> For the complete documentation index, see [llms.txt](https://docs.urbit.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.urbit.org/build-on-urbit/runtime/u3.md).

# U3

The Urbit interpreter is built on a Nock runtime system written in C, `u3`. This section is a relatively complete description.

You should keep reading if (a) you're planning to work on the Urbit interpreter; (b) you're a language implementation geek; or (c) you don't really understand anything until you've seen the actual structs.

## u3: Noun processing in C <a href="#u3-noun-processing-in-c" id="u3-noun-processing-in-c"></a>

`u3` is the C library that makes Urbit work. If it wasn't called `u3`, it might be called `libnoun` - it's a library for making and storing nouns.

What's a noun? A noun is either a cell or an atom. A cell is an ordered pair of any two nouns. An atom is an unsigned integer of any size.

To the C programmer, this is not a terribly complicated data structure, so why do you need a library for it?

One: nouns have a well-defined computation kernel, Nock, whose spec fits on a page and gzips to 340 bytes. But the only arithmetic operation in Nock is increment. So it's nontrivial to compute both efficiently and correctly.

Two: `u3` is designed to be a "solid-state interpreter," ie, a single-level store which is transparently snapshotted. This implies a specialized memory-management model, etc, etc.

(Does `u3` depend on the higher levels of Urbit, Arvo and Hoon? Yes and no. `u3` expects you to load something shaped like an Arvo kernel, and use it as an event-processing function. But you don't need to use this feature if you don't want, and your kernel doesn't have to be Arvo proper - just Arvo-compatible. Think of `u3` as the BIOS and Arvo as the boot kernel. And there are no dependencies at all between Hoon the language and `u3`.)


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