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CIS 725

CIS 725. Lecture 2. Finite State Machine Model. FSM = (A, S, T, s 0 ) A = set of actions S = set of states s 0 = initial states T = transition relation S * A  S. Communicating FSM. send actions: -m or !m q ! m = send message m to q

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CIS 725

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  1. CIS 725 Lecture 2

  2. Finite State Machine Model • FSM = (A, S, T, s0) A = set of actions S = set of states s0 = initial states T = transition relation S * A  S

  3. Communicating FSM • send actions: -m or !m q ! m = send message m to q • receive actions: +m or ? m q ? m = receive message from q • internal actions

  4. Protocol • A collection of CFSMs

  5. Asynchronous coupling • Channels ch1 = channel from M to N ch2 = channel from N to M • Global state: (q1, q2, x, y) q1 = state of M q2 = state of N x = messages in transit to M y = messages in transit to N

  6. Protocol actions • Each protocol action results in a change in the global state

  7. Error states • Deadlock states: - state from which no transition is enabled • Unspecified reception: - a state in which a message exists in a channel but no transition is enabled in the state to receive it.

  8. studying studying !meeting waiting Meeting appointment • - Initial state: studying • Send a message for meeting • May decide to cancel anytime ?OK meeting

  9. Two-process mutual exclusion: Token ring protocol • Single token circulates among two nodes • Node with the token can enter the critical section

  10. Request-based Token ring protocol - To get the token, send a request to the other node

  11. no_token no_token !req !req ?token ?token !token req_sent req_sent enter enter enter exit exit exit ?req token & req_recd token token ?req in_cs in_cs& req_recd in_cs Request-based Token ring protocol

  12. Programming language style notation • Guarded actions • en(a)  a en(a): guard of the action boolean condition or boolean condition + receive statement

  13. Normal form • init; do en(a1)  a1 [] en(a2)  a2 : : od

  14. Token-based system • P1: hold = false; in_cs = false do ? token  hold = true [] hold /\ not in_cs  ! Token; hold =false [] hold  in_cs = true [] in_cs  in_cs = false od

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