Computer Systems Organization (Tietokoneen toiminta ) Autumn -99

Exercise 6 (29.11.3.12.)

1. Answer the following questions:
a) For what purpose are relocation tables needed? What information is stored in the relocation table?
b) Consider the process states:

Why is there no direct switch from WAIT into TERMINATED?
Why is there no direct switch from WAIT into RUN?
In what kinds of situations is it possible that a process is switched directly from RUN into READY?
What happens in the computer when a process is switched from READY to RUN?

c) Where is the TLB (Translation Lookaside Buffer) located? What information is stored in the TLB?
d) Explain when and how the translation look aside buffer is used. What benefits does the use of TLB bring? Could virtual memory work without TLB? It is possible that TLB could cause some harm?
e) Would it make any sense to substitute the validity bit (V-bit) with the process identifier (PID) and to add an extra register PidR into MMU for storing the PID?

2. Compile the following recursive program into TTK-91 machine language.

        static long Duo(int n) { 
            if n < 1 then return 1 else 
            return Duo(n-1)+n+1; 
         }

Write also a main program, that calls method Duo.

3. Write an assembly program which consists of a function MValue and a main program. The function MValue has two parameters. The first one is the address of the first word of an array and the second is the size of the array. The array has been stored in the static data part of the program. The function returns the mean value of the integers in the array. The main program reserves 100 words for the array, reads the size of the array ( < 100) and the elements and calculates the mean value of the elements with the help of the function and outputs it.

4. Explain in detail the tasks of the CPU and the operating system when the time slice for the running process terminates. Your explanation should answer at least to the following questions:

How does the CPU know when the time slice is terminated?
How is the CPU directed to run the operating system instead of a user process?
How is the CPU taken away from the old process?
How is the CPU given to a new process? Which one?

5. When an interrupt has been handled, it's possible that the process which was running before the interrupt is not continued, at least not immediately. Give examples of this kinds of situations. On the other hand, when is it sensible to continue running the same process?

6. Expand the semaphore example below to cover situations where there are three processes executing and none of them is allowed to proceed before the other two processes have arrived to a certain point.

Example of the synchronization of two processes: establishing a normal phone connection. process A: process B: ..... ... dial number wait for a call up(proceed-B) down(proceed-B) wait for the other to answer answer the phone down(proceed-A) up(proceed-A) talk, talk, talk talk, talk, talk

The problems of parallel processes and semaphores are discussed in Tanenbaum's book on page 436-445.