Abstract:
A tag monitoring system for assigning tags to instructions embodied in software on a tangible computer-readable storage medium. A source supplies instructions to be executed by a functional unit. A queue having a plurality of slots containing tags which are used for tagging instructions. A register file stores information required for the execution of each instruction at a location in the register file defined by the tag assigned to that instruction. A control unit monitors the completion of executed instructions and advances the tags in the queue upon completion of an executed instruction. The register file also contains a plurality of read address enable ports and corresponding read output ports. Each of the slots from the queue is coupled to a corresponding one of the read address enable ports. Thus, the information for each instruction can be read out of the register file in program order.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/338,817, filed Jan. 25, 2006, now U.S. Pat. No. 7,430,651, which is a continuation of U.S. patent application Ser. No. 10/847,460, filed May 18, 2004, now U.S. Pat. No. 7,043,624, which is a continuation of U.S. patent application Ser. No. 10/034,252, filed Jan. 3, 2002, now U.S. Pat. No. 6,757,808, which is a continuation of U.S. patent application Ser. No. 09/574,251, filed May 19, 2000, now U.S. Pat. No. 6,360,309, which is a continuation of U.S. patent application Ser. No. 09/252,655, filed Feb. 19, 1999, now U.S. Pat. No. 6,092,176, which is a continuation of U.S. patent application Ser. No. 08/811,237, filed Mar. 3, 1997, now U.S. Pat. No. 5,896,542, which is a continuation of U.S. patent application Ser. No. 08/224,328, filed Apr. 4, 1994, now U.S. Pat. No. 5,628,021, which is a continuation-in-part of U.S. patent application Ser. No. 07/999,648 filed Dec. 31, 1992, now U.S. Pat. No. 5,604,912. The entirety of each of the foregoing applications is incorporated by reference herein. 
     This application is related to U.S. patent application Ser. No. 08/799,462, filed Feb. 13, 1997, now U.S. Pat. No. 5,892,963, the entirety of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to superscalar computers, and more particularly, a system and method for using tags to control instruction execution in a superscalar reduced instruction set computer (RISC). 
     2. Related Art 
     Processors used in conventional computer systems typically execute program instructions one at a time, in sequential order. The process of executing a single instruction involves several sequential steps. The first step generally involves fetching the instruction from a memory device. The second step generally involves decoding the instruction, and assembling any operands. 
     The third step generally involves executing the instruction, and storing the results. Some processors are designed to perform each step in a single cycle of the processor clock. Alternatively, the processor may be designed so that the number of processor clock cycles per step depends on the particular instruction. 
     To improve performance, modern computers commonly use a technique known as pipelining. Pipelining involves the overlapping of the sequential steps of the execution process. For example, while the processor is performing the execution step for one instruction, it might simultaneously perform the decode step for a second instruction, and perform a fetch of a third instruction. Pipelining can thus decrease the execution time for a sequence of instructions. 
     Another class of processors improve performance by overlapping the sub-steps of the three sequential steps discussed above are called superpipelined processors. 
     Still another technique for improving performance involves executing multiple instructions simultaneously. Processors which utilize this technique are generally referred to as superscalar processors. The ability of a superscalar processor to execute two or more instructions simultaneously depends on the particular instructions being executed. For example, two instructions which both require use of the same, limited processor resource (such as the floating point unit) cannot be executed simultaneously. This type of conflict is known as a resource dependency. Additionally, an instruction which uses the result produced by the execution of another instruction cannot be executed at the same time as the other instruction. An instruction which depends on the result of another instruction is said to have a data dependency on the other instruction. Similarly, an instruction set may specify that particular types of instructions must execute in a certain order relative to each other. These instructions are said to have procedural dependencies. 
     A third technique for improving performance involves executing instructions out of program order. Processors which utilize this technique are generally referred to as out-of-order processors. Usually, out-of-order processors are also superscalar processors. Data dependencies and procedural dependencies limit out-of-order execution in the same way that they limit superscalar execution. 
     From here on, the term “superscalar processor” will be used to refer to a processor that is: capable of executing multiple instructions simultaneously, or capable of executing instructions out of program order, or capable of doing both. 
