the input a, input ci and output f, respectively, of the ALU to the East of the switching section  14 . Moreover, the busses m, vci, an are connected to the output f, input ci and input a, respectively, of the ALU to the south of the switching section  14 . 
     In addition to these connections, the busses vregw, vrege are connected via respective programmable switches  18  to 4-bit connection points vtsw, vtse, respectively, (shown by crosses in FIG. 2) in the area  12  of the ALU to the North of the switching section  14 . Also, the busses hregs, hregn are connected via respective programmable switches  18  to 4-bit connection points htse, htne, respectively, in the area  12  of the ALU to the West of the switching section  14 . Furthermore, the busses hregs, hregn are connected via respective programmable switches  18  to 4-bit connection points htsw, htnw, respectively, in the area  12  of the ALU to the East of the switching section  14 . Moreover, the busses vregw, vrege are connected via respective programmable switches  18  to 4-bit connection points vtnw, vtne, respectively, in the area  12  of the ALU to the south of the switching section  14 . These connection points vtnw, vtne, htne, htse, vtse, vtsw, htsw, htnw will be described below in further detail with reference to FIGS. 3 to  5 . 
     Also, as shown in FIG. 2, the busses hregn, vrege, hregs, vregw have respective 4-bit connection points  22  (shown by small squares in FIG. 2) which will be described below in further detail with reference to FIG.  9 . 
     FIG. 3 shows one level of interconnections between the locations of the arithmetic logic units, which are illustrated by squares with rounded corners. A group of four 4-bit busses v 8 , v 4   w,  v 4   e,  v 16  extend vertically across each column of ALU locations  12 . The leftmost bus v 8  in each group is in segments, each having a length generally of eight tiles. The leftmost but one bus v 4   w  in each group is in segments, each having a length generally of four tiles. The rightmost but one bus v 4   e  in each group is in segments, again each having a length generally of four tiles, but offset by two tiles from the leftmost but one bus v 4   w.  The rightmost bus v 16  in each group is in segments, each having a length generally of sixteen tiles. At the top edge of the array, which is at the top of FIG. 4, and at the bottom edge the lengths of the segments may be slightly greater than or shorter than specified above. 
     Referring to FIGS. 3 and 5, where each group of four busses v 8 , v 4   w,  v 4   e,  v 16  crosses each ALU location  12 , four 4-bit tap connections are made at the connection points htnw, htsw, htse, htne. The ends of the bus segments take priority in being so connected over a connection to a bus segment which crosses the ALU location. 
     Similarly, as shown in FIGS. 4 and 5, a group of four 4-bit busses h 8 , h 4   n,  h 4   s,  h 16  extend horizontally across each row of ALU locations  12 . The uppermost bus h 8  in each group is in segments, each having a length generally of eight tiles. The uppermost but one bus h 4   n  in each group is in segments, each having a length generally of four tiles. 
     The lowermost but one bus h 4   s  in each group is in segments, again each having a length generally of four tiles, but offset by two tiles from the uppermost but one bus h 4   n . The lowermost bus h 16  in each group is in segments, each having a length generally of sixteen tiles. At the left hand edge of the array, which is at the left of FIG. 4, and at the right hand edge the lengths of the segments may be slightly greater than or shorter than specified above. Where each group of busses h 8 , h 4   n,  h 4   s,  h 16  crosses each ALU location  12 , a further four 4-bit tap connections are made at the connection points vtnw, vtsw, vtse, vtne. The ends of the bus segments take priority in being so connected over a connection to a bus segment which crosses the ALU location. 
     As shown in FIG. 5, the connection points htnw, htsw, htne, htse are connected via programmable switches to the busses hregn, hregs of the switching sections to the West and the East of the ALU location. Also, the connection points vtnw, vtne, vtsw, vtse are connected via programmable switches to the busses vregw, vrege of the switching sections to the North and the South of the ALU location. 
