Why RISC Was Developed: A Comprehensive Examination

As humans, our cognitive limits mean that manually handling complex CPUs with an abundance of intricate instructions is beyond the grasp of most individuals. Historically, complexity in CPUs was driven by the high cost of memory resources such as RAM, ROM, and disks, making it worthwhile to design CPUs capable of running dense and versatile code with arbitrary data alignment. However, the advent of cheaper memory and the realization that code/data density was less of an issue laid the foundation for the development of Reduced Instruction Set Computing (RISC).

The Shift to RISC

Let's delve into the details and see how these changes brought about the development of RISC.

The Shift Away from Assembly to High-Level Language Programming

One significant factor that contributed to this transition was the shift from assembly languages to high-level programming languages. High-level languages provided more abstraction, making it easier to write and maintain code, and allowed for the development of more powerful compilers. These compilers could more effectively utilize registers and optimize code, leading to more efficient instruction sets.

The Discovery of Efficient Subsets

A pivotal moment in the development of RISC was the realization that programs compiled to a simple subset of certain complex instruction sets, such as IBM 370, ran faster than those compiled to the full instruction set. This discovery highlighted the inefficiency of complex instruction sets and paved the way for simpler designs.

Processor Design and VLSI Technology

Computer scientists working on processor design played a crucial role in this transformation. Strategic use of Very Large Scale Integration (VLSI) technology allowed for the integration of more transistors onto a single chip, but with limitations. CISC (Complex Instruction Set Computing) designs, which were characterized by their hardware specialized for many operations, faced challenges with memory and transistor usage.

For example, to add the contents of two addresses together, a CISC CPU could perform this in a single instruction. In contrast, a RISC CPU might require multiple instructions, including load, addition, and store operations. It was observed that while this hardware complexity seemed beneficial, it did not significantly improve performance due to the infrequent use of many complex instructions. This inefficiency led to the realization that simpler tasks could often be accomplished with a combination of basic operations.

Expressed another way, RISC designs focus on using simpler operations to achieve the same results, with the benefit of reusable components. By focusing on a subset of common operations, RISC architectures made it easier to implement in a single chip and improve clock speeds due to fewer steps required for each instruction.

Efficiency and Performance

The shift to RISC led to a more efficient use of transistors and power. By using one set of general-purpose hardware to perform a wide range of tasks, manufacturers could allocate extra die space and power to other enhancements such as increased register counts, superscalar technology, and improved pipeline designs. The simplicity of RISC instructions also made them faster to execute, enabling higher clock speeds. As a result, even complex operations in RISC architecture can be broken down into simpler parts, leading to better overall performance.

Today, while some specialized systems like IBM's System Z incorporate elements of both, the vast majority of modern CPU architectures are RISC-based. Notable examples include x86, ARM, and MIPS, which all utilize RISC principles internally, even if their external interfaces or historical practices might suggest otherwise.

Conclusion

The development of RISC was driven by the need to simplify CPU design and improve performance in the face of cheaper memory and more efficient compiler technology. By focusing on basic operations and reusing components, RISC designs became more efficient and power-friendly, leading to widespread adoption across various computing platforms.