Vere is the Nock runtime environment and Urbit VM. It's written in C, runs on Unix, and is the intermediate layer between your urbit and Unix. As noted earlier, Unix system calls are made by Vere, not Arvo; Vere must also encode and deliver relevant external events to Arvo. Vere is also responsible for implementing jets and maintaining the persistent state of each urbit.
In principle, Vere keeps a comprehensive log of every event from the time you initially booted your urbit. What happens if the physical machine loses power and your urbit's state is 'lost' from memory? When your urbit restarts it will replay its entire event history and totally recover its latest state from scratch.
In practice, event logs become large and unwieldy over time. Periodically a snapshot of the permanent state is taken and the logs are pruned. You're still able to rebuild your state in case of power outage, down to the last keystroke.
Vere is not essential to the Urbit stack; one can imagine using Urbit on a hypervisor, or even bare metal. One member of the community is even working on an independent implementation of Urbit using Graal/Truffle on the JVM.
The Urbit stack (compiler, standard library, kernel, modules, and applications, but excluding Vere) is about 30,000 lines of Hoon. Urbit is patent-free and MIT licensed.
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