Operating System

PRINCIPLES OF I/O SOFTWARE
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PRINCIPLES OF I/O SOFTWARE

Let us now turn our attention from the I/O hardware to the I/O software. First we will consider the goals of the I/O software and then at the various ways I/O can be done from the point of view of the operating system.

I/O SOFTWARE LAYERS
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I/O SOFTWARE LAYERS

I/O software is normally organized in four layers, as illustrated in Figure 1. Each layer has a well-defined function to perform and a well-defined interface to the adjacent layers. The functionality and interfaces differ from system to system, so the discussion that follows,

Device Drivers
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Device Drivers

Earlier in this section we examined what device controllers do. We saw that each controller has some device registers used to give it commands or some device registers used to read out its status or both. The number of device registers and the nature of the commands vary

Device-Independent I/O Software
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Device-Independent I/O Software

Though some of the I/O software is device specific, other parts of it are device independent. The exact boundary between the drivers and the device-independent software is system (and device) dependent, because some functions that could be done in a device-independent way

User-Space I/O Software
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User-Space I/O Software

Though most of the I/O software is within the operating system, a small portion of it comprises libraries linked together with user programs, and even whole programs running outside the kernel. System calls, including the I/O system calls, are usually made by library procedures.

Disk Arm Scheduling Algorithms
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Disk Arm Scheduling Algorithms

Now we will consider some issues related to disk drivers in general. First, look at how long it takes to read or write a disk block. The time needed is determined by three factors:For most disks, the seek time dominates the other two times, so reducing the mean seek time can improve system performance considerably.

Error Handling
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Error Handling

Disk manufacturers are constantly pushing the limits of the technology by increasing linear bit densities. A track midway out on a 5.25-inch disk has a circumference of about 300 mm. If the track holds 300 sectors of 512 bytes, the linear recording density may be about 5000

Clock Software
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Clock Software

All the clock hardware generate interrupts at known intervals. Everything else involving time must be done by the software, the clock driver. The exact duties of the clock driver vary among operating systems, but generally include most of the following:

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