From CISC to RISC: Why Did Processor Design Shift from Complex to Simple Instructions?

Introduction

When discussing processor architecture, two key terms often arise in the history of computer engineering:

• CISC: Complex Instruction Set Computer

• RISC: Reduced Instruction Set Computer

Each of these design philosophies represents a different approach to how a processor handles machine instructions. Over time, the industry witnessed a gradual shift from complex instruction sets to simplified ones. In this article, we will explore the historical and technical motivations behind this shift, highlight the advantages and disadvantages of each approach, and explain how they continue to influence modern processor design.

First: What is CISC, and Why Did It Emerge First?

In the early days of computing, during the 1960s and 1970s, processors like the Intel 8086 and Motorola 68000 were based on the CISC philosophy. The primary goal at that time was:

To reduce the number of instructions needed to perform a task.

Engineers were facing several limitations:

• Memory was expensive and slow.

• Processor speeds were relatively low.

• Compilers were primitive, and programmers often wrote assembly code manually.

The CISC idea was straightforward:

"Let’s design complex instructions that can perform multiple operations in one go." This would make the code shorter, reduce the number of instructions fetched from memory, and optimize the use of limited resources.

Real-world example: Instead of writing five instructions to store a computed value in memory, CISC could handle it in one instruction such as:

MUL AX, [BX]   ; Multiply the value in AX by the value in memory pointed to by BX

This single instruction handles memory access, multiplication, and result storage in one step.

Second: Why Did the RISC Philosophy Emerge?

In the early 1980s, researchers like John Hennessy and David Patterson introduced a new design model called:

RISC - Reduced Instruction Set Computer

Motivations behind RISC:

1. Most complex instructions were rarely used

Studies showed that a small set of instructions accounted for the majority of program execution, while many complex instructions were rarely called.

2. Simpler instructions were faster to execute

Since RISC instructions were simple, most could be executed in a single clock cycle, which made it easier to implement:

• Pipelining: executing multiple instructions simultaneously in different stages.

• Superscalar execution: running more than one instruction at the same time.

3. Hardware improved significantly

Memory became faster and cheaper, making compact code less of a priority than efficient execution.

4. Compilers became smarter

Modern compilers were able to generate efficient machine code using only simple instructions, eliminating the need for overly complex hardware-level logic.

Third: Detailed Comparison Between CISC and RISC

Fourth: Did RISC Win? What Happened Next?

By the 1990s and beyond, major companies like ARM, MIPS, Apple, and IBM began adopting RISC-based designs, especially in mobile and embedded systems.

• ARM, for instance, has always used RISC and now dominates the smartphone and tablet markets.

• RISC-V is an open-source architecture rapidly gaining popularity in academia and research.

• x86 processors from Intel and AMD, while officially CISC, internally translate complex instructions into simpler micro-operations that follow RISC principles.

This hybrid model is often referred to as:

CISC frontend with a RISC backend

Fifth: When to Use RISC vs. CISC?

• If you're working with mobile, embedded, or low-power systems, RISC (e.g., ARM) is the preferred choice.

• If your domain involves desktop software, games, or servers, you're likely using CISC (e.g., x86).

However, in practice, the distinction is increasingly blurred. Many modern processors incorporate features from both schools of design, forming hybrid architectures.

Conclusion

Processor design initially favored complex instructions due to the limitations of early memory and CPU technologies. As technology advanced, the RISC approach emerged as a more efficient and scalable alternative. Today, even traditionally CISC architectures rely heavily on internal RISC principles.

Understanding this evolution helps programmers and engineers:

• Write more efficient and optimized code.

• Understand the underlying behavior of modern processors.

• Design better systems, especially in performance-critical or embedded environments.