State Transitions: All state transitions occur with the clock signal and each main state $($ fetch, decode, indirect, execute, trap$)$ cycles through two steps coincident with the clock $($so the machine transitions to the fetch state at S$1($written fetch$/$S$1)$ and them moves to the fetch state at S$2$ where it transitions as per the informal specification given below:

fetch

S$1.-$

S$2.$ goto decode$/$S$1$

decode

S$1.-$

S$2.$ if indirect then goto indirect$/$S$1$

else goto execute$/$S$1$

indirect

S$1.-$

S$2.$ goto execute$/$S$1$

execute

S$1.-$

S$2.$ if program$-$check then goto trap$/$S$1$ else goto fetch$/$S$1$

trap

S$1.-$

S$2.$ goto fetch$/$S$1$

Note however, the following exceptions:

If condition $C1$ is true at the end of fetch then the machine transitions back to state fetch$/$S$1.$