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RISC and CISC in Computer Organization

By Akash Kumar, on November 10, 2018

Complex Instruction Set Architecture (CISC)

The basic idea behind is to make hardware complex as a single instruction will do all the operation such as loading, evaluating and storing operations just like a division command will do stuff like loading data, evaluating and storing it.

Characteristic of CISC

1. As the name suggests Complex instruction should have complex instruction decoding.
2. These Instruction sizes are greater than one-word size.
3. Unlike RISC, CISC can take more than one clock cycle to execute Instruction.
4. Unlike RISC, in CISC we basically require Less number of general purpose register as operation get performed in memory itself.
5. It comprises complex Addressing Modes.

Reduced Set Instruction Set Architecture (RISC)

The basic idea behind this is to make hardware simpler by using an instruction set composed of very few basic steps for many operation such as loading, evaluating and storing operations just like an subtraction command will be composed of loading data, evaluating and storing.

Characteristic of RISC

1. It basically consists of simple instruction which basically leads to simple instruction decoding.
2. This Instruction basically comes undersize of one word.
3. This instruction basically takes one clock cycle to get executed.
4. These basically require a large number of general purpose register.
5. It comprises simple Addressing Modes.

Both approaches basically try to increase the CPU performance.

• CISC: The CISC approach basically tries to minimize the number of instructions per program but it leads to theincrease in the cost of number of cycles per instruction.
• RISC: Role of RISC is basically to reduce the cycles per instruction at the cost of the number of instructions per program.