     For executing instructions either simultaneously or out of order, a superscalar processor must contain a system called an Execution unit. The Execution Unit contains multiple functional units for executing instructions (e.g., floating point multiplier, adder, etc.). Scheduling control is needed to dispatch instructions to the multiple functional units. With in-order issue, the processor stops decoding instructions whenever a decoded instruction creates a resource conflict or has a true dependency or an output dependency on a uncompleted instruction. As a result, the processor is not able to look ahead beyond the instructions with the conflict or dependency, even though one or more subsequent instructions might be executable. To overcome this limitation, processors isolate the decoder from the execution stage, so that it continues to decode instructions regardless of whether they can be executed immediately. This isolation is accomplished by a buffer between the decode and execute stages, called an instruction window. 
     To take advantage of lookahead, the processor decodes instructions and places them into the window as long as there is room in the window and, at the same time, examines instructions in the window to find instructions that can be executed (that is, instructions that do not have resource conflicts or dependencies). The instruction window serves as a pool of instructions, giving the processor lookahead ability that is constrained only by the size of the window and the capability of the instruction source. Thus, out-of-order issue requires a buffer, called an instruction window between the decoder and functional units; and the instruction window provides a snap-shot of a piece of the program that the computer is executing. 
     After the instructions have finished executing, instructions must be removed from the window so that new instructions can take their place. Current designs employ an instruction window that utilizes a First In First Out queue (FIFO). In certain designs, the new instructions enter the window and completed instructions leave the window in fixed size groups. For example, an instruction window might contain eight instructions (I 0 -I 7 ) and instructions may be changed in groups of four. In this case, after instructions I 0 , I 1 , I 2  and I 3  have executed, they are removed from the window at the same time four new instructions are advanced into the window. Instruction windows where instructions enter and leave in fixed size groups are called “Fixed Advance Instruction Windows.” 
     In other types of designs, the new instructions enter the window and completed instructions leave the window in groups of various sizes. For example, an instruction window might contain eight instructions (I 0 -I 7 ) and may be changed in groups of one, two or three. In this case, after any of instructions I 0 , I 1  or I 2  have executed, they can be removed from the window and new instructions can be advanced into the window. Instruction windows where instructions enter and leave in groups of various sizes are called “Variable Advance Instruction Windows.” 
     Processors that use Variable Advance Instruction Windows (VAIW) tend to have higher performance than processors that have Fixed Advance Instruction Windows (FAIW). However, fixed advance instruction windows are easier for a processor to manage since a particular instruction can only occupy a fixed number of locations in the window. For example, in an instruction window that contains eight instructions (I 0 -I 7 ) and where instructions can be added or removed in groups of four, an instruction can occupy only one of two locations in the window (e.g., I 0  and I 4 ). In a variable advance instruction windows, that instruction could occupy all of the locations in the window at different times, thus a processor that has a variable advance instruction window must have more resources to track each instruction&#39;s position than a processor that has a fixed advance instruction window. 
     Current designs use large queues to implement the instruction window. The idea of using queues is disadvantageous, for many reasons including: a large amount of chip area resources are dedicated to a plurality of queues especially when implementing a variable advance instruction window; there is limited flexibility in designing a system with more than one queue; and control logic for directing data in queues is complex and inflexible. 
     Therefore, what is needed is a technique to “track” or monitor instructions as they move through the window. The system must be flexible and require a small area on a chip. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a technique for monitoring instruction execution of multiple instructions in parallel and out of program order using a system that assigns tags to the multiple instructions and maintains an instruction window that contains the multiple instructions. The system is a component of a superscalar unit which is coupled between a source of instructions and functional units which execute the instructions. The superscalar unit is in charge of maintaining the instruction window, directing instructions to the various functional units in the execution unit, and, after the instructions are executed, receiving new instructions from the source. 
     The present invention employs a tag monitor system, which is a part of the superscalar unit. The tag monitor system includes: a register file and a queue that operates on a First-In-First-Out basis (the queue is a multiple-advance, multiple output, recycling FIFO). The queue is coupled to the register file. The register file is coupled to the instruction source and is used to store instruction information (i.e., the resource requirements of each instruction). When an instruction is sent from the instruction source to the register file it is assigned a tag that is not currently assigned to any other instruction. The instruction information is then stored in the register file at an address location indicated by the tag of the instruction. Once an instruction&#39;s information is stored in the register file, it is said to be “in the instruction window.” The tags of each instruction in the instruction window are stored in the queue. The tags are arranged in the queue in the same order as their corresponding instructions are arranged in the program. 
