Assignment #3 Memory Management

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Operating Systems COMP 310 – ECSE 427
Assignment #3
Memory Management

Labs 3 to 6 will provide some background for this assignment.
This assignment has two parts: a programming part and a question/answer part from the online
The question is presented from a Linux point of view using the computer science server, which you can reach remotely using ssh or putty from your laptop (see lab 1). If you
do this assignment from an MS Windows machine, then make sure to provide the DLL libraries your
program uses (if any) so that the TA can run it from their MS Windows machine. It is not the TA’s
responsibility to make your program run. The TAs will not debug your program.
You must write this assignment in the C Programming language.
Build your assignment #3 using your solution from Assignment #2 or the official solution posted on
PART 1: Online Textbook Problems
Answer the following problems from the online textbook (you can find it on myCourses Content area):
• Page 254 #4
• Page 264 #7
• Page 257 #28
• Page 258 #38
PART 2: Programming Problem
Building Virtual Memory for the Kernel
Assignment #3 adds the following new elements to your OS:
– The OS Boot Sequence to create some necessary OS structures
o Prepare the Backing Store
o Prepare RAM for paging
– The Memory Manager to handle memory allocation for processes
o Launcher
▪ Loads new programs to the run-time environment
o PCB Modifications
▪ Addition of the page table
o Page Fault
▪ Find page, swap in, select victim, we will not do a swap out
o Task Switch additions
▪ Generate Page Fault and properly assigns addresses
Operating Systems COMP 310 – ECSE 427 McGill University
Vybihal Assignment #3 Page 2 of 6
The boot sequence occurs as the very first task begun by the OS. In our simulation, this
corresponds to the first thing in your main() function. Basically:
int main() {
int error=0;
boot(); // First : actions performed by boot
error = kernel(); // Second: actions performed by kernel
return error;
Notice that the main() function is very simple. The function boot() is a one time call invoked
by the main() at the start to initialize and acquire the resources it needs to run. Place this
function in kernel.c. The function kernel() contains the kernel main function code from
ass#2. This is also in kernel.c.
The boot operation performs many activities, but for us, it will perform only two activities.
1. It assumes that RAM is a global array of size 40 strings. This means it is not instantiated (not
malloced). It assumes that each 4 cells of the array are a frame. At boot time there are no
other programs running except the kernel, so it initializes every cell of the array to NULL.
This indicates that there are no pages of code in RAM.
2. It prepares the Backing Store. This means that it clears the Backing Store. It makes sure
there is nothing in the Backing Store. A Backing Store is a partition of the hard disk. For us,
this will be simulated by a directory. Use the C system() command to delete the old
backing store directory and then create a new directory. Name the directory
BackingStore. Note that the directory is only deleted when you run your kernel. This
means, when you exit your kernel the directory will still be present for the TA to look at.
Create a new C module called memorymanager.c. You may need to create a .h file.
The launcher procedure is associated only with your command-line exec command (not the
run command). Create a function called int launcher(FILE *p) in
memorymanager.c. Place the function call within the exec() function after successfully
opening a file. Important: your exec command can open the same file name multiple times
(unlike assignment #2). The launcher() function returns a 1 if it was successful launching
the program, otherwise it returns 0. Launching a program consists of the following steps:
1. Copy the entire file into the backing store.
2. Close the file pointer pointing to the original file.
3. Open the file in the backing store.
4. Our launch paging technique defaults to loading two pages of the program into RAM when it
is first launched. A page is 4 lines of code. If the program has 4 or less lines of code, then
only one page is loaded. If the program has more than 8 lines of code, then only the first
Operating Systems COMP 310 – ECSE 427 McGill University
Vybihal Assignment #3 Page 3 of 6
two pages are loaded. To do this, implement the following helper functions that exist in the
memorymanager.c file:
a. Function: int countTotalPages(FILE *f)
It returns the total number of pages needed by the program. For example if the
program has 4 lines or less of code, it returns 1. If greater than 4 and less that or
equal to 8, it returns 2. Etc.
b. Function: void loadPage(int pageNumber, FILE *f, int
FILE *f points to the beginning of the file in the backing store. The variable
pageNumber is the desired page from the backing store. The function loads the 4
lines of code from the page into the frame in ram[].
c. Function: int findFrame()
Use the FIFO technique to search ram[] for a frame (not equal to NULL). If one
exists then return its index number, otherwise return -1.
d. Function: int findVictim(PCB *p)
This function is only invoke when findFrame() returns a -1. Use a random
number generator to pick a random frame number. If the frame number does not
belong to the pages of the active PCB (ie. it is not in its page table) then return that
frame number as the victim, otherwise, starting from the randomly selected frame,
iteratively increment the frame number (modulo-wise) until you come to a frame
number not belong to the PCB’s pages, and return that number.
e. Function: int updatePageTable(PCB *p, int pageNumber, int
frameNumber, int victimFrame)
The page tables must also be updated to reflect the changes. If a victim was
selected then the PCB page table of the victim must be updated. We do this once for
the PCB asking for the page fault, and we might do it again for the victim PCB (if
there was one).
p->pageTable[pageNumber] = frameNumber (or = victimFrame).
5. Modify the PCB by adding an array: int pageTable[10]; The index of the array is the
page number. The values stored in the cell is the frame number. The array is size 10
because RAM is size 10 in our simulator (4 lines of code per page). The PC must be the
offset from the beginning of a frame, where offset is the line count starting from zero. Keep
