CS 2734 Computer Organization II

Read pages 49-54 (Section 2.2.5) of Tanenbaum

Characteristics of secondary storage devices:

• Tapes
  • Strictly sequential access (on the order of minutes to get to correct position).
  • Medium of choice for backups.
  • Each linear bit consists of 8 data bits plus parity.
  • Typical densities 1600 bpi.
  • Typical length 2400 feet.
  • Each physical write to tape is followed by a gap which is typically .25 inches in length.
  • Question: If you write 80 character physical records, what percentage of tape is data.

• Magnetic Disks
  • Information is organized on concentric circles called tracks.
  • If more than one platter, the tracks are organized into cylinders.
  • Tracks are divided into sectors. Transfers start at beginning of sectors.
  • Typical capacity 2 to 9 gigabytes.
  • The three key times:
    • Seek time -- position head at correct cylinder (Typical time 10ms)
    • Rotational latency - wait for disk to rotate to correct position under head (Typical time 10 ms - Note 3600 rpm = 16.67 ms).
    • Transfer rate (Fast SCSI 2 Bbytes/Sec) Raids give 20 MBytes/second.)

• Floppy Disks
  • See Figure 2-17 for typical times.
  • Heads actually touch the disk
  • Spin down when not being used, so it requires a 1/2 second spin up time when being accessed.

• CD Rom
  • Constant linear velocity - 75 inches/second = 153.60 Kbytes/sec
  • Capacity = 500 Meg
  • Detector measures energy off surface when laser is shone. (Bits are pits and lands that have different reflectivity.)

Revision Date: 4/11/97