1. Principles of Operating Systems Abhishek Dubey Daniel Balasubramanian Fall 2014 Slides Based On Power points and book material from William Stallings

2. Operating System Defined 2 common meanings: 1.Entire package consisting of the central software managing a computer’s resources and all of the accompanying software tools 2.The software that manages and allocates computer resources (CPU, RAM, DVD, display, etc) The second is also called a kernel; this is the definition we will use The kernel itself is an executable; on Linux, this executable is located at /boot/vmlinuz

3. Operating System A program that controls the execution of application programs and manages the computer, which is a set of resources for the movement, storage, and processing of data An interface between applications and hardware Main objectives of an OS: Convenience Efficiency Ability to evolve

4. Functions in the same way as ordinary computer software Program, or suite of programs, executed by the processor Frequently relinquishes control and must depend on the processor to allow it to regain control

5. What does a kernel do? Process scheduling Memory management Provides file systems Creates and terminates processes Provides device access Networking Provides a system call interface

6. Process Scheduling The CPUs execute the instructions of programs Linux is a preemptive, multitasking OS: – Multiple processes simultaneously reside in memory – The rules governing which process receives the CPU and for how long are determined by the OS process scheduler. In other words, the kernel decides when to run a process and how long to run it

7. Key Elements of an Operating System

8. Memory Management A process must reside in main memory to run – But RAM is limited, so the OS has to manage how memory is allocated to processes Linux virtual memory management provides 2 things – Processes are isolated from one another and from the kernel – Only part of a process is kept in memory at a time, which allows more processes in RAM simultaneously

9. Provides File Systems File system: an organized collection of regular files and directories A file system provides a way to store, retrieve, update and delete files The kernel must understand a variety of file systems – In Linux, you can see which file-system types the kernel knows about by viewing the file /proc/filesystems

10. Create and terminate processes The kernel loads new programs into memory and gives them resources – Memory, CPU, file access An instance of a running program is called a process Once a process is finished, the kernel reclaims its resources for subsequent reuse

11. Device access The kernel provides a standardized interface to simplify access to devices Also arbitrates access to each device by many processes

12. Networking The kernel transmits and receives network messages for user processes. – Includes routing network packets to the target system

13. Provides a system call interface A system call is the way processes can request the kernel to perform tasks on their behalf Equivalently, a system call is the entry point for a process to request the kernel to perform tasks on its behalf

14. Processor Modes Modern processors have at least 2 CPU modes: user mode and kernel mode – Hardware instructions switch between the two Areas of virtual memory are marked as user or kernel space. – When in user space, can only access memory in user space; kernel mode can access both. Some operations are only available in kernel mode – Halt instruction to stop system, accessing memory management hardware. This hardware design prevents user programs from affecting the kernel and executing certain instructions.

15. The shell Special program that reads commands you type and executes commands in response – Also called a command interpreter User level process – Therefore it is not considered part of the OS kernel – Several different shells: Bourne shell (sh), C shell (csh), Korn shell (ksh), Bourne again shell (bash)

16. Different Architectural Approaches Demands on operating systems require new ways of organizing the OS Microkernel architecture Multithreading Symmetric multiprocessing Distributed operating systems Object-oriented design Different approaches and design elements have been tried:

17. Microkernel Architecture vs the monolithic kernel approach Assigns only a few essential functions to the kernel: – The approach: address spaces interprocess communication (IPC) basic scheduling simplifies implementation provides flexibility is well suited to a distributed environment

18. Key Interfaces Instruction set architecture (ISA) Application binary interface (ABI) Application programming interface (API)

19. Evolution of Operating Systems  A major OS will evolve over time for a number of reasons: Hardware upgrades New types of hardware New services Fixes

20. Evolution of Operating Systems Serial Processing Simple Batch Systems Multiprogrammed Batch Systems Time Sharing Systems

21. Earliest Computers: No operating system programmers interacted directly with the computer hardware Computers ran from a console with display lights, toggle switches, some form of input device, and a printer Users have access to the computer in “series” Problems: Scheduling: – most installations used a hardcopy sign-up sheet to reserve computer time – time allocations could run short or long, resulting in wasted computer time – Setup time – a considerable amount of time was spent just on setting up the program to run

22. Simple Batch Systems: 1950s – 1960s Early computers were very expensive – important to maximize processor utilization Monitor – user no longer has direct access to processor – job is submitted to computer operator who batches them together and places them on an input device – program branches back to the monitor when finished

23. Monitor controls the sequence of events Resident Monitor is software always in memory Monitor reads in job and gives control Job returns control to monitor

24. Processor executes instruction from the memory containing the monitor Executes the instructions in the user program until it encounters an ending or error condition “control is passed to a job” means processor is fetching and executing instructions in a user program “control is returned to the monitor” means that the processor is fetching and executing instructions from the monitor program

25. while the user program is executing, it must not alter the memory area containing the monitor Memory protection for monitor prevents a job from monopolizing the system Timer can only be executed by the monitor Privileged instructions gives OS more flexibility in controlling user programs Interrupts

26. Simple Batch System Overhead Processor time alternates between execution of user programs and execution of the monitor Sacrifices: – some main memory is now given over to the monitor – some processor time is consumed by the monitor – Despite overhead, the simple batch system improves utilization of the computer

27. Processor is often idle even with automatic job sequencing I/O devices are slow compared to processor

28. The processor spends a certain amount of time executing, until it reaches an I/O instruction; it must then wait until that I/O instruction concludes before proceeding

29. There must be enough memory to hold the OS (resident monitor) and one user program When one job needs to wait for I/O, the processor can switch to the other job, which is likely not waiting for I/O

30. Effects on Resource Utilization Table 2.2 Effects of Multiprogramming on Resource Utilization

31. Can be used to handle multiple interactive jobs Processor time is shared among multiple users Multiple users simultaneously access the system through terminals, with the OS interleaving the execution of each user program in a short burst or quantum of computation

32. Table 2.3 Batch Multiprogramming versus Time Sharing

33. Operating Systems are among the most complex pieces of software ever developed Major advances in development include: Processes/Multi Threading Memory management Information protection and security Scheduling and resource management System structure

34. Technique in which a process, executing an application, is divided into threads that can run concurrently Thread dispatchable unit of work includes a processor context and its own data area to enable subroutine branching executes sequentially and is interruptible Process a collection of one or more threads and associated system resources programmer has greater control over the modularity of the application and the timing of application related events

35. Causes of Errors Nondeterminate program operation – program execution is interleaved by the processor when memory is shared – the order in which programs are scheduled may affect their outcome Deadlocks – it is possible for two or more programs to be hung up waiting for each other – may depend on the chance timing of resource allocation and release Improper synchronization – a program must wait until the data are available in a buffer – improper design of the signaling mechanism can result in loss or duplication Failed mutual exclusion – more than one user or program attempts to make use of a shared resource at the same time – only one routine at at time allowed to perform an update against the file