Assignment 3

due at 23:59 on Wed 29 Feb

In this short assignment, you will experiment with cooperative task switching in GeekOS. To retrieve the files, just do a git pull as shown below, and the a3 folder should appear. Build and run the project as it is:

Note: I have altered the format for commands in this handout, so that the prompt is not just a dollar sign, but reveals the user name (liucs) and the current directory. The part you type is still what comes after the dollar sign, in green. This can help orient you if you are not in the correct directory for a particular command.

liucs:~$ cd ~/cs643
liucs:~/cs643$ git pull
liucs:~/cs643$ cd a3/build
liucs:~/cs643/a3/build$ make

Task 1

When you run the kernel in VirtualBox, it should print out something like this:

8192KB memory detected, 1679 pages in freelist, 1048576 bytes in kernel heap
Initializing IDT...
Initializing keyboard...
Welcome to GeekOS!
Shiny happy people...

Now take a look at a3/src/geekos/main.c. You should see two functions at the top, called my_thread_1() and my_thread_2(), and within main() you will see two calls to Start_Kernel_Thread().

The two thread functions cooperate to print out the message. Explain what happens during the Yield() call in my_thread_1(). [Please type your answer into a program comment in main.c, just before the my_thread_1 function.]

Task 2

Now, just by moving the Yield() call to a different position, force the threads to print out “happy Shiny people…” instead. do not change or adjust the order of the Print() statements!

Task 3

Now, you will add a for loop to each of the thread functions. In my_thread_1(), it should loop from 0 to 99 and print out numbers preceded by A, as in:

A00 A01 A02 A03 A04 A05 A06 A07 A08 A09 A10 A11 A12 ...

To do this, it’s helpful to know that the Print() function works very much like the standard C printf() function: it takes a format string which can contain percent codes, like this:

    Print("A%02d ", i);

In this case, the %02d tells it to print a decimal (base 10) integer, in a field of width 2, right-aligned with zeroes. (That’s how I get A00 instead of just A0.) Eventually the numbers will wrap around to the next line; that’s fine.

Do the same thing for my_thread_2(), only precede the numbers by B. Neither thread should output their numbers until after the “happy Shiny people…” message is finished; this may require adding one more Yield() statement somewhere.

Task 4

Now, you will position Yield() statements inside the for loops in such a way that my_thread_1() will yield after printing any odd number, and my_thread_2() will yield after printing any number divisible by 3. Remember: the C mod operator (%) is useful for checking these conditions.

The output should be something like this:

    happy Shiny people...
    A00 A01 B00 A02 A03 B01 B02 B03 A04 A05 B04 B05 B06 ...etc.

Task 5

Finally, add a new thread function my_thread_3() that prints the numbers 0 through 4 preceded by C. Position calls to Yield() in such a way that the C numbers appear between B03 and A04. At the very least, they should not disrupt the “happy Shiny people…” message. You’ll have to start your new thread in Main using Start_Kernel_Thread, as is done for threads 1 and 2.

That’s it, commit your work!

liucs:~/cs643/a3/build$ git commit -am "my assignment 3"
liucs:~/cs643/a3/build$ git push

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