a) Consider four concurrent processes A, B, C, and D, synchronized by two semaphores ‘s1’…

a) Consider four concurrent processes A, B, C, and D, synchronized by two

semaphores 's1' and 's2'. Initially, s1 = 1 and s2 = 0.

process A = process B = process C = process D =

wait ( s1 ) wait ( s1 ) wait ( s2 ) wait ( s1 )

print 'a' print 'b' print 'c' print 'd'

signal( s1 ) signal( s2 ) signal( s1 ) signal( s1 )

i) Suppose 'a' is printed first. List the characters that can be printed

as the last character of the string. Justify your answer.

ii) Suppose 'b' is printed first. List the characters that can be printed

as the last character of the string. Justify your answer.

b) Consider a different example. Initially, atom = 1 and goB = 0.

process A = process B =

'do' j := 1 to m —> 'do' j := 1 to n —>

wait (atom) wait (atom)

… …

signal (goB) wait (goB)

… …

signal (atom) signal (atom)

'od' 'od'

i) Let m > n. In this case, does there exist an execution scenario in

which both processes block permanently? Does there exist an execution

scenario in which neither process blocks permanently? Justify your answers.

ii) Let m

which both processes block permanently? Does there exist an execution

scenario in which neither process blocks permanently? Justify your answers.