The document discusses processes in an operating system. It defines a process as a program in execution that includes the program code and current activity. Processes can be in different states like running, ready, waiting, terminated. Each process is represented by a process control block that stores information like process state, program counter, registers, scheduling details. The OS maintains queues for processes in different states. Context switching requires saving and restoring the state of processes.

1. Operating System
Chapter 3: Process

2. Process
A process is a program in execution.
A process is more than the program
code, which is sometimes known as the
text section. It also includes the
current activity, as represented by
the value of the program counter and
the contents of the processor's
registers.

3. Process states
As a process executes, it changes state. The state of a process is defined
in part by the current activity of that process. Each process may be in one
of the following states:
Instructions are being
executed.
The process is waiting
to be assigned to a
processor.
The process is being
created.
The process is waiting
for some event to occur
(such as an I/O
completion or reception
of a signal)
The process has finished
execution.
01
New
02
Running
03
Waiting
04
Ready
05
Terminated

4. Process states
These names are arbitrary, and they vary across operating systems. The states
that they represent are found on all systems, however. Certain operating
systems also more finely delineate process states. It is important to realize
that only one process can be running on any processor at any instant. Many
processes may be ready and limiting, however. The state diagram corresponding
to these states is presented in bellow figure:
Ready Run TerminatedNew
Wait/Block
SuspendWait
Suspend Ready
Schedule/Dispatch
Priority/Time quantum
Completion
I/O completion
Suspend
Resume
I/O completed but still in suspend
Resume
Suspend

5. Process control block (PCB)
Each process is represented in the operating system by a process control
block (PCB)—also called a task control block. A PCB is shown in bellow
figure:

6. Information of PCB
PCB contains many pieces of information associated with a specific process,
including these:
The state may be new, ready, running, waiting, halted, and so on.
The counter indicates the address of the next instruction to
be executed for this process.
The registers vary in number and type, depending on
the computer architecture.They include accumulators,
index registers, stack pointers, and general-purpose
registers, plus any condition-code information. .
This information includes a process priority,
pointers to scheduling queues, and any other
scheduling parameters.
This information may include such information as the
value of the base and limit registers, the page tables,
or the segment tables, depending on the memory
system used by the operating system.
This information includes the amount of CPU and
real time used, time limits, account members, job
or process numbers, and so on.
This information includes the list of I/O devices allocated to the process,
a list of open files, and so on. In brief, the PCB simply serves as the
repository for any information that may vary from process to process.
Process state01
Program counter02
CPU registers03
CPU-scheduling
information
04
Memory-management
information
05
Accounting information06
I/O status
information
07
PCB
PCB
Highlights

7. Process scheduling queue
The OS maintains all PCBs in Process Scheduling Queues. The OS maintains a separate
queue for each of the process states and PCBs of all processes in the same execution
state are placed in the same queue.
The Operating System maintains the following important process scheduling queues −

8. Process scheduling
Partially executed
swapped-out processes
Ready queue CPU
I/O I/O queue I/O queue
Time slice expired
Child
executes
Fork a child
Interrupt
occurs
Wait for an
interrupt
Swap in Swap out
end

9. Classification of scheduling or scheduler
1
2
3
Medium-term
scheduler
The key idea behind a medium-term scheduler is that
sometimes it can be advantageous to remove
processes from memory (and from active contention
for the CPU) and thus reduce the degree of
multiprogramming.
Long-term
scheduler
The long-term scheduler, or job scheduler, selects
processes from this pool and loads them into
memory for execution.
Short-term
scheduler
The short-term scheduler, or CPU scheduler,
selects from among the processes that are
ready to execute and allocates the CPU to
one of them.

10. Kinds of process
I/O-bound processYourText Here
An I/O-bound process is one
that spends more of its time
doing I/O than it spends doing
computations.
A CPU-bound process, in
contrast, generates I/O
requests infrequently, using
more of its time doing
computations.
2
1
CPU-bound process

11. Process states
Switching the CPU to another process requires performing a state save of the
current process and a state restore of a different process. This task is known
as a context switch.
Context-switch times are highly dependent on hardware support. A context switch
here simply requires changing the pointer to the current register set.
process Po operating system process P1
Save into PCBo
Save into PCB1
Save into PCB1
Save into PCBo
.
.
.
.
.
.
Interrupt or system call
Interrupt or system call
idle
idle
idle executing
executing
executing

12. Prepared by:
Ankon Gopal Banik
Front End Developer | WWF
(Web With Friends)
m: +8801771981989
p: 027730402
e: agbanik2081@outlook.com
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Dhaka, Bangladesh
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