     When an instruction is finished, the queue advances and the tag of the instruction is effectively pushed out the bottom of the queue. The tag can then be reassigned to a new instruction that enters the instruction window. Accordingly, the tag is sent back to the top of the queue (in other words, it is recycled). It is also possible for several tags to be recycled at the same time when several instructions finish at the same time. In a preferred embodiment, instructions are required to finish in order. This is often necessary to prevent an instruction from incorrectly overwriting the result of another instruction. For example, if a program contains two instructions that write to the same location of memory, then the instruction that comes first in the program should write to the memory before the second. Thus, the results of instructions that are executed out of order must be held in some temporary storage area and the instructions themselves must remain in the instruction window until all previous instruction have been executed. When a group of instructions is completed, all of their results are moved from the temporary storage area to their real destinations. Then the instructions are removed from the window and their tags are recycled. 
     The register file has write ports where new instruction information is received from the instruction source. The register file has a number of write ports equal to the number of new instructions that can be added to the window at one time. The register file has one entry for each instruction in the window. The register file also has one output port for every instruction in the window. Associated with each output port is an address port. The address port is used to select which register file entry&#39;s contents will be output on its corresponding output port. 
     The queue has an output for each slot (e.g., specific buffer location in the queue) that shows the value of the tag stored in that slot. These outputs are connected to the read address ports of the register file. This connection causes the register file to provide an entry&#39;s contents on its corresponding output port when a tag value is presented by the queue to the read address ports. The outputs of the register file are sent to various locations in the superscalar unit and execution units where the instruction information is used for instruction scheduling, instruction execution, and the like. 
     It is possible that some of the locations in the instruction window may be empty at any given time. These empty window locations are called “bubbles.” Bubbles sometimes occur when an instruction leaves the window and the instruction source cannot immediately send another instruction to replace it. If there are bubbles in the window, then some of the entries in the register file will contain old or bogus instruction information. Since all of the data in the register file is always available, there needs to be some way to qualify the data in the register file. 
     According to the present invention, a “validity bit” is associated with each entry in the instruction window to indicate if the corresponding instruction information in the register file is valid. These validity bits can be held in the tag FIFO with the tags. There is one validity bit for each tag in the FIFO. These bits are updated each time a tag is recycled. If, when a tag is recycled, it gets assigned to a valid instruction, then the bit is asserted. Otherwise it is deasserted. 
     The validity bits are output from the tag monitor system along with the outputs of the register file. They are sent to the same locations as the outputs of the register file so that the superscalar unit or execution units will know if they can use the instruction information. 
     A feature of the present invention is that an instruction window can be maintained without storing instruction information in large queues. This simplifies design and increases operational flexibility. For example, for a window containing n instructions, the tag monitor system would contain a queue with n entries and a register file with n entries and n output ports. If each output of the queue is connected to its corresponding read address port on the register file (e.g., output  0  connected to read address port  0 , output  1  connected to read address port  1 , etc.) then the register file outputs will “display” (i.e., make available at the output ports) the information for each instruction in the window in program order (e.g., output port  0  will show instruction  0 &#39;s information, output port  1  will show instruction  1 &#39;s information, etc.). When the window advances, the queue advances and the addresses on the read address ports change. This causes the outputs of the register file to change to reflect the new arrangement of instructions in the window. It is necessary for the instruction information to be displayed in order on the register file outputs so that it can be sent to the rest of the superscalar unit in order. The superscalar unit needs to know the order of the instructions in the window so that it can schedule their execution and their completion. 
     Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
         FIG. 1  shows a representative block diagram of a superscalar environment of the present invention. 
         FIG. 2  shows a representative block diagram of a tag monitoring system of the present invention. 
         FIG. 3  shows a representative operational flowchart for tag monitoring according to the tag monitoring system of  FIG. 2 . 
         FIG. 4  shows a tag monitoring system that contains two register files. 
         FIG. 5  shows a diagram of a simple FIFO. 
         FIG. 6  shows a diagram of a simple FIFO with multiple outputs. 
         FIG. 7  is a FIFO with multiple output terminals. 
         FIG. 8  shows a recycling FIFO. 
         FIG. 9  shows a multiple advance FIFO. 
