The V operating system (sometimes written V-System, not to be confused with System V) is a microkernel operating system that was developed by faculty and students in the Distributed Systems Group at Stanford University in the 1980s, led primarily by Prof. David Cheriton. V was the successor to the Thoth and Verax operating systems that Cheriton had worked on previously.

The original V terminology uses "process" for what is now commonly called a "thread", and "team" for what is now commonly called a "process" consisting of multiple threads sharing an address space, but this article will use modern terminology.

The key concepts in V are multithreading and synchronous message passing. Communication between threads in V uses synchronous message passing, with short, fixed-length messages that can include access rights for the receiver to read or write part of the sender's address space before replying. The same message-passing interface is used both between threads within one process, between threads of different processes within one machine, and between threads on different machines connected by a local Ethernet. A thread receiving a message is not required to reply to it before receiving other messages; this distinguishes the model from Ada rendezvous.

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X Window System branch
Preceded by
Thoth and Verax
V (operating system) Followed by
X Window System