CS372H Spring 2011 Homework 1

Problem 1

Define three styles of switching from user mode to supervisor mode.

Problem 2

Which of the following components is responsible for loading the initial value in the program counter for an application program before it starts running:

        * Compiler
        * Linker
        * Loader
        * Boot module or boot ROM

Problem 3

A hardware designer argues that there are enough transistors on the chip to provide 1024 integer registers and 512 floating point registers. You have been invited as the operating system guru to give opinion about the new design.

        1. What is the effect of having such a large number of registers on the operating system?
        2. What additional hardware features you would recommend added to the design above.
        3. What happens if the hardware designer also wants to add a 16-station pipeline into the CPU. How would that affect the context
            switching overhead?

Problem 4

Given the following piece of code:
    main(int argc, char ** argv)
    { }
     
How many different copies of the variable care there? What are their values?

Problem 5

Given the following piece of code How many processes are created if the above piece of code is run? Hint: It may be easier to solve this problem by induction.

Problem 6

What is the output of the following programs (inspect the manual for the system calls if you need more information, but please solve the problem without compiling and running the program).

Program 1:
main()
{
    val = 5;
    if(fork())
        wait(&val);
    val++;
    printf("%d\n", val);
    return val;
}

Program 2:
main()
{
    val = 5;
    if(fork())
        wait(&val);
    else
        exit(val);
    val++;
    printf("%d\n", val);
    return val;
}
 

Problem 7

A typical hardware architecture provides an instruction called return from interrupt, and abbreviated by something like iret. This instruction switches the mode of operation from supervisor mode to user mode. This instruction is usually only available while the machine is running in supervisor mode.

    1. Explain where in the operating system this instruction would be used.
    2. What happens if an application program executes this instruction?

Problem 8

System Calls vs. Procedure Calls: How much more expensive is a system call than a procedure call? Write a simple test program to compare the cost of a simple procedure call to a simple system call ("getpid()" is a good candidate on UNIX; see the man page.) (Note: be careful to prevent the optimizing compiler from "optimizing out" your procedure calls. Do not compile with optimization on.)

Hint: You should use system calls such as gethrtime() or gettimeofday() for time measurements. Design your code such that the measurement overhead is negligible. Also, be aware that timer values in some systems have limited resolution (e.g., millisecond resolution).

Problem 9 (this one is tough)

When an operating system receives a system call from a program, a switch to the operating system code occurs with the help of the hardware. In such a switch, the hardware sets the mode of operation to supervisor mode, calls the operating system trap handler at a location specified by the operating system, and allows the operating system to return back to user mode after it finishes its trap handling. Now, consider the stack on which the operating system must run when it receives the system call. Should this be a different stack from the one that the application uses, or could it use the same stack as the application program? Assume that the application program is blocked while the system call runs.