Difference between RISC and CISC architecture

In the ever-evolving landscape of computer architecture, two predominant paradigms have shaped how processors are designed:

• Reduced Instruction Set Computing (RISC)

• Complex Instruction Set Computing (CISC)

These two approaches represent divergent strategies for achieving efficient and high-performance computing. Each architecture has its own principles, advantages, and trade-offs, making it essential to comprehend their differences to make informed decisions regarding hardware and software design.

What is RISC architecture?

Reduced Instruction Set Computing, commonly called RISC, is an architectural philosophy that emphasizes simplicity and efficiency in processor design. The hallmark of RISC architecture lies in its streamlined instruction set, which comprises a small number of simple and atomic instructions such as Add (ADD), subtract (SUB), Load (LW), and Store (SW). These instructions are typically executed in a single clock cycle, fostering faster execution and enabling higher throughput.

RISC processors often feature a load-store architecture, where data is loaded from memory to registers for manipulation and then stored back to memory. This approach reduces memory access times and enhances performance.

RISC architecture examples

Here are a few examples of RISC architecture-based processors:

1. ARM (Advanced RISC Machines)

2. MIPS (Microprocessor without Interlocked Pipeline Stages)

3. Power Architecture (used in IBM's PowerPC processors)

4. SPARC (Scalable Processor Architecture)

5. RISC-V (an open-source RISC architecture)

What is CISC architecture?

Complex Instruction Set Computing, or CISC, takes a contrasting approach by incorporating diverse complex instructions. CISC architecture strives to provide a comprehensive range of instructions that can perform multi-step operations in a single instruction, potentially reducing the number of instructions needed to accomplish a task.

This approach was initially conceived to relieve the burden on programmers by offloading complex tasks to the hardware. CISC processors might perform operations directly between memory and registers, potentially saving a few instructions. However, this advantage can come at the cost of increased cycle times and reduced clock speed due to the complexity of the instructions.

CISC architecture examples

Here are a few examples of CISC architecture-based processors:

1. x86 (Intel and AMD processors)

2. Motorola 68k (Motorola 68000 series)

3. DEC Alpha (Digital Equipment Corporation Alpha)

4. VAX (Virtual Address eXtension)

5. IBM System/360 and its successors

Understanding the concept

To grasp the concept, let's explore a computer-related real-life analogy to distinguish between RISC and CISC architectures:

Downloading files from the internet

Imagine downloading files from the internet using two different download managers—one representing RISC and the other representing CISC architectures.

RISC download manager (simplicity and speed)

In the RISC download manager, you have a straightforward interface with three options: start, pause, and cancel. With a single click on "start," your download begins immediately. This minimalist approach ensures fast downloading without the fuss of numerous settings.

CISC download manager (complexity and versatility)

Now, let's look at the CISC download manager. Here, you're greeted with a multitude of choices. You can adjust download speeds, set bandwidth limits, specify locations, and schedule downloads. While configuring takes longer, the CISC-style manager offers a customized experience. It tailors downloads to your preferences, optimizes bandwidth usage, and even divides files for efficient downloading.

Explanation

In this simplified analogy, the download manager scenario mirrors the difference between RISC and CISC architectures. Like the RISC-style download manager focusing on speed by streamlining options, RISC processors prioritize quick execution of simple instructions. On the other hand, the CISC-style download manager's versatility parallels CISC processors, offering complex instructions for a broader range of tasks, even if it involves more setup time. As you'd choose the download manager based on your needs, hardware designers choose RISC or CISC architectures based on their computing requirements.

Differences between RISC and CISC architectures

A comparison between RISC and CISC architecture is presented:

Real-time example: ARM vs x86

A concrete example of RISC and CISC architectures can be observed in the ARM (RISC) and x86 (CISC) processor families.

• ARM processors, commonly found in mobile devices, follow RISC principles to balance performance and power efficiency.

• x86 processors, prevalent in desktop and server environments, employ CISC principles to provide a wide range of instructions catering to diverse workloads.

Conclusion

Choosing between RISC and CISC architectures depends on the specific use case and performance requirements. RISC architecture excels in simplicity and fast execution, while CISC architecture offers a more comprehensive instruction set for diverse tasks. As technology advances, both architectures continue to evolve, borrowing elements from each other to optimize performance and efficiency in an ever-changing computing landscape.