RISC vs CISC Architectures

What is RISC?

It is known as the Reduced Instruction Set Computer. It is a type of microprocessor that has a limited number of instructions. They can execute their instructions very fast because the instructions are very small and simple.

RISC chips require fewer transistors which makes them cheaper to design and produce. In RISC, the instruction set contains simple and basic instructions from which more complex instruction can be produced. Most instructions complete in one cycle, which allows the processor to handle many instructions at the same time.

In these instructions are register-based and data transfer takes place from register to register.

What is CISC?

It is referred to as the Complex Instruction Set Computer. it's a kind of microprocessor that features a large number of complex instructions. It can have special instructions also as for instructions that take more than one cycle to execute.

They are chips that are easy to program that makes efficient use of memory. CISC eliminates the necessity for generating machine instructions for the processor.

Characteristics of RISC

• Simple Instructions are used in RISC architecture.
• RISC helps and supports few simple data types and synthesizes complex data types.
• RISC utilizes simple addressing modes and fixed-length instructions for pipelining.
• One Cycle Execution Time.
• RISC contains Large Number of Registers in order to prevent various number of interactions with memory.
• Reduced instructions need a less number of transistors in RISC.
• A compiler is used to perform the conversion operation means to convert a high-level language statement into the code of its form.

Characteristics of CISC

• Instruction-decoding logic will be Complex.
• One instruction is required to support multiple addressing modes.
• Less chip space is enough for general purpose registers for the instructions that are 0operated directly on memory.
• Various CISC designs are set up with two special registers for the stack pointer, handling interrupts, etc.
• MUL is referred to as a “complex instruction” and requires the programmer for storing functions.

Difference between RISC and CISC

Architectural Characteristics	Reduced Instruction Set Computer(RISC)	Complex Instruction Set Computer(CISC)
Instruction size and format	A small set of instructions with fixed format (32 bit)	A large set of instructions with variable formats (16-64 bits per instruction)
Data transfer	Register to register	Memory to memory
CPU control	Mostly hardwired without control memory	Most micro coded using control memory (ROM) but modern CISC use hardwired control
Instruction type	Register-based instructions	Not register-based instructions
Memory access	Less memory access	More memory access
Clocks	Includes single clock	Includes multi-clocks
Instruction nature	Instructions are reduced and simple	Instructions are complex

Table-1: Difference between RISC and CISC

Pros and Cons of RISC

Pros of RISC

• Complex and efficient machine instructions.
• It offers extensive addressing capabilities for memory management.
• Relatively few registers when compared with RISC processors.
• It helps you to reduce the instruction set.
• Offers limited addressing schemes for memory operands.

Cons of RISC

• The performance of the RISC processors depends on the programmer or compiler. The compiler plays an important role while converting the CISC code to a RISC code.
• RISC processors have large memory caches on the chip itself.
• RISC architecture necessitates on-chip hardware to be continuously reprogrammed.

Pros and Cons of CISC

Pros of CISC

• In CISC it is easy to add new commands into the chip without need to change the structure of the instruction set.
• This architecture allows you to make efficient use of main memory.
• The compiler should not be very complicated, as with the case of CISC. The instruction sets can be written to match the structures of high-level languages.

Cons of CISC

• Earlier generations of a processor family mostly contained as a subset in every new version. Hence, instruction sets & chip hardware becomes complex with each generation of computers.
• The performance of the machine slows down because of clock time taken by different instructions will never be similar.
• They are larger as they require more transistors.