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8 May 2021

CISC vs RISC | Difference | Comparison

Both CISC and RISC are the architectural design of the CPU based on instruction set variation. The two are differentiated on various factors such as a programming unit, addressing mode, clock, implementation, application, etc  The main key difference between RISC and CISC is that RISC utilizes a hardwired programming unit, while the CISC has a microprogramming unit. So here this article gives the difference between CISC and RISC to better understand this topic.

What is RISC?

The key concept is to make hardware simpler by using an instruction set that consists of a few basic steps for loading, evaluating, and storing operations, similar to how a load command loads data and a store command stores data.

What is CISC?

The key concept is that a single instruction will handle all loading, evaluating, and storing operations, similar to how a multiplication command will handle loading, evaluating, and storing data, which is why it is complicated.

Difference between CISC and RISC:

  • A hardwired programming unit is used in RISC, while microprogramming is used in CISC.
  • CISC is an original microprocessor ISA, while the RISC has redesigned ISA that emerged in the early 1980s.
  • The Instruction set optimization of RISC is software-based while that of CISC is hardware-based.
  • The instruction decoding of RISC is comparatively simpler than that of CISC.
  • The simple nature of the RISC instrument set, offers a short execution time than the CISC instruction set, as CISC posses complex nature.
  • For the purpose of storing instruction, multiple register sets are needed by the RISC processor while the same requires a single resistor set in the case of a CISC processor.
  • The program designed for the RISC needs, large memory space for its storage. CISC architecture the design program requires comparatively less space.
  • For the purpose of calculation, no external memory is used by RISC whereas CISC requires external.
  • CISC is used more efficient RAM than RISC, RISC is heavy use RAM.
  • For the purpose of the calculation, no external memory is used by RISC whereas CISC requires external memory for the same.
  • The operation is supported by a small number of addressing modes in RISC, while the operation is supported by compound addressing modes in CISC.
  • The size of the instrument format in the case of RISC is fixed and is 32 bits whereas CISC offers a variable instruction set format whose size ranges between 16 to 64 bits for each instruction.
  • RISC can perform the only register to register arithmetic operations, while the CISC can perform REG to REG, or REG to MEM, 
  • In a RISC processor, both data and instruction required a separate cache. while it is a combined cache for data and instruments in CISC.
  • In RISC, there might be some issues with code expansion, but in CISC, the code expansion causes no problems.
  • The average close required per cycle for operation in RISC is generally single while CISC requires multiple clocks for operation execution.
  • In RISC the complexity is associated with the compiler whereas in CISC the complexity is associated with the microprogram.
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