Operating system


Operating System
Interrupt Handler
An interrupt handler, also known as an interrupt service routine (ISR), is a callback subroutine in an operating system or device driverwhose execution is triggered by the reception of an interrupt. Interrupt handlers have a multitude of functions, which vary based on the reason the interrupt was generated and the speed at which the interrupt handler completes its task.
An interrupt handler is a low-level counterpart of event handlers. These handlers are initiated by either hardware interrupts or interrupt instructions in software, and are used for servicing hardware devices and transitions between protected modes of operation such as system calls.
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A routine which is executed when an interrupt occurs. Interrupt handlers typically deal with low-level events in the hardware of a computer system such as a character arriving at aserial port or a tick of a real-time clock. Special care is required when writing an interrupt handler to ensure that either the interrupt which triggered the handler's execution is masked out (inhibitted) until the handler exits, or the handler is re-entrant so that multiple concurrent invocations will not interfere with each other.

If interrupts are masked then the handler must execute as quickly as possible so that important events are not missed. This is often arranged by splitting the processing associated with the event into "upper" and "lower" halves. The lower part is the interrupt handler which masks out further interrupts as required, checks that the appropriate event has occurred (this may be necessary if several events share the same interrupt), services the interrupt, e.g. by reading a character from a
UART and writing it to a queue, and re-enabling interrupts.

The upper half executes as part of a user process. It waits until the interrupt handler has run. Normally the 
operating system is responsible for reactivating a process which is waiting for some low-level event. It detects this by a shared flag or by inspecting a shared queue or by some other synchronisation mechanism. It is important that the upper and lower halves do not interfere if an interrupt occurs during the execution of upper half code. This is usually ensured by disabling interrupts during critical sections of code such as removing a character from a queue.
2.Device driver

In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device.
A driver typically communicates with the device through the computer bus or communications subsystem to which the hardware connects. When a calling program invokes a routine in the driver, the driver issues commands to the device. Once the device sends data back to the driver, the driver may invoke routines in the original calling program. Drivers are hardware-dependent and operating-system-specific. They usually provide the interrupt handling required for any necessary asynchronous time-dependent hardware interface.
A device driver simplifies programming by acting as translator between a hardware device and the applications or operating systems that use it. Programmers can write the higher-level application code independently of whatever specific hardware device.

Device drivers can be abstracted into logical and physical layers. Logical layers process data for a class of devices such as Ethernet ports or disk drives. Physical layers communicate with specific device instances. For example, a serial port needs to handle standard communication protocols such as XON/XOFF that are common for all serial port hardware. This would be managed by a serial port logical layer. However, the physical layer needs to communicate with a particular serial port chip. 16550 UART hardware differs from PL-011. The physical layer addresses these chip-specific variations. Conventionally, OS requests go to the logical layer first. In turn, the logical layer calls upon the physical layer to implement OS requests in terms understandable by the hardware. Inversely, when a hardware device needs to respond to the OS, it uses the physical layer to speak to the logical layer.
In Linux environments, programmers can build device drivers either as parts of the kernel or separately as loadable modules. Makedevincludes a list of the devices in Linux: ttyS (terminal), lp (parallel port), hd (disk), loop (loopback disk device), sound (these include mixer,sequencer, dsp, and audio)...[1]
The Microsoft Windows .sys files and Linux .ko modules contain loadable device drivers. The advantage of loadable device drivers is that they can be loaded only when necessary and then unloaded, thus saving kernel memory.
##A device driver is a program that controls a particular type of device that is attached to your computer. There are device drivers for printers, displays, CD-ROM readers, diskette drives, and so on. When you buy an operating system
, many device drivers are built into the product. However, if you later buy a new type of device that the operating system didn't anticipate, you'll have to install the new device driver. A device driver essentially converts the more general input/output instructions of the operating system to messages that the device type can understand.
Some Windows programs are virtual device drivers. These programs interface with the Windows Virtual Machine Manager. There is a virtual device driver for each main hardware device in the system, including the hard disk drive controller, keyboard, and serial and parallel ports. They're used to maintain the status of a hardware device that has changeable settings. Virtual device drivers handle software interrupts from the system rather than hardware interrupts.
In Windows operating systems, a device driver file usually has a file name suffix of DLL or EXE. A virtual device driver usually has the suffix of VXD.

3.Kernel
In computing, the kernel is the central component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources (the communication between hardware andsoftware components).[1] Usually as a basic component of an operating system, a kernel can provide the lowest-level abstraction layer for the resources (especially processors and I/O devices) that application software must control to perform its function. It typically makes these facilities available toapplication processes through inter-process communication mechanisms and system calls.

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