OS Organization

Basic OS Responsibilities 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

Basic OS Functions 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

Basic OS Organization Processor(s) Main Memory Devices Process, Thread & Resource Manager Memory Manager Device File

Device Management 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

Device Management – cont. Device-Independent Part Device-Dependent Device …

Device Management – cont. Application Process File Manager Device Controller Command Status Data Hardware Interface System Interface Device-Independent Device-Dependent

Device Management – cont. read(device, …); 9 1 8b Data System Interface Device Status Table 4 7 Device Handler read driver 2 write driver 6 8a Interrupt Handler 3 Hardware Interface 5 Command Status Data Device Controller

Device Management – cont.

Life Cycle of an I/O Request

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

Process, Thread, and Resource Management … Processor Primary Memory Abstract Resources Multiprogramming Thread Abstraction Process Generic Resource Manager Other

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

Memory Management – cont.

Memory Management – cont.

Memory Management – cont.

File System Management

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

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

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

Performance 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

Typical Computer at 1980 and 2000

Exclusive Use of Resources 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

Exclusive Use of Resources – cont 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

Processor Modes 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

Processor Modes – cont 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

Processor Modes – cont Supervisor mode User 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

Privileged Instructions 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

Exclusive Access to a Resource Process A Supervisor Program A’s Protected Object Processor Process B When A is using processor, register points to its object When B is using processor, register does not point to A’s object

Kernels Kernels Extensions to the OS execute in user mode 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

Trap Instruction S Trusted Code Mode Branch Table User Supervisor 1 2 3 Trusted Code User Supervisor

Trap Instruction – cont. … fork(); fork() { trap N_SYS_FORK() } sys_fork() sys_fork() { /* system function */ return; Kernel Trap Table

Requesting Services from O.S. Two techniques System call Message passing call(…); trap return;

How to Make a System Call 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.

System Call Flow of Control … fork(); fork() { trap N_SYS_FORK() } sys_fork() sys_fork() { /* system function */ return; Kernel Trap Table

How to Make a System Call – cont. 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.

A Thread Performing a System Call User Space Kernel Space fork(); sys_fork() { } Thread

Requesting Services – cont. Two techniques System call Message passing send(…, A, …); receive(…, B, …); receive(…A, …); … send(…, B, …); send/receive

Basic OS Organization Processor(s) Main Memory Devices Process, Thread & Resource Manager Memory Manager Device File

Operating System Examples UNIX MACH MS-DOS Windows NT OS/2 MacOS

UNIX 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

History of UNIX Versions

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

UNIX Design Principles 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.

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

Current Status UNIX is copyrighted – now supported by SCO long litigious story there! Open source variations freebsd www.freebsd.org/availability.html Linux many distributions

Windows NT 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

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

History of Windows NT 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 3.0.

Windows NT Design Principles 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.

Window NT Design Principles - Cont. 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.

Window NT Design Principles - Cont. Compatibility — applications that follow the IEEE 1003.1 (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.

Windows NT Architecture 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.