Giles' blog

Writing an LLM from scratch, part 19 -- wrapping up Chapter 4

I've now finished chapter 4 in Sebastian Raschka's book "Build a Large Language Model (from Scratch)", having worked through shortcut connections in my last post. The remainder of the chapter doesn't introduce any new concepts -- instead, it shows how to put all of the code we've worked through so far into a full GPT-type LLM. You can see my code here, in the file gpt.py -- though I strongly recommend that if you're also working through the book, you type it in yourself -- I found that even the mechanical process of typing really helped me to solidify the concepts.

So instead of writing a post about the rather boring process of typing in code, I decided that I wanted to put together something in the spirit of writing the post that I wished I'd found when I started reading the book. I would summarise everything I've learned, with links back to the other posts in this series. As I wrote it, I realised that the best way to describe things was to try to explain things to myself as I was before ChatGPT came out, say mid-2022 -- a techie, yes, but with minimal understanding of how modern AI works.

Some 6,000 words in, I started thinking that perhaps I was trying to pack a little bit too much into it. So, coming up next, three "state of play" posts, targeting people with 2022-Giles' level of knowledge.

• "What AI chatbots are actually doing under the hood" This is a high-level post, including stuff that I already knew when I started the book, giving enough information for anyone -- hopefully even non-techies -- to understand how we get from next-word completion to something you can have a conversation with.
• "The maths you need to start understanding LLMs". This one is more techie-focused. For understanding LLMs at a fairly good level, you don't need much beyond high-school maths. So this is kind of a bridging section, which fills in the mathematical concepts that they don't teach at school. Nothing particularly difficult, though, at least if you remember matrices and similar stuff from your schooldays.
• "How do LLMs work?". This one actually explains how these AIs work, starting with a high-level description, then zooming in on the building blocks that make it up.

Next, it's time to move on to the next chapter, training. Hopefully all the time I spent fine-tuning LLMs last year will turn out to be useful there!

If you want to jump straight forward to that, here's the first post on training.

@misc{thomas2025aug-llm-from-scratch-19-wrapping-up-chapter-4,
  author       = {Thomas, Giles},
  title        = {{Writing an LLM from scratch, part 19 -- wrapping up Chapter 4}},
  year         = {2025},
  month        = aug,
  howpublished = {Blog post},
  url          = {https://www.gilesthomas.com/2025/08/llm-from-scratch-19-wrapping-up-chapter-4},
}