Memory

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Example Exam Questions

Describe the role of the following in a computer system: [8]

  • (a) Random Access Memory (RAM)
  • (b) Read Only Memory (ROM)
  • (c) Flash Memory
  • (d) Cache Memory

The diagram below [not included] shows three components of a Central Processing Unit (CPU).

  • (a) Describe the purpose of these components.
    • (i) Controller
    • (ii) ALU
    • (iii) Internal memory

Source: GCSE COMPUTER SCIENCE Specimen Assessment Materials, WJEC/CBAC (2012)

Exam board specific requirements

Students should:

  • know the differences between non-volatile and volatile memory
  • understand the purpose of both types of memory and when each should be used
  • be able to explain the purpose of virtual memory and cache memory
  • be able to explain the concept that data and instructions are stored in memory and processed by the CPU

Definition of Memory

A computer's memory holds instructions and data that the CPU can process.

Memory is stored either physically on magnetic disks, optical disks, flash chips, or tapes. Depending on the physical components memory can be accessed at different rates.

Factors in selecting physical storage devices are:

  • Access time -- the sum of seek time and latency delay in physical movement of parts on the device (rotation of disc, for example)
  • Transfer rate -- the rate at which data can be transferred from the device
  • Volatility
  • Capacity
  • Cost
  • Physical format

Many "types" of storage are differentiated because of their function, not because of any differences in the above factors.

Difference between Volatile and Permanent Memory

"Volatile memory is computer storage that only maintains its data while the device is powered. Most RAM (random access memory) used for primary storage in personal computers is volatile memory."

Volatile memory - WhatIs

"Non-volatile memory, nonvolatile memory, NVM or non-volatile storage is computer memory that can retrieve stored information even after having been power cycled. Examples of non-volatile memory include read-only memory, flash memory, ferroelectric RAM (F-RAM), most types of magnetic computer storage devices (e.g. hard disks, floppy disks, and magnetic tape), optical discs, and early computer storage methods such as paper tape and punched cards."

Non-volatile memory - Wikipedia

Primary, Secondary, Tertiary, and Cloud Storage

  • Primary storage usually refers to the storage of data either on the CPU chip or very close to it in the RAM. It is the data that is either being processed or about to be processed.
  • Second storage refers to large capacity storage devices that are part of the computer, for example a large Hard Disk Drive. This memory can be accessed rapidly, though not as fast as Cache or RAM.
  • Tertiary storage is data that has been saved to removable tapes, optical disks or disk drives. The storage items are often stored off site from the computer as a backup or archive that will not be damaged by a fire or flood where the main computers are.
  • Cloud storage is beginning to complicate the above definitions. It is off-site and not part of the main computer, so it is tertiary, but it is accessible via an internet link almost as quickly as a hard drive on the computer.

Formats of storage - based on physical characteristics

  • Magnetic disks
  • Optical disks
  • Flash disks
  • Tapes
  • Cards

Types of storage - based on function

Random Access Memory (RAM)

These are chips that can only store data when the computer is switched on. The chips provide very fast access to data, but are expensive to produce and have a relatively small capacity compared to large volume disk or tape storage. They store data as small chargers of electrical energy.

Read Only Memory (ROM)

ROM is a concept rather than a storage format. For example CD's and other optical disks are read only. ROM provides large volume non-volatile storage capacity that is relatively slow to access. The memory is read only because the data is physically burnt onto the surface of the optical disks, this leads some people to define the storage as "permanent". An optical disk is not prone to damage from magnetic fields and are hard to break compared to a delicate hard drive or tape.

Flash Memory

Flash memory is non-volatile storage on chips. It stores bits of data as small units of electrical energy which are held in capacitors that keep their charge even after the power source has been switched off. Flash is not easily damaged as there are no moving parts. Sometimes Flash memory is referred to as EEPROM.

Many small mobile devices like tablets and mobile phone now use Flash memory exclusively, enabling a very fast access rate with a physically stable storage.

Cache Memory

Data in cache is stored on a chip that is part of the CPU, and can be accessed the quicker than any other data storage device by the CPU. Data stored in Cache is being processed or being queued to be processed.