Comparing CISC and RISC-V Architectures: Insights for Modern Computing and Design Choices

Note: This article aims to clarify the differences between CISC and RISC architectures, focusing on the modern relevance of these concepts, especially in the context of RISC-V, ARM, and X86 architecture choices. We will explore the technical nuances and the reasons behind certain design decisions in the contemporary computing landscape.

Introduction to CISC and RISC Architectures

A common misunderstanding arises when attempting to compare CISC and RISC architectures directly with RISC-V (a specific implementation of a RISC design philosophy). Let's start by understanding the basics: CISC stands for Complex Instruction Set Computer, and it is a design philosophy where instructions are complex and can perform various tasks in a single operation. On the other hand, RISC stands for Reduced Instruction Set Computer, characterized by a simpler set of instructions focusing on core operations.

RISC-V, however, is a specific open-source instruction set architecture (ISA) that adheres to the RISC principles. It is not a generic implementation of RISC, but a standardized, free ISA with various extensions.

Comparison Between CISC and RISC

The core distinction lies in the complexity and efficiency of the instruction sets. A key point of confusion often arises from the fact that while CISC and RISC architectures share a superscalar microarchitecture (and thus, more powerful cores), the intrinsic differences in their instruction sets make them fundamentally different in terms of performance and resource utilization.

Anatomy of Instruction Sets

To understand the differences, let's count the number of instructions:

CISC architectures, like x86 x64, feature an extensive instruction set with over 4500 instructions. Furthermore, the ADD instruction in x86 has 22 different versions, supporting operations on 8, 16, 32, and 64 bits, including direct memory operations. Contrast this with RISC architectures, such as ARM or RISC-V, which typically have a simpler instruction set with only a few hundred instructions. For instance, the ADD operation in RISC-V or ARM is a simple math/logical operation on internal registers, not directly working with memory.

The simplicity of the RISC architecture is what allows it to have a higher number of registers (GPRs, General Purpose Registers), whereas CISC architectures require more complex instructions to handle different data types and memory operations.

RISC-V Code Example

To illustrate, here is a simple C code and its corresponding RISC-V assembly code:

C Code:

int Variable  12345678

Variable  Variable  9876543

RISC-V Assembly Code:

// int Variable  12345678LUI r10, HI(Variable)ADDI r10, r0, LO(Variable)LUI r11, HI(12345678)ADDI r11, r0, LO(12345678)SW r11, [r10]// Variable  Variable  9876543LUI r12, HI(9876543)ADDI r12, r0, LO(9876543)LW r13, [r10]ADD r13, r13, r12SW r13, [r10]

This example clearly demonstrates why RISC architectures have a higher number of instructions: each operation must be broken down into simpler, direct register-to-register operations, leading to a more complex overall instruction set compared to CISC architectures.

Modern Computing Trends: Why ARM Over X86

Now, let's discuss why many companies are favoring ARM architecture over X86 and other CISC architectures. There are several factors influencing this trend:

Performance per Watt

One of the primary reasons is performance per watt. ARM architecture is designed with power consumption in mind, a critical factor in mobile and energy-efficient computing. X86, known for its complex instruction set and strong performance, tends to consume more power, making it less suitable for battery-operated devices.

Instruction Set Flexibility

ARM's instruction set is optimized for modern computing tasks, supporting a wide range of operations efficiently. This flexibility, combined with ARM's focus on low power consumption, has made it the preferred architecture for smartphones, tablets, and embedded systems.

Intellectual Property and Market Pie

X86 architecture, owned by Intel, has a significant market share due to its legacy and performance capabilities. However, ARM's proprietary models (both ARM LLC and RISC-V) are more accessible and flexible. ARM LLC sells ISA and core schematics, making it easier for companies to design and optimize their own ARM cores. RISC-V is fully free and open-source, attracting a broad community of developers and designers.

Performance and Power Consumption

When it comes to performance and power consumption, the landscape is nuanced:

Low Power RISC: In low-power environments, RISC-based architectures like RISC-V consume less power. This is particularly evident in modern mobile devices like the latest iPhone 15, which despite using advanced 3nm technology, still requires throttling due to power consumption. High Performance RISC: More powerful RISC-based cores, when designed with a wider microarchitecture, can match or even surpass the performance of CISC architectures. For instance, the ARM X4 core, being 8-wide, can outperform x86 cores using fewer transistors and lower power consumption.

The choice between CISC and RISC is not just about performance but also about the specific needs of the application and the trade-offs between power consumption, cost, and design flexibility.

Conclusion

While CISC and RISC architectures share a common internal architecture like superscalar microarchitecture, their external characteristics and instruction sets make them fundamentally different. Modern computing trends favor architectures like ARM and RISC-V due to their performance-to-power ratio, flexibility, and the availability of open-source designs. Ultimately, the choice of architecture depends on the application's requirements, market needs, and the balance between power consumption and performance.