CISC [Complex Instruction Set Computer]

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  • The primary goal of CISC architecture is to complete a task in as few lines of assembly as possible.
  • This is achieved by building processor hardware that is capable of understanding and executing a series of operations. 
  • CISC processor would come prepared with a specific instruction (we'll call it "MULT"). 
  • When executed, this instruction loads the two values into separate registers, multiplies the operands in the execution unit, and then stores the product in the appropriate register.
  • The entire task of multiplying two numbers can be completed with one instruction.
  • MULT is what is known as a "complex instruction." It operates directly on the computer's memory banks and does not require the programmer to explicitly call any loading or storing functions. 
  • If the control unit contains a number of micro-electronic circuitry to generate a set of control signals and each micro-circuitry is activated by a micro-code 
  • For example, instead of having to make a compiler write long machine instructions to calculate a square-root, a CISC processor would have a built-in ability to do this. 
  • Examples of CISC processors are: 
    • Intel 386, 486, Pentium, Pentium Pro, Pentium II, Pentium III 

Features of CISC Processors: 

  • CISC chips have a large amount of different and complex instructions.
  • CISC machines generally make use of complex addressing modes. 
  • CISC machines uses micro-program control unit. 
  • CISC processors are having limited number of registers. 

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