text: fig1 shows a representative high level block diagram of an instruction execution unit ( ieu ) 100 associated with the present invention . the goal of ieu 100 is to execute as many instructions as possible in the shortest amount of time . there are two basic ways to accomplish this : optimize ieu 100 so that each instruction takes as little time as possible or optimize ieu 100 so that it can execute several instructions at the same time . an ieu for use with the present invention is disclosed in commonly owned , patents titled , “ high performance risc microprocessor architecture ”. u . s . pat . no . 5 , 539 , 911 , and “ extensible risc microprocessor architecture ”, u . s . pat . no . 5 , 560 , 032 , the disclosures of which are incorporated herein by reference . instructions are sent to ieu 100 from an instruction fetch unit ( ifu , not shown ) through an instruction fifo ( first - in - first - out register stack storage device ) 101 in groups of four called “ buckets .” ieu 100 can decode and schedule up to two buckets of instructions at one time . fifo 101 stores 16 total instructions in four buckets labeled 0 - 3 . ieu 100 looks at an instruction window 102 . in one embodiment of the present invention , window 102 comprises eight instructions ( buckets 0 and 1 ). every cycle ieu 100 tries to issue a maximum number of instructions from window 102 . window 102 functions as a instruction buffer register . once the instructions in a bucket are executed and their results stored in the processor &# 39 ; s register file ( see block 117 ), the bucket is flushed out a bottom 104 and a new bucket is dropped in at a top 106 . in order to execute instructions in parallel or out of order , care must be taken so that the data that each instruction needs is available when the instruction needs it and also so that the result of each instruction is available for any future instructions that might need it . a register rename circuit ( rrc ), which is part of the scheduling logic of the computer &# 39 ; s ieu performs this function by locating dependencies between current instructions and then renaming the sources ( inputs ) of the instruction . as noted above , there are three types of dependencies : input dependencies , output dependencies and anti - dependencies . input dependencies occur when an instruction , call it a , that performs an operation on the result of a previous instruction , call it b . output dependencies occur when the outputs of a and b are to be stored in the same place . anti - dependencies occur when instruction a comes before b in the instruction stream and b &# 39 ; s result will be stored in the same place as one of a &# 39 ; s inputs . input dependencies are handled by not executing instructions until their inputs are available . rrc 112 is used to locate the input dependencies between current instructions and then to signal an instruction scheduleror issuer 118 when all inputs for a particular instruction are ready . in order to locate these dependencies , rrc 112 compares the register file addresses of each instruction &# 39 ; s inputs with the addresses of each previous instruction &# 39 ; s output using a data dependency circuit ( ddc ) 108 . if one instruction &# 39 ; s input comes from a register where a previous instruction &# 39 ; s output will be stored , then the latter instruction must wait for the former to finish . this implementation of rrc 112 can check eight instructions at the same time , so a current instruction is defined as any one of those eight from window 102 . it should become evident to those skilled in the art that the present invention can easily be adapted to check more or less instructions . in one embodiment of the present invention , instructions can have from 0 to 3 inputs and 0 or 1 outputs . most instructions &# 39 ; inputs and outputs come from , or are stored in , one of several register files . each register file 117 ( e . g ., separate integer , floating and boolean register files ) has 32 real entries plus the group of 8 temporary buffers 116 . when an instruction completes ( the term “ complete ” means that the operation is complete and the operand is ready to be written to its destination register ), its result is stored in its preassigned location in the temporary buffers 116 . its result is later moved to the appropriate place in register file 117 after all previous instructions &# 39 ; results have been moved to their places in the register file . this movement of results from temporary buffers 116 to register file 117 is called “ retirement ” and is controlled by termination logic , as should become evident to those skilled in the art . more than one instruction may be retired at a time . retirement comprises updating the “ official state ” of the machine including the computer &# 39 ; s program counter , as will become evident to those skilled in the art . for example , if instruction i 0 happens to complete directly before instruction i 1 , both results can be stored directly into register file 117 . but if instruction i 3 then completes , its result must be stored in temporary buffer 116 until instruction 12 completes . by having ieu 100 store each instruction &# 39 ; s result in its preassigned place in the temporary buffers 116 . ieu 100 can execute instructions out of program order and still avoid the problems caused by output and anti - dependencies . rrc 112 sends a bit map to an issuer 118 via a bus 120 indicating which instructions in window 102 are ready for issuing . instruction decode logic ( not shown ) indicates to issuer 118 the resource requirements for each instruction