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78-514: The traditional means to improve performance in processors include dividing instructions into sub steps so the instructions can be executed partly at the same time (termed pipelining ), dispatching individual instructions to be executed independently, in different parts of the processor ( superscalar architectures ), and even executing instructions in an order different from the program ( out-of-order execution ). These methods all complicate hardware (larger circuits, higher cost and energy use) because

156-494: A VLIW mode. In the VLIW mode, the processor always fetched two instructions and assumed that one was an integer instruction and the other floating-point. The i860's VLIW mode was used extensively in embedded digital signal processor (DSP) applications since the application execution and datasets were simple, well ordered and predictable, allowing designers to fully exploit the parallel execution advantages enabled by VLIW. In VLIW mode,

234-420: A VLIW processor must be codesigned. This was inspired partly by the difficulty Fisher observed at Yale of compiling for architectures like Floating Point Systems ' FPS164, which had a complex instruction set computing (CISC) architecture that separated instruction initiation from the instructions that saved the result, needing very complex scheduling algorithms. Fisher developed a set of principles characterizing

312-524: A VLIW, the compiler uses heuristics or profile information to guess the direction of a branch. This allows it to move and preschedule operations speculatively before the branch is taken, favoring the most likely path it expects through the branch. If the branch takes an unexpected way, the compiler has already generated compensating code to discard speculative results to preserve program semantics. Vector processor cores (designed for large one-dimensional arrays of data called vectors ) can be combined with

390-406: A calculation that has not yet occurred. Various processors may stall, may attempt branch prediction , and may be able to begin to execute two different program sequences ( eager execution ), each assuming the branch is or is not taken, discarding all work that pertains to the incorrect guess. A processor with an implementation of branch prediction that usually makes correct predictions can minimize

468-401: A core if the processor is a multi-core processor ), but an execution resource within a single CPU such as an arithmetic logic unit . While a superscalar CPU is typically also pipelined , superscalar and pipelining execution are considered different performance enhancement techniques. The former (superscalar) executes multiple instructions in parallel by using multiple execution units, whereas

546-420: A fixed maximum speed, a pipelined computer can be made faster or slower by varying the number of stages in the pipeline. With more stages, each stage does less work, and so the stage has fewer delays from the logic gates and could run at a higher clock rate. A pipelined model of computer is often the most economical, when cost is measured as logic gates per instruction per second. At each instant, an instruction

624-658: A given CPU): Seymour Cray 's CDC 6600 from 1964 is often mentioned as the first superscalar design. The 1967 IBM System/360 Model 91 was another superscalar mainframe. The Intel i960 CA (1989), the AMD 29000 -series 29050 (1990), and the Motorola MC88110 (1991), microprocessors were the first commercial single-chip superscalar microprocessors. RISC microprocessors like these were the first to have superscalar execution, because RISC architectures free transistors and die area which can be used to include multiple execution units and

702-703: A much longer opcode (termed very long ) to specify what executes on a given cycle. Examples of contemporary VLIW CPUs include the TriMedia media processors by NXP (formerly Philips Semiconductors), the Super Harvard Architecture Single-Chip Computer (SHARC) DSP by Analog Devices, the ST200 family by STMicroelectronics based on the Lx architecture (designed in Josh Fisher's HP lab by Paolo Faraboschi),

780-404: A pipelined computer is usually more complex and more costly than a comparable multicycle computer. It typically has more logic gates, registers and a more complex control unit. In a like way, it might use more total energy, while using less energy per instruction. Out of order CPUs can usually do more instructions per second because they can do several instructions at once. In a pipelined computer,

858-452: A program, termed out-of-order execution . These three methods all raise hardware complexity. Before executing any operations in parallel, the processor must verify that the instructions have no interdependencies . For example, if a first instruction's result is used as a second instruction's input, then they cannot execute at the same time and the second instruction cannot execute before the first. Modern out-of-order processors have increased

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936-474: A proper VLIW design, such as self-draining pipelines, wide multi-port register files , and memory architectures . These principles made it easier for compilers to emit fast code. The first VLIW compiler was described in a Ph.D. thesis by John Ellis, supervised by Fisher. The compiler was named Bulldog, after Yale's mascot. Fisher left Yale in 1984 to found a startup company, Multiflow , along with cofounders John O'Donnell and John Ruttenberg. Multiflow produced

