CISC

• CISC: Complex Instruction Set Computer
• CISC aims to complete the task in as few lines of assembly as possible - this means the processor hardware and circuitry has to be more complicated so it can understand and execute a series of operations
• A CISC processor could include a specific instruction for multiplying two numbers - e.g. MULT. When executed, this instruction would load the two values into the register, multiply them together and store the result back out
• MULT is an example of a complex instruction. It resembles the original command from a high-level language: product = num1 + num2. The computer has to do very little work to translate the high-level language statement into assembly. However, this complex instruction might take more than one machine/clock cycle to execute.
• Although less common today than it used to be, CISC architecture is mainly found in desktop computers and laptops. Intel's x86 processors still use CISC architecture, although more recent changes beyond the scope of this course mean they operate in a RISC fashion, as they utilise microcodes to leverage many RISC benefits
• CISC:

  • Used in laptops/desktops computers
  • Has more complex hardware
  • Multiple machine/clock cycles per instruction
  • More expensive
  • Greater energy consumption
  • Can't make use of pipelining
  • Compiler (software) has to do less work
  • Higher cycles per second
  • Makes more efficient use of RAM than RISC
• One advantage of CISC is its ability to execute complex instructions, while a disadvantage is the complexity of the hardware.
• CISC uses complex instructions with a higher number of cycles per instruction.