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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.

Disk Formatting
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Disk Formatting

A hard disk is made of a stack of aluminum, alloy, or glass platters 5.25 inch or 3.5 inch in diameter (or even smaller on notebook computers). On each platter is deposited a thin magnetizable metal oxide. After manufacturing, there is no information whatsoever on the disk.

CD-Recordables
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CD-Recordables

In the beginning, the equipment required to produce a master CD-ROM (or audio CD, for that matter) was very expensive. But as usual in the computer industry, nothing stays expensive for long. By the mid 1990s, CD recorders no bigger than a CD player were a common

CD-ROMs
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CD-ROMs

In recent years, optical (as opposed to magnetic) disks have become available. They have much higher recording densities than conventional magnetic disks. Optical disks were originally developed for recording television programs, but they can be put to more esthetic use as

RAID
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RAID

CPU performance has been increasing exponentially over the past decade, roughly doubling every 18 months. Not so with disk performance. In the 1970s, average seek times on minicomputer disks were 50 to 100 msec. Now seek times are slightly under 10 msec. In most technical

DISKS
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DISKS

Let's begin studying some real I/O devices. We will begin with disks, which are conceptually simple, yet very important. After that we will look at clocks, keyboards, and displays.

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.

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

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

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,