GCSE Computer Science

Component 1

1.2a Primary and secondary storage

The need for primary storage

Primary storage includes RAM, ROM, cache and the registers inside the CPU.

Primary storage can be directly accessed by the CPU. This is important is it either contains the instructions that for programs that are currently running or the instructions needed when the computer first turns on.

primary storage




  • Stands for Random Access Memory
  • Stores instructions and data for programs currently in use.
  • Volatile - loses its data when the power is switched off.
  • Two types:
    • DRAM (Dynamic RAM) - less expensive, not as fast, needs constant refreshing. Used for the main RAM in your computer.
    • SRAM (Static RAM) - much more expensive, faster, doesn’t need lots of refreshing. Used for cache (see Topic 1.1).



  • Stands for Read Only Memory
  • Stores the instructions needed to start up the computer including the BIOS.
  • Non-volatile - keeps its content when the power is switched off.
  • Originally it was read only meaning data could not be written to by the user. However over the years different types of ROM have been developed that allow the user to write some data e.g. PROM, EPROM, EEPROM.

Virtual memory

Often there isn’t enough space in RAM for all the programs that are currently being run.

The operating system can use a special section of the hard drive to deal with the overflow if necessary. This is called a page file and the process is known as virtual memory.

Data and instructions must be in RAM to be processed by the CPU so they need to be swapped in and out from the hard drive before they can be processed. This means that although there is more overall space, using virtual memory is much slower than using RAM.

virtual memory

Modern ROM vs flash

Advances in ROM technology now mean that ROM can be made to be fully rewritable. This is called flash memory.

Flash memory is useful as it is very fast compared to a hard disk drive and can be used as secondary storage in USB memory sticks, SD cards and solid state drives. (see the next topic).

Should ROM still be called Read Only Memory? You decide….

flash rom

The need for secondary storage

Secondary storage uses magnetic, optical or flash technology. It cannot be directly accessed by the CPU and data must be copied into RAM first. Secondary storage stores data permanently.

Optical, magnetic and solid state technologies

Magnetic Storage

hard disk drive
A type of storage that uses a magnet to make a magnetised segment on a disk platter.

Includes hard disk drives and the outdated floppy disk.

Hard disk drives feature in most modern computers as they offer a lot of storage at a cheap price and are relatively fast.

Optical Storage

optical disc
A type of storage that uses lasers to burn marks in a reflective disc.

There are three types: CD-ROM, DVD-ROM and Blu-Ray ROM.

Often used whenever something cheap to produce and portable is needed. Also, most people have access to the equipment to read the discs.

Flash Storage

flash drive
A type of storage that uses electricity to open and close gates on a circuit board.

Includes USB memory sticks, SD cards and solid state drives.

Very fast, portable and not affected by moving parts. This technology is slowly overtaking the storage world as it becomes cheaper and cheaper

secondary storage characteristics

Characteristics of secondary storage technologies

    • How much data the media can store. The higher the capacity the better.
    • A typical SD card might hold 32GB. A hard drive might be about 2TB. Old floppy disks were 1.44MB.
    • How quickly the device can read and write data.
    • Old floppy disks were very slow as are tape drives. Optical drives are quite slow. Hard drives are pretty fast. Solid state drives are very fast.
    • Can the device be carried around easily?
    • Internal hard disk drives can’t be carried around easily. Neither can solid state drives. Optical disks, flash memory sticks and SD cards are very portable.
    • Is the device easy to damage?
    • Floppy disks and optical discs are very fragile. Internal hard disks have moving parts which can go wrong if shaken. Flash memory is much more durable.
    • How long does it last before it starts to break?
    • Most storage devices are quite reliable and will suit most uses. However, flash memory degrades over time and eventually wears out. Optical discs can’t be rewritten forever. Magnetic storage like hard disks last a very long time.

Units of data

UnitAbbreviationNumber of BytesNotes
Bit1/8either a 0 or 1
Nibble1/2e.g. 1010
Byte1e.g. 11001100
KilobyteKB1,000written as 1KB
MegabyteMB1,000,000or 1000KB
GigabyteGB1,000,000,000or 1000MB
TerabyteTB1,000,000,000,000or 1000GB
PetabytePB1,000,000,000,000,000or 1000TB