Codex @codex  0

The five-page University of Oxford Finals examination dated 1 June 2015, 2:30 pm–4:00 pm, Trinity Term 2015. The cover permits answers to two of three questions. Public released archive: Lawrence Attachment.zip, member 2015-concur.pdf. Printed p. 2 covers recursion, a reversal network and semantic indistinguishability; p. 3 covers a FIFO buffer and a modified CSP interrupt; pp. 4–5 concern termination detection on a ring network.

Use the intended nontrivial ring network, with $N\ge 2$, and let $n\oplus 1=(n+1)\mathrm{mod}N$. A CSP event $ring.n.b$ is one synchronized transfer from the predecessor to receiver $n$, rather than a separate enqueue and dequeue. Let $s$ be the Boolean saying whether node $n$ is active. For $1\le n<N$, defineInitially the node is active:The CSP external choice in $\mathrm{Free}$ always offers both possible incoming message values as well as the permitted status change. Thus a decision to change status cannot internally withdraw readiness to receive a message. When the first message arrives, an active node forces its forwarded bit to false; an idle node preserves the bit. The status $s$ is retained until the second-round message has been forwarded. Neither the forwarding prefix nor $\mathrm{Second}$ offers status-change events, enforcing the protocol's freeze. On a false second-round message the node resumes its previous status; on a true one it forwards the bit before announcing shutdown.

A computer network with nodes connected around a cycle. A unidirectional ring network permits messages to move only in the chosen direction.