It is often possible to separate the two portions to provide a more modular and

It is often possible to separate the two portions to

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I/O units typically consist of a mechanical component and an electronic component. It is often possible to separate the two portions to provide a more modular and general design. The electronic component is called the device controller or adapter. On personal computers, it often takes the form of a printed circuit card that can be inserted into an expansion slot. The mechanical component is the device itself. This arrangement is shown in Fig. 3-2 Figure 3-2. A model for connecting the CPU, memory, controllers, and I/O devices.
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(This item is displayed on page 224 in the print version) [View full size image] The controller card usually has a connector on it, into which a cable leading to the device itself can be plugged. Many controllers can handle two, four, or even eight identical devices. If the interface between the controller and device is a standard interface, either an official ANSI, IEEE, or ISO standard or a de facto one, then companies can make controllers or devices that fit that interface. Many companies, for example, make disk drives that match the IDE (Integrated Drive Electronics) and SCSI (Small Computer System Interface) interfaces. [Page 224] We mention this distinction between controller and device because the operating system nearly always deals with the controller, not the device. Most personal computers and servers use the bus model of Fig. 3-2 for communication between the CPU and the controllers. Large mainframes often use a different model, with specialized I/O computers called I/O channels taking some of the load off the main CPU. The interface between the controller and the device is often low-level. A disk, for example, might be formatted with 1024 sectors of 512 bytes per track. What actually comes off the drive, however, is a serial bit stream, starting with a preamble, then the 4096 bits in a sector, and finally a checksum, also called an Error-Correcting Code (ECC). The preamble is written when the disk is formatted and contains the cylinder and sector number, the sector size, and similar data. The controller's job is to convert the serial bit stream into a block of bytes and perform any error correction necessary. The block of bytes is typically first assembled, bit by bit, in a buffer inside the controller. After its checksum has been verified and the block declared to be free of errors, it can then be copied to main memory.
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The controller for a monitor also works as a bit serial device at an equally low level. It reads bytes containing the characters to be displayed from memory and generates the signals used to modulate the CRT beam. The controller also generates the signals for making a CRT beam do a horizontal retrace after it has finished a scan line, as well as the signals for making it do a vertical retrace after the entire screen has been scanned. On an LCD screen these signals select individual pixels and control their brightness, simulating the effect of the electron beam in a CRT. If it were not for the video controller, the operating system programmer would have to program the scanning explicitly. With the
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