Lecture 1 and 2 processes

Rushdi Shams, Dept of CSE, KUET 1
“There are 10 types of people in the world- who understands binary
and who does not”

Process Concept
 Process is a program in execution; process
execution must progress in sequential fashion
 A process includes:
1. Text Section: Program code. Text files have a
program code which is different than program
codes of E-mail programs
2. Program counter: Represents the current state
of the process
3. Stack: Contains temporary data like method
parameters, return addresses
4. Data section: Contains global variables

Process State
 As a process executes, it changes state. This
state is noticed by the program counter.
new: The process is being created
running: Instructions are being executed
waiting: The process is waiting for some event to
occur
ready: The process is waiting to be assigned to a
processor
terminated: The process has finished execution

Diagram of Process State

Process Control Block (PCB)
Information associated with each
process
 Process state
 Program counter
 CPU registers
 CPU scheduling information
 Memory-management information
 I/O status information

Process Control Block (PCB)
6Rushdi Shams, Dept of CSE, KUET

CPU Switch From Process to
Process

Process Scheduling Queues
 Job queue – set of all processes in the
system
 Ready queue – set of all processes residing
in main memory, ready and waiting to execute
 Device queues – set of processes waiting for
an I/O device
 Processes migrate among the various queues

Representation of Process
Scheduling

Schedulers
 Long-term scheduler (or job
scheduler) – selects which processes
should be brought into the ready queue
 Short-term scheduler (or CPU
scheduler) – selects which process
should be executed next and allocates
CPU

Schedulers (Cont.)
 Short-term scheduler is invoked very
frequently
milliseconds
must be fast
 Long-term scheduler is invoked very
infrequently
seconds, minutes
may be slow
 The long-term scheduler controls the
degree of multiprogramming

Mathematics!
 If a process is required 10 milliseconds to be
decided for a GO and it takes 100 milliseconds
to execute then the CPU usage for that
particular task is-
110 ms process requires 10 ms to be decided
1 ms process requires 10/110 ms to be decided
100 ms process requires ??? ms to be decided
9% of the CPU was used to accomplish that
process

Mathematics!
 If a process is required 100 seconds to be
decided for a GO and it takes 1000 seconds to
execute then the CPU usage for that particular
task is-
1100 s process requires 100 s to be decided
1 s process requires 100/1100 s to be decided
100 s process requires ??? s to be decided
9% of the CPU was used to accomplish that
process

Mathematics!
 Which process should be scheduled
by Long term scheduler? Justify your
statement

The Process Model
 Conceptually, each process has its own
virtual CPU
 In reality, the real CPU switches back
and forth from process to process
 This procedure is called
pseudoparallelism.

 A Computer Engineer is making cake
today.
 In his kitchen, he has a book- “how to
bake cake”
 He also has ingredients like- flour,
baking soda, custard powder, etc.

 The book is the program (algorithm)
 The computer engineer is the CPU
 The ingredients are input data
(remember global and local variables??)
 The process is the activity consisting of
our baker reading the book, fetching the
ingredient and baking the cake!

 Now, his youngest son came to the
kitchen
 He alleged to his father that he has been
attacked by cockroaches and they had a
bite on him!
 His father stops baking
 The computer engineer took the first aid
box and aided his son

 The son is the interrupt program
 The first aid box is a different program
 The injection, medicines in the box are
data need to run the program
 And of course, aiding son is more high
prioritized process than baking cakes!
 The engineer after aiding his son comes
back to the kitchen and then again
bakes cakes!

Process Creation
 Three principal reasons for which a
process is created.
1. System initialization
when the OS is booted, several
processes are created. Some of them
are foreground processes, others are
background processes.

 Background processes designed to deal
with incoming mails sleep. Whenever a
mail comes, they wake.
 Processes that stay in background to
handle some activity like email, web
pages, printing are called Daemons.

Process Creation
2. Process created by other process
mostly done by system calls. In
network sharing, one process may
collect the stream of bytes and store
them, other maintains the buffer while
introduced by the first one.

Process Creation
3. Process created by a user request
Whatever you click and do on Windows
platform, with every single click, there
is a possibility to create a new thread!!


Process Creation
 In UNIX, there is only one system call to
create new process: FORK
 This call creates an exact clone of the
calling process.
 After the FORK, two processes- child and
parent have the same memory image,
environment strings, same everything!
 The child process then executes EXECVE
to change its memory image and run a
new program

Process Creation
 The reason for this two step process is
like a reservation in bus ticket. The OS
is alerted that with the procedure.
Secondly, the child process gets plenty
of time to decide whether it’s going to
purchase or decline that ticket!
 Windows does not have such
FORKING. It uses CreateProcess
system call and the child is created
straight away!

Process Termination
 Sooner or later the new process will
terminate-
1. Normal Exit (voluntary)
Simply, when you close a window on
Windows, the process(es) related to it
have their normal exit

Process Termination
2. Error Exit (voluntary)
 How many times your compiler
reported error to you during compiling?
 Screen oriented platforms like
Windows, however, uses a hybrid of
Normal Exit and Error Exit in such
cases!

Process Termination
3. Fatal Error (Non-voluntary)
this error is caused by the process
itself, normally due to the program bug.
Referencing non-existing memory
location or divide by zero problems
cause such errors and process
termination

Process Termination
4. Killed by another process
In UNIX, the system call is KILL. In
Windows, the call is TerminateProcess.
In both cases, the KILLER must have
the proper authorization to kill the
KILLEE.
 In some systems, when a process
terminates, all processes it creates die.
UNIX and Windows-none of them
works in this fashion.

Lecture 1 and 2 processes

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