int PC as the pointer to the current position in the process. Add int PC_page,
PC_offset, pages_max. This tracks which page and offset the program is currently at,
and the total number of pages in this program.
A page fault occurs when we run out of lines of code in our frame. Each frame stores a pointer
to 4 lines of code. When the quanta are done a task-switch occurs. The quanta are 2
instructions (as from assignment 2).
The CPU is modified in the following way: IP stays the same, but we add int offset. The
new address is IP+offset. To complete the simulation, increment the offset to the next
address (instruction), offset++. When the offset reaches 4 generate a pseudo-interrupt:
Operating Systems COMP 310 – ECSE 427 McGill University
Vybihal Assignment #3 Page 4 of 6
regardless at what quanta count the program is at, execution stops because the CPU is at the
end of the frame, and the page fault operation must happen.
The page fault operation follows these steps:
1. Determine the next page: PC_page++. (our scripts never loop)
2. If PC_page is beyond pages_max then the program terminates, otherwise we check to
see if the frame for that page exists in the pageTable array.
3. If pageTable[PC_page] is valid then we have the frame number and can do
PC=ram[frame] and reset PC_offset to zero.
4. If pageTable[PC_page] is NOT valid then we need to find the page on disk and update
the PCB page table and possibly the victim’s page table. Start by (a) finding the page in the
backing store, then (b) finding space in RAM (either find a free cell or select a victim), finally
(c) update the page tables, (d) update RAM frame with instructions, and do (e)
PC=ram[frame] and (f) reset PC_offset to zero.
5. Since the PCB was interrupted, it has lost it quanta, even when there were some quanta left.
Store everything back into the PCB. Place the PCB at the back of the Ready queue.
Program termination is like assignment 2 with one addition: the file in the backing store is
Testing your kernel:
This assignment uses the same testfile and scripts as assignment #2.
The TAs will use and modify the provided text file to test your kernel. This text file will contain
the same tests from assignment 2. Since we are not implementing threads, we will not be
testing recursive exec calls. You can also use this file to test your kernel or you can test it the
old fashion way by typing input from the keyboard. To use the provided text file, you will need
to run your program from the OS command line as follows:
$ ./mykernel < testfile.txt
Each line from testfile.txt will be used as input for each prompt your program displays to the
user. Instead of typing the input from the keyboard the program will take the next line from the
file testfile.txt as the keyboard input.
When testfile.txt is exhausted of input the shell command line prompt is displayed to the user
(unless testfile.txt had a quit command, in that case mykernel terminates).
Make sure your program works in the above way.
Operating Systems COMP 310 – ECSE 427 McGill University
Vybihal Assignment #3 Page 5 of 6
Your assignment has a due date plus two late days. If you choose to submit your assignment during the
late days, then your grade will be reduced by -5% per day. Submit your assignment to the assignment
#3 submission box in myCourses. You need to submit the following:
• Make sure to ZIP your assignment submission into a single file called
• A README.TXT file stating what OS you used: or MS Windows and any other
special instructions you think the TA needs to know to run your program.
• Your version of TESTFILE.TXT. This will be you telling the TA that you know for sure that your
program can at least do the following. The TA will run your program with this file and they will
also run it with their own version of the file.
• Submit all the .c file described in the assignment (you may want to create .h files, if so, please
hand those in as well)
• Submit a bash file with the gcc command to compile your program (for mimi).
• Submit the executable (for Windows).
• If you used MS Windows and you used a DLL then upload those as well.
You must submit your own work. You can speak to each other for help but copied code will be handled
as to McGill regulations.
Your assignment is graded out of 20 points and it will follow this rubric:
• The student is responsible to provide a working solution for every requirement.
• The TA grades each requirement proportionally. This means, if the requirement is only 40%
correct (for instance), then the student receives only 40% of the points assigned to that
• Your program must run to be graded. If it does not run, then the student receives zero for the
entire assignment. If your program only runs partially or sometimes, you should still hand it in.
You will receive partial points.
• The TA looks at your source code only if the program runs (correctly or not). The TA looks at
your code to verify that you (A) implemented the requirement as requested, and (B) to check if
the submission was copied.
• Mark breakdown:
o PART 1
▪ Each question is worth 2 points
o PART 2
▪ 1 point – Source file names. The TA must verify that the student created the
specified source files as the only source files in the application. In addition, the
source files must be populated with at least what was specified in the
assignment description for each file. The student is permitted to supply
additional helper functions as they see fit, if it does not hinder the assignment
Operating Systems COMP 310 – ECSE 427 McGill University
Vybihal Assignment #3 Page 6 of 6
▪ 1 points – Modular programming. If the student wrote their application using
modular programming techniques as described in lab 2 (or seen from another
course) then they receive these points.
▪ 1 point – The student’s compiled program must be named mykernel.
▪ 1 point – A fully working assignment 3.
▪ 1 point – Ability to use the shell to input the exec command (and see error
▪ 1 point – The ram[]
▪ 2 points – The PCB
▪ 1 point – The CPU
▪ 2 points – Task Switch
▪ 3 points – Page Fault
▪ 2 point – Backing Store
▪ 1 point – Program Termination
▪ 1 point – Boot operation
▪ 2 points – Program can be tested with the TESTFILE.TXT file
o Final grade calculations
▪ PART 1 + PART 2 = 28 points. Final grade weighted down to 20 points.
You have three weeks for this assignment. Please use that time to your advantage.

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