1014-427: A sequence of RISC-like instructions. The compiler analyzes this code for dependence relationships and resource requirements. It then schedules the instructions according to those constraints. In this process, independent instructions can be scheduled in parallel. Because VLIWs typically represent instructions scheduled in parallel with a longer instruction word that incorporates the individual instructions, this results in

1092-504: A single processor. Thus a multicore CPU is possible where each core is an independent processor containing multiple parallel pipelines, each pipeline being superscalar. Some processors also include vector capability. Instruction pipelining In the fourth clock cycle (the green column), the earliest instruction is in MEM stage, and the latest instruction has not yet entered the pipeline. In computer engineering , instruction pipelining

1170-436: A superscalar CPU the dispatcher reads instructions from memory and decides which ones can be run in parallel, dispatching each to one of the several execution units contained inside a single CPU. Therefore, a superscalar processor can be envisioned as having multiple parallel pipelines, each of which is processing instructions simultaneously from a single instruction thread. Most modern superscalar CPUs also have logic to reorder

1248-399: A superscalar processor can execute more than one instruction during a clock cycle by simultaneously dispatching multiple instructions to different execution units on the processor. It therefore allows more throughput (the number of instructions that can be executed in a unit of time) than would otherwise be possible at a given clock rate . Each execution unit is not a separate processor (or

1326-575: A technology named Flexible Length Instruction eXtensions (FLIX) that allows multi-operation instructions. The Xtensa C/C++ compiler can freely intermix 32- or 64-bit FLIX instructions with the Xtensa processor's one-operation RISC instructions, which are 16 or 24 bits wide. By packing multiple operations into a wide 32- or 64-bit instruction word and allowing these multi-operation instructions to intermix with shorter RISC instructions, FLIX allows SoC designers to realize VLIW's performance advantages while eliminating

1404-578: Is a VLIW microarchitecture. In December 2015, the first shipment of PCs based on VLIW CPU Elbrus-4s was made in Russia. The Neo by REX Computing is a processor consisting of a 2D mesh of VLIW cores aimed at power efficiency. The Elbrus 2000 ( Russian : Эльбрус 2000 ) and its successors are Russian 512-bit wide VLIW microprocessors developed by Moscow Center of SPARC Technologies (MCST) and fabricated by TSMC . When silicon technology allowed for wider implementations (with more execution units) to be built,

1482-426: Is a technique for implementing instruction-level parallelism within a single processor. Pipelining attempts to keep every part of the processor busy with some instruction by dividing incoming instructions into a series of sequential steps (the eponymous " pipeline ") performed by different processor units with different parts of instructions processed in parallel. In a pipelined computer, instructions flow through

1560-399: Is in only one pipeline stage, and on average, a pipeline stage is less costly than a multicycle computer. Also, when made well, most of the pipelined computer's logic is in use most of the time. In contrast, out of order computers usually have large amounts of idle logic at any given instant. Similar calculations usually show that a pipelined computer uses less energy per instruction. However,

1638-426: Is no assurance otherwise and failure to detect a dependency would produce incorrect results. No matter how advanced the semiconductor process or how fast the switching speed, this places a practical limit on how many instructions can be simultaneously dispatched. While process advances will allow ever greater numbers of execution units (e.g. ALUs), the burden of checking instruction dependencies grows rapidly, as does

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1716-454: Is removed and delegated to the compiler . Explicitly parallel instruction computing (EPIC) is like VLIW with extra cache prefetching instructions. Simultaneous multithreading (SMT) is a technique for improving the overall efficiency of superscalar processors. SMT permits multiple independent threads of execution to better utilize the resources provided by modern processor architectures. The fact that they are independent means that we know that

1794-613: Is said to be fully pipelined if it can fetch an instruction on every cycle. Thus, if some instructions or conditions require delays that inhibit fetching new instructions, the processor is not fully pipelined. Seminal uses of pipelining were in the ILLIAC II project and the IBM Stretch project, though a simple version was used earlier in the Z1 in 1939 and the Z3 in 1941. Pipelining began in earnest in

