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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.

Interrupts Revisited
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Interrupts Revisited

We briefly introduced interrupts in "Handheld Computer Operating Systems" but there is more to be said. In a typical personal computer system, the interrupt structure is as illustrated in Figure 1. At the hardware level, interrupts work as follows. When an I/O device has finished

Direct Memory Access (DMA)
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Direct Memory Access (DMA)

Regardless of whether a CPU does or does not have memory-mapped I/O, it needs to address the device controllers to exchange data with them. The CPU can request data from an I/O controller one byte at a time but doing so wastes the CPU's time, so a different scheme, called

Memory-Mapped I/O
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Memory-Mapped I/O

Each controller has a few registers that are used for communicating with the CPU. By writing into these registers, the operating system can command the device to deliver data, accept data, switch itself on or off, or otherwise perform some action. By reading from these registers,

INPUT/OUTPUT
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INPUT/OUTPUT

In addition to providing abstractions such as processes (and threads), address spaces, and files, an operating system also controls all the computer's I/O (Input/Output) devices. It must issue commands to the devices, catch interrupts, and handle errors. It should also provide an

The UNIX V7 File System
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The UNIX V7 File System

The early versions of UNIX had a fairly sophisticated multiuser file system since it was derived from MULTICS. Below we will talk about the V7 file system, the one for the PDP-11 that made UNIX famous. The file system is in the form of a tree starting at the root directory, with the

The MS-DOS File System
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The MS-DOS File System

The MS-DOS file system is the one the first IBM PCs came with. It was the main file system up through Windows 98 and Windows ME. It is still supported on Windows 2000, Windows XP, and Windows Vista, although it is no longer standard on new PCs now except for floppy disks.

EXAMPLE FILE SYSTEMS
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EXAMPLE FILE SYSTEMS

In the next sections we will discuss numerous example file systems, ranging from quite simple to more sophisticated. Since modern UNIX file systems and Windows Vista's native file system are covered in "UNIX" and "Windows Vista" we will not cover those systems here. We will,

Defragmenting Disks
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Defragmenting Disks

When the operating system is at first installed, the programs and files it requires are installed consecutively starting at the beginning of the disk, each one directly following the previous one. All free disk space is in a single contiguous unit following the installed files. On the other

File System Performance
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File System Performance

Access to disk is much slower than access to memory. Reading a 32-bit memory word might take 10 nsec. Reading from a hard disk might proceed at 100 MB/sec, which is four times slower per 32-bit word, but to this must be added 5-10 msec to seek to the track and then wait for