CS 231 - Micro Electronics

Chapter 5: Data Storage Technology

Objectives:

This lesson discusses various technologies used for data storage. Objectives important to this lesson:

  1. Primary and secondary storage
  2. Primary storage devices
  3. Secondary storage devices
  4. Factors that affect magnetic storage
  5. Choosing appropriate secondary storage
Concepts:

This chapter begins with generic discussion of storage devices, which it means as a category that covers all the media covered in the chapter. A storage device of any sort, we are told, must have a read/write mechanism and a storage medium.

The chapter gives us a list of characteristics that separate storage devices from each other. The two most important are speed and volatility.

Speed refers to the speed with which a device can be used. The text gives us an example of a processor with a 1 GHz clock rate, and remarks that it would be best to use storage devices that can supply new instructions at that rate. Otherwise, the storage devices introduce wait states for the processor. In general, there is no wait for anything already stored in a register, a short wait for anything stored in RAM, and a longer wait for any other kind of storage. The text discusses the fact that access time is the same for all addresses in RAM, but it can vary from one part of a hard drive to another.

We see a short exercise at the bottom on page 160, showing us a method for calculating the data transfer rate for a de-ice. The calculation is done by taking the inverse of the access time of the device and multiplying it by the word size of the device in bytes.

Volatility has to do with loss of data over time. It is usually related to whether a device needs to be powered in order to retain its data. As I have stated earlier in the notes, the text tells us that primary storage is generally volatile, and secondary storage is generally nonvolatile. It also tells us that devices tend to suffer from a decay of their stored data over time, which makes them all somewhat volatile if they are kept around long enough.

 The text continues with a discussion of three access methods that might be used, regardless of the type of storage device:

  • serial access - data elements are stored in a linear sequence, and must be accessed the same way, which makes this the slowest of the three access methods; it is an older method that was primarily used on magnetic tape and on some old magnetic drives
  • random access - also called direct access, it allows the device to access any location in any order, and generally takes the same amount of time to access any location; often used in RAM and in hard drives
  • parallel access - this method allows the device to access multiple storage locations at the same time; the text argues that any device that allows us to access one or more bytes at once is allowing us to access multiple bits at the same time

The text also discusses portability and the relationship between cost and capacity. It should be obvious to anyone who has purchased system components that they cost more the more they can do. Portability is in the eye of the beholder. If we can put a component in a machine, we can take it back out.

The text amplifies on its thoughts about primary storage for a few pages. It classes RAM in two main groups, and a variation:

  • Static RAM (SRAM) - each bit is stored in two transistors; requires a continuous supply of power to maintain its data
  • Dynamic RAM (DRAM) - each bit is stored in a transistor and a capacitor; requires a refresh of power several thousand times per second
  • Synchronous DRAM (SDRAM) - similar to DRAM, but uses the system clock and can be read very quickly after the first read

The text also discusses a few types of nonvolatile memory (NVM). The reader may be familiar with ROM (read only memory) and EEPROM (electronically erasable programmable ROM). These are often used for functions on a motherboard. The text also mentions flash RAM, most often used in memory sticks.

The text discusses several features of hard drives and magnetic tape that have already been mentioned.

  • magnetic storage is made by affecting the orientation of magnetic sensitive particles in a medium with a magnetic field
  • magnetic decay means that eventually the stored orientation will not be readable
  • magnetic leakage occurs when the charges on adjacent point in the storage medium affect each other, making them unreadable
Hard drives are discussed in some detail. Note the illustration on page 177 that shows several standard features:
  • platters - flat, circular disks that are covered with a magnetic sensitive medium
  • read/write heads - electronic storage and reading mechanisms mounted on moving arms mounted on hinges between the platters
  • tracks - numbered concentric circular paths on the platters where data is stored and read
  • sector - a section of a track that typically holds 512 bytes; typically data is read and written one sector at a time
  • cylinder - all tracks with the same number across all platters in a device

The text mentions some facts about solid state drives on page 183. They are better for causing fewer wait states, but still cost a great deal more than hard drives of comparable storage capacity.

The chapter ends with a discussion of optical storage devices, which are read with reflected laser light. The basic information is summarized on page 190:

  • CD, DVD, and CD-ROM - these are discs holding data that was recorded by a manufacturing process
  • CD-R, DVD+/-R, and BD-R - these are discs that can be written once by a consumer
  • CD-RW, rewritable forms of DVD and Blu-ray discs - these are discs that can be written, erased, and overwritten multiple times

many things that happen in the Central Processing Unit of a computer. First, we are reminded that a CPU will contain at least three components: the Arithmetic Logic Unit (which does the computation and comparisons), the Control Unit (which moves data and instructions to and from secondary storage, RAM, and registers), and registers (volatile memory in the CPU itself).

The chapter spends the next several pages discussing electronic notation and physical factors that affect all electrical equipment, such as heat, electrical resistance, and circuit length. It also discusses some possible improvements in computing that have in fact been discussed for years but have yet to be realized.