         FIG. 10  shows a recycling, multiple-advance FIFO. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     1.0 System Environment 
       FIG. 1  is a block diagram of a superscalar environment  101 . Superscalar environment  101  includes: an instruction source  102 , a superscalar unit  104  and a functional unit  106 . Superscalar unit  104  controls the execution of instructions by functional unit  106 . Functional unit  106  may include a floating point unit (not shown), an integer unit (not shown), a load/store unit (not shown) and other such hardware commonly used by processors depending on the desired application. Specific implementations of instruction source  102  and functional unit  106  would be apparent to a person skilled in the relevant art. 
     Instruction source  102  sends instruction information to superscalar unit  104  via a bus  103 . The superscalar unit  104  then issues the instructions to functional unit  106 . Generally, superscalar unit  104  monitors functional unit  106  availability and checks for dependencies between instructions. Once the instructions are completed, instruction source  102  sends more instruction information to superscalar unit  104 . 
     The buses shown in  FIG. 1  represent data and control signals. Bus and instruction size may vary depending on the application. The remaining discussion will be focused on a tag monitor system, which tracks instructions for superscalar unit  104 . 
     2.0 Structure and Operation of the Tag Monitor System 
     A. Structure 
       FIG. 2  shows a block diagram of tag monitor system  222  located within a portion of superscalar unit  104  (shown as the inner dashed line in  FIG. 2 ). Tag monitor system  222  includes: a register file  202 , a tag FIFO  204  and control logic  207 . 
     Tag FIFO  204  is a multiple advance, multiple output, recycling FIFO that stores tags in a plurality of slots  206 . The term “multiple advance” means that the FIFO can be advanced any number of slots at a time. For example, a multiple advance 4-slot FIFO can be advanced 0-3 slots at a time. The term “multiple output” means that the contents of each slot of the FIFO are available. A tag is a unique label that superscalar unit  104  assigns to each instruction as it enters the instruction window. Tag FIFO  204  has one slot  206  for each instruction in the window. Each slot  206  has an output  232  that indicates (i.e., outputs) the value of the tag in the corresponding slot  206 . Each slot  206  also has a validity bit that indicates whether the instruction assigned to the tag in the slot  206  is valid. In a preferred embodiment, tag FIFO  204  contains eight slots  206 . Each of these slots  206  contains a unique binary number (tag) ranging from 0 to 7. For example a tag is three bits (e.g., 000, 001, 010, etc.) which, with the validity bit, causes each slot to hold four bits. Thus each output  232  is four bits wide. Each slot  206  of tag FIFO  204  is loaded with a unique tag when the chip is powered-on or reset. 
     Once a tag is assigned to an instruction, it will remain with that instruction until the instruction is removed from the window. Once an instruction is removed from the window, its tag is sent back to the top  212  of tag FIFO  204 . The tag sent to top  212  can be reassigned to a new instruction that enters the window. In this fashion, tags are “recycled” or are recirculated in tag FIFO  204 . Generally, tags advance through the tag FIFO  204  from top  212  to bottom  210 . Thus, FIFO  204  is called a recycling queue. 
     Register file  202  is coupled to tag FIFO  204  and instruction source  102 . Register file  202  stores instruction information sent by instruction source  102 . The following are examples of the type of information that can be sent from instruction source  102  to register file  202 : decoded instruction information; instruction functional unit requirements; the type of operation to be performed by the instruction; information specifying a storage location where instruction results are to be stored; information specifying a storage location where instruction operands are stored; information specifying a target address of a control flow instruction; and information specifying immediate data to be used in an operation specified by the instruction. 
     Register file  202  includes: a write data port  214 , a write address port  216 , a write enable port  218 , a read address port  220 , and a read data port  224 . 
     Write data port  214  receives instruction information from instruction source  102  via bus  103 . Write address ports  216  specify what addressable location in register file  202  the instruction information that is received through write data ports  214  is to be stored. Write address ports  216  are coupled to control logic  207  via a bus  226 . Write enable ports  218  indicate when to write data from instruction source  102  into register file  202 . Write enable ports are coupled to control logic  207  via bus  228 . In a preferred embodiment (shown in  FIG. 2 ) register file  202  has four write data ports  214  labeled A through D. Write data ports  214  have corresponding write address ports  216  labeled A through D, and corresponding write enable ports  218  also labeled A through D. 
     Read address port  220  is coupled to tag FIFO  204  via bus  230 . Bus  230  carries outputs  232  of each slot  206  of tag FIFO  204 . Read address ports  220  select the instruction information that will be accessed through read data ports  224 . Each read address port  220  has a corresponding read data port  224 . In a preferred embodiment (shown in  FIG. 2 ), the instruction window has eight entries (i.e., the depth of tag FIFO  204 ) and register file  202  has one read address port  220  and one read data port  224  for each instruction in the window. Read address ports  220  are labeled  0  through  7  and their corresponding read data ports  224  are also labeled  0  through  7 . 