1872-567: Is sometimes distinguished from a pure VLIW architecture, since EPIC advocates full instruction predication, rotating register files, and a very long instruction word that can encode non-parallel instruction groups. VLIWs also gained significant consumer penetration in the graphics processing unit (GPU) market, though both Nvidia and AMD have since moved to RISC architectures to improve performance on non-graphics workloads. ATI Technologies ' (ATI) and Advanced Micro Devices ' (AMD) TeraScale microarchitecture for graphics processing units (GPUs)

1950-478: Is stalled for one cycle, as is the red instruction after it. Because of the bubble (the blue ovals in the illustration), the processor's Decode circuitry is idle during cycle 3. Its Execute circuitry is idle during cycle 4 and its Write-back circuitry is idle during cycle 5. When the bubble moves out of the pipeline (at cycle 6), normal execution resumes. But everything now is one cycle late. It will take 8 cycles (cycle 1 through 8) rather than 7 to completely execute

2028-412: Is such a method, and involves scheduling the most likely path of basic blocks first, inserting compensating code to deal with speculative motions, scheduling the second most likely trace, and so on, until the schedule is complete. Fisher's second innovation was the notion that the target CPU architecture should be designed to be a reasonable target for a compiler; that the compiler and the architecture for

2106-479: Is the code bloat that occurs when one or more execution unit(s) have no useful work to do and thus must execute No Operation NOP instructions. This occurs when there are dependencies in the code and the instruction pipelines must be allowed to drain before later operations can proceed. Since the number of transistors on a chip has grown, the perceived disadvantages of the VLIW have diminished in importance. VLIW architectures are growing in popularity, especially in

2184-480: Is the difference between scalar and vector arithmetic. A superscalar processor is a mixture of the two. Each instruction processes one data item, but there are multiple execution units within each CPU thus multiple instructions can be processing separate data items concurrently. Superscalar CPU design emphasizes improving the instruction dispatcher accuracy and allowing it to keep the multiple execution units in use at all times. This has become increasingly important as

2262-419: Is the list of instructions waiting to be executed, the bottom gray box is the list of instructions that have had their execution completed, and the middle white box is the pipeline. The execution is as follows: A pipelined processor may deal with hazards by stalling and creating a bubble in the pipeline, resulting in one or more cycles in which nothing useful happens. In the illustration at right, in cycle 3,

2340-641: The ALU , integer multiplier , integer shifter, FPU , etc. There may be multiple versions of each execution unit to enable the execution of many instructions in parallel. This differs from a multi-core processor that concurrently processes instructions from multiple threads, one thread per processing unit (called "core"). It also differs from a pipelined processor , where the multiple instructions can concurrently be in various stages of execution, assembly-line fashion. The various alternative techniques are not mutually exclusive—they can be (and frequently are) combined in

2418-935: The FR-V from Fujitsu , the BSP15/16 from Pixelworks , the CEVA-X DSP from CEVA, the Jazz DSP from Improv Systems, the HiveFlex series from Silicon Hive, and the MPPA Manycore family by Kalray. The Texas Instruments TMS320 DSP line has evolved, in its C6000 family, to look more like a VLIW, in contrast to the earlier C5000 family. These contemporary VLIW CPUs are mainly successful as embedded media processors for consumer electronic devices. VLIW features have also been added to configurable processor cores for system-on-a-chip (SoC) designs. For example, Tensilica's Xtensa LX2 processor incorporates

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2496-431: The central processing unit (CPU) in stages. For example, it might have one stage for each step of the von Neumann cycle : Fetch the instruction, fetch the operands, do the instruction, write the results. A pipelined computer usually has "pipeline registers" after each stage. These store information from the instruction and calculations so that the logic gates of the next stage can do the next step. This arrangement lets

2574-448: The code bloat of early VLIW architectures. The Infineon Carmel DSP is another VLIW processor core intended for SoC. It uses a similar code density improvement method called configurable long instruction word (CLIW). Outside embedded processing markets, Intel's Itanium IA-64 explicitly parallel instruction computing (EPIC) and Elbrus 2000 appear as the only examples of a widely used VLIW CPU architectures. However, EPIC architecture

