What does the CPU think? A great introductory article on building a 16-bit CPU directly in C.
language
eng
date
Mar 15, 2026
slug
building-a-16-bit-cpu-from-scratch-in-c-review
author
status
Public
tags
CPU
Computer Architecture
C language
Emulator
summary
This is a text that helps you understand registers, memory, ALU, flags, branching, stack, and assembler while directly implementing a 16-bit CPU in C code. It is especially recommended for those who want to connect computer architecture not through abstract explanations but through working code.
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Post
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๐ค Computer Science
updatedAt
Mar 16, 2026 02:03 PM
Personally, if I have a belief about technology as an engineer rather than a scientist, it is that if one can truly understand and utilize technology and theory, then evaluation and simulation of that technology and theory must be possible, and a virtuous cycle of this process should be achievable. I believe that evaluation and simulation can provide positive feedback to each other, allowing for a deeper understanding of the technology and theory.
While studying computer architecture, there is a lot of learning through simulation (here, since it mimics hardware, I will use the term Emulation). Implementing an ALU using AND and OR gates or creating registers is a well-known learning method. However, beyond the circuit characteristics, it is often unclear how various elements within an actual CPU connect and operate. Recently, I came across an article on Twitter titled,Building a 16-bit CPU from scratch in Cwhich serves as a good resource to fill that gap. It does not explain the CPU theoretically alone but provides practical operations and explanations of flags, stacks, jump instructions, etc., along with C code. The full code is available on GitHub, and the article includes code blocks in between explanations, allowing readers to follow along with the code.
In the original article, the internal operation structure of the CPU is explained using an office analogy. Registers are likened to sticky notes on a desk, memory is compared to a filing cabinet, and the ALU is described as a calculator. Using these three items, the process of continuously reading, interpreting, executing commands, and moving on to the next command is depicted. The core cycle of the CPU,fetch โ decode โ executeis described in a very intuitive manner.
While explaining the basic elements that make up a CPU through the office analogy, the article also builds each component with code.
- 8 16-bit registers
- 64KB memory
- Program Counter (PC)
- Stack Pointer (SP)
- Zero / Negative / Overflow flags
- ALU responsible for arithmetic/logical operations
- Jump and conditional branching
- PUSH / POP / CALL / RET
- A simple assembler
The core of the CPU is covered comprehensively. Although the office analogy may seem rough, it excels in aiding intuitive understanding. For example, when explaining the difference between registers and memory, the worker describes that calculations can only be made with what is on the desk, making it easy to understand that memory is storage space and that actual calculations occur in the registers. The explanation of how conditional statements operate in hardware also helps to understand how code like if (a == b)actually works, as the CPU cannot recognize the concept of equality and determines if the result is zero or not through subtraction, flags, and jump instructions.Additionally, unlike merely explaining the CPU's circuit configuration with AND and OR gates, the article transitions into explanations about stacks and function calls. It brings along the essential flow for a program to run, allowing for a more detailed understanding of how software operates on the actual hardware of the CPU.
In the process of directly designing instruction encoding, the implementation of a 16-bit CPU explains the trade-off between 16-bit opcodes and instruction and information bits, and by implementing an assembler, it concludes with a final example program that sums numbers from 1 to 100, showcasing how all the previously explained elements connect and operate.
Of course, there are many important elements missing in CPU operation. Aspects like pipelining, interrupts, and cache are either mentioned briefly or not covered at all. Nevertheless, since the focus is on introduction and understanding, directly dealing with or understanding complex modern CPUs falls outside the goal. This article serves as a good starting point for those like me who have purchased computer architecture books or courses but have not been able to start.
This article helps to understand registers, memory, ALU, flags, branching, stacks, and assemblers by directly implementing a 16-bit CPU in C. It is especially recommended for those who want to connect computer architecture to working code rather than abstract explanations.
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