CISC architecture and RISC architecture


February 8, 2013 by Ozgur Ozden

When engineers design processors, performance is the first factor to be considered. There Imageare three issues mainly affects the performance of a processor and (Krad and Al-Taie, 2007) explains these factors as:
“1- How fast you can crank up the clock.
2- How much work you can do per cycle.
3- How many instructions you need to perform a task”

Keeping these factors in mind we have two different approach to processor architecture today. Complex Instruction Set Computer  (CISC) and Reduced Instruction Set Computer (RISC). In both case main idea is to increase performance. Both have advantages and disadvantages so in this article we will try to illustrate these differences and predict the future trends for processor architecture .

CISC runs and includes full set of instructions in the computer. Pentium processors are examples of such design. Main advantages and disadvantages of CISC architecture can be summarized as:

  • High power consumption
  • More suitable for High-Level Languages
  • Complex instructions limits the memory use
  • Complex instructions may take single step to complete
  • Instructions can be completed in many clock cycles
  • Expensive to produce
  • Instructions are different in size
  • Less space left for registers
  • More transistors required

RISC runs minimum number of machine instructions so it can operate at very high speeds. It is first proved by John Cocke , a researcher from IBM, at 1974 that computers actually performs 80% of the tasks by only using 20% of the total instructions. PowerPC processors used in IBM computers or Macintosh computers are examples for RISC designed processors. There are some certain design considerations for RICS processors as Farhat Mashood (2011) explained.

  1. Cycles Per Instruction: This is one important feature of a CPU design.  It can be explained as time needed for the execution of a single instruction.
  2. Pipelining (Fetch, Decode, and Execute):  executions of the instructions or parts at the same time.
  3. Registers:  Keep large number of registers available to prevent interactions with memory.

Advantages and disadvantages of RICS design can be illustrated as:

  • Effective, fast
  • Easy to design  and cheap to produce
  • Short design period for engineers
  • Low power consumption
  • Instructions are the same length so it can be executed in one clock cycle
  • Since the instructions are completed in one clock cycle, this allows processor to perform multiple tasks at the same time.
  • Higher clock speeds
  • Large number of registers
  • Needs to access memory only for LOAD and STORE instructions
  • Fixed instructions length
  • Complex instructions may complete in many steps and this may increase the coding size
  • Less transistors needed

In today’s developments which processor is better argument is little bit unclear. Because both RISC and CISC designs are changed in time. In addition to this what is “Complex” and “Reduced” definitions are changed as well.  RISC processors have become more complex and CISC designs have become more efficient today. PowerPc 601 ,as an example of RISC design contains more instruction than Pentium processor which is considered to be CISC design.
I believe that the future of processor architecture lies in the Hybrid system which is a combination of CISC and RISC. Intel described its Pentium II processor as CRISC (Complex-Reduce Instruction Set Computer). Intel core 2 Duo again might be a very good example of Hybrid systems and we can expect more developments in this direction.

1- RISC (reduced instruction set computer [Online]. Available from: (Accessed: 28 December 2012)

2- Farhad  Mashood, 2011, RISC and CISC Computer Architecture.   [Online]. Available from: (Accessed: 28 December 2012)
3- Hasan Krad and Aws Yousif Al-Taie , 2007. A New Trend for CISC and RISC Architectures . Asian Journal of Information Technology, 6: 1125-1131. [Online]. Available from: (Accessed: 29 December 2012)
4- Brookshear, J.G. (2011) Computer Science An Overview. 11th ed. New York: Addison-Wesley, pp.77-78.



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