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# CSE 330 – Operating Systems –Assignment: #2 Solution

Q1] Suppose a system has an atomic hardware instruction SHIFT, that does the
follows:
SHIFT ( int *A, *B ) {
*B =*A; // ATOMICALLY
*A = 0
}

A] Implement Dijkstra style semaphores with the shift instruction, that is,
semaphores which utilize busy waiting.

B] Implement blocking semaphores using the shift instructions.

Q2] There are 4 processes, executing concurrently. Process P0 is in an infinite loop, incrementing the value of the variable x (x is initialized to 0). P0 is the only process that changes the value of x.

The rest of the processes Pi (1<= i <=3) monitor the value
of x. Whenever x reaches a value such that it is divisible by i, Pi prints the value
of x. For example, P3 will print the sequence 3 6 9 12 ….. as the value of x reaches 3, 6, 9, 12 and so on.

Write the code for all the 4 processes using semaphores. Note that P1 - P3 should be identical; also Pi determines whether x is to be printed, and this decision is not
Q3] A synchronization mechanism consists of 2 atomic routines, ENQ(r) and DEQ(r).
"r" is a resource variable that has two fields, inuse (boolean) and queue (a queue)
which is a queue of processes waiting to acquire the resource. The definitions of

ENQ and DEQ are:
ENQ(r) : if (r.inuse==1) then begin
insert current process in r.queue
block
end
else r.inuse = 1;
DEQ(r) : if r.queue == nil then inuse = false
else delete a process from r.queue and activate it.

Construct an implementation of ENQ/DEQ using semaphores. You can use other
variables, etc that you need, but no other atomic code or synchronization constructs other than P or V can be used.

Q4]
Do the reverse of Q3, that is implement Semaphores using ENQ/DEQ.