This blog post details a simple 16-bit CPU design implemented in Logisim, a free and open-source educational tool. The author breaks down the CPU's architecture into manageable components, explaining the function of each part, including the Arithmetic Logic Unit (ALU), registers, memory, instruction set, and control unit. The post covers the design process from initial concept to a functional CPU capable of running basic programs, providing a practical introduction to fundamental computer architecture concepts. It emphasizes a hands-on approach, encouraging readers to experiment with the provided Logisim files and modify the design themselves.
This blog post, titled "Simple CPU Design," meticulously details the process of designing a rudimentary Central Processing Unit (CPU) using readily available, cost-effective components like an Arduino Mega. The author emphasizes the educational value of the project, highlighting its potential to provide a practical understanding of fundamental computer architecture principles. The design centers around a simplified Harvard architecture, which means the CPU uses separate memory spaces for instructions and data. This separation simplifies the design and allows for concurrent access, potentially increasing processing speed.
The core functionality of the CPU is explained through a series of interconnected modules, including an Arithmetic Logic Unit (ALU), responsible for performing arithmetic and logical operations; a Control Unit (CU), which fetches instructions from memory and decodes them to control the other components; program memory, holding the instructions to be executed; data memory, for storing data used in computations; and registers, which serve as fast, temporary storage locations within the CPU. The interplay between these modules is illustrated through detailed diagrams and explanations of the data flow.
The ALU, a crucial component, supports a limited set of arithmetic and logical operations, including addition, subtraction, bitwise AND, and bitwise OR. The Control Unit, designed using a finite state machine approach, fetches instructions from program memory and decodes them into control signals that dictate the operation of the ALU, data memory, and registers. The instruction set architecture (ISA) is purposely kept simple, with a small number of instructions that encompass basic arithmetic, logical, memory access, and control flow operations.
The blog post provides comprehensive schematics, illustrating the connections between the various components and the flow of data within the CPU. It also includes the Arduino code used to emulate the CPU's functionality, demonstrating the logic behind each operation. The code serves as a concrete implementation of the theoretical design principles discussed. Furthermore, the author emphasizes the modularity of the design, suggesting possibilities for expansion and improvement, such as increasing the size of memory or adding more complex instructions to the ISA. This iterative approach reinforces the learning process, encouraging experimentation and further exploration of CPU design principles.
The author acknowledges the limitations of the simplified design compared to modern CPUs, particularly in terms of performance and complexity. However, they stress the project’s pedagogical value, arguing that it offers a tangible and accessible way to grasp the core concepts of computer architecture. This simplicity allows for a focused understanding of the essential building blocks of a CPU without the overwhelming complexity of modern processors. The project is presented as a stepping stone towards more advanced exploration of computer architecture and digital design.
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https://news.ycombinator.com/item?id=42793597
HN commenters largely praised the Simple CPU Design project for its clarity, accessibility, and educational value. Several pointed out its usefulness for beginners looking to understand computer architecture fundamentals, with some even suggesting its use as a teaching tool. A few commenters discussed the limitations of the simplified design and potential extensions, like adding interrupts or expanding the instruction set. Others shared their own experiences with similar projects or learning resources, further emphasizing the importance of hands-on learning in this field. The project's open-source nature and use of Verilog also received positive mentions.
The Hacker News post titled "Simple CPU Design" linking to simplecpudesign.com has generated a moderate discussion with a number of insightful comments. Several commenters praise the clarity and accessibility of the resource, finding it a valuable introduction to CPU architecture. One user appreciates its focus on the fundamentals, contrasting it with more complex designs often encountered in university settings. They highlight how the tutorial breaks down the concepts into manageable steps, making it easier to grasp the overall picture.
Several users discuss their own experiences with similar projects, often mentioning their use of FPGAs and VHDL or Verilog for implementation. They share specific challenges and solutions encountered during their learning process, creating a sense of shared experience among those interested in building their own CPUs. One commenter recounts their project of building a CPU on an FPGA and connecting it to a PS/2 keyboard, emphasizing the rewarding feeling of seeing their creation interact with physical hardware.
The practicality of the design is also a point of discussion. Some commenters note the limitations of such a simple CPU, particularly its lack of pipelining and other performance-enhancing features. However, others argue that the simplicity is the point, allowing for a deeper understanding of the core principles before moving on to more complex designs. This echoes the sentiment that the tutorial is an excellent starting point, laying a solid foundation for further exploration.
There's also some discussion around potential enhancements and modifications to the simple CPU design. Ideas include adding interrupts, implementing a more complex instruction set, and exploring different memory architectures. This demonstrates the engagement of the commenters and their interest in pushing the design further.
A recurring theme is the educational value of the resource. Many users express their enthusiasm for finding a clear and concise explanation of CPU design, often contrasting it with more academic or overly technical resources. They appreciate the author's approach of starting with the basics and gradually building complexity. One user even suggests using the tutorial as a teaching tool for introductory computer architecture courses.
Finally, there are a few comments discussing the choice of Logisim, the digital logic simulator used in the tutorial. While some find it suitable for the purpose, others suggest alternative simulators like Digital, pointing to their advantages in terms of features and usability. This discussion highlights the variety of tools available for those interested in exploring digital logic design.