     Typically, register file  202  is connected to other elements (e.g. an issuer not shown) located within superscalar environment  101 . 
     Control logic  207  is comprised of logic circuits. Control logic  207  monitors functional unit  106  via a bus  234  and bus  230  from tag FIFO  204 . Control logic  207  signals instruction source  102  via bus  238  to send new instruction information to register file  202  as instructions leave the window. Control logic  207  indicates how many new instructions that instruction source  102  should send. In a preferred embodiment (shown in  FIG. 2 ), the maximum number of instructions that can be sent is four, which corresponds to the total number of write data ports  214  in register file  202 . Control logic  207  will also synchronize tag FIFO  204  via a bus  236  to advance as instructions leave the window. Thus, under command of control logic  207 , tag FIFO  204  advances by as many steps as the number of instructions that leave the window at one time. The control logic  207  also maintains the validity bits stored in tag FIFO  204  via bus  236 . The circuit implementation for control logic  207  would be apparent to a person skilled in the relevant art. For example, currently well known and commercially available logic synthesis and layout systems can be used to convert a behavioral description (e.g., Verilog, manufactured by Cadence Design Systems, San Jose, Calif.) to a silicon or chip design. 
     Note that the bit width of the various buses disclosed herein may support parallel or serial address or data transfer, the selection of which is implementation specific, as would be apparent to a person skilled in the relevant art. 
     It is also possible for the tag monitor system to contain more than one register file. In a preferred embodiment, the instruction information is distributed among many register files. For example, one register file contains the destination register addresses of each instruction. Another contains the functional unit requirements of each instruction and so on. One advantage to using multiple register files is that it allows the designer to use smaller register files which can be located near where their contents are used. This can make the physical design of the processor easier. The register files&#39; read and write addresses are all connected together and come from the same source. The write data of the register files still comes from the instruction source. However, not all of the register files have to hold all of the information for each instruction. The outputs of each register file only go to where the data held in that register file is needed. 
       FIG. 4  shows a tag monitor system  222  that contains two register files  202   a  and  202   b . In a preferred embodiment, only a portion of each instruction&#39;s information is stored in each register file  202   a  and  202   b . So the data sent on bus  103  from the instruction source  102  is divided. One portion  103   a  is sent to register file  202   a  and the other  103   b  is sent to register file  202   b . Both register files  202   a  and  202   b  are connected to buses  226  and  228  that provide control signals from the control logic  207  and to bus  230  that provides the outputs from tag FIFO  204 . The outputs of register files  202   a  and  202   b  are provided on separate buses  240   a  and  240   b  to different locations throughout the superscalar unit  104 . 
     The tag FIFO  204  will now be described with the reference to example embodiments. 
       FIG. 5  shows a diagram of a FIFO  500 . FIFO  500  holds four pieces of data in its four slots  504 ,  508 ,  512 , and  516 . The four slots are connected via buses  506 ,  510  and  514 . FIFO  500  has an input  502  and an output  518  through which data enters and leaves the FIFO  500 . 
     FIFO  500  behaves like a queue with four positions. When FIFO  500  advances, any data in slot  516  leaves FIFO  500  through output  518 . Data in slot  512  moves to slot  516  via bus  514 . Data in slot  508  moves to slot  512  via bus  510 . Data in slot  504  moves to slot  508  via bus  506 , and data on the input  502  moves into slot  504 . Each of these data transfers happens whenever FIFO  500  advances. 
       FIG. 6  shows a diagram of a FIFO  600  with multiple outputs. FIFO  600  is structured much like FIFO  500  in  FIG. 5 . Data enters FIFO  600  through an input  602 , moves through four slots  604 ,  610 ,  616  and  622  and then out through an output  626 . The difference between FIFO  500  and FIFO  600  is that the data stored in each slot  604 ,  610 ,  616  and  622  is visible on (i.e., can be read four) corresponding buses  606 ,  612 ,  618  or  624  from the time that it enters a respective slot until FIFO  600  advances again. Outputs  606 ,  612 ,  618  or  624  allow the user to know what data is stored in FIFO  600  at any given time. 
