1. Avani R.Vasant V.V.P. Engineering College
Concepts of OS
Some of the Operating Systems Concepts
Process
Deadlocks
Memory Management
Input / Output
Files
Security
The Shell
System Calls
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2. Avani R.Vasant V.V.P. Engineering College
Process
It is a Key Concept in OS.
A Process is a Program in execution.
Each process has its address space. List of memory locations, which process can read and write.
Address Space Contains
The executable program & its data
Its stack
Each process has set of registers
program counters
Stack Pointer
Other H/W registers & Information to run the program.
Ex. Of process
Batch Job
Time-Shared user program
Spooling output to a printer
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3. Avani R.Vasant V.V.P. Engineering College
Process Cont…
Time-Sharing System
OS decides to stop running one process and start another process (CPU Sharing)
Suspended process must later be re-started in the same state, where it was stopped
All the Information must be stored somewhere before suspension
Process may have
several files open for the reading
Associated with each file a pointer giving the current position (no. of bytes to be read next)
All these pointers must be saved before suspension so that a read call can read the proper data at the time of resumption.
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4. Avani R.Vasant V.V.P. Engineering College
Process Cont…
Process Table:
In many OS the Information about process (other than address space) is stored in an OS table called the process table
The Key process mgmt system calls are those dealing with the creation and termination of process.
Process can create another process (Child process)
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5. Avani R.Vasant V.V.P. Engineering College
Process Cont…
Ex: Shell reads command from a terminal
A programmer types a command requesting that a program be compiled
Shell must create new process that will run the compiler
When process has finished it executes a system call to terminated itself.
Processes that cooperate to get some Job done often need to communicate with one another and synchronize their activities
Communication is called IPC ( Inter Process Communication.)
UID User Identification
PID Process Identification (child has the same UID as its parent)
GID Group Identification
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6. Avani R.Vasant V.V.P. Engineering College
Deadlock
When two or more processes are interacting, they sometimes get into a stalemate situation they cannot get out of
Such a situation is called Deadlock.
Deadlock in traffic
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7. Avani R.Vasant V.V.P. Engineering College
Deadlock
Processes in a computer can experience an analogous situation in which they cannot make any progress.
Ex Tape Drive and CD-ROM
Process 1 requests for Tape Drive and gets it.
Process 2 requests for CD-ROM and gets it.
Now, Process1 wants CD-ROM drive so suspended
Process 2 wants Tape Drive so suspended
DEADLOCK FOREVER
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Avani R.Vasant V.V.P. Engineering College

8. Avani R.Vasant V.V.P. Engineering College
Memory Management
Every Computer has Main Memory and it is Central to every Computer
It holds programs which is executing
Repository of quickly accessible data shared by CPU & I/O devices
CPU & I/O reads & Writes data to and from memory
In Simple OS one program in memory if second needs then one has to be to be removed
Development in OS handles multiple programs in memory, programs do not interfere each other so some protection mechanism is required.
Managing & Protecting the Computer Memory
Managing & Protecting the address space of each process
What if Process needs more address space than the memory size?
In early days was not possible, but now Virtual Memory is the answer
Part of process is on disk and part of process is in memory so OS handles the things using different algorithms.
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9. Avani R.Vasant V.V.P. Engineering College
Memory Management
OS is responsible for
Keeps track of which part of memory is currently in use and by whom
Decides which processes to be loaded in the memory when the space is available
Allocate & Deallocate memory space as needed
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10. Avani R.Vasant V.V.P. Engineering College
Input / Output
All Computers have devices for I/P and O/P
Ex. Keyboard, Monitors, Printers etc.
OS manages all this devices.
OS hides the peculiarities of Hardware devices from the users.
Device driver Interface
Drivers specific to each H/W devices
Buffering, caching & spooling
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11. Avani R.Vasant V.V.P. Engineering College
Files
Another Key Concept supported by all OS is file system
To hide everything
System Calls are needed to create files, remove files, read files & write files.
Before a file is read, it must be located on the disk & opened, after that it has been read & it should be closed.
Calls are used to do these things also.
Directory
To provide a place to keep files, A concept used by the OS is the directory as a way of grouping files together
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12. Files Figure 1. A file system for a university department. 6/15/2018
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13. Avani R.Vasant V.V.P. Engineering College
Files
System calls are needed to create & remove directories
Calls are needed to put an existing files in a directory & remove from a directory
Directory entries are directory or files
Process VS File (both can be shown by trees)
Process hierarchies are not deep(2-3) where as files can be
Process hierarchies are short-lived (few mins) where as files/ Dir exists for the years
Parent/Child can only access the process where files can be accessible to wider groups
Pathname Root directory to the file
/faculty/Avani/ppts/os is a Path
If working directory is /faculty/Avani then req path is
/ppts/os
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14. Avani R.Vasant V.V.P. Engineering College
Files
Before a file can be read or written it must be opened and checked for the permissions
If Access is permitted then the system returns a small integer called a File Descriptor to use in subsequent operations
Otherwise error code will be generated.
Mount Call
For Floppies, CD-ROMs , Pen Drives etc
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15. Avani R.Vasant V.V.P. Engineering College
Mount Call
Figure 2. (a) Before mounting, the files on the CD-ROM are not accessible. (b) After mounting, they are part of the file hierarchy.
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Avani R.Vasant V.V.P. Engineering College

