three subsections we will provide a little general background on IO hardware as

Three subsections we will provide a little general

This preview shows page 76 - 78 out of 140 pages.

three subsections we will provide a little general background on I/O hardware as it relates to programming. 3.1.1. I/O Devices I/O devices can be roughly divided into two categories: block devices and character devices. A block device is one that stores information in fixed-size blocks, each one with its own address. Common block sizes range from 512 bytes to 32,768 bytes. The essential property of a block device is that it is possible to read or write each block independently of all the other ones. Disks are the most common block devices. If you look closely, the boundary between devices that are block addressable and those that are not is not well defined. Everyone agrees that a disk is a block addressable device because no matter where the arm currently is, it is always possible to seek to another cylinder and then wait for the required block to rotate under the head. Now consider a tape drive used for making disk backups. Tapes contain a sequence of blocks. If the tape drive is given a command to read block N, it can always rewind the tape and go forward until it comes to block N. This operation is analogous to a disk doing a seek, except that it takes much longer. Also, it may or may not be possible to rewrite one block in the middle of a tape. Even if it were possible to use tapes as random access block devices, that is stretching the point somewhat: they are not normally used that way. The other type of I/O device is the character device. A character device delivers or accepts a stream of characters, without regard to any block structure. It is not addressable and does not have any seek operation. Printers, network interfaces, mice (for pointing), rats (for psychology lab experiments), and most other devices that are not disk-like can be seen as character devices. This classification scheme is not perfect. Some devices just do not fit in. Clocks, for example, are not block addressable. Nor do they generate or accept character streams. All they do is cause interrupts at well-defined intervals. Still, the model of block and character devices is general enough that it can be used as a basis for making some of the operating system software dealing with I/O device independent. The file system, for example, deals only with abstract block devices and leaves the device-dependent part to lower-level software called device drivers.
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[Page 223] I/O devices cover a huge range in speeds, which puts considerable pressure on the software to perform well over many orders of magnitude in data rates. Fig. 3-1 shows the data rates of some common devices. Most of these devices tend to get faster as time goes on. Figure 3-1. Some typical device, network, and bus data rates. Device Data rate Keyboard 10 bytes/sec Mouse 100 bytes/sec 56K modem 7 KB/sec Scanner 400 KB/sec Digital camcorder 4 MB/sec 52x CD-ROM 8 MB/sec FireWire (IEEE 1394) 50 MB/sec USB 2.0 60 MB/sec XGA Monitor 60 MB/sec SONET OC-12 network 78 MB/sec Gigabit Ethernet 125 MB/sec Serial ATA disk 200 MB/sec SCSI Ultrawide 4 disk 320 MB/sec PCI bus 528 MB/sec 3.1.2. Device Controllers I/O units typically consist of a mechanical component and an electronic component. It is
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