over a bus 123 . for each resource in ieu 100 ( e . g ., each functional unit being an adder , multiplier , shifter , or the like ), issuer 118 scans this information and selects the first and subsequent instructions for issuing by sending issue signals over bus 121 . the issue signals select a group of register file port muxes ( rpms ) 124 inside rrc 112 whose inputs are the addresses of each instruction &# 39 ; s inputs . because the results may stay in temporary buffer 116 several cycles before going to register file 117 , a mechanism is provided to get results from temporary buffer 116 before they go to register file 117 , so the information can be used as operands for other instructions . this mechanism is called “ result forwarding ,” and without it , issuer 118 would not be able to issue instructions out of order . this result forwarding is done in register file 117 and is controlled by rrc 112 . the control signals necessary for performing the result forwarding will be evident to those skilled in the art , as should the random logic used for generating such control signals . if an instruction is not dependent on any of the current instructions result forwarding is not necessary since the instruction &# 39 ; s inputs are already in register file 117 . when issuer 118 decides to execute that instruction , rrc 112 tells register file 117 to output its data . rrc 112 contains three subsections : a data dependency checker ( ddc ) 108 , tag assign logic ( tal ) 122 and register file port muxes ( rpm ) 124 . ddc 108 determines where the input dependencies are between the current instructions . tal 122 monitors the dependencies for issuer 118 and controls result forwarding . rpm 124 is controlled by issuer 118 and directs the outputs of tal 122 to the appropriate register file address ports 119 . instructions are passed to ddc 108 via bus 110 . all source registers are compared with all previous destination registers for each instruction in window 102 . each instruction has only one destination , which may be a double register in one embodiment . an instruction can only depend on a previous instruction and may have up to three source registers . there are various register file source and destination addresses that need to be checked against each other for any dependencies . as noted above , the eight bottom instructions corresponding to the lower two buckets are checked by ddc 108 . all source register addresses are compared with all previous destination register addresses for the instructions in window 102 . for example , let &# 39 ; s say a program has the following instruction sequence : add r 0 , r 1 , r 2 ( 0 ) add r 0 , r 2 , r 3 ( 1 ) add r 4 , r 5 , r 2 ( 2 ) add r 2 , r 3 , r 4 ( 3 ) the first two registers in each instruction 0 - 3 are the source registers , and the last listed register in each instruction is the destination register . for example , r 0 and r 1 are the source registers for instruction 0 and r 2 is the destination register . instruction 0 adds the contents of registers 0 and 1 and stores the result in r 2 . for instructions 1 - 3 in this example , the following are the comparisons needed to evaluate all of the dependencies : i 1 s 1 , i 1 s 2 vs . i 0 d , i 2 s 1 , i 2 s 2 vs . i 1 d , i 0 d , i 3 s 1 , i 3 s 2 vs . i 2 d , i 1 d , i 0 d . the key to the above is as follows : ixrs 1 is the address of source ( input ) number 1 of instruction x ; ixrs 2 is the address of source ( input ) number 2 of instruction x ; and ixd is the address of the destination ( output ) of instruction x . note also that rrc 112 can ignore the fact that instruction 2 is output dependent on instruction 0 , because the processor has a temporary buffer where instruction 2 &# 39 ; s result can be stored without interfering with instruction 0 &# 39 ; s result . as discussed before , instruction 2 &# 39 ; s result will not be moved from temporary buffers 116 to register file 117 until instructions 0 and 1 &# 39 ; s results are moved to register file 117 . the number of instructions that can be checked by rrc 112 is easily scalable . in order to check eight instructions at a time instead of four , the following additional comparisons would also need to be made : i 4 s 1 , i 4 s 2 vs . i 3 d , i 2 d , i 1 d , i 0 d , i 5 s 1 , i 5 s 2 vs . i 4 d , i 3 d , i 2 d , i 1 d , i 0 d , i 6 s 1 , i 6 s 2 vs . i 5 d , i 4 d , i 3 d , i 2 d , i 1 d , i 0 d , i 7 s 1 , i 7 s 2 vs . i 6 d , i 5 d , i 4 d , i 3 d , i 2 d , i 1 d , i 0 d . there are several special cases that rrc 112 must handle in order to do the dependency check . first , there are some instructions that use the same register as an input and an output . thus , rrc 112 must compare this source / destination register address with the destination register addresses of all previous instructions . so for instruction 7 , the following comparisons would be necessary : i 7 s 1 , i 7 s 2 , i 7 s / d vs . i 6 d , i 5 d , i 4 d , i 3 d , i 2 d , i 1 d , i 0 d . another special case occurs when a program contains instructions that generate 64 bit outputs ( called long - word operations ). these instructions need two registers in which to store their results . in this embodiment , these registers must be sequential . thus if rrc 112 is checking instruction 4 &# 39 ; s dependencies and instruction 1 is a long - word operation then it must do the following comparisons : i 4 s 1 , i 4 s 2 vs . i 3 d , i 2 d , i 1 d , i 1 d + 1 . i 0 d . sometimes , instructions do not have destination registers . thus rrc 112 must ignore any dependencies between instructions without destination registers and any future instructions . also , instructions may not have only one valid source register , so rrc 112 must ignore any dependencies between the unused source register ( usually s 2 ) and any previous instructions . rrc 112 is also capable of dealing with multiple register files . when using multiple register files , dependencies only occur when one instruction &# 39 ; s source register has the same address and is in the same register file as some other instruction &# 39 ; s destination register . rrc 112 treats the information regarding which register file a particular address is from as part of the address . for example , in an implementation using four 32 bit register files , rrc 112 would do 7 bit compares stead of 5 bit compares ( 5 for the address and 2 for the register file ). signals indicating which instructions are long - word operations or have invalid source or destination registers are sent to rrc 112 from instruction decode logic ( idl ; not shown ). a straight forward , representative floorplan for laying out dcl 108 , tal 122 and rpm 124 for rrc 112 is shown in fig2 . ddc 108 has two sets of inputs . the first set includes source address signals from fifo 101 for all eight instructions of window 102 ; these inputs are shown at reference number 202 . inputs 202 are also supplied to tal blocks 220 , as shown by reference number 222 . the second set of inputs includes long - word load operation flags , register file decode signals , invalid destination register flags , destination address signals and addressing mode flags for all eight instructions ; these inputs are shown at reference number 203 . ddc 108 comprises 28 data dependency blocks 204 . each block 204 eceives 3 inputs , ixs 1 , ixs 2 and ixs / d . ixs 1 is the address of source ( input ) number 1 of instruction x , ixs 2 is the address of source ( input ) number 2 of instruction x ; and ixs / d is the address of the source / destination ( input ) of instruction x . each block 204 also receives input iys / d , which is the destination register address for instruction y . a first column 208 , for example , receives i 0 s / d , which is the destination register address for instruction 0 . each block 204 outputs the data dependency results to one of a corresponding bus line 214 to a tal block 220 . in this example , the address of i 2 s / d must be checked with operand addresses s 1 , s 2 and s / d of instructions 7 , 6 , 5 , 4 , and 3 . each tag assignment logic block 220 receives the corresponding data dependency results via buses 214 , as well as further signals that come from the computer &# 39 ; s idl ( not shown ) via a set of input lines 226 . a bkt bit forms the least significant bit of the tag . a set of done [ x ] flags for instructions 0 through 6 indicate if the instruction is done . a set of dblreg [ x ] flags indicates which , if any , of the instructions is a double ( long ) word . each tal block 220 also receives its own instruction &# 39 ; s register addresses as inputs ; this input is indicated by reference number 222 . the miscellaneous signals dblreg and bkt signals are all implementation dependent control signals . each tal block 220 outputs 0 - 3 tags 126 labeled ixs 1 , ixs 2 and ixs / d , which are 6 bits . tals 220 also outputs the least significant 5 bits of each tag signal to rpms 124 via output buses 224 which form a main bus 126 , and the most significant tag bit to isl 218 via bus 120 . the floorplan arrangement shown in fig2 has two major limitations : it requires a large area , and some of the outputs 214 of ddc 108 have to travel a long distance to tal 122 , which limits the performance of rrc 112 . a second floorplan embodiment is shown at fig3 . in this arrangement , tal blocks 220 are placed ( e . g ., integrated with ) between compare blocks 204 of ddc 108 , as shown generally at reference number 302 . this arrangement does , however , have one limitation . the most efficient arrangement of ddc 108 and tal 122 requires that tal 122 outputs 224 exit near the middle of rows 4 , 5 , 6 and 7 , which is shown at a dashed box 304 . this creates a wiring problem , because tal 122 outputs 224 now must travel a long distance to rpm 124 , especially in the case of 17 . to resolve this problem , the tal outputs of the rows furthest away from rpm 124 must be channeled through the rows closest to rpm 124 . one method would be to expand rows 4 , 5 and 6 enough to get all of the wires through . since compare blocks 204 must be lined up vertically , row 7 would also need to be expanded . this would increase the width of rrc 112 . a preferred floorplan embodiment of the present invention is shown in fig4 . in the floorplan layout shown in fig4 , the left sides of rows 4 , 5 , 6 , and 7 have been flipped . in other words , referring to the vertically aligned comparators 204 and their associated tal logic as columns , columns 3 , 4 , 5 and 6 have been flipped . this creates gap in rows 4 , 5 and 6 without increasing the length of row 7 . ( the gap is also called a center channel and is shown as a dashed box 402 .) tal outputs 224 of rows 4 - 7 are laid - out in center channel 402 and are fed directly to rpm 124 in essentially a straight path . the overall area of rrc 112 therefore remains the same . while various embodiments of the present invention have been described above , it should be understood that they have been presented by way of example , and not limitation . thus the breadth and scope of the present invention should not be limited by any of the above - described exemplary embodiments , but should be defined only in accordance with the following claims and their equivalents .