Brief introduction about unix kernel , levels, file system and so on

1. Advanced Operating Systems
Unit I Introduction to UNIX/Linux Kernel
By,
Ashvini Chaudhari

2. Prerequisites:
1. Working knowledge of C programming.
2. Basic Computer Architecture concepts.
3. Basic algorithms and data structure concepts.

3. Prof.Prasad Sawant
,Assitiant Professor
,Dept. Of CS PCCCS
Chichwad
History
• 1965, AT&T Bell Lab & GE (General Electric Company) &
Project MAC of MIT : Multics
• Ken Thompson & Dennis Ritchie :UNICS
• 1971, UNIX was tried on more powerful machine PDP-1
• 1972, Dennis Ritchie developed ‘C’ language
• 1977, it was first ported (or say installed) on Non-PDP
machine called Interdata 8/32.
• 1982, AT&T itself created UNIX System III (for commercial
purpose), UNIX System IV (only for its internal use)
• Jan 1983, the UNIX System V Last Version
• Due to availability of source code, University California in
Berkeley, developed their own UNIX standard for VAX
machine, which was called as UNIX 4.3 BSD (Berkeley
Software Distribution).
•

4. Prof.Prasad Sawant
,Assitiant Professor
,Dept. Of CS PCCCS
Chichwad
Why Unix is popular
• easy to read, easy to understand, easy to move to other machines.
• It has easy user interface,
• Complex programs can be built by combining existing simple programs
(obviously by using pipe).
• It uses Hierarchical file system, which is easy to maintain & implement.
• In UNIX, everything storable is “file”, means device, terminals, even
directories are also files. UNIX considers “file” as stream of byte.
Thus system programming becomes easy.
• It is Multi-user, Multi-tasking, Multi-threading operating system.
• It hides hardware (i.e. machine architecture) from the programmer & end
user.
• Though system was written in C, it provides environmental support to
other languages like Basic, Fortran, Pascal, ADA, Cobol, Lisp, Prolog,
C++, Java, etc.

5. The Basic System Architecture

6. Prof.Prasad Sawant
,Assitiant Professor
,Dept. Of CS PCCCS
Chichwad
• This interaction (i.e. communication of application
program with the Kernel) is done by a set of functions
known as System Calls. In UNIX System Release V,
there are such 64 system calls, out of which 32 system
calls are sued much frequently.
•

7. Prof.Prasad Sawant
,Assitiant Professor
,Dept. Of CS PCCCS
Chichwad
Features
• The high-level features of UNIX system, mainly includes
3 points :
• The File System.
• The Processing Environment.
• The Building Block Primitives.

8. Sample File System Tree
Absolute Path
Relative Path

9. Prof.Prasad Sawant
,Assitiant Professor
,Dept. Of CS PCCCS
Chichwad
Characteristics
▫ A hierarchical (i.e. tree like) structure.
▫ Consistent treatment to file data (i.e. all files are
stream of bytes).
▫ Ability to create new & to delete old files.
▫ Dynamic growth of files.
▫ The protection of file data (by access permissions)
▫ Peripheral devices & directories are also
considered as files.
•

10. Prof.Prasad Sawant
,Assitiant Professor
,Dept. Of CS PCCCS
Chichwad
• “Files in UNIX are unformatted stream of bytes” and
system treats them with same system calls assessment.
• Thus “copy oldfile newfile” command will
operate on any type of file, whether it is a regular file
or directory file or device file.
•

11. Processing Environment
• A program is an executable file, and a process is an instance of
the program in execution .Many process can execute
simultaneously on UNIX system with no logical limit to their
number ,and many instances of a program can exist
simultaneously in the system .

12. Prof.Prasad Sawant
,Assitiant Professor
,Dept. Of CS PCCCS
Chichwad
• The major 4 system calls for process management are
fork ( ),
• exec ( ),
• wait ( ) and
• exit ( ), where fork ( ) is used to create a new process,
exec ( ) is used to execute the new process, wait ( ) is
used to allow the old process to wait for the completion
of the execution of new process and exit ( ) is used for
exiting the program

13. Prof.Prasad Sawant
,Assitiant Professor
,Dept. Of CS PCCCS
Chichwad
• When exec ( ) executes a child process, the child
process overlaps (or more precisely overlays) the
address space (memory) of parent process. And thus
there remains no memory for parent process to run, thus
parent process must wait for the completion of execution
of child.

14. User Perspective
• the fork system call to create a
new process. The new process,
called the child process, gets a 0
return value from fork and
invokes execl
• The execl call overlays the address
space of the child process with the
file "copy“.
• If the execl call succeeds, it never
returns because the process executes
in a new address space meanwhile,
the process that had invoked fork (the
parent) receives a non-0 return from
the call, calls wait:, suspending its
execution until copy finishes, prints
the message "copy done," and exits

15. User perspective: Shell
• The shell allows three types of commands.
• First, a command can be an executable file that contains object code
produced by compilation of source code (a C program for example).
• Second, a command can be an executable file that contains a sequence of
shell command lines
• The internal commands make the shell a programming language in
addition to a command interpreter and include commands for looping

16. User Perspective :
Building Block Primitives
• redirect I/O
• Processes conventionally have access to three files: they read
from their standard input file, write to their standard output
file, and write error messages to their standard error file.