     In a preferred embodiment, data stored in slots  604 ,  610 ,  616  and  622  is continuously visible on each slot&#39;s output bus (i.e., on buses  608 ,  614 ,  620  and  626 ). In this situation, buses  606 ,  612 ,  618  or  624  are unnecessary. An example of this embodiment is shown in  FIG. 7 . Buses  706 ,  710  and  714  are used to convey data between slots  1  and  4  ( 704 ,  708 ,  712  and  716 , respectively) and also indicate the contents of slots  1 ,  2  and  3 ,  704 ,  708  and  712  respectively. Output bus  718  always permits the contents of slot  716  to be read. 
       FIG. 8  shows a recycling FIFO  800 . Recycling FIFO  800  also functions much like FIFO  500  in  FIG. 5 . Recycle FIFO  800  comprises four slots  804 ,  808 ,  812  and  816 . The main difference is that when FIFO  800  advances, data in slot  816  moves to slot  804 . Since FIFO  800  has no means for inputting new data into slot  804 , it must be designed so that when turned on or reset, each slot  804 ,  808 ,  812  and  816  is initialized with some value. These initial values then circulate through FIFO  800  until reinitialized in a known manner. 
     Sometimes it is necessary to advance a FIFO by more than one step at a time. Since the FIFO inputs one piece of data each time the FIFO advances on step, the FIFO must also have as many inputs as the maximum number of steps that the FIFO can advance. The FIFO must have some means besides buses to carry the data from each slot or input to the correct destination. 
       FIG. 9  shows a multiple advance FIFO  900 . FIFO  900  is capable of advancing  1 ,  2 ,  3 , or  4  steps (i.e., slots) at one time. FIFO  900  has four inputs  902 ,  904 ,  906  and  908 , and four slots  914 ,  922 ,  930  and  938 . When FIFO  900  advances by four steps, the data on input  902  goes to slot  938 , input  904  goes to slot  930 , input  906  goes to slot  922  and input  908  goes to slot  914 . When FIFO  900  advances by three steps, data in slot  914  goes to slot  938 , input  902  goes to slot  930 , input  904  goes to slot  922  and input  906  goes to slot  914 . In this case, the data on input  908  does not enter FIFO  900 . When FIFO  900  advances by two steps, data in slot  922  goes to slot  938 , data in slot  914  goes to slot  930 , input  902  goes to slot  922  and input  904  goes to slot  914 . Finally, as in the simple FIFO case, when the FIFO advances by one step, the data in slot  930  goes to slot  938 , the data in slot  922  goes to slot  930 , the data in slot  914  goes to slot  922  and the data on input  902  goes to slot  914 . 
     In order to advance more than one step at a time, the inputs must be switchably connected to each slot and the outputs of some slots must go to more than one other slot. Therefore, FIFO  900  has four multiplexers: MUX 1 , MUX 2 , MUX 3  and MUX 4 , shown at  910 ,  918 ,  926  and  934 , respectively. These multiplexers are used to select the data that goes into each slot when FIFO  900  advances. Inputs to each multiplexer are the data that might need to go to its corresponding slot. For example, depending on the number of steps that FIFO  900  advances, the data from slot  914 , slot  922 , slot  930  or input  902  might go to slot  938 . Thus the inputs to  934  are the outputs from slot  916 , slot  924 , slot  932  and input  902 . The structure and operation of the logic circuits necessary to control the multiplexers  910 ,  918 ,  926  and  934  would be apparent to a person skilled in the relevant art. 
     It is also possible to design a multiple advance FIFO that recycles its contents. This FIFO is a combination of the FIFOs shown in  FIGS. 8 and 9 . A diagram of recycling, multiple advance FIFO  1000  is shown in  FIG. 10 . FIFO  1000  is capable of being advanced one, two or three steps at a time. Since FIFO  1000  has four stages (slots  1 - 4 , labeled  1006 ,  1014 ,  1022  and  1030 , respectively), advancing by four steps is logically the same as not advancing at all. Thus, since it never has to advance by four steps, the structure of the multiplexers in the recycling, multiple advance FIFO  1000  is different from that shown in the multiple advance FIFO  900 . FIFO  1000  is also a multiple output FIFO like FIFO  700  shown in  FIG. 7 . Furthermore, like the recycling FIFO  800  in  FIG. 8 , FIFO  1000  must also have some means for initialization. 