16. Special Files Pipe Figure 3 Two processes connected by a pipe.
I/O devices look like files
Block Special Files Ex disks
Character Special Files Ex Printers, Modems etc.
/ dev directory
/dev/lp
Pipe
A pipe is a sort of pseudo file that can be used to connect two processes
Figure 3 Two processes connected by a pipe.
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17. Avani R.Vasant V.V.P. Engineering College
Files
OS is responsible for
Creation & deletion of Files
Creation & deletion of Directories
Support for manipulating Files
Mapping files on secondary storage
Back up of files on storage media
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18. Avani R.Vasant V.V.P. Engineering College
Security
Computer Contains large amount of information that user wants to keep confidential
s, business plans, tax returns etc
OS manages security so that it is only accessible to authorized users
Unix uses 9-bit binary code for protection mechanism
rwxr-x—x
Owner, group, others(everyone)
Protection from users and viruses also to be handled
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19. Avani R.Vasant V.V.P. Engineering College
The Shell
Command Interpreter or called The Shell
It is a primary interface between a user sitting at his terminal and the OS
Many shells exists
Sh, csh, ksh, bash
When user logs in a shell is started up
Shell has the terminal as standard I/P and standard O/P
When u get dollar sign means shell is working
It tell the user that shell is waiting to accept a command
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20. Avani R.Vasant V.V.P. Engineering College
The Shell
Ex date
Shell creates a child process and runs the date program as child
While child is running shell waits for it to terminate
When it finishes, shell returns the prompt & again tries to read the next I/P
Ex date > file
Ex sort < file1 > file2
Ex cat file1 file2 file3 | sort > /dev/lp
Ex cat file1 file2 file3 | sort > /dev/lp &
Runs the job in background and user can get the prompt to work
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21. Avani R.Vasant V.V.P. Engineering College
System Calls
The Interface between the OS and the user program is defined by the set of system calls.
System Calls vary from OS to OS
We will look at the UNIX System Calls
If a process is running a user program in user mode and needs a system services, such as reading data from a file, it has to execute a trap or system call instruction to transfer control to the OS
Then OS checks the parameters and carries out the system call
Then returns control to the instruction following the system call
It is like a procedure call
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22. Avani R.Vasant V.V.P. Engineering College
System Calls
Figure 4. The 11 steps in making the system call read (fd, buffer, nbytes).
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23. Avani R.Vasant V.V.P. Engineering College
Read System Call
Calling program pushes the parameters onto the stack (Step 1-3)
Then actual call to the library procedure is made (Step 4)
Library procedures are in assembly language, typically puts the system call number in register (step 5)
This executes TRAP instruction to switch from user mode to kernel mode. Start execution at a fixed address within the kernel (step 6)
Kernel code starts examining the system call number & dispatches to the correct system call handler via a table with Handlers (step 7)
Then system call handler runs (step 8)
Once it is done control returned back to the user space following the TRAP (step 9)
This procedure runs the user program (step 10)
Then to finish the Job user program cleans up the stack (step 11)
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Avani R.Vasant V.V.P. Engineering College

24. System Calls for Process Management
Some of the major POSIX system calls.
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25. System Calls for File Management (1)
Some of the major POSIX system calls.
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Avani R.Vasant V.V.P. Engineering College

26. System Calls for File Management (2)
Some of the major POSIX system calls.
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Avani R.Vasant V.V.P. Engineering College

27. Miscellaneous System Calls
Some of the major POSIX system calls.
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