17. User Perspective :
Building Block Primitives
• Pipe
• The pipe, a mechanism that allows a stream of data to be
passed between reader and writer processes. Processes can
redirect their standard output to a pipe to be read by other
processes that have redirected their standard input to come
from the pipe.
• The data that the first processes write into the pipe is the input
for the second processes. The second processes could also
redirect their output, and so on, depending on programming
need. Again, the processes need not know what type of file
their standard output is;they work regardless of whether
their standard output is a regular file, a pipe, or a device

18. Operating System Functions
▫ Process creation, termination, suspension and
inter-process communication.
▫ CPU scheduling for multiple processes in “time-
shared” manner. (here it is assumed that system
has one CPU)
▫ Memory allocation (either by swapping or paging)
for executing process/processes.
▫ File I/O, by creating file system.
▫ Insulates processes from hardware of the machine.

19. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Assumptions about h/w
• A process (i.e. running program) runs in two modes : - Kernel
Mode & User Mode.
• Kernel Mode : When a process makes a system call, then
it said that, now the process is running in Kernel Mode.
• User Mode : When the process is doing operations like
assignments, comparisons, looping, then it is said that, now the
process is running in User Mode.

20. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Interrupts :
• While handing the interrupts, Kernel gives first preference to
high priority interrupts and then to low priority interrupts.
• The priority of the interrupt is usually decided by the hardware
itself.

21. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Exceptions :
• Exception is an un-excepted event caused by a process. E.g. –
Addressing illegal memory by using pointer, execution of un-
allowed instruction (like dividing by 0) etc.
• Interrupts occur by the events which are external to the process
while exception occurs “in the middle” of the process due to
code instruction.
• It is also said that, interrupt occurs between the two
instructions and exception occurs at the instruction (obviously
at illegal instruction).
• For interrupt, system starts the next instruction normally after
handling the interrupt.
• But for exception, system handles the exception, and then
either re-starts the same instruction

22. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Processor Execution Level
• If a processor execution level is set to “Disk Interrupt
Level” (see in figure), then only “System Clock” &
“Machine Error” interrupts are processed and remaining
all lower level interrupts (including the set level) are
prevented
• During & Critical Activity
• Privileged Instructions :

23. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Memory Management :
• If sufficient memory is available, then Kernel can assign
memory to the program’s process by methods of
memory management, like swapping or demand paging.

24. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Unix System Architecture

25. Assumptions about Hardware
user VS kernel
1. Processes in user mode can access their own instructions and
data but not kernel instructions and data (or those of other
processes). Processes in kernel mode,however, can access
kernel and user addresses.
2. Some machine instructions are privileged and result in an
error when executed in user mode.

26. Concepts of Linux Programming
Files and the File system
1. The file is the most basic and fundamental abstraction in Linux. Linux
follows the everything-is-a-file
2. In order to be accessed, a file must first be opened. Files can be opened for
reading, writing, or both
3. An open file is referenced via a unique descriptor, a mapping from the
metadata associated with the open file back to the specific file itself. Inside
the Linux kernel, this descriptor is handled by an integer (of the C type int)
called the file descriptor, abbreviated fd.

27. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Files and the Filesystem
• Regular files
• What most of us call “files” are what Linux labels regular files. A regular file contains
bytes of data, organized into a linear array called a byte stream. In Linux, no further
organization or formatting is specified for a file.
• Directories and links
• Accessing a file via its inode number is cumbersome so files are always opened
from user space by a name, not an inode number.
• Directories are used to provide the names with which to access files. A directory
acts
• as a mapping of human-readable names to inode numbers. A name and inode pair
is
• called a link. The physical on-disk form of this mapping a simple table, a hash, or
• Whatever is implemented and managed by the kernel code that supports a given
• filesystem. Conceptually, a directory is viewed like any normal file, with the
difference
• that it contains only a mapping of names to inodes. The kernel directly uses this
• mapping to perform name-to-inode resolutions.

28. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Files and the File system
• Hard links
• Conceptually, nothing covered thus far would prevent multiple names
resolving to the same inode. Indeed, this is allowed. When multiple
links map different names to the same inode, we call them hard links.
• Hard links allow for complex filesystem structures with multiple
pathnames pointing to the same data. The hard links can be in the
same directory, or in two or more different directories.

29. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Files and the File system
• Symbolic links
• Hard links cannot span filesystems because an inode number is
meaningless outside of the inode’s own filesystem. To allow
links that can span filesystems, and that are a bit simpler and
less transparent, Unix systems also implement symbolic links
(often shortened to symlinks).
• Symbolic links look like regular files. A symlink has its own
inode and data chunk, which contains the complete pathname
of the linked-to file. This means symbolic links can point
anywhere, including to files and directories that reside on
different filesystems, and even to files and directories that do
not exist. A symbolic link that points to a nonexistent file is
called a broken link.

30. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Users and Groups
• Authorization in Linux is provided by users and groups. Each user is
associated with a unique positive integer called the user ID (uid). Each
process is in turn associated with exactly one uid, which identifies the user
running the process, and is called the process’ real uid. Inside the Linux
kernel, the uid is the only concept of a user. Users themselves, however,
refer to themselves and other users through usernames, not numerical values.
Usernames and their corresponding uids are stored in /etc/passwd, and
library routines map user-supplied usernames to the corresponding uids.

31. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Permissions

32. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Signals
• Signals are a mechanism for one-way asynchronous
notifications. A signal may be sent from the kernel to a
process, from a process to another process, or from a
process to itself. Signals typically alert a process to
some event, such as a segmentation fault, or the user
pressing Ctrl-C.

33. Prof.Prasad Sawant ,Assitiant Professor ,Dept. Of CS PCCCS Chichwad
Inter process Communication
• Allowing processes to exchange information and notify
each other of events is one of an operating system’s
most important jobs.