2652-423: The embedded system market, where it is possible to customize a processor for an application in a system-on-a-chip . Superscalar A superscalar processor (or multiple-issue processor ) is a CPU that implements a form of parallelism called instruction-level parallelism within a single processor. In contrast to a scalar processor , which can execute at most one single instruction per clock cycle,

2730-458: The 1990s. Along with the above systems, during the same time (1989–1990), Intel implemented VLIW in the Intel i860 , their first 64-bit microprocessor, and the first processor to implement VLIW on one chip. This processor could operate in both simple RISC mode and VLIW mode: In the early 1990s, Intel introduced the i860 RISC microprocessor. This simple chip had two modes of operation: a scalar mode and

2808-456: The CPU complete an instruction on each clock cycle. It is common for even-numbered stages to operate on one edge of the square-wave clock, while odd-numbered stages operate on the other edge. This allows more CPU throughput than a multicycle computer at a given clock rate , but may increase latency due to the added overhead of the pipelining process itself. Also, even though the electronic logic has

2886-454: The CPU guesses wrong, all of these instructions and their context need to be flushed and the correct ones loaded, which takes time. This has led to increasingly complex instruction-dispatch logic that attempts to guess correctly , and the simplicity of the original reduced instruction set computing (RISC) designs has been eroded. VLIW lacks this logic, and thus lacks its energy use, possible design defects, and other negative aspects. In

2964-472: The CPU's internal machine code. Thus, the Transmeta chip is internally a VLIW processor, effectively decoupled from the x86 CISC instruction set that it executes. Intel's Itanium architecture (among others) solved the backward-compatibility problem with a more general mechanism. Within each of the multiple-opcode instructions, a bit field is allocated to denote dependency on the prior VLIW instruction within

3042-452: The TRACE series of VLIW minisupercomputers , shipping their first machines in 1987. Multiflow's VLIW could issue 28 operations in parallel per instruction. The TRACE system was implemented in a mix of medium-scale integration (MSI), large-scale integration (LSI), and very large-scale integration (VLSI) , packaged in cabinets, a technology obsoleted as it grew more cost-effective to integrate all of

3120-699: The VLIW architecture such as in the Fujitsu FR-V microprocessor, further increasing throughput and speed . Cydrome was a company producing VLIW numeric processors using emitter-coupled logic (ECL) integrated circuits in the same timeframe (late 1980s). This company, like Multiflow, failed after a few years. One of the licensees of the Multiflow technology is Hewlett-Packard , which Josh Fisher joined after Multiflow's demise. Bob Rau , founder of Cydrome, also joined HP after Cydrome failed. These two would lead computer architecture research at Hewlett-Packard during

3198-482: The West. Fisher's innovations involved developing a compiler that could target horizontal microcode from programs written in an ordinary programming language . He realized that to get good performance and target a wide-issue machine, it would be necessary to find parallelism beyond that generally within a basic block . He also developed region scheduling methods to identify parallelism beyond basic blocks. Trace scheduling

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3276-473: The compiled programs for the earlier generation would not run on the wider implementations, as the encoding of binary instructions depended on the number of execution units of the machine. Transmeta addressed this issue by including a binary-to-binary software compiler layer (termed code morphing ) in their Crusoe implementation of the x86 architecture. This mechanism was advertised to basically recompile, optimize, and translate x86 opcodes at runtime into

3354-407: The compiler could be designed to generate machine code that avoids hazards. In some early DSP and RISC processors, the documentation advises programmers to avoid such dependencies in adjacent and nearly adjacent instructions (called delay slots ), or declares that the second instruction uses an old value rather than the desired value (in the example above, the processor might counter-intuitively copy

3432-501: The compiler. Compilers of the day were far more complex than those of the 1980s, so the added complexity in the compiler was considered to be a small cost. VLIW CPUs are usually made of multiple RISC-like execution units that operate independently. Contemporary VLIWs usually have four to eight main execution units. Compilers generate initial instruction sequences for the VLIW CPU in roughly the same manner as for traditional CPUs, generating

