Primary and secondary storage

What is primary storage?

• holds the data and instructions that the CPU needs to access whilst the computer is turned on

What is secondary storage?

• Long term/non-volatile storage of data/files

• External/auxiliary storage of data

• large capacity

Why does a computer need primary storage?

• access times are considerably faster than secondary

• This means the time taken to complete operations such as the Fetch-Execute Cycle is dramatically reduced

• Due to the fast access times, primary storage is used as short term, working memory, in hardware that is directly connected to the CPU such as RAM, and components that reside inside the CPU such as Cache and Registers

• Performance of primary storage means a much higher cost which limits the amount that is used

What is RAM (Random Access Memory)?

• primary storage directly connected to the CPU

• This stores the instructions and data that are currently being used

Characteristics of RAM

• volatile

• read/write

• faster than secondary storage

• larger capacity than ROM

How does RAM obtain data instructions that are currently in use?

For the CPU to access the data and instructions they must be copied from secondary storage into RAM

What does volatile mean?

contents of RAM are lost when the power is turned off

what does read/write mean?

data can be read from and written to

What is ROM (Read Only Memory)?

• primary storage that holds the first instructions a computer needs to start up (Bootstrap)

• contains the BIOS (Basic Input Output System)

Characteristics of ROM:

• non-volatile

• read only

• fast memory, much faster than secondary storage but slower than RAM

• smaller capacity than RAM

What is virtual memory?

• an extension of primary storage (RAM) located on secondary storage

• If the computer does not have enough RAM to run a process or if the RAM is almost full it can makes use of virtual memory.

• This frees up room in RAM for other programs and data

What is swapping?

• The process of moving data between RAM and secondary storage when using virtual memory.

• When the data in hard disk is needed again, any other unused data is transferred to hard disk before the original data is transferred back to RAM.

why is virtual memory beneficial?

allows more programs to be run at the same time with less system slowdown

what is a problem with virtual memory?

Virtual memory is much slower than RAM and it's use will impact negatively on system performance

How to avoid using virtual memory?

increase the size of RAM

What is secondary storage?

• Secondary storage is non-volatile storage used to save and store data that can be accessed repeatedly.

• ​Secondary storage is not directly embedded on the motherboard (and possibly even external) and therefore further away from the CPU so it is slower to access then primary storage. 

characteristics to be considered when choosing a secondary storage device?

• Capacity/size

• Access Speed

• Portability

• Durability

• Reliability

• Cost

What is capacity?

The maximum amount of data that can be stored on the device

What is access speed?

How quickly data on the device can be read or edited

What is portability?

How easy it is to carry the device around

What is durability?

The strength of the device, to last without breaking

What is reliability?

The likelihood of the device continuing to perform well over time

what is cost?

The average price it costs to purchase the storage device

3 types of secondary storage

• optical

• magnetic

• solid state

Optical storage

• type of non-volatile media

• uses lasers to burn the surface of a disk, creating pits and lands suitable for storing binary 0s and 1s

Examples of optical storage devices

• CDs

• DVDs

• Blu-ray

Characteristics of optical storage:

• slowest access speed of all types of secondary storage

• thin and very portable

• cheap to buy in bulk

• not durable because discs are very fragile and can break or scratch easily

Capacities of optical devices:

• CD = 700 MB

• DVD = 4.7 GB

• Blu-ray = 25 GB

Magnetic storage

• Type of non volatile media

• Uses magnets (polarity) to store binary 1s and 0s

Examples of magnetic storage

• hard disk drive (HDD)

• Floppy disk (obsolete) - replaced by solid state devices

• Magnetic tape

Capacity of HDD

500GB - 8TB

Characteristics of magnetic storage

• large capacity and cheaper cost per gigabyte than solid state

• Modern external HDDs are small and well protected so they are durable and portable, however because of the moving parts, it should not be moved when powered on because it can damage the device

• Slower access speed than solid state but faster than optical

Solid state storage

• Type of non volatile media

• Uses electronic circuits to store binary 1s and 0s

Example of solid state storage

• USB flash drives

• Memory cards e.g SD card

• Micro-SD cards

• Solid state drives (SSD)

Capacity of solid state storage

• SSD = 128GB - 2TB

• USB flash drive = 8GB - 256GB

Characteristics of solid state storage

• More expensive cost per GB than magnetic

• Usually durable but cheap USB sticks can snap or break

• The small size of USB sticks and memory cards mean that they are very portable and can fit easily in a bag or pocket

• Solid state storage have a high capacity and the fastest access speed because they have no moving parts