It’s not easy to compare and debate on Complex instruction set architecture with its counterpart, Reduced Instruction Set, until and unless we a have thorough knowledge of both of these computer architectures but the first question that arises is "What actually is instruction set architecture”. An Instruction set architecture or ISA is the architecture of a micro-processor. Instructions Set is the syntax or more generally is the language that a microprocessor understands and follows whereas an instruction set architecture is the vocabulary of that language.

An instruction set architecture has been standardized due to its importance in the manufacturing of computer microprocessors industry. Many standards are followed nowadays but the most common and widely accepted are termed as RISC ISA and CISC ISA. RISC is an acronym for Reduced Instruction Set Computer whereas CISC is an acronym for Complex Instruction Set Computer.

The concept of a reduced instruction set was given by IBM in 1974 but the people, who are really responsible for implementing it, are John Hennessey and David. A. Patterson. The concept of development of this instructions set is based on the principle of simplicity as computers only use 20% of their total instructions while remaining are made superfluous to the original requirement. A microprocessor following this principle would use lesser electronic components i.e. encoders, decoders, multiplexers, switches and transistors which would ultimately result in fewer instructions. This model was first adopted and implemented on IBM and Sun SPARC machines. Because of its success, MIPS technologies was founded which developed the famous M.I.P.S (microprocessor without interlocked pipeline stage) microprocessor.

MIPS processors use a very simple architecture consisting of a very simple yet optimized set of instruction. There are many famous families including ALPHA, ARC, Nintendo, SPARC and MIPS which use the RISC architecture. In fact, Apple IPOD’s are using this architecture because of higher precision, lower cost and low cost embedment with other devices. It is also being used in high class gaming consoles such as Nintendo, XBOX 360 and Sony play station 3.

RISC is based on four design principles which are stated below:

  • Simplicity favors regularity i.e. lesser no. of instructions is faster to process.
  • Smaller is faster i.e. smaller instructions are faster to process.
  • Good design demands good compromises i.e. it’s not easy to design a good processor without trade-offs.
  • Make the common case fast i.e. common instructions should be used via registers.

A register is the part of a microprocessor which is a memory location. Instructions in RISC processors are carried through registers. Data can be both exported and imported to the main memory through these registers.
 
CISC is an acronym for complex instruction set computer which are now widely used in nearly every type of computers that are available. Let it be a desktop, palmtop, notebook or a main frame. The concept of this type of architecture was developed in 1964 by IBM. The basis of CISC processors is the Intel 8086 architecture with several registers, flags. Data bus, Control Bus, Address Bus and Input/output Bus. The size of address bus is 20-bits. Data and Input/output are of 16-bits. That’s the reason that we can attach 65,536 peripherals on Intel 8086 board. The control bus is of 4 bits. Each bit is reserved for read data, read memory, write data and write memory.

There are eight addressing schemes of Intel 8086 processors based on CISC principle. They are register direct, register indirect, register, immediate, register relative, base plus index, register relative plus base and scaled index. These processors are based on highly complex and enriched instruction set. Because of this high functionality and performance, CISC processors are normally preferred over RISC and this is also the reason of their wide-spread use.