     The programmable connections  16  between pairs of 4-bit busses which cross at right angles will now be described with reference to FIG.  6 A. The conductors of the horizontal busses are denoted as x 0 , x 1 , x 2 , x 3 , and the conductors of the vertical busses are denoted as y 0 , y 1 , y 2 , y 3 . Between each pair of conductors of the same bit significance, a respective transistor  160 ,  161 ,  162 ,  163  is provided. The gates of the transistors  160 ,  161 ,  162 ,  163  are connected in common to the output of a NOR gate  16 g, which receives as its two inputs an inverted ENABLE signal from a single bit memory cell, which may be shared by a group of the switches, and the inverted content of a single bit memory cell  24 . Accordingly, only when the ENABLE signal is high and the content of the memory cell  24  is high, the conductors x 0 , x 1 , x 2 , x 3  are connected by the transistors  160 ,  161 ,  162 ,  163 , respectively, to the conductors y 0 , y 1 , y 2 , y 3 , respectively. 
     The programmable connections  18  between pairs of 4-bit busses which meet each other end to end in line will now be described with reference to FIG.  6 B. The conductors of one bus are denoted as x 10 , x 11 , x 12 , x 13 , and the conductors of the other bus are denoted as x 20 , x 21 , x 22 , x 23 . Between each pair of conductors of the same bit significance, a respective transistor  180 ,  181 ,  182 ,  183  is provided. The gates of the transistors  180 ,  181 ,  182 ,  183  are connected in common to the output of a NOR gate  18   g,  which receives as its two inputs an inverted ENABLE signal from a single bit memory cell, which may be shared by a group of the switches, and the inverted content of a single bit memory cell  24 . Accordingly, only when the ENABLE signal is high and the content of the memory cell  24  is high, the conductors x 10 , x 11 , x 12 , x 13  are connected by the transistors  180 ,  181 ,  182 ,  183 , respectively, to the conductors x 20 , x 21 , x 22 , x 23 , respectively. 
     The programmable connections  20  between the carry conductors hco,vco,hci,vci will now be described with reference to FIG.  6 C. The horizontal carry output conductor hco is connected to the horizontal carry input conductor hci and the vertical carry input conductor vci via transistors  20   hh,    20   hv,  respectively. Furthermore, the vertical carry output conductor vco is connected to the vertical carry input conductor vci and the horizontal carry input conductor hci via transistors  20   vv,    20   vh,  respectively. The gates of the transistors  20   hh,    20   w  are connected in common to the output of an inverter  20   i,  and the gates of the transistors  20   hv,    20   vh  and the input to the inverter  20   i  are connected to the output of a NOR gate  20   g.  The NOR gate  20   g  receives as its two inputs an inverted ENABLE signal from a single bit memory cell, which may be shared by a group of the switches, and the inverted content of a single bit memory cell  24 . 
     Accordingly, when the ENABLE signal is high, the conductors hco, vco are connected to the conductors hci, vci, respectively, or to the conductors vci, hci, respectively, in dependence upon the content of the memory cell  24 . 
     It will be noted that each of the switchable connections  16 ,  18 ,  20  described with reference to FIGS. 6A to  6 C includes a NOR gate  16   g,    18   g,    20   g.  As shown in FIG. 7, a NOR gate  16   g  is typically formed by four transistors  16   g   1 ,  16   g   2 ,  16   g   3 ,  16   g   4 , two  16   g   1 ,  16   g   3  of which are responsive to the inverted ENABLE signal, and two  16   g   2 ,  16   g   4  of which are responsive to the inverted content of the memory cell  24 . In the embodiment of the invention, it is desirable that a group of the switchable collections  16 ,  18 ,  20  may be disabled in common, without any need for only part of such a group to be disabled. Such a group might consist of all of the switchable connections in one switching section  14 , all of the switchable connections in the two switching sections  14  in a particular tile, or all of the switchable connections in a larger area of the array. In this case, the transistor  16   g   1  may be made common to all of the switchable connections  16 ,  18 ,  20  in the group, as shown in FIG.  8 . This enables a 25% less one saving in the number of transistors required for the gates, but does require a further conductor linking the gate, as shown in FIG.  8 . 
