CS 537 Notes, Section #22: I/O Devices

I/O Device Characteristics:

• Terminal.
  • One character (8 bits of data or control function) is sent at a time, one interrupt per character.
  • 10-1800 characters per second.
  • Keyboard and display are independent in most systems (no automatic echo, full duplex).
  • Usually handled with one interrupt per character, but sometimes DMA nowadays.
• Raster Printers.
  • Bit mapped (one bit for each dot).
  • Typically about 300 to 1200 dots per inch.
  • Impact, ink jet, or laser printing.
• Tape.
  • 8mm wide by 750 feet long, or 1/2" wide by 2500 feet long..
  • Multi-track, helical, serpentine.
  • Variable length records.
  • Capacities go up to 70 GB/tape.
  • Densities of more than 30 KB/cm.
  • Inter-record gaps ofs about 10mm.
  • Transfer rates up to 5-500 MB/second.
  • Can read or write, but cannot write in middle. Can skip records.
  • Tapes are DMA devices, not one interrupt per character.

• Disk.
  • Draw picture of spindle, platters, read/write-arm.
  • Typically about 1024 cylinders, 20 tracks per cylinder, 32 sectors per track, 512 bytes per sector.
  • This is a total of about 1 GB per disk, or 500,000 double-spaced typewritten pages. Typically sectors are fixed length (512-4096 bytes are popular sizes).
  • Sectors can be read and written individually, or in adjacent groups.
  • Seek time = 5-100 ms, latency is 0-15 ms (drive spins at 3600-7200 RPM). This is the standard for medium term computer storage. Transfer times are about 5-20 MB/second. These times depend on drive, controller, and interface standard (IDE, SCSI).

• CD-ROMs.
  • Current CD-ROMS's can hold up to 720 MB (650 MB is more typical) of data or 74 minutes of uncompressed audio. The data is organized in one (or a few) continuous spirals. The block size is 2K and "tracks" vary in size from 8 to 23 blocks There is extensive error correction information encoded with the data.
  • Seek time = 100-300 ms. Transfer times are about .3-1.5 MB/second (for 2X-10X). 1X (150 KB/second) is needed for audio CD's.
  • Next generation CD (DVD) should store about 5-10 GB and be fully readable and writable. These drives will handle fully integrated data, audio, and video. There are currently several competing standards (ala the VHS vs. Beta competition of the 80's).

Disks and tapes read and write blocks of information rather than single bytes:

• Storage efficiency: give example for tape. At 1600 bpi, 80-byte records use .05 inch, gaps use .6 inch, tape is all gap. However, 8000-byte records use 5 inches so gaps are only about 11% of the tape. In the case of disks, there are a couple of thousand bits of leader at the beginning of each sector used to identify the sector and to synchronize when reading. For 1000-byte sectors, 20% of the disk space is wasted.
• Access efficiency: on disk it typically takes 25ms overhead before transfer begins. The actual transfer is only about 1 microsecond per byte. Thus one-byte transfers take 25ms total time/byte, 1000-byte transfers take about 25 us total time/byte. Re-iterate the importance of eliminating seeks.

Thus most software that deals with disks and other I/O devices attempts to process information in large blocks (usually sequentially).