CISC vs RISC Differences & Comparison

CISC:

Complex instruction set computer (CISC) is a processor design where single instructions can execute several low-level operations or are capable of multi-step operations or addressing modes within single instructions.

CISC contains large number of complex instructions. It facilitates the extensive manipulation of low-level computational elements and events such as memory, binary arithmetic, and addressing.

RISC:                                     

Reduced instruction set computer (RISC) is a microprocessor that is designed to perform a smaller number of types of computer instructions so that it can operate at a higher speed.

RISC retains only those instructions that can be overlapped and made to execute in one machine cycle or less. RISC has small number of instructions and the instruction size is constant.

Difference:

The main difference between RISC and CISC is in the number of computing cycles each of their instructions take. With CISC, each instruction may utilize a much greater number of cycles before completion than in RISC.

The reason behind the difference in number of cycles utilized is the complexity and goal of their instructions. In RISC, each instruction is only meant to achieve a very small task. So if you want a
complex task done, then you need a lot of these instructions strung together.

Performance:                           

         The CISC approach attempts to minimize the number of instructions per program, sacrificing the number of cycles per instruction. RISC does the opposite, reducing the cycles per instruction at the cost of the number of instructions per program.

In terms of the list of available instructions, RISC has the longer one over CISC. This is because each small step may need a separate instruction, unlike in CISC where a single instruction would already cover multiple steps. Although CISC may be easier for programmers, it also has its downside. Using CISC may not be as efficient as when you use RISC. This is because inefficiencies in the CISC code will then be used again and again, leading to wasted cycles. Using RISC allows the programmer to remove unnecessary code and prevent wasting cycles.

Comparison:

CISC

l  Increases instruction size of instruction sets.

l  Designs ever more complex instructions.

l  Provides more addressing modes.

RISC

l  Attempts to make architecture simpler.

l  Reduced number of instructions.

l  Reduces the number of memory accesses required by increasing the number of registers.

l  Reduces the number of addressing modes

CISC instructions utilize more cycles than RISC

CISC has way more complex instructions than RISC

CISC typically has fewer instructions than RISC

CISC implementations tend to be slower than RISC implementations

Computers typically use CISC while tablets, smartphones and other devices use RISC.