     The man skilled in the art will appreciate that the structures depicted in FIGS. 7 and 8 can be modified for optimisation. For example, the arrangement of FIGS. 7 and 8 would not fully exploit memory cells  24  designed to return both a stored value and a complement of that stored value. Use of the complement obtained from such cells  24  could be used to obviate any need for both the ENABLE and inverted ENABLE signals to be carried to all of the switchable connections in a group, as is the case in FIG.  8 . 
     As mentioned above with reference to FIGS. 1 and 2, at each switching section  14 , the busses hregn, hregs, vregw, vrege are connected by respective 4-bit connections  22  to a register or buffer circuit, and this circuit will now be described in more detail with reference to FIG.  9 . The four connections  22  are each connected to respective inputs of a multiplexer  26 . The multiplexer  26   th  selects one of the inputs as an output, which is supplied to a register or buffer  28 . The output of the register or buffer  28  is supplied to four tri-state buffers  30   s,    30   w,    30   n,    30   e,  which are connected back to the connections  22  to the busses hregs, vregw, hregn, vrege, respectively. In the case where a buffer  28  is used, the 4-bit signal on a selected one of the busses hregs, vregw, hregn, vrege is amplified and supplied to another selected one of the busses hregs, vregw, hregn, vrege. In the case where a register  28  is used, the 4-bit signal on a selected one of the busses hregs, vregw, hregn, vrege is amplified and supplied to any selected one of the busses hregs, vregw, hregn, vrege after the next active clock edge. 
     It will be appreciated that the arrangement described above provides great flexibility in the routing of signals around and across the array. With appropriate setting of the switches  16 ,  18 ,  20  using the memory cells  24  and with appropriate setting of the multiplexers  26  and registers or buffers  28 , signals can been sent over large distances, primarily using the busses v 16 , h 16 , v 8 , h 8 , v 4   e,  v 4   w,  h 4   n,  h 4   s  from the edge of the array to a particular ALU, between ALUs, and from a particular ALU to the edge of the array. These busses can be joined together in line, or at right angles, by the switching sections  14 , with amplification by the registers or buffers  28  in order to reduce propagation delays, and with pipeline stages introduced by the registers  28 . Also, these busses can be tapped part way along their lengths, so that the siting of the ALUs to perform a particular processing operation is not completely dictated by the lengths of the busses, and so that signals can be distributed to more than one ALU. Furthermore, the shorter length busses described with reference to FIGS. 1 and 2 can be used to route signals between the switching sections  14  and the ALUs, and to send signals primarily over shorter distances, for example from one ALU to an adjacent ALU in the same row or column, or diagonally adjacent, even though the busses extend horizontally or vertically. Again, the registers or buffers  28  can be used to amplify the signals or introduce programmable delays into them. 
     In the arrangement described above, the memory cells  24  are distributed across the array to the same extent as the switching sections  14  and the ALU locations  12 . Each memory cell  24  is disposed adjacent the switch or switches, multiplexer, register or buffer which it controls. This enables a high circuit density be achieved. 
     A description will now be made of the manner in which data is written to or read from the memory cells  24 , the way in which the ENABLE signals for the programmable switches  16 ,  18 ,  20  are written to their memory cells, the way in which instructions, and possibly constants, are distributed to the ALUs, and the way in which other control signals, such as a clock signal, are transmitted across the array. For all of these functions, an “H-tree” structure (which is known per se) may be employed, as shown in FIG.  10 . Referring to FIGS. 10 and 11, in order to distribute an ENABLE signal to any of 64 locations in the example shown, the ENABLE signal  30   a  and a 6-bit address  32   a  for it are supplied to a decoder  34   a.  The decoder  34   a  determines which of the four branches from it leads to the address and supplies an ENABLE signal  30   b  to a further decoder  34   b  in that branch, together with a 4-bit address  32   b  to the decoders  34   b  in all four branches. 
