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Very Long Instruction Word (VLIW) Architecture

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  • Difficulty Level : Easy
  • Last Updated : 07 Dec, 2020
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The limitations of the Superscalar processor are prominent as the difficulty of scheduling instruction becomes complex. The intrinsic parallelism in the instruction stream, complexity, cost, and the branch instruction issue get resolved by a higher instruction set architecture called the Very Long Instruction Word (VLIW) or VLIW Machines.

VLIW uses Instruction Level Parallelism, i.e. it has programs to control the parallel execution of the instructions. In other architectures, the performance of the processor is improved by using either of the following methods: pipelining (break the instruction into subparts), superscalar processor (independently execute the instructions in different parts of the processor), out-of-order-execution (execute orders differently to the program) but each of these methods add to the complexity of the hardware very much. VLIW Architecture deals with it by depending on the compiler. The programs decide the parallel flow of the instructions and to resolve conflicts. This increases compiler complexity but decreases hardware complexity by a lot.

Features :

  • The processors in this architecture have multiple functional units, fetch from the Instruction cache that have the Very Long Instruction Word.
  • Multiple independent operations are grouped together in a single VLIW Instruction. They are initialized in the same clock cycle.
  • Each operation is assigned an independent functional unit.
  • All the functional units share a common register file.
  • Instruction words are typically of the length 64-1024 bits depending on the number of execution unit and the code length required to control each unit.
  • Instruction scheduling and parallel dispatch of the word is done statically by the compiler.
  • The compiler checks for dependencies before scheduling parallel execution of the instructions.

Block Diagram of VLIW Architecture

Time Space Diagram of VLIW Processor where 4 instructions are executed in parallel in a single instruction word

Advantages :

  • Reduces hardware complexity.
  • Reduces power consumption because of reduction of hardware complexity.
  • Since compiler takes care of data dependency check, decoding, instruction issues, it becomes a lot simpler.
  • Increases potential clock rate.
  • Functional units are positioned corresponding to the instruction pocket by compiler.

Disadvantages :

  • Complex compilers are required which are hard to design.
  • Increased program code size.
  • Larger memory bandwidth and register-file bandwidth.
  • Unscheduled events, for example a cache miss could lead to a stall which will stall the entire processor.
  • In case of un-filled opcodes in a VLIW, there is waste of memory space and instruction bandwidth.
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