Chapter Two -- Operating-System Structures -- Lecture Notes

• 2.0 Chapter Objectives
  • Describe OS Services
  • Discuss Design of OS Structure
  • Explain Installation, Customization, and Booting
  
  
• 2.1 Operating-System Services
  • An OS provides: user interfaces, program execution, control of I/O devices, file-system manipulation, process communication facilities, error detection, resource allocation services, accounting, protection, and security.
  
  
• 2.2 User Operating-System Interface
  • An OS must provide users with an interface for making requests for system services - i.e. command interpreter, shell, or GUI.
  • The user-interface process may perform a requested operations directly or may start up a child process to run a separate program which performs the service. The second approach is more modular.
  • The user-interface is not considered fundamental to the study of operating systems. We study mostly what exists from the system call level and down to the interface with the hardware.
  
  
• 2.3 System Calls
  • A system call is like a function call. It is a request for an OS service.
  • Typically the OS is in charge of controlling I/O, so an application that wants to copy or move data in either direction between the RAM and a peripheral device must call upon the OS by making a system call.
  • Typically a computer executes thousands of system calls per second.
  • The run-time support functions in libraries provide the system-call interface for many programming languages - including C and Java.
  • A process can pass parameters to a system call by leaving them in CPU registers, by leaving them in a RAM block (pointed to by an address in a register), or by pushing them onto the runtime stack.
  
  
• 2.4 Types of System Calls
  • There are system calls for process control, file manipulation, device manipulation, information maintenance, and communication.
  • Process control: halt, dump, abort, load, execute, create process, get and set process attributes, wait, wait event, signal event, allocate and free memory.
  • File Management: create file, delete file, open, close, read, write, reposition, get & set file attributes.
  • Device Management: request device, release device, read write, reposition, get & set device attributes, logically attach or detach device.
  • Information Maintenance: get or set time or date, get or set system data (e.g. number of current processes or amount of free memory), get or set process, file, or device attributes.
  • Communications: create or delete communications connection, send or receive message, transfer status information, attach or detach remote devices.
  
  
• 2.5 System Programs
  • Typically there are system programs which organize access to system calls for the convenience of users. There may be system programs for file management, status information, file modification, programming-language support, program loading and execution, and/or communication.
  • Because of the heavy use of API's, system programs, and higher level applications, most computer users are seldom if ever directly concerned with the system-call interface.
  
  
• 2.6 Operating-System Design and Implementation
  • An OS is software and design of the OS is a problem in software engineering. The principles of good software design certainly apply to OS development.
  • Separation of Policy from Mechanism is an important design principle important for flexibility.
  • Typically A few parts of an OS have to be written in assembly language.
  • However modern operating systems are written almost entirely in a high-level programming language such as C. Advantages: code is developed faster; is smaller, easier to understand, debug and change; and easier to port.
  • There was a time when writing more of the code in assembly could be justified because assembly programmers could use 'tricks' that made routines run faster by some constant factor compared with compiled high level code. Modern optimizing compilers have eliminated that advantage almost entirely, although there are still a few bottlenecks in systems that are 'hand-coded' in assembly language in order to achieve 'the absolute maximum' efficiency.
  • As the authors say: "As is true in other systems, major performance improvements in operating systems are more likely to be the result of better data structures and algorithms than of excellent assembly-language code."
  • It is very important to study existing systems in order to learn how to build better systems. For example, this was one of the 'secrets of success' of the developers of Berkeley Unix. It is important to build system-monitoring code into the OS. Such systems produce "traces" of their operation - they write records of all 'interesting' system events in log files which can be analyzed later.
  
  
• 2.7 Operating-System Structure
  • As with any software project, an OS should be designed with appropriate attention to information hiding and modularity.
  • One way to achieve the goals of information hiding and modularity is to create a layered design, in which each layer depends only on the functionality of layers beneath it. A layered system may be built from the bottom up. If there are problems, one can always be certain that they are in the layer currently under construction. The main disadvantages of this approach are first the difficulty of conceiving of layers without any circular dependencies, and second the fact that each layer adds overhead which slows down the operation of the system.
  • There is a microkernel approach to OS design. The idea is to remove all functions possible from the kernel and implement them as system and user-level applications. For example, file systems can be implemented at the user level.
  • Typically the remaining microkernel provides some process control, memory management,and process communication facilities.
  • Microkernel systems tend to be bug-free, secure, easy to modify and easy to port.
  • The main disadvantage seems to be the overhead created by the need for user-level parts of the system to communicate through the kernel.
  • The design style that is currently popular is similar to the microkernel except that it employs dynamically loadable user-level modules which are not constrained to communicate through the kernel.
  
  
• 2.8 Virtual Machines
  • It is difficult, but not impossible, to design an OS so that it makes available several virtual operating system environments on a single computer. For example you can have a microkernel system on which there are running a virtual Windows XP computer, a virtual Linux computer, and a virtual Macintosh. These can operate as if they were three completely independent computers.
  • Designers of operating systems can use the virtual computer idea to test their designs. If a design fails, it won't crash other VM's hosted on the same hardware.
  • One can also test application software on different OS's by using a system that offers all the OS's as virtual machines.
  • The Java virtual machine is another example - here the object is portability - a java program can run safely (relatively) on any computer that has the JVM.
  
  
• 2.9 Operating-System Generation
  
  
• 2.10 System Boot
  • Modern computers have ROM that begins executing automatically at system start-up time.
  • The ROM runs diagnostics, initializes the hardware, and loads the OS from disk. (It might load a secondary boot program, which in turn loads the OS.)