 Create an abstract machine environment  A nicer environment than bare hardware  Consists of multiple, autonomous abstract components  Components may be in use concurrently  Coordinate the use of the components  Resource manager  Manage according to the policies of the machine’s administrator 2

Exact set of functions required depends on engineering and marketing choices but each function falls in one of these categories:  Device management  Process, thread, and resource management  Memory management  File management 3

4 Processor(s)Main MemoryDevices Process, Thread & Resource Manager Memory Manager Device Manager File Manager

 OS uses policies chosen by designer or system administrator to manage  Allocation  Isolation  Sharing  Device manager in two parts  Device independent – provides unified interface  Device dependent – device driver: handles those aspects unique to a device 5

6 Device-Independent Part Device-Independent Part Device-Dependent Part Device-Dependent Part Device … Device-Dependent Part Device-Dependent Part Device-Dependent Part Device-Dependent Part

7 Application Process Application Process File Manager File Manager Device Controller Command Status Data Hardware Interface System Interface Device-Independent Device-Dependent

8 read(device, …); Data Device Controller Command Status Data read driver write driver Hardware Interface System Interface Device Status Table Device Handler Device Handler Interrupt Handler Interrupt Handler 6 7 8a 8b 9 3

9

10

11 Protection Deadlock Synchronization Process Description Process Description Resource Manager Resource Manager Resource Manager Resource Manager Resource Manager Resource Manager Process Mgr Scheduler CPU Other H/W

12 … Processor Primary Memory Abstract Resources Multiprogramming Thread Abstraction Thread Abstraction Process Abstraction Process Abstraction Generic Resource Manager Generic Resource Manager Other

13 Primary Memory Process Manager Process Manager Block Allocation Block Allocation Virtual Memory Virtual Memory Isolation & Sharing Isolation & Sharing Storage Devices

14

15

16

17

18 Scheduler IPC Process/Thread Admin Synchronization Memory Allocation Virtual Memory File Management Device Management Resource Management Deadlock Management Protection Mechanisms Interrupt Handler

19 Application Software Application Software Other System Software Other System Software Other OS Functions Kernel Functions Application Software Application Software Other System Software Other System Software Other OS Functions Nucleus Functions Application Software Application Software Other System Software Other System Software Other OS Functions Nucleus Functions Application Software Application Software Other System Software Other System Software Other OS Functions Skeletal Nucleus Nucleus Functions (a) Monolithic (b) Modular (microkernel) (c) Extensible(d) Layered

 Two recurring issues in design  Performance  Exclusive use of resources  Three basic implementation mechanisms  Processor modes  Kernels  Method of invoking system service 20

 Must be as efficient as possible in use of resources (especially processor and memory)  Every design issue MUST be evaluated wrt its contribution to functionality of system AND its impact on performance  Seek to minimize “overhead” of the system wrt the applications running on the system  Increased hardware performance does allow added functionality in spite of inefficiency 21

22

 Multiprogramming  resource sharing  Therefore, need software-controlled resource isolation  Security policy : Sharing strategy chosen by computer’s owner  Protection mechanism : Tool to implement a family of security policies 23

 Security depends on correct operation of software  trusted vs. untrusted software  Need to insure that untrusted software cannot change trusted software  Can limit the function of the OS  Guiding a manned spaceship  Managing a nuclear reactor 24

 Mode bit: Supervisor or User mode  Some processors may have more than one mode  Supervisor mode (privileged, protected)  Can execute all machine instructions  Can reference all memory locations  User mode  Can only execute a subset of instructions  Can only reference a subset of memory locations 25

 Ensures proper operation of a computer system  Protect the operating system and all other programs and their data from any malfunctioning program  Protection is needed for any shared resource  Trusted OS software runs in supervisor mode  All other software runs in user mode 26

 Supervisor mode  all instructions are legal  all addresses are absolute physical addresses (base and bound are not used)  User mode  instructions that modify control registers are illegal  all addresses must be less than bound and have base added to them 27

 Instructions that can only be executed in the supervisor mode are called supervisor, privileged, or protected instructions  I/O instructions are privileged instructions  A user program in user mode cannot perform its own I/O  Instruction to change the mode is a privileged instruction  Instruction to set the halt flag is a privileged instruction 28

 When A is using processor, register points to its object  When B is using processor, register does not point to A’s object 29 Process A Supervisor Program Supervisor Program A’s Protected Object A’s Protected Object Processor Process B

 Kernels  Critical parts of OS that run in supervisor mode  Have access to other parts of the kernel  Trusted software  Extensions to the OS execute in user mode  The trap instruction is used to switch from user to supervisor mode, entering the OS 30