     The FIFOs shown in  FIGS. 5 ,  6 ,  7 ,  8 ,  9  and  10  are all shown with four stages as an example. It is, of course, possible to modify these designs so that they contain a number of slots other than four. These modifications would be apparent to a person skilled in the relevant art. 
     B. Operation 
       FIG. 3  is a flowchart illustrating the operation of tag monitor system  222 . Operational steps  310 - 312  will be described with reference to hardware elements of  FIGS. 1 and 2 . 
     Operation starts at a step  301 . In a step  302 , control logic  207  sends a request data signal  238  requesting instruction source  102  to send instruction information. Control logic  207  requests information for a number of instructions equal to the number of empty spaces in the instruction window. In a preferred embodiment, in effect, control logic  207  determines how many new instructions can be added to the instruction window, and then requests sufficient instruction information from instruction source  102  to refill the empty top slots of the queue. There is a maximum number of instructions whose information can be sent that is less than the number of spaces in the window. 
     In a step  304 , actuate write enable and write address, assign tag and update validity bits. Control logic  207  sends an enable signal on bus  226  and an address signal on bus  228  to write enable port  218  and write address port  216 , respectively. The addresses on each port  216  specify where the instruction information on the corresponding data port  214  should be stored in register file  202  during a step  306 . Instruction information is sent from instruction source  102  to register file  202  via bus  103 . Typically, the total number of enable bits on bus  226  equals the maximum number of instructions whose information can be sent at one time, which in the preferred embodiment is four. 
     The address where each instruction&#39;s information is stored in register file  202  is specified by the tag of that instruction. Since the data on write data ports  214  does not always need to be stored in register file  202 , control logic  207  uses enable signals on bus  228  to select only the data that needs to be written. For example, if there is only one empty space at the top of the instruction window, then control logic  207  will send the tag contained in top slot  212  of the queue on bus  228  to write address port  216 A and assert write enable port  218 A via bus  226 . This operation causes only the instruction information on write data port  214 A to be stored in register file  202  in a location specified by the tag in top slot  212  of tag FIFO  204 . If there are two empty spaces in the instruction window, then control logic  207  will send two enables to ports  218 A and  218 B and the two tags at the top of the window will be sent to write address ports  216 A and  216 B (the tag in top slot  212  going to  216 B), thus causing the instruction information on ports  214 A and  214 B to be stored in register file  202 . When an instruction&#39;s information is stored in a location in register file  202  specified by a tag, the instruction is said to have been “assigned” that tag. Control logic  207  also updates the validity bits in tag FIFO  204  during step  304 . If instruction source  102  cannot supply an instruction for every request made in step  302 , control logic  207  will only assert the validity bits of the tags that were assigned to valid instructions in step  304 . For those tags that do not get assigned, their validity bits will remain unasserted until they are assigned to a valid instruction. 
     In a step  308 , all of the contents of register file  202  are read through read data ports  224 . It is contemplated to use less than all the contents of register file  202 . The data that is to be read from register file  202  is specified by the addresses presented to register file  202  through read address ports  220 . The data is then used in the execution of some or all of the instructions in the window. In a preferred embodiment, read address  220  is always asserted. In other words, there is always a tag in each slot  206 . 
     In a decisional step  310 , control logic  207  determines if any of the instructions executed in step  308  are ready to retire. If no instruction retires, data will continue to be read out of register file  202  and the instructions in the window will continue to be executed, as indicated by the “NO” path  311  of decisional step  310 . If an instruction does retire, control logic  207  will receive information indicating the number of instructions that are retiring via bus  234  as shown in a step  312 . The information received on bus  234  comes from a retirement unit (not shown). The details of the retirement unit are not relevant to carry out the present invention. (An example, however, of an instruction retirement unit is disclosed in U.S. Pat. No. 5,826,055). Control logic  207  then indicates, via bus  236 , how many steps tag FIFO  204  should advance. 
     Referring to  FIG. 2 , if one instruction retires, then tag FIFO  204  will advance by one step. Tag I will move from bottom  210  to top  212  into Tag O&#39;s current location, and all other tags will be advanced accordingly. When Tag  1  is moved from the bottom  210  to the top  212 , its validity bit is deasserted. Tag  1  will be reassigned to the next new instruction to enter the instruction window. Tag  2  should be located at bottom  210  of tag FIFO  204  after step  312 . The operation of tag monitor system  222  will continue by returning to operational step  302  discussed above via branch  314 . 
     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.