CISC : Complex Instruction Set Computers  

RISC : Reduced Insruction Set Computers

comparision of CISC machines and RISC machines :

Characteristics of RISC:
1. Relatively few instructions
2. Relatively few addressing modes
3. Memory access limited to load and store instructions
4. All operations done within the registers of the CPU
5. Fixed-length, easily decoded instruction format
6. Single-cycle instruction execution
7. Hardwired rather than microprogrammed control
8. A relatively large number of registers in the processor unit
9. Use of overlapped register windows to speed-up procedure call and return
10. Efficient instruction pipeline
11. Compiler support for efficient translation of high-level language programs into machine 
language programs
Characteristics of CISC:
1. A larger number of instructions – typically from 100 to 250 instructions
2. Some instructions that perform specialized tasks and are used infrequently
3. A large variety of addressing modes – typically from 5 to 20 different modes
4. Variable-length instruction formats
5. Instructions that manipulate operands in memory