31 S Mode Trusted Code trap UserSupervisor Branch Table 2 31

32 … fork(); … fork() { … trapN_SYS_FORK() … } sys_fork() sys_fork() { /* system function */ … return; } Kernel Trap Table

 Two techniques  System call  Message passing 33 call(…); trap return;

 For the system  through a trap instruction which causes an interrupt  Hardware saves PC and current status information  Hardware changes mode to system mode  Hardware loads PC from system call interrupt vector location.  Execute the system call interrupt handler  return from the handler, restores PC and other saved status information  User process continues. 34

35 … fork(); … fork() { … trapN_SYS_FORK() … } sys_fork() sys_fork() { /* system function */ … return; } Kernel Trap Table

 Parameter passing  Through registers  System call number passed through register  Parameters are passed through registers  Returned value is also passed through a register to C/C++  Through a table in memory  Pass the address of the table in a register  Through the stack  Push the parameters on the stack by the user program  Pop the parameters off the stack by the O.S. 36

37 User SpaceKernel Space fork(); sys_fork() { } Thread

 Two techniques  System call  Message passing 38 send(…, A, …); receive(…, B, …); receive(…A, …); … send(…, B, …); send/receive

39 Processor(s)Main MemoryDevices Process, Thread & Resource Manager Memory Manager Device Manager File Manager

 UNIX  MACH  MS-DOS  Windows NT  OS/2  MacOS 40

 One of the most popular operating systems  First version released in 1969  By Ken Thompson & Dennis Ritchie at Bell Labs  ACM Turing Award – 1983  National Medal of Technology – 1999  Japan Prize for Information and Communications – 2011  Widely used in universities and research organizations  Time-sharing system  Supports multiple processes  Disk files and I/O devices are treated similarly 41

42

 Written in a high-level language.  Distributed in source form.  Provided powerful operating-system primitives on an inexpensive platform.  Small size, modular, clean design. 43

 Designed to be a time-sharing system  Has a simple standard user interface that can be replaced.  File system with multilevel tree-structured directories.  Files are supported by the kernel as unstructured sequences of bytes.  Supports multiple processes; a process can easily create new processes.  High priority given to making system interactive, and providing facilities for program development. 44

45 Libraries Commands Device Driver Interactive User Application Programs Application Programs OS System Call Interface Device Driver Driver Interface … Monolithic Kernel Module Process Management Memory Management File Management Device Mgmt Infrastructure Trap Table …

 UNIX is copyrighted – now supported by SCO  long litigious story there!  Open source variations  freebsd   Linux  many distributions 46

 32-bit preemptive multitasking operating system for modern microprocessors.  Key goals for the system:  portability  security  POSIX compliance  multiprocessor support  extensibility  international support  compatibility with MS-DOS and MS-Windows applications.  Uses a micro-kernel architecture.  Available in two versions, Windows NT Workstation and Windows NT Server.  In 1996, more NT server licenses were sold than UNIX licenses 47

48 Processor(s)Main MemoryDevices Libraries Process Subsystem User Subsystem Hardware Abstraction Layer NT Kernel NT Executive I/O Subsystem T T T T T T TT T Process Management Memory Management File Management Device Mgmt Infrastructure

 In 1988, Microsoft decided to develop a “new technology” (NT) portable operating system that supported both the OS/2 and POSIX APIs.  Originally, NT was supposed to use the OS/2 API as its native environment but during development NT was changed to use the Win32 API, reflecting the popularity of Windows

 Extensibility — layered architecture.  NT executive, which runs in protected mode, provides the basic system services.  On top of the executive, several server subsystems operate in user mode.  Modular structure allows additional environmental subsystems to be added without affecting the executive. 50

 Portability — NT can be moved from one hardware architecture to another with relatively few changes.  Written in C and C++.  Processor-dependent code is isolated in a dynamic link library (DLL) called the “hardware abstraction layer” (HAL).  Reliability — NT uses hardware protection for virtual memory, and software protection mechanisms for operating system resources. 51

 Compatibility — applications that follow the IEEE (POSIX) standard can be complied to run on NT without changing the source code.  Performance — NT subsystems can communicate with one another via high- performance message passing.  Preemption of low priority threads enables the system to respond quickly to external events.  Designed for symmetrical multiprocessing.  International support — supports different locales via the national language support (NLS) API. 52

 Layered system of modules.  Protected mode — HAL, kernel, executive.  User mode — collection of subsystems  Environmental subsystems emulate different operating systems.  Protection subsystems provide security functions. 53