Characteristics (see Table 2) and Differences in I/O Devices

Table 2: Characteristics of I/O Devices

aspect	variation	example
data transfer mode	character, block	terminal, disk
access method	sequential, random	modem, CD-ROM
transfer schedule	synchronous, asynchronous	tape, keyboard
sharing	dedicated, sharable	tape, keyboard
device speed	latency, seek time, transfer rate, delay between operations
I/O direction	read only, write only, readwrite	CD-ROM, graphics controller, disk

• Application
  • Disk used to store files requires file management software
  • Disk used to store virtual memory pages needs special hardware and software to support it
  • Terminal used by system administrator may have a higher priority
• Complexity of control;
  • Unit of transfer; Data may be transferred as a stream of bytes for a terminal or in larger blocks for a disk
  • Data representation; Encoding schemes
  • Error conditions; Devices respond to errors differently
• Blocking; process suspended until I/O completed
  • Easy to use and understand
  • Insufficient for some needs
• Nonblocking; I/O call returns as much as available
  • user interface, data copy (buffered I/O)
  • implemented via multi-threading code for I/O call
  • returns quickly with count of bytes transferred
• Asynchronous; process runs while I/O executes
  • difficult to use
  • I/O subsystem signals process when I/O completed, e.g., callbacks: pointer to completion code