3510-449: The complexity of instruction scheduling is moved into the compiler, complexity of hardware can be reduced substantially. A similar problem occurs when the result of a parallelizable instruction is used as input for a branch. Most modern CPUs guess which branch will be taken even before the calculation is complete, so that they can load the instructions for the branch, or (in some architectures) even start to compute them speculatively . If

3588-402: The complexity of register renaming circuitry to mitigate some dependencies. Collectively the power consumption , complexity and gate delay costs limit the achievable superscalar speedup. However even given infinitely fast dependency checking logic on an otherwise conventional superscalar CPU, if the instruction stream itself has many dependencies, this would also limit the possible speedup. Thus

3666-412: The components of a processor (excluding memory) on one chip. Multiflow was too early to catch the following wave, when chip architectures began to allow multiple-issue CPUs. The major semiconductor companies recognized the value of Multiflow technology in this context, so the compiler and architecture were subsequently licensed to most of these firms. A processor that executes every instruction one after

3744-406: The control unit arranges for the flow to start, continue, and stop as a program commands. The instruction data is usually passed in pipeline registers from one stage to the next, with a somewhat separated piece of control logic for each stage. The control unit also assures that the instruction in each stage does not harm the operation of instructions in other stages. For example, if two stages must use

3822-491: The data in process and restart. This is called a "stall." Much of the design of a pipelined computer prevents interference between the stages and reduces stalls. The number of dependent steps varies with the machine architecture. For example: As the pipeline is made "deeper" (with a greater number of dependent steps), a given step can be implemented with simpler circuitry, which may let the processor clock run faster. Such pipelines may be called superpipelines. A processor

3900-419: The degree of intrinsic parallelism in the code stream forms a second limitation. Collectively, these limits drive investigation into alternative architectural changes such as very long instruction word (VLIW), explicitly parallel instruction computing (EPIC), simultaneous multithreading (SMT), and multi-core computing . With VLIW, the burdensome task of dependency checking by hardware logic at run time

3978-647: The following is an instruction for the Super Harvard Architecture Single-Chip Computer (SHARC). In one cycle, it does a floating-point multiply, a floating-point add, and two autoincrement loads. All of this fits in one 48-bit instruction: f12 = f0 * f4, f8 = f8 + f12, f0 = dm(i0, m3), f4 = pm(i8, m9); Since the earliest days of computer architecture, some CPUs have added several arithmetic logic units (ALUs) to run in parallel. Superscalar CPUs use hardware to decide which operations can run in parallel at runtime, while VLIW CPUs use software (the compiler) to decide which operations can run in parallel in advance. Because

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4056-400: The hardware resources which schedule instructions and determine interdependencies. In contrast, VLIW executes operations in parallel, based on a fixed schedule, determined when programs are compiled . Since determining the order of execution of operations (including which operations can execute simultaneously) is handled by the compiler, the processor does not need the scheduling hardware that

4134-417: The i860 could maintain floating-point performance in the range of 20-40 double-precision MFLOPS; a very high value for its time and for a processor running at 25-50Mhz. In the 1990s, Hewlett-Packard researched this problem as a side effect of ongoing work on their PA-RISC processor family. They found that the CPU could be greatly simplified by removing the complex dispatch logic from the CPU and placing it in

4212-435: The incremented number into R5 as its fifth step (register write back) at t 5 . But the second instruction might get the number from R5 (to copy to R6) in its second step (instruction decode and register fetch) at time t 3 . It seems that the first instruction would not have incremented the value by then. The above code invokes a hazard. Writing computer programs in a compiled language might not raise these concerns, as

4290-448: The instruction of one thread can be executed out of order and/or in parallel with the instruction of a different one. Also, one independent thread will not produce a pipeline bubble in the code stream of a different one, for example, due to a branch. Superscalar processors differ from multi-core processors in that the several execution units are not entire processors. A single processor is composed of finer-grained execution units such as

