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Operating System Concepts – 9th^ Edition Silberschatz, Galvin and Gagne ©
Chapter 2: Operating-System
Structures
Operating System Concepts – 9th^ Edition 2.2 Silberschatz, Galvin and Gagne ©
Chapter 2: Operating-System Structures
(^) Operating System Services (^) User Operating System Interface (^) System Calls (^) Types of System Calls (^) System Programs (^) Operating System Design and Implementation (^) Operating System Structure (^) Operating System Debugging (^) Operating System Generation (^) System Boot Operating System Concepts – 9th^ Edition 2.3 Silberschatz, Galvin and Gagne ©
Objectives
(^) To describe the services an operating system provides to users, processes, and other systems (^) To discuss the various ways of structuring an operating system (^) To explain how operating systems are installed and customized and how they boot Operating System Concepts – 9th^ Edition 2.4 Silberschatz, Galvin and Gagne ©
Operating System Services
(^) Operating systems provide an environment for execution of programs and services to programs and users (^) One set of operating-system services provides functions that are helpful to the user: (^) User interface - Almost all operating systems have a user interface (UI). (^) Varies between Command-Line (CLI), Graphics User Interface (GUI), Batch (^) Program execution - The system must be able to load a program into memory and to run that program, end execution, either normally or abnormally (indicating error) (^) I/O operations - A running program may require I/O, which may involve a file or an I/O device
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Operating System Services (Cont.)
(^) One set of operating-system services provides functions that are helpful to the user (Cont.): (^) File-system manipulation - The file system is of particular interest. Programs need to read and write files and directories, create and delete them, search them, list file Information, permission management. (^) Communications – Processes may exchange information, on the same computer or between computers over a network (^) Communications may be via shared memory or through message passing (packets moved by the OS) (^) Error detection – OS needs to be constantly aware of possible errors (^) May occur in the CPU and memory hardware, in I/O devices, in user program (^) For each type of error, OS should take the appropriate action to ensure correct and consistent computing (^) Debugging facilities can greatly enhance the user’s and programmer’s abilities to efficiently use the system Operating System Concepts – 9th^ Edition 2.6 Silberschatz, Galvin and Gagne ©
Operating System Services (Cont.)
(^) Another set of OS functions exists for ensuring the efficient operation of the system itself via resource sharing (^) Resource allocation - When multiple users or multiple jobs running concurrently, resources must be allocated to each of them (^) Many types of resources - CPU cycles, main memory, file storage, I/O devices. (^) Accounting - To keep track of which users use how much and what kinds of computer resources (^) Protection and security - The owners of information stored in a multiuser or networked computer system may want to control use of that information, concurrent processes should not interfere with each other (^) Protection involves ensuring that all access to system resources is controlled (^) Security of the system from outsiders requires user authentication, extends to defending external I/O devices from invalid access attempts Operating System Concepts – 9th^ Edition 2.7 Silberschatz, Galvin and Gagne ©
A View of Operating System Services
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User Operating System Interface - CLI
CLI or command interpreter allows direct command entry (^) Sometimes implemented in kernel, sometimes by systems program (^) Sometimes multiple flavors implemented – shells (^) Primarily fetches a command from user and executes it (^) Sometimes commands built-in, sometimes just names of programs (^) If the latter, adding new features doesn’t require shell modification
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Example of System Calls
(^) System call sequence to copy the contents of one file to another file Operating System Concepts – 9th^ Edition 2.14 Silberschatz, Galvin and Gagne ©
Example of Standard API
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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 invokes the intended system call in OS kernel and returns status of the system call and any return values (^) The caller need know nothing about how the system call is implemented (^) Just needs to obey API and understand what OS will do as a result call (^) Most details of OS interface hidden from programmer by API (^) Managed by run-time support library (set of functions built into libraries included with compiler) Operating System Concepts – 9th^ Edition 2.16 Silberschatz, Galvin and Gagne ©
API – System Call – OS Relationship
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System Call Parameter Passing
