Internal security on the other hand mainly deals with

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Unformatted text preview: e to memory (in DMA transfer). This interrupt is actually caused by the interrupt handler software. The interrupt handler sends an interrupt signal to the appropriate device driver to wake it up so that it can take the further course of action. 2. Device Drivers. There is a device driver for each type of device connected to the system. For example, a system normally has a disk driver for disks, a printer driver for printers, a tape driver for tapes, and so on. The device driver contains all device-dependent code. A device driver accepts abstract I/O requests from the device-independent software layer above it, and translates it into commands for the appropriate device controller. It then writes these commands and their associated parameters into the controller's registers. 3. Device-independent Software. This layer of software performs the I/O functions that are common to all devices, and provides a uniform interface to the user-level software. It takes care of mapping symbolic device names on to the appropriate device driver. Some I/O devices, such as disks, are sharable in the sense that they can be used by many users at the same time. Other devices, such as tapes and printers, have to be dedicated to a single user until that user has finished using it. The device-independent software layer handles both shared and dedicated devices in such a manner that avoids problems. 4. User-level Software. This layer of software contains the standard I/O system calls in the form of library procedures. The users use these system calls, such as read, write, print, send, and receive for performing the desired I/O operations. SECURITY We saw towards the beginning of this chapter that security in computer systems deals with protecting the various resources and information of a computer system against destruction and unauthorized access. A total approach to computer security involves both external and internal security. External security deals with securing the computer system against exte...
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This document was uploaded on 04/07/2014.

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