     The decoder  34   b  receiving the ENABLE signal  30   b  determines which of the four branches from it leads to the required address and supplies an ENABLE signal  30   c  to a farther decoder  34   c  in that branch, together with a 4-bit address  32   c  to the decoders  34   c  in all four branches. The decoder  34   c  receiving the ENABLE signal  30   c  then supplies the ENABLE signal  34   d  to the required address where it can be stored in a single bit memory cell. An advantage of the H-tree structure is that the lengths of the signal paths to all of the destinations are approximately equal, which is particularly advantageous in the case of the clock signal. 
     A great advantage of the arrangement described above is that groups of the memory cells  24  in for example one switching section  14 , or in the two switching sections in one tile, or in the switching sections in a sub-array of the tiles may be disabled en bloc by the inverted ENABLE signals so that the contents of those memory cells do not affect the associated switches. It is then possible for those memory cells  24  to be used as “user” memory by an application, rather than being used for configuring the wiring of the array. 
     The embodiment of the invention has been described merely by way of example, and many modifications and developments may be made in keeping with the present invention. For example, the embodiment employs ALUs as the processing units, but other processing units may additionally or alternatively be used, for example look-up tables, programmable logic arrays and/or self-contained CPUs which are able to fetch their own instructions. 
     Furthermore, the embodiment has been described as if the whole array is covered by ALUs and switching sections. However, other types of section may be included in the array. For example, a sub-array might be composed of a 4×4 arrangement of tiles of ALUs and switching sections as described above, and the array might be composed of such sub-arrays and memory in a 4×4 array, or such sub-arrays and RISC CPUs in a 4×4 array. 
     In the embodiment described above, each ALU location is square, and each switching section is square and of the same size as the ALU locations, but it should be noted that the controllable switches  18  in the register busses vregw, vrege, hregn, hregs encroach into the square outline of the ALU locations. The ALU locations need not be of the same size as the switching sections, and in particular may be smaller, thus permitting one or more busses to pass horizontally or vertically directly from one switching section  14  to a diagonally adjacent switching section  14 , for example running between the busses h 2 s, h 2   n  or between the busses v 2   e,  v 2   w.    
     In the embodiment described above, each ALU has two independent carry inputs vci, hci and a connected pair of carry outputs co. If required, the ALUs may be arranged to deal with two types of carry: a fast carry between adjacent ALUs which may be of particular use for multi-bit adding operations; and a slow carry which can be routed more flexibly and may be of particular use for digital serial arithmetic. The fast carry might be arranged in a similar manner to that described above with reference to the drawings, whereas the slow carry might employ programmable switches in the switching sections between the carry conductor and particular bits of the 4-bit busses. 
     In the embodiment described above, particular bit widths, sizes of switching section and sizes of array have been mentioned, but it should be noted that all of these values may be changed as appropriate. Also, the programmable switches  16 ,  18 ,  20  have been described as being disposed at particular locations in each switching section  14 , but other locations may be used as required and desired. 
     In the embodiment described above, the array is two-dimensional, but the principles of the invention are also applicable to three-dimensional arrays, for example by providing a stack of the arrays described above, with the switching sections in adjacent layers staggered with respect to each other. The stack might include just two layers, but preferably at least three layers, and the number of layers is preferably a power of two. 
     In the embodiment described above, the memory cells  24  can be isolated by the gates  16   g,    18   g,    20   g  from the switches which they control so that the memory cells can be used for other purposes, that is put in the “user plane”. The ENABLE signal memory cells, however, cannot be transferred to the user plane. In an alternative embodiment, the switches in a particular switching section  14  may be disconnectable from the remainder of the array by further switches in the busses at the boundary of that switching section  14 , with the further switches being controlled by a further memory cell which cannot be transferred to the user plane. 
     Many other modifications and developments may also be made.