(^) Often, more information is required than simply identity of desired system call (^) Exact type and amount of information vary according to OS and call (^) 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 (^) Block and stack methods do not limit the number or length of parameters being passed Operating System Concepts – 9th^ Edition 2.18 Silberschatz, Galvin and Gagne ©
Parameter Passing via Table
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Types of System Calls
(^) Process control (^) create process, terminate process (^) end, abort (^) load, execute (^) get process attributes, set process attributes (^) wait for time (^) wait event, signal event (^) allocate and free memory (^) Dump memory if error (^) Debugger for determining bugs, single step execution (^) Locks for managing access to shared data between processes Operating System Concepts – 9th^ Edition 2.20 Silberschatz, Galvin and Gagne ©
Types of System Calls
(^) File management (^) create file, delete file (^) open, close file (^) read, write, reposition (^) get and set file attributes (^) Device management (^) request device, release device (^) read, write, reposition (^) get device attributes, set device attributes (^) logically attach or detach devices
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Example: MS-DOS
(^) Single-tasking (^) Shell invoked when system booted (^) Simple method to run program (^) No process created (^) Single memory space (^) Loads program into memory, overwriting all but the kernel (^) Program exit -> shell reloaded At system startup running a program Operating System Concepts – 9th^ Edition 2.26 Silberschatz, Galvin and Gagne ©
Example: FreeBSD
(^) Unix variant (^) Multitasking (^) User login -> invoke user’s choice of shell (^) Shell executes fork() system call to create process (^) Executes exec() to load program into process (^) Shell waits for process to terminate or continues with user commands (^) Process exits with: (^) code = 0 – no error (^) code > 0 – error code Operating System Concepts – 9th^ Edition 2.27 Silberschatz, Galvin and Gagne ©
System Programs
(^) System programs provide a convenient environment for program development and execution. They can be divided into: (^) File manipulation (^) Status information sometimes stored in a File modification (^) Programming language support (^) Program loading and execution (^) Communications (^) Background services (^) Application programs Operating System Concepts – 9th^ Edition 2.28 Silberschatz, Galvin and Gagne ©
System Programs
(^) Provide a convenient environment for program development and execution (^) Some of them are simply user interfaces to system calls; others are considerably more complex (^) File management - Create, delete, copy, rename, print, dump, list, and generally manipulate files and directories (^) Status information (^) Some ask the system for info - date, time, amount of available memory, disk space, number of users (^) Others provide detailed performance, logging, and debugging information (^) Typically, these programs format and print the output to the terminal or other output devices (^) Some systems implement a registry - used to store and retrieve configuration information
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System Programs (Cont.)
(^) File modification (^) Text editors to create and modify files (^) Special commands to search contents of files or perform transformations of the text (^) Programming-language support - Compilers, assemblers, debuggers and interpreters sometimes provided (^) Program loading and execution- Absolute loaders, relocatable loaders, linkage editors, and overlay-loaders, debugging systems for higher-level and machine language (^) Communications - Provide the mechanism for creating virtual connections among processes, users, and computer systems (^) Allow users to send messages to one another’s screens, browse web pages, send electronic-mail messages, log in remotely, transfer files from one machine to another Operating System Concepts – 9th^ Edition 2.30 Silberschatz, Galvin and Gagne ©
System Programs (Cont.)
(^) Background Services (^) Launch at boot time (^) Some for system startup, then terminate (^) Some from system boot to shutdown (^) Provide facilities like disk checking, process scheduling, error logging, printing (^) Run in user context not kernel context (^) Known as services, subsystems, daemons (^) Application programs (^) Don’t pertain to system (^) Run by users (^) Not typically considered part of OS (^) Launched by command line, mouse click, finger poke Operating System Concepts – 9th^ Edition 2.31 Silberschatz, Galvin and Gagne ©
Operating System Design and Implementation
(^) Design and Implementation of OS not “solvable”, but some approaches have proven successful (^) Internal structure of different Operating Systems can vary widely (^) Start the design by defining goals and specifications (^) Affected by choice of hardware, type of system (^) User goals and System goals (^) User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast (^) System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient Operating System Concepts – 9th^ Edition 2.32 Silberschatz, Galvin and Gagne ©
Operating System Design and Implementation (Cont.)