4368-523: The instruction width is 32 bits or fewer. In contrast, one VLIW instruction encodes multiple operations, at least one operation for each execution unit of a device. For example, if a VLIW device has five execution units, then a VLIW instruction for the device has five operation fields, each field specifying what operation should be done on that corresponding execution unit. To accommodate these operation fields, VLIW instructions are usually at least 64 bits wide, and far wider on some architectures. For example,

4446-452: The instructions to try to avoid pipeline stalls and increase parallel execution. Available performance improvement from superscalar techniques is limited by three key areas: Existing binary executable programs have varying degrees of intrinsic parallelism. In some cases instructions are not dependent on each other and can be executed simultaneously. In other cases they are inter-dependent: one instruction impacts either resources or results of

4524-591: The late 1970s in supercomputers such as vector processors and array processors. One of the early supercomputers was the Cyber series built by Control Data Corporation. Its main architect, Seymour Cray , later headed Cray Research. Cray developed the XMP line of supercomputers, using pipelining for both multiply and add/subtract functions. Later, Star Technologies added parallelism (several pipelined functions working in parallel), developed by Roger Chen. In 1984, Star Technologies added

4602-441: The latter (pipeline) executes multiple instructions in the same execution unit in parallel by dividing the execution unit into different phases. In the "Simple superscalar pipeline" figure, fetching two instructions at the same time is superscaling, and fetching the next two before the first pair has been written back is pipelining. The superscalar technique is traditionally associated with several identifying characteristics (within

4680-463: The more rigid methods used in the simpler P5 Pentium ; it also simplified speculative execution and allowed higher clock frequencies compared to designs such as the advanced Cyrix 6x86 . The simplest processors are scalar processors. Each instruction executed by a scalar processor typically manipulates one or two data items at a time. By contrast, each instruction executed by a vector processor operates simultaneously on many data items. An analogy

4758-459: The next one begins: A branch out of the normal instruction sequence often involves a hazard. Unless the processor can give effect to the branch in a single time cycle, the pipeline will continue fetching instructions sequentially. Such instructions cannot be allowed to take effect because the programmer has diverted control to another part of the program. A conditional branch is even more problematic. The processor may or may not branch, depending on

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4836-572: The next one begins; this assumption is not true on a pipelined processor. A situation where the expected result is problematic is known as a hazard . Imagine the following two register instructions to a hypothetical processor: If the processor has the 5 steps listed in the initial illustration (the 'Basic five-stage pipeline' at the start of the article), instruction 1 would be fetched at time t 1 and its execution would be complete at t 5 . Instruction 2 would be fetched at t 2 and would be complete at t 6 . The first instruction might deposit

4914-748: The number of units has increased. While early superscalar CPUs would have two ALUs and a single FPU , a later design such as the PowerPC 970 includes four ALUs, two FPUs, and two SIMD units. If the dispatcher is ineffective at keeping all of these units fed with instructions, the performance of the system will be no better than that of a simpler, cheaper design. A superscalar processor usually sustains an execution rate in excess of one instruction per machine cycle . But merely processing multiple instructions concurrently does not make an architecture superscalar, since pipelined , multiprocessor or multi-core architectures also achieve that, but with different methods. In

4992-480: The other (i.e., a non- pipelined scalar architecture) may use processor resources inefficiently, yielding potential poor performance. The performance can be improved by executing different substeps of sequential instructions simultaneously (termed pipelining ), or even executing multiple instructions entirely simultaneously as in superscalar architectures. Further improvement can be achieved by executing instructions in an order different from that in which they occur in

5070-468: The other. The instructions a = b + c; d = e + f can be run in parallel because none of the results depend on other calculations. However, the instructions a = b + c; b = e + f might not be runnable in parallel, depending on the order in which the instructions complete while they move through the units. Although the instruction stream may contain no inter-instruction dependencies, a superscalar CPU must nonetheless check for that possibility, since there

5148-411: The performance penalty from branching. However, if branches are predicted poorly, it may create more work for the processor, such as flushing from the pipeline the incorrect code path that has begun execution before resuming execution at the correct location. Programs written for a pipelined processor deliberately avoid branching to minimize possible loss of speed. For example, the programmer can handle

