Designing a New Instruction Set for a Processor: A Comprehensive Guide

When faced with the challenge of designing a new instruction set for a processor, various approaches can be taken. This article explores several methods, from building a custom CPU to using existing microcode to creating your own instruction set in microcode. We will also discuss building a software emulator, and the considerations involved in this process.

Building a Custom CPU vs. Using Microcode

The answer to designing a new instruction set for a processor can often be summarized by the statement: "Build your own CPU." This approach, however, is not without its challenges and costs. Building a physical CPU typically requires a significant investment of time and money across all stages of design, testing, and manufacturing. An alternative method involves creating a software simulation of your processor, which can be achieved using bit-slice processors or microcode.

Bit-Slice Processors and Microcode

Bit-slice processors, such as those found in the Burroughs B1700/B1800/B1900 series, offer a practical solution for time-sharing multiple microcode sets. If you can obtain access to one of these processors or a modern equivalent, along with comprehensive documentation, it is possible to write your own instruction set in microcode and run it on these machines.

While this method might be more viable than building a physical CPU, it still poses challenges such as the need for adequate documentation and the constraints of the existing microcode architecture. Nevertheless, it provides a more feasible and cost-effective path compared to starting from scratch with a physical CPU design.

Creating a Software Emulator

A more accessible approach is to design a machine architecture that includes the instruction set and create a software emulator for it using a preferred programming language. This emulator will allow you to run programs written in your custom instruction set. An even more ambitious step would be to modify an open-source compiler to emit your instruction set and potentially port an open-source operating system to run on your emulated machine.

This method offers a significant advantage in terms of time and cost. While the performance of the emulated system will be relatively slow, you will not need to dedicate thousands of hours and substantial financial resources to build a physical machine. The process involves designing the architecture, writing the emulator, and possibly optimizing the environment to suit your needs.

Starting Points for Custom Modifications

For those interested in creating a custom instruction set, there are several starting points available for modification. There are small Reduced Instruction Set Computing (RISC) CPUs designed for field-programmable gate arrays (FPGAs), which can be a great starting point for custom modifications. Alternatively, creating a software simulation of your intended CPU is another option.

You can consider designing a subset of the 68000, known for its symmetrical register-based instruction set, or a stack-based instruction set reminiscent of the Forth language. The primary challenge lies in the lack of tools, which means you may need to either create your own tools or program directly in machine code. This process requires careful consideration and planning, involving a multidisciplinary team of chip designers, software engineers, and testing specialists.

The Process of Instruction Set Design

The process of designing a new instruction set is complex and fraught with challenges. It involves listening to chip designers, understanding their estimates for the required circuitry and implementation times, and ultimately making compromises. In the final stages, chip layout specialists have the final say in determining the feasibility of the design.

Conclusion

In conclusion, designing a new instruction set for a processor is a multifaceted task that requires careful planning, resourcefulness, and technical expertise. Whether you choose to build a custom CPU, use microcode, or create a software emulator, each approach offers its own set of benefits and challenges. By following the steps outlined in this guide, you can embark on this exciting journey and potentially revolutionize the way processors are designed and used.