(^) In an OS, it is important to separate policy from mechanism to keep the system flexible and adaptable to changes. Policy: What will be done? Mechanism: How to do it? (^) Policy Defines what should be done (^) A user can only run a program for a maximum of 2 hours. (^) Mechanism Defines how it will be done (^) A timer is implemented to track execution time and stop the program if it exceeds 2 hours. Why Separate Policy from Mechanism? (^) Flexibility – Policies may change over time, but mechanisms should remain reusable. (^) Better Design – Helps keep the OS modular and easier to maintain. (^) Scalability – Different systems can implement different policies without modifying core mechanisms.
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Traditional UNIX System Structure
Beyond simple but not fully layered Operating System Concepts – 9th^ Edition 2.38 Silberschatz, Galvin and Gagne ©
Layered Approach
(^) The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface. (^) With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers Operating System Concepts – 9th^ Edition 2.39 Silberschatz, Galvin and Gagne ©
Microkernel System Structure
(^) Moves as much from the kernel into user space (^) Mach example of microkernel (^) Mac OS X kernel (Darwin) partly based on Mach (^) Communication takes place between user modules using message passing (^) Benefits: (^) Easier to extend a microkernel (^) Easier to port the operating system to new architectures (^) More reliable (less code is running in kernel mode) (^) More secure (^) Detriments: (^) Performance overhead of user space to kernel space communication
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Modules
(^) Many modern operating systems implement loadable kernel modules (^) Uses object-oriented approach (^) Each core component is separate (^) Each talks to the others over known interfaces (^) Each is loadable as needed within the kernel (^) Overall, similar to layers but with more flexible (^) Linux, Solaris, etc Operating System Concepts – 9th^ Edition 2.42 Silberschatz, Galvin and Gagne ©
Solaris Modular Approach
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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 kernels in kernel address space, so monolithic, plus modular for dynamic loading of functionality (^) Windows mostly monolithic, plus microkernel for different subsystem personalities (^) Apple Mac OS X hybrid, layered, Aqua UI plus Cocoa programming environment (^) Below is kernel consisting of Mach microkernel and BSD Unix parts, plus I/O kit and dynamically loadable modules (called kernel extensions)
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Performance Tuning
(^) Improve performance by removing bottlenecks (^) OS must provide means of computing and displaying measures of system behavior (^) For example, “top” program or Windows Task Manager Operating System Concepts – 9th^ Edition 2.50 Silberschatz, Galvin and Gagne © DTrace (^) DTrace tool in Solaris, FreeBSD, Mac OS X allows live instrumentation on production systems (^) Probes fire when code is executed within a provider, capturing state data and sending it to consumers of those probes (^) Example of following XEventsQueued system call move from libc library to kernel and back Operating System Concepts – 9th^ Edition 2.51 Silberschatz, Galvin and Gagne © Dtrace (Cont.) (^) DTrace code to record amount of time each process with UserID 101 is in running mode (on CPU) in nanoseconds Operating System Concepts – 9th^ Edition 2.52 Silberschatz, Galvin and Gagne ©
Operating System Generation
(^) Operating systems are designed to run on any of a class of machines; the system must be configured for each specific computer site (^) SYSGEN program obtains information concerning the specific configuration of the hardware system (^) Used to build system-specific compiled kernel or system- tuned (^) Can general more efficient code than one general kernel
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System Boot
(^) When power initialized on system, execution starts at a fixed memory location (^) Firmware ROM used to hold initial boot code (^) Operating system must be made available to hardware so hardware can start it (^) Small piece of code – bootstrap loader, stored in ROM or EEPROM locates the kernel, loads it into memory, and starts it (^) Sometimes two-step process where boot block at fixed location loaded by ROM code, which loads bootstrap loader from disk (^) Common bootstrap loader, GRUB, allows selection of kernel from multiple disks, versions, kernel options (^) Kernel loads and system is then running Operating System Concepts – 9th^ Edition^ Silberschatz, Galvin and Gagne ©
End of Chapter 2
