No other process could print anything Instead what is done is to create a

No other process could print anything instead what is

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printer, suppose a process opened it and then did nothing for hours? No other process could print anything. Instead what is done is to create a special process, called a daemon, and a special directory, called a spooling directory. To print a file, a process first generates the entire file to be printed and puts it in the spooling directory. It is up to the daemon, which is the only process having permission to use the printer's special file, to print the files in the
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directory. By protecting the special file against direct use by users, the problem of having someone keeping it open unnecessarily long is eliminated. Spooling is used not only for printers, but also in various other situations. For example, electronic mail usually uses a daemon. When a message is submitted it is put in a mail spool directory. Later on the mail daemon tries to send it. At any given instant of time a particular destination may be temporarily unreachable, so the daemon leaves the message in the spool with status information indicating it should be tried again in a while. The daemon may also send a message back to the sender saying delivery is delayed, or, after a delay of hours or days, saying the message cannot be delivered. All of this is outside the operating system. [Page 237] Figure 3-8 summarizes the I/O system, showing the layers and principal functions of each layer. Starting at the bottom, the layers are the hardware, interrupt handlers, device drivers, device-independent software, and the user processes. Figure 3-8. Layers of the I/O system and the main functions of each layer. [View full size image] The arrows in Fig. 3-8 show the flow of control. When a user program tries to read a block from a file, for example, the operating system is invoked to carry out the call. The device-independent software looks for it in the buffer cache, for example. If the needed block is not there, it calls the device driver to issue the request to the hardware to go get it from the disk. The process is then blocked until the disk operation has been completed. When the disk is finished, the hardware generates an interrupt. The interrupt handler is run to discover what has happened, that is, which device wants attention right now. It then extracts the status from the device and wakes up the sleeping process to finish off the I/O request and let the user process continue.
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3.1. Principles of I/O Hardware Different people look at I/O hardware in different ways. Electrical engineers look at it in terms of chips, wires, power supplies, motors, and all the other physical components that make up the hardware. Programmers look at the interface presented to the softwarethe commands the hardware accepts, the functions it carries out, and the errors that can be reported back. In this book we are concerned with programming I/O devices, not designing, building, or maintaining them, so our interest will be restricted to how the hardware is programmed, not how it works inside. Nevertheless, the programming of many I/O devices is often intimately connected with their internal operation. In the next
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