CMPE322 project series is a set of projects that teaches how to construct an operating
system, named BUnix (written in institutional colors of Boğaziçi University J ), from scratch. In the
first project, we learn how to manage processes (tasks) in an operating system for a single CPU (with
a single core). For this purpose, we expect from you to implement a round robin scheduler in C
Round Robin Algorithm
Round robin algorithm is a scheduling algorithm in which the CPU can be interrupted by the
operating system while it executes a process and is forced to switch to the execution of another
process in the ready queue. The interruption occurs after the process consumes its assigned time
called “time slot”. Then, the scheduler sends this process to the end of the ready queue and executes
the process at the head of this queue. For example, assume that the time slot is 100 milliseconds in
BUnix and the operating system has to schedule the following processes using round robin algorithm.
You should have noticed that the “total execution time” of a process is not a given parameter; it
depends on the execution of the program code which is explained in the next section.
PROCESS NAME ARRIVAL TIME (ms) TOTAL EXECUTION TIME (ms)
P1 0 210
P2 50 120
P3 120 240
P4 450 30
Table 1 – Example for BUnix Operating System with four processes
The actions we are expect from your scheduler are:
Time The Scheduler Action The Ready Queue
0 Execute P1 P1
50 Add P2 to the end of the queue P1 P2
100 P1 consumes its time slot; reschedule it and execute P2 P2 P1
120 Add P3 to the end of the queue P2 P1 P3
200 P2 consumes its time slot; reschedule it and execute P1 P1 P3 P2
300 P1 consumes its time slot; reschedule it and execute P3 P3 P2 P1
400 P3 consumes its time slot; reschedule and execute P2 P2 P1 P3
420 P2 is finished; remove it from the queue and execute P1 P1 P3
430 P1 is finished; remove it from the queue and execute P3 P3
450 Add P4 to the end of the queue P3 P4
530 P3 consumes its time slot; reschedule and execute P4 P4 P3
560 P4 is finished; remove it from the queue and execute P3 P3
600 P3 is finished
Table 2 – Round robin scheduler
The Process Structure and the Code Files
As you learned in the lectures, a process contains the program code and its current activity
(context). Normally, a program code is a binary file, which contains the instructions that are
executable by the CPU. However, for the sake of simplicity, we will use text files (*.code) that
represent the executable code of the processes in this project. The structure of the text file is shown
in Table 3. Each row of the text file represents an instruction in which the first column presents the
name of this instruction and the second column is the execution time required by the CPU to
complete this instruction.
INSTRUCTION NAME INSTRUCTION EXECUTION TIME (ms)
Table 3 – A Text Based Program Code File Example
Each of the rows is an atomic operation, which means if the CPU starts to execute this
instruction, it cannot be interrupted to schedule another process until the execution of that
instruction is completed. Assume that the “time slot” is 100 ms in BUnix and we start executing the
process with the program code in Table 3. The scheduler should stop the process after
“instruction_3” (execution time 120ms), add this process to the end of ready queue, and start
executing the first process in the queue. Finally, you might have noticed that the last instruction
name is “exit” which means that this process should be finalized and it should be removed from the
system after the exit instruction is executed.
In addition to the program code, we need to store the current activity (context) of a process
to reschedule this process and continue to execute it with the CPU. Normally, the current activity of a
process contains different elements such as the registers in the CPU and stack of the process. In
BUnix, we just need to store the line number of the last executed instruction before sending this
process to the ready queue.
Don’t get confused by the names of the instructions. These are not real machine codes and YOU WILL
NOT REALLY EXECUTE. The only important thing is that you have to calculate the execution times of
these instructions correctly. So, you can stop a process, send it back to the ready queue, and run
another process at the right time.
The Process Definition File
The process definition file (definition.ini) is a text file that provides the initial information of
the processes in the system. The following table shows the structure of this file.
Process Name Program Code File Arrival Time (ms)
P1 1.code 0
P2 3.code 20
P3 1.code 130
P4 1.code 170
Table 4 – The Process Definition File Structure
Each row represents a process in the system. You should have noticed that process names are
unique, but more than one process may be assigned to the same program code.
The Scheduler Code
This component is a C code that parses the process definition file and schedules these processes
using the round robin algorithm. The design should have at least the following parts:
• The parsing of the “process definition file” and the programming code files.
• The process data structure, which includes at least the programming code address (or the
filename) and the last executed line information.
• A FIFO queue to implement the ready queue.
• A round robin scheduler that maintains the queue.
• An output mechanism that generates a file to show the ready queue whenever it is changed
by the algorithm. The output file should have the format that is shown in Table 5.
Table 5 – The Output File Format
You have to implement your design in the Linux Platform with GCC/G++ Compiler. We
strongly advise you to install Ubuntu which has pre-installed GCC/G++ compiling tools. We will test
your code in this platform and if your compiled code is not compatible with the test platform. You
might be invited for a demo, if necessary.
The following files are given together with the project:
• The process definition file (number of processes and their arrival times will be modified
to test your code).
• Four program code files (number of instructions, their names and their execution times
will be modified to test your code. Only exception is the last instruction name is always
“exit” and its execution time is 10 milliseconds).
• The expected output file.
• Note: The length of the time slot is a constant value in BUnix operating system (100
• Your project should have generated the expected output file for modified process definition
file and program code files. (80%)
• The source code documentation. The documentation depends on your design style but at
least having comments that explain your code is strongly advised. (20%)
• You have to submit your source code and your documentation (if it is not only comments in
your source code) to the Moodle as a zip file.
• The name of the zip file should be in [STUDENT_ID].zip format (such as 2015800054.zip).
• Deadline is 17 November 2017, 23:55.
• Good Luck.