What is the difference between RISC and CISC machines? Write some points about advantages and disadvantages of CISC machine.

The instruction set of a computer specifies the primitive commands or machine instructions that a programmer can use in programming the machine. The complexity of an instruction set is attributed to the instruction format, data format, addressing modes, general purpose registers, opcode specifications and flow control mechanisms used. The difference between CISC and RISC machines are listed below:

Architectural characteristics CISC RISC
Instruction set size and instruction format Large set of instruction variable formats (16-64 bits/inst) Small set of instruction with fixed (32 bit) format and most register based instruction
Addressing modes 12-24 3-5
General purpose register and cache design 8-24 GPRs , mostly with unified cache 32-192 GPRs, with split cache(data and instruction)
Clock rate and CPI 33-50 MHzCPI   2-15 50-150MHzCPI <1.5
CPU control Microprogrammed control unit Hardwired control unit

 
                                                 Fig: CISC architecture with unified cache
 

Fig: RISC architecture with split cache
Reduced Instruction Set Computer(RISC):
Key features:

  • large number of general purpose register used
  • use of compiler technology to optimize register use
  • limited and simple instruction set
  • emphasis on optimizing the instruction pipeline

Characteristics:

  • one instruction per cycle
  • register to register operations
  • few, simple addressing modes
  • few, simple instruction formats
  • hardwired design (no microcode)
  • fixed instruction format
  • more compile time/effort

Complex Instruction Set Computer (CISC):
Characteristics:

  • to attempt to provide a single machine instruction for each statement that is written in a high level language
  • variable length instruction format
  • provides direct manipulation of operands residing in memory
  • a large number of instruction typically from 100 to 250
  • some instructions that perform specialized tasks and are used frequently
  • a large variety of addressing modes typically from 5 to 20

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