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Chapter 4: Model Checking of Finite State SystemsPowerPoint Presentation

Chapter 4: Model Checking of Finite State Systems

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### Chapter 4: Model Checking of Finite State Systems

Albert M. K. Cheng

Model Checking

Is the finite-state graph

a model of the temporal

logic formula?

Specification

represented as

a labeled

finite-state

Graph (Kripke

structure)

Safety assertion

written as temporal

logic formula

Propositional, branching-time temporal logic

Next-time operator X, Until operator U

A(E)X f : f holds in every (some) immediate successor of current state

A(E)[f1 U f2] : for every (some) computation path, there exists an initial prefix of the path such that f2 holds at the last state of the prefix and f1 holds at all other states along the prefix

Computation Tree Logic CTLf holds in the future along every path from the initial state s0, so f is inevitable

EG(f) =

NOT AF(NOT f)

EF(f) = E[True U f]: there is some path from the initial state s0 that leads to a state at which f holds, so f potentially holds

AG(f) =

NOT EF(NOT f)

CTL abbreviationsfor (fi=flength; fi >= 1; fi--)

labelgraph(fi,s,&correct);

labelgraph (fi,s,b)

short fi, s;

Boolean *b;

{

short i;

switch(nf[fi-1][0].opcode)

{

case atomic:

atf(fi,s,b);

break;

case nt:

ntf(fi,s,b);

break;

case ad:

adf(fi,s,b);

break;

case ax:

axf(fi,s,b);

break;

case ex:

exf(fi,s,b);

break;

Explicit-State Model CheckingExplicit-State Model Checking

case au:

for (i=0; i <= numstates; i++)

marked[i] = false;

for (i=0; i <= numstates; i++)

if (!marked[i])

auf(fi,s,b);

break;

case eu:

euf(fi,s,b);

break;

}

}

Symbolic Model Checking

- Transition relation between the values of the variables in the current and the next states can be stated as a Boolean formula
- Use Binary Decision Diagrams (BDDs) to present this Boolean formula
- Apply model checker to finite-state graph represented as BBDs

Real-Time CTL

- Existentially Bounded Until operator:
E[f_1 U[x,y] f_2] at state s_0 means there exists a path beginning at s_0 and some i such that x <= i <= y and f_2 holds at state s_i and forall j < i, f_1 holds at state s_j

- Min/max delays
- Min/max number of condition occurrences

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