RISC vs CISC

What is a Processor?

  • Processor is same as Central Processing Unit (CPU).
  • Its primary functions include Fetching, Decoding, Executing, Writeback.
  • There are two types of architecture: 1. RISC, 2.CISC.
  • Each architecture has machine code that corresponds with their own unique assembly language.

CISC stands for Complex Instruction Set Computing and RISC stands for Reduced Instruction Set Computing.

Topics of discussion are:

  • What is RISC?
  • What is RISC?
  • RISC Architecture
  • CISC Architecture
  • Characteristics of RISC and CISC Architecture
  • RISC vs CISC (Main Differences)

 

What is RISC?

It is designed to reduce the execution time by simplifying the instruction set computer. In RISC processors each instruction requires one clock cycle (each clock cycle consist of fetching the instruction, decoding the instruction, and executing the instruction) to execute the resulting uniform execution time, which reduces the efficiency of the machine with RISC processor whenever there is a large number of instructions. In RISC memory is accessed through specific instructions rather than as a part of most instructions. RISC reduce the cycles per instruction at the cost of the number of instructions per program.

The ARM architecture is used in smartphones (Android, IOS), tablets which provide a wide user base for RISC-based systems. RISC processors are also used in supercomputers one of them is Summit, which was the fastest supercomputer of the year 2018. ARC,  M88000, MIPS, PA-RISC, Atmel AVR, Power ISA (including PowerPC), RISC-V, Alpha, Am29000, ARM, i960, SuperH, Blackfin, and SPARC are some of the RISC designs.

 

RISC Architecture

It uses a highly optimized set of instructions. Most RISC architectures have fixed-length instructions (commonly 32 bits) and a simple encoding, which simplifies fetch, decode, and issue logic considerably. RISC reduces the cycles per instruction at the cost of the number of instructions per program. Pipelining is one of the unique features of RISC. Extensive use of pipeline architecture is involved in order to reach one instruction per clock cycle. It is performed by overlapping the execution of several instructions in a pipeline fashion.

Also Read: Comparison of Mobile Processors

Characteristics of RISC Architecture

  • Relatively fewer instructions, same instructions can be used for different operations.
  • Relatively few addressing modes.
  • Memory access is limited to load and storage.
  • All operations are done within the registers of the CPU that is why RISC architecture contains a large number of CPU registers.
  • Fixed length and easily decoded instruction format.
  • Single-cycle instruction execution.
  • Hardwired rather than microprogrammed controlled.
  • Overlapped register windows.
  • Every instruction has the same number of pipeline stages

 

 

What is CISC?

CISC executes larger and more complex assembly instructions at a time. The main idea is to make hardware complex as a single instruction will do all loading, evaluating and storing operations just like a multiplication command will do stuff like loading data, evaluating and storing it. The CISC approach attempts to minimize the number of instructions per program but at the cost of an increase in the number of cycles per instruction.

CISC is used in personal computers, laptops, which processes heavy graphics for games, heavy computation for the equation. Examples of CISC processors are:-

  • IBM 370/168
  • VAX 11/780
  • Intel 80486
  • Intel x86
  • Motorola 68000 family

Reasons for complex instruction set computers.

—more powerful instructions.

•e.g. floating point arithmetic instructions.

—Assembly language instructions were closer to the high-level language instructions.

•e.g. loops, complex conditional jumps.

Benefits

  • easier programming in assembly language.
  • fewer instructions needed to write an application.
  • easier compilation of high-level language.
  • support for complex data structures.

Also read: What are Mobile GPUs? Comparing various Mobile GPUs

 

CISC Architecture

The CISC approach attempts to minimize the number of instructions per program but at the cost of an increase in the number of cycles per instruction. CISC architecture was designed to decrease the memory cost. As the large program needs more storage thereby increasing the memory cost, therefore in CISC architecture the number of instructions per program can be reduced by embedding the number of operations in a single instruction, thereby making the instructions more complex.

 

Characteristics of CISC Architecture

  • Minimize the number of instruction per program, hence making them complex.
  • Less RAM is required to store instructions.
  • Variety of addressing modes are used in CISC processor.
  • Variable length of instruction format.
  • Many cycles may be required to execute a single instruction.
  • Instruction decoding logic is complex.

 

Risc vs Cisc (Main Differences)

S.No.                                   CISC                                    RISC
 1.The primary goal is to complete a task in as few lines of code as possible.The primary goal is to speed-up individual instruction.
 2.Emphasis on hardware.Emphasis on software.
 3.Includes multi-clock, complex instructions.Single-clock, reduced instructions only.
 4.Memory-to-memory:

“Load” and “Store”

incorporated in instructions.

Register to register:

“Load” and “Store”

are independent instructions.

 5.Difficult to apply pipelining.Easy to apply pipelining.
 6.Small code sizes, high cycles per second.Low cycles per second, large code sizes.

 

Also read: Detailed explanation of Mobile applications

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