Example: Three processes run independently. Not all possible

reading sequences are valid

Assignment $=$ WRITE operation

$,$ Print $=$ READ operation

Assumption: $x,y,z$ all initialized to $0$

Figure $7-6.$ Three concurrently$-$executing processes.

Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, $2$e$,($c$)2007$ Prentice$-$Hall, Inc. Example: Four valid execution sequences

Prints $-$ concatenation of outputs in order

$$ How many valid

execution sequences &

signature sequences?

Signature $-$ concatenation of outputs in order of P$1,$ P$2$ & P$3$

Signature $000000$ and $001001$ are invalid $($why$?)$

Figure $7-7.$ Four valid execution sequences for the processes of Fig. $7-6.$ The vertical axis is time.Assume that all three variables, $x,y$ and $z$ have been initialized to $0.$ Consider three concurrently executing processes, as shown below.

Referring to lecture slides No$.16$ and No$.17$ in Module $7,$ you may find that there are many valid execution sequences. For each valid execution sequence, a "Prints" $($i$.$e$.,$ the concatenation of

outputs in order$)$ and a "Signature" $($i$.$e$.,$ the concatenation of outputs in order P$1,$ P$2,$ & P$3)$ can be generated.

For each of the following $($potential$)$ signature sequences,

$000011,$

$000111,$

$001111,$

determine if it is a valid signature. If it is a valid signature, please write $($or generate$)$ ONE execution sequence for it $($and also list the corresponding Prints and Signature$).$ If it is an invalid

signature, briefly explain why it is invalid.