5226-471: The pipelined divide circuit developed by James Bradley. By the mid-1980s, pipelining was used by many different companies around the world. Pipelining was not limited to supercomputers. In 1976, the Amdahl Corporation 's 470 series general purpose mainframe had a 7-step pipeline, and a patented branch prediction circuit. The model of sequential execution assumes that each instruction completes before

5304-568: The processor cannot decode the purple instruction, perhaps because the processor determines that decoding depends on results produced by the execution of the green instruction. The green instruction can proceed to the Execute stage and then to the Write-back stage as scheduled, but the purple instruction is stalled for one cycle at the Fetch stage. The blue instruction, which was due to be fetched during cycle 3,

5382-480: The processor must make all of the decisions internally for these methods to work. In contrast, the VLIW method depends on the programs providing all the decisions regarding which instructions to execute simultaneously and how to resolve conflicts. As a practical matter, this means that the compiler (software used to create the final programs) becomes more complex, but the hardware is simpler than in many other means of parallelism. The concept of VLIW architecture, and

5460-453: The program instruction stream. These bits are set at compile time , thus relieving the hardware from calculating this dependency information. Having this dependency information encoded in the instruction stream allows wider implementations to issue multiple non-dependent VLIW instructions in parallel per cycle, while narrower implementations would issue a smaller number of VLIW instructions per cycle. Another perceived deficiency of VLIW designs

5538-424: The same piece of data, the control logic assures that the uses are done in the correct sequence. When operating efficiently, a pipelined computer will have an instruction in each stage. It is then working on all of those instructions at the same time. It can finish about one instruction for each cycle of its clock. But when a program switches to a different sequence of instructions, the pipeline sometimes must discard

5616-453: The term VLIW , were invented by Josh Fisher in his research group at Yale University in the early 1980s. His original development of trace scheduling as a compiling method for VLIW was developed when he was a graduate student at New York University . Before VLIW, the notion of prescheduling execution units and instruction-level parallelism in software was well established in the practice of developing horizontal microcode . Before Fisher

5694-528: The theoretical aspects of what would be later called VLIW were developed by the Soviet computer scientist Mikhail Kartsev based on his Sixties work on military-oriented M-9 and M-10 computers. His ideas were later developed and published as a part of a textbook two years before Fisher's seminal paper, but because of the Iron Curtain and because Kartsev's work was mostly military-related it remained largely unknown in

5772-483: The three methods described above require. Thus, VLIW CPUs offer more computing with less hardware complexity (but greater compiler complexity) than do most superscalar CPUs. This is also complementary to the idea that as many computations as possible should be done before the program is executed, at compile time. In superscalar designs, the number of execution units is invisible to the instruction set. Each instruction encodes one operation only. For most superscalar designs,

5850-477: The traditional uniformity of the instruction set favors superscalar dispatch (this was why RISC designs were faster than CISC designs through the 1980s and into the 1990s, and it's far more complicated to do multiple dispatch when instructions have variable bit length). Except for CPUs used in low-power applications, embedded systems , and battery -powered devices, essentially all general-purpose CPUs developed since about 1998 are superscalar. The P5 Pentium

5928-421: The unincremented value), or declares that the value it uses is undefined. The programmer may have unrelated work that the processor can do in the meantime; or, to ensure correct results, the programmer may insert NOPs into the code, partly negating the advantages of pipelining. Pipelined processors commonly use three techniques to work as expected when the programmer assumes that each instruction completes before

6006-483: The usual case with sequential execution and branch only on detecting unusual cases. Using programs such as gcov to analyze code coverage lets the programmer measure how often particular branches are actually executed and gain insight with which to optimize the code. In some cases, a programmer can handle both the usual case and unusual case with branch-free code . To the right is a generic pipeline with four stages: fetch, decode, execute and write-back. The top gray box

6084-455: Was the first superscalar x86 processor; the Nx586 , P6 Pentium Pro and AMD K5 were among the first designs which decode x86 -instructions asynchronously into dynamic microcode -like micro-op sequences prior to actual execution on a superscalar microarchitecture ; this opened up for dynamic scheduling of buffered partial instructions and enabled more parallelism to be extracted compared to

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