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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

made by P0.

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.

You'll get a 14.9KB .DOCX file.