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Chapter: Operating Systems : I/O System

Requirements For Linux Ssytem Administrator

Key operating-system data structures are managed as objects using common facilities for allocation, reference counting, and security.



v           Hardware-Abstraction Layer


   The HAL is the layer of software that hides hardware chipset differences from upper levels of the operating system. The HAL exports a virtual hardware drivers. Only a single version of each device driver is required for each CPU architecture, no matter what support chips might be present. Device drivers map devices and access them directly, but the chipset-specific details of mapping memory, configuring I/O buses, setting up DMA, and coping with motherboard-specific facilities are all provided by the HAL interfaces.


v           Kernel


   The kernel layer ofWindows has four main responsibilities: thread scheduling, low-level processor synchronization, interrupt and exception handling, and switching between user mode and kernel mode. The kernel is implemented in the C language, using assembly language only where absolutely necessary to interface with the lowest level of the hardware architecture.


Kernel Dispatcher

   The kernel dispatcher provides the foundation for the executive and the subsystems. Most of the dispatcher is never paged out of memory, and its execution is never preempted. Its main responsibilities are thread scheduling and context switching, implementation of synchronization primitives, timer management, software interrupts (asynchronous and deferred procedure calls), and exception dispatching.


ü           Threads and Scheduling


   Like many other modern operating systems, Windows uses processes and threads for executable code. Each process has one or more threads, and each thread has its own scheduling state, including actual priority, processor affinity, and CPU usage information.


   There are six possible thread states: ready, standby, running, waiting, transition, and terminated. Ready indicates that the thread is waiting to run. The highest-priority ready thread is moved to the standby state, which means it is the next thread to run. In a multiprocessor system, each processor keeps one thread in a standby state. A thread is running when it is executing on a processor. It runs until it is preempted by a higher-priority thread, until it terminates, until its allotted execution time (quantum) ends, or until it waits on a dispatcher object, such as an event signaling I/O completion. A thread is in the waiting state when it is waiting for a dispatcher object to be signaled. A thread is in the transition state while it waits for resources necessary for execution; for example, it may be waiting for its kernel stack to be swapped in from disk. A thread enters the terminated state when it finishes execution.


ü           Implementation of Synchronization Primitives


   Key operating-system data structures are managed as objects using common facilities for allocation, reference counting, and security. Dispatcher objects control dispatching and synchronization in the system. Examples of these objects include the following:


        The event object is used to record an event occurrence and to synchronize this occurrence with some action. Notification events signal all waiting threads, and synchronization events signal a single waiting thread.


      The mutant provides kernel-mode or user-mode mutual exclusion associated with the notion of ownership.


     The mutex, available only in kernel mode, provides deadlock-free mutual exclusion.


      The semaphore object acts as a counter or gate to control the number of threads that access a resource.


     The thread object is the entity that is scheduled by the kernel dispatcher. It is associated with a process object, which encapsulates a virtual address space. The thread object is signaled when the thread exits, and the process object, when the process exits.


      The timer object is used to keep track of time and to signal timeouts when operations take too long and need to be interrupted or when a periodic activity needs to be scheduled.



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