CITS2002 - Second Project
A simple simulation of virtual memory
• This project is worth 10% of the marks in the unit.
• The project can be done in groups of two.
• The due date of the project is October 17, 11:59 pm.
• The project description is long, but the coding is simple. We will
discuss the project in the workshops on Fridays.
1 A simple simulation of virtual memory
The aim of this project is to simulate a simple virtual memory system using
an array as the RAM of a hypothetical machine. The project will also require
some C programming skills of using structures and pointers.
We have a computer whose RAM is an array of size 16. It is an array
of pointers. There are 8 page frames in the RAM, each consisting of two
contiguous locations in the array. Hence, the page size of this computer is 2.
The virtual memory of this computer is an array of pointers of size 32
(We will pretend it is on disc, but actually it is an array in the RAM of
our computer). There are 4 processes in this computer, and each proces can
have 4 pages, and obviously all the pages of all the processes cannot be in
the main memory at the same time. Some pages will be in the main memory
and some pages will be in the virtual memory at any time. The processes are
numbered 0 . . . 3. Each process has a page table, which is an integer array,
entry of a process page table indicates whether the page is in RAM or in the
virtual memory (on disc), k if the page is in RAM (k is the frame number,
between 0 . . . 7), and 99 if the page is in disc (99 cannot be a frame number).
You have to define a structure that will consist of three fields, a process
id, a page number of the process, and the last time this page was accessed
if it is in the RAM. Time in the simulation is not real time, rather a time
step. Time increases in simulation steps, as explained below. The simulation
starts (at time 0) by initializing the virtual memory with all the 4 pages of
each process. You have to do the following steps before the simulation starts:
1• Define a structure whose pointer will be stored in each array location
of the RAM and the virtual memory. The structure may look like this:
struct {
int process_id;
int page_num;
int last_accessed;
} memory;
Initialise the process id and page num with the id of the process (a
number between 0 . . . 3) and a page number of that process (a number
between 0 . . . 3). Initialise all last access to 0.
• Create each page and store pointers in the array for the virtual memory.
 Note that the process id and page num of two consecutive array
locations will be the same since each page occupies two array locations.
The simulation starts by reading a file where there is a single line of
integers separated by blanks, for example:
0 2 1 3 3 2 2 0 2 1 0 2 3 0
Each integer indicates a process id. For example, the first number 0 indicates
 that the next page of process 0 has to be brought in from virtual
memory to the RAM. The process table of process 0 and the RAM have to
be updated accordingly. You can keep the content of the virtual memory
unchanged, as that is how virtual memory systems work. Our processes do
not do any computation, they just request the next page and later may write
a page back to virtual memory. You can assume for simplicity that all the
pages are always in the virtual memory and nothing needs to be written
back, as no page is updated by doing any computation. The last accessed
time of a page will be the time step when you brought the page to RAM.
For example, after reading this file, the first (or 0th page of process 0 will
be brought to RAM), the last accessed time of this page will be 0, as the
simulation starts now and time is 0. Time will increase by 1 for each entry
in the file.
The RAM may become full sometime, you have to use the local Least
Recently Used (LRU) algorithm for evicting a page and bringing a new page.
2local means you have to evict the least recently used page of the same
process for accommodating the new page. If there is no page of the process
whose page you want to bring in, use a global LRU policy, evict the page
that is least recently used among all pages in the RAM.
2 Submission
You have to write a C program in a single file called simulation.c, and
compiled as an executable called simulation. It will read two file names from
the command line, in.txt and out.txt. The first file is the one mentioned
above, for reading process ids. The second file is an output file where you
should print the following information at the end of the simulation. Your
submission will be executed as:
simulation in.txt out.txt
• The page tables of the four processes in separate lines. For example,
the page table for process 0 may look like this:
3, 2, 1, 99
This means there are three pages of process 0 in the RAM, pages 0, 1
and 2, in frames 3, 2 and 1, and page 3 is in the disc.
You have to also print the content of the RAM, each location separated
by a ’;’. For example, the RAM may look like this:
0,0,5; 0,0,5; 2,0,1; 2,0,1; etc. (16 entries)
Note that, the first two locations of the RAM stores page 0 of process
0, as each page occupies two array locations of the RAM. Also, this
page was brought to RAM at time step 5.
Amitava Datta
September 2024


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