Operating Systems. Lecture2: System Calls, OS Structure

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1 Operating Systems Lecture2: System Calls, OS Structure

2 System Calls Interface between the OS and user programs is defined by the set of system calls provided by OS Request to kernel Is like making a function call, but the difference is entering the kernel Varies from OS to OS We will be talking about POSIX standard which is adopted by UNIX versions, Linux, and Mac OS X. Win32 API for Windows

3 System Calls Since the actual system call procedure itself is machine dependent often expressed in assembly code A procedure library is provided to let making system calls from C (or other languages) programs libc for C programmers procedure has usually the same name as the system call itself E.g. system call: read Library procedure call from C: count = read(fd, buffer, nbytes) How many bytes successfully read Set to -1 if error in reading File descriptor where the read data will be put how many bytes to read

4 System Calls The 11 steps in making the system call read(fd, buffer, nbytes). Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved : calling program pushes parameters (fd, buffer,nbytes) onto stack (in reverse order for historical reasons) 4. Library procedure call: call read 5. Library procedure puts system call number in a register for OS to get 6. Executes TRAP instruction: switches to kernel to a fixed location. 7. Uses the system call number to index a table containing the address of the system call handler for this system call 8. System call handler runs 9. Returns to the library procedure 10. Return to caller user program 11. Pop the stack (the parameters)

5 System Call Parameter Passing Often, more information is required than simply identity of desired system call E.g. parameters passed Three general methods used to pass parameters to the OS Simplest: pass the parameters in registers In some cases, may be more parameters than registers Parameters stored in a block, or table, in memory, and address of block passed as a parameter in a register This approach taken by Linux and Solaris Parameters placed, or pushed, onto the stack by the program and popped off the stack by the operating system

6 Parameter Passing via Table

10 Linking link( /usr/jim/memo, /user/ast/note ) (a) Two directories before linking /usr/jim/memo to ast s directory. (b) The same directories after linking. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

11 Mounting Name of the block special file for USB drive 0 The directory where it is to be mounted mount( /dev/sdb0, /mnt,0); Read-write or readonly (a) File system before the mount. (b) File system after the mount. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

13 Operating System Structure General-purpose OS is very large program Various ways to structure ones Monolithic Layered Microkernel Client-Server Systems Virtual Machines

14 Monolithic Kernel Most common A single program A collection of procedures linked into a single executable any procedure can call any other no information hiding provides the file system, CPU scheduling, memory management, and other operating-system functions; Disadvantage: a large number of functions for one level Difficult to implement and maintain Performance advantage: minimum overhead for communication within the kernel

15 Modules UNIX/Linux/Solaris are considered monolithic However, they also support loadable extensions (modules) Links in additional modules during boot time or runtime on demand Dynamic linking as the kernel is running

16 Layered Systems Modular approach: The operating system is divided into a number of layers, each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface. Each layer takes care of a well-defined set of functions and provides a clear interface to above layers A layer can use functions of only lower layers Advantage: Simplicity of construction and debugging Difficulty: appropriately defining separation of various layers Disadvantage: added overhead by each layer Each layer calls the next lower E.g. THE, MULTICS

17 Microkernels Objective: Reliability Minimal kernel: Nonessential components removed from the kernel and implemented as system or user programs. Kernel contains minimal process and memory management and communication facility. Provide communication between client programs and various services also running in user-space Communication takes place between user modules using message passing E.g. Mach, MINIX 3 Mac OS X kernel (Darwin) partly based on Mach

18 Microkernel System Structure Application Program File System Device Driver user mode messages messages Interprocess Communication memory managment CPU scheduling kernel mode microkernel hardware

19 Microkernel Benefits: MORE RELIABLE: e.g., running a device driver or file system as a user-space process, a bug in them will NOT crash the entire system. Easier to extend a microkernel, new services added to user space, no or minimal modification to the kernel Detriments: Performance overhead of user space to kernel space communication

20 Client-Server Model A slight variation of microkernel Lowest layer is the microkernel Two classes of user processes servers that provides some service clients which use these services Clients and servers communicate via message passing

21 Client-Server Model: an abstraction Generalization: Clients and servers run on different machines connected by a network. E.g. home PCs as clients, web servers elsewhere

22 Hybrid Systems Most modern operating systems are actually not one pure model Hybrid combines multiple approaches to address performance, security, usability needs Linux and Solaris are monolithic, plus modular for dynamic loading of functionality Windows mostly monolithic, plus retains some microkernel behaviour providing support for user-space different subsystems

23 Hybrid Systems Apple Mac OS X Somewhat layered and microkernel behaviour At the top: Aqua UI plus a set of application environments (such as Cocoa programming environment) Below is kernel consisting of Mach microkernel Memory management IPC (interprocess communication), message passing and BSD Unix kernel parts, Command-line interface Networking File system plus I/O kit and dynamically loadable modules (called kernel extensions)

24 Mac OS X Structure graphical user interface Aqua application environments and services Java Cocoa Quicktime BSD kernel environment Mach BSD I/O kit kernel extensions

25 Virtual Machines (VM) Allows operating systems to run as applications within other operating systems. Vast and growing industry Virtualization OS natively compiled for CPU, running guest OSes also natively compiled Several guest operating systems (execution environment) Giving the illusion that each guest execution environment is running its own private computer

26 Virtualization

27 Virtual Machines (VM) VMware application: ran one or more guest copies of Windows or other OSes, each running applications, all on native Windows host OS VMM (Virtual Machine Manager) runs the guest OSes, manages their resources and protects each guest from others. Use cases involve laptops and desktops running multiple OSes for exploration or compatibility Developing apps for multiple OSes without having multiple systems QA testing applications without having multiple systems

Example of Standard API

16 Example of Standard API System Call Implementation Typically, a number associated with each system call System call interface maintains a table indexed according to these numbers The system call interface