Basic tradeoff of concerns for instruction set design, Computer Engineering

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Q. Basic Tradeoff of concerns for instruction set design?

Basic Tradeoff: Smaller instruction (less space) vs. desire for more powerful instruction repertoire.

Generally programmer desire:

  • More operands andop-code: as it results in smaller programs
  • More addressing modes: for greater flexibility in implementing functions such as multiple branching,table manipulations.

Though a 32 bit instruction however will occupy double the space and can be fetched at double the rate of a 16 bit instruction thoughcan't be doubly useful. 

Factors which are must to be considered for deciding about instruction length:

  • Memory size: if larger memory range is to be addressed then more bits can be needed in address field.
  • Memory organization: if addressed memory is virtual memory then memory range that is to be addressed by instruction is larger than physical memory size.
  • Memory transfer length: Instruction length mustgenerally be equal to data bus length or multiple of it.
  • Memory transfer: Data transfer rate from memory preferablymust be equivalent to processor speed. It may become a bottleneck if processor executes instructions faster than rate of fetching the instructions. One solution for this problem is to use cache memory or another solution can be to keep instruction short.

Generally an instruction length is kept as a multiple of length of a character (which is 8 bits), and equal to length of fixed-point number. The term word is commonly used in this context. Generally the word size is equal to length of fixed point number or equal to memory-transfer size. Additionally a word must store integral number of characters. So word size of 16 bit, 32 bit, 64 bit are to be coming very common and henceforth the similar length of instructions are normally being used.

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