• The space where programs and data are stored. Data in memory is accessed through instructions from the CPU.
  • Classified into primary and secondary.

Primary Memory

  • Directly accessed by the CPU, ie RAM, ROM, Cache

Primary memory types listed in order of closeness to the CPU:

  • Registers
  • Cache
  • RAM
  • Virtual Memory

The closer to the chip, the faster the memory, however the more expensive the memory is.

Secondary memory

  • External devices such as hard drives, SSDs, flash drives


  • Latency is the time taken by components to respond to the request.
  • To avoid short delays between the CPU’s request for data and finding the data in memory some instructions and data are copied to the cache.

Random Access Memory (RAM)

  • Temporary memory which stores data, files and parts of the operating system
  • All data is loaded from secondary storage into RAM.
  • Is volatile.

Dynamic RAM (DRAM)

  • DRAM consists of transistors that act as switches and capacitors that hold binary data.
  • DRAM has to be constantly refreshed (every 15microseconds) to maintain capacitor charge.
  • DRAM is used for main RAM memory.
  • Is volatile

Static RAM (SRAM)

  • SRAM uses “flip-flops” which hold each bit of memory.
  • SRAM need not to be refreshed constantly.
  • SRAM is faster than DRAM.
  • SRAM is used in cache memory.
  • SRAM is more complex to build and therefore more expensive than DRAM.
  • Comes in smaller capacities than SRAM.
  • Non-volatile

Read-Only Memory (ROM)

  • Read only memory is a permanent memory that is used to store the instructions that are executed once a computer is switched ON. This set of instructions is called a boot process.
  • This is responsible for initializing the hardware and operating system soon after the power is switched ON.
  • The contents of ROM are not erased, even when the power is switched OFF.
  • The contents of ROM can only be read and cannot be changed.
  • ROM is made by interconnecting several transistors. It is an example of non-volatile memory.

Flash memory

  • A type of ROM
  • Programmed by applying a slightly larger electric current that forces an electron through a barrier.
  • Can be rewritten

Hard disk drives (HDD)

  • Also called magnetic disk drives, used in computers and laptops.

  • It provides high storage capacity and is cost-effective.

  • Large storage facilities also use this technology.

  • In a computer, it can store operating systems, user data and programs.

  • The disk is made of a magnetic surface, known as a platter

  • Digital data is stored in these magnetic platters

  • The disk can spin at around 7000 revolutions per second

  • Data can be accessed by a number of read-write heads on the surface

  • Moves from centre of disk to edge and back

Latency of HDD

  • Relatively high
  • Latency is defined as the time taken for a specific block of data to rotate around to the read-write head.
  • High latency leads to “not responding” and “please wait” messages increasing in frequency.

Solid State Drives (SSD)

  • The latency is reduced in SSD compared to HDD as there is no read/write head that needs to be moved.
  • Data is stored and retrieved using the electronic properties of NAND chips.
  • This type of memory is used in USB devices to transfer information from one device to another.
  • Data stored in millions of transistors within the chip.


  • SSD have faster speeds but cost more
  • A combination of both can be used, with frequently needed data store on an SSD and less frequently loaded data stored on an HDD.

Optical Storage Systems

  • CD, DVD, Blu-ray discs use optics to store data.

  • The surface of CDs and DVDs are made of light-sensitive organic dyes or metal alloys.

  • Data is read and written using a laser light.

  • DVDs (4.7 GB) can store large amounts of data compared to CDs (800 MB).

  • Blu-ray discs use blue laser light and can hold up to 50 GB. Used to store music, movies, and games.

  • The time taken to transfer information to these devices is comparatively higher than HDD.

  • After data is stored, it cannot be overwritten. Some do support being overwritten, however it is very limited.

  • The clear plastic layer on the surface allows the laser light to pass through.

  • The colour changes in the dye layer when light falls on it.

Types of memory access

  • Memory can be accessed sequentially or directly. Different devices work in different ways.

Sequential Memory Access

  • Allows user to access data one by one in a sequence.
  • Sequential memory access is slower than direct memory access.
  • Eg, film reel

Direct (Random) Memory Access

  • Any storage location can be accessed at any moment—such as a DVD.