1. External Memory

<aside> 📖 Characteristics

• Store large scale data, which may not be always used
• Not volatile </aside>

<aside> 📖 Types

• Magnetic disk
• Optical memory
• Magnetic tape
• USB flash disk, SSD </aside>

2. Magnetic Disk - Hard Disk

<aside> 📖 A circular platter constructed of nonmagnetic material (substrate) coated with a magnetizable material

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2.1 Read and write mechanism

• Read and write operation conduct coil named the head
• During a read or write operation, the head is stationary while the platter rotates beneath it
• Number of head
  • Single: read and write share one head (floppy disk, older rigid disk)
  • Two: use a separate read head (contemporary rigid disk)

<aside> 📖 Write mechanism

• Electric pulses are sent to the write head
• Electric current induces a magnetic field across the gap
• The resulting magnetic patterns are recorded on the surface below
• Reversing the direction of the current reverses the direction of the magnetization on the recording medium </aside>

<aside> 📖 Read mechanism

• The read head consists of a partially shielded magnetoresistive (MR) sensor, whose electrical resistance depends on the direction of the magnetization of the medium moving under it
• Pass a current through MR sensor, and detect resistance changes by voltage signals
• Higher-frequency operation leads to greater storage densities and operating speeds </aside>

<aside> 📖 Head mechanism

• The head must generate or sense an electromagnetic field of sufficient magnitude to write and read properly
• The narrower head requires the closer distance to the platter
• Greater data density requires a narrower head and narrower tracks, which will lead to the greater risk of error
• Winchester head
  • The head is actually an aerodynamic foil that rests lightly on the platter’s surface when the disk is motionless
  • The air pressure generated by a spinning disk is enough to make the foil rise above the surface </aside>

2.2 Data organization

Adjacent tracks are separated by intertrack gaps, and adjacent sectors are separated by intersector gaps

<aside> 📖 Constant angular velocity (CAV)

• Increase the space between bits to make the disk be scanned at a fixed speed
• Advantage
  • individual blocks of data can be directly addressed by track and sector
• Disadvantage
  • disk storage capacity is limited by the maximum recording density achieved on the innermost track </aside>

<aside> 📖 Multiple zone recording

• Divide the surface into a number of concentric zones, placing more sectors per zone on outer tracks than on inner tracks.
• Advantage
  • increase storage capacity
• Disadvantage
  • require more complex circuitry </aside>

<aside> 📖 Formatting

• There must be some starting point on the track and a way of identifying the start and end of each sector
• Format
  • add some extra data used only by the disk drive and not accessible to the user

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2.3 Timing of a disk transfer

<aside> 📖 Components

• $\text{Seek time }T_{seek}$
  • To read the contents of some target sector, the arm first positions the head over the track that contains the target sector. The time required to move the arm is called the seek time.
• $\text{Rotational latency }T_{rotation}$
  • The time it takes for the beginning of the sector to reach the head
  • $T_{avg~rotation}=\frac12T_{max~rotation}$
• $\text{Transfer time }T_{transfer}$
  • The time required for data transfer
  • $\begin{aligned} T_{avg~transfer} &= \frac{60~secs\times b}{RPM\times b_{sum}} \end{aligned}$ </aside>