COMPLEX INSTRUCTION SET COMPUTER (CISC)

• A complex instruction set computer  is a computer where single instructions can execute several low-level operations (such as a load from memory, an arithmetic operation, and a memory store) and/or are capable of multi-step operations or addressing modes within single instructions
• Generally, the instruction set of a CISC system is made efficient by incorporating a large number of powerful instructions
• Instructions are executed by micro-program
• Variable format instructions
• Supports complex addressing mode
• CISC systems have following drawbacks:

1. CPU COMPLEXITY : The control unit design (mainly instruction decoding) becomes complex since the instruction set is large with heavily encoded instructions
2. SYSTEM SIZE & COST: There is a lot of hardware circuitry due to complexity of the CPU. This increases the hardware cost of the system and also the power requirements
3. CLOCK SPEED: due to increase circuits the propagation delays are more and the CPU cycle time is large & hence the effective clock speed is reduced
4. RELIABILITY: The heavy hardware is prone to frequent failures
5. MAINTAINABILITY: Troubleshooting & detecting a fault is a big task since there are a large number of huge circuits. The invention  of microprogramming has reduced this burden to some extent. In built diagnostic microcodes were also provided in many CISC systems giving a helping hand to the hardware engineer in case of system failure.