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System Calls • System calls provide the interface between a running program and the operating system • Generally available as assembly-language (trap) instructions • Languages defined to replace assembly language for systems programming allow system calls to be made directly • Methods used to pass parameters to the operating system • Pass parameters in registers • Store the parameters in a table in memory, and the table address is passed as a parameter in a register • Push (store) the parameters onto the stack by the program, and pop off the stack by operating system
System Design Goals • User goals – operating system should be convenient to use, easy to learn, reliable, safe, fair, and fast • System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient • System goals and user goals may conflict • efficiency vs. fairness • easy vs. flexible
System Structure – Layered Approach • The operating system is divided into a number of layers or levels, each built on top of lower layers. The lowest layer is the hardware; the highest is the user interface. • Layers are selected such that each uses functions and services of only lower-level layers.
An Operating System Layer Functions implemented at layer M layer M layer M -1 Functions hidden by layer M Functions passed by layer M
Monolithic vs. Microkernel OS • Monolithic • Every thing resides in the kernel • process and memory management • Interrupts, I/O, device drivers, file systems • Networking • Other functions and system calls • The Kernel is big • Everything is part of every application's address space Process in a monolithic OS OS KERNELProcess mngmt Memory mngmt I/O File systems Networking Interrupts All system calls BIG Shared Libraries App1 code App1 data Not used App1 Stack
Monolithic vs. Microkernel OS Memory Server Client File Server Client • Microkernel • The kernel contains code for the most basic OS services • All other OS services are provided by separate processes running in user space • Can modify components while system is running, don’t need to recompile to change a driver or other component • kernel, drivers export function tables to each other so they can interact Microkernel Microkernel Microkernel Microkernel Message from client to server
Mechanisms and Policies • Mechanisms determine how to do something, policies decide what will be done • The separation of policy from mechanism is a very important principle: it allows maximum flexibility if policy decisions are to be changed later • Micro-kernel design supports this separation • Easy to replace system components • Monolithic design encourages combined mechanism and policy at a low level
Unix System Structure • The UNIX OS consists of two parts: • Systems programs • The kernel • Consists of everything below the system-call interface and above the physical hardware • Provides the file system, CPU scheduling, memory management, and other OS functions
Windows Architecture • Interaction among executive components similar to a microkernel system • But not all components run as user level services • Executive components communicate by passing messages through a component called the kernel • Hardware abstraction layer (HAL) maps generic hardware interaction to specific platforms. HAL isolates the kernel from architecture-specific details.
What is a Process • A program in execution • A sequentially executing piece of code that runs on one processing unit of the system • An asynchronous computational activity (i.e., it proceeds at its own pace, independent of another process) • The locus of control of a program in execution • A process is: • an encapsulation of program, data, resources and a virtual processor • controlled and supported by the underlying OS kernel • Which can interact with other processes and I/O devices through the OS kernel
The System View of Processes • Processes are an abstraction • The computer hardware executes instruction • Instructions are grouped into multiple programs in execution by software constructs • Processes do not exist when the computer starts • The instructions initially run set up the data structures needed to implement the process abstraction • The operating system is responsible more maintaining the data structures that implement the process model • The OS needs a system that supports exceptions to do this
In the Beginning • Hardware defines a location from which the initial instruction is fetched after system start-up • This bootstrap program loads the operating system code and initializes operating system data • Then “real” processes can be created • Though the process abstraction is not yet defined when it runs, the bootstrap program can be viewed as the initial process
Address Space • In the process abstraction, an address space is more than just memory • Inter-process communication objects • Files • Other resources • Any entity which can be referenced by a process is part of its address space
Process-OS interaction • Program (text) • Data (global variables) • Stack (temporary data) • Heap (dynamic memory) Inter-process Communication File & I/O Operations System Calls OS/Kernel The operating system can handle all interaction between a process and other system entities; each communication requires a system call
Process-OS interaction • Program (text) • Data (global variables) • Stack (temporary data) • Heap (dynamic memory) Inter-process Communication File & I/O Operations System Calls OS/Kernel The operating system can set up communication channels between a process and other system entities; once the channel is established, further interaction does not require a system call
A UNIX Process a.out Object instructions Static variables Temporary variables Program Text Data Resources Stack Process Status Files Abstract Machine Environment
Listing unix processes • list processes (Solaris 2): • ps — my processes, little detail • ps -l — my processes, more detail • ps -al — all processes, more detail miller.cs: ps -l F UID PID PPID %C PRI NI SZ RSS WCHAN S TT TIME COMMAND 8 1004 16628 16626 0 58 20 1724 1200 ??? S pts/103 0:00 -csh 8 1004 16780 16628 3 58 20 1264 724 ??? S pts/103 0:03 find / 8 1004 16884 16882 0 58 20 1724 1200 ??? S pts/108 0:00 -csh
terminated Process states create dispatch running activate terminate ready inactive pre-empt block suspend waiting wakeup