1.2 - Memory and storage

Why do computers need primary storage?

• To store data and instructions that are currently in use by the CPU.

• It also provides fast access

Give the differences between RAM and ROM

Feature	RAM (Random Access Memory)	ROM (Read-Only Memory)

Volatility

Volatile – loses its data when the computer is turned off

Non-volatile – retains data even when the computer is off

Purpose

Stores data and instructions currently in use by the CPU

ROM stores permanent instructions that the computer needs to start up (eg. BIOS/ bootstrap loader and firmware)

Read/Write

Can be read from and written to

Usually read-only, cannot be easily modified

Speed

Fast access for the CPU

Slower compared to RAM

What is the purpose of ROM in a computer system?

To store permanent instructions or data that the computer needs to start up; for example, the bootstrap loader/ BIOS and firmware.

Give examples of what RAM and ROM store

RAM (RAM is made up of a large number of storage locations, each can be identified by a unique address):

• data and instructions currently in use by the CPU

• parts of the operating system

ROM:

• bootstrap loader/ BIOS

What is the purpose of RAM in a computer system?

To store data/instructions/part of the operating system that is currently in use by the CPU

How does virtual memory work and why may it be needed in a computer system?

Virtual memory is needed when the computer has run out of main memory/ RAM; without virtual memory the computer system would crash or refuse to open programs when RAM is full.

• Programs must be stored in RAM to be accessed by the CPU

• Virtual memory is a process where secondary storage is used as an extension of RAM

• If new data needs to be stored in RAM, unused data is moved to the hard drive so that the new data has space so that it can be transferred into RAM

• If the original data is required again, it will be moved back from virtual memory (secondary storage) into RAM

What is cache memory?

Very small, very fast type of main memory located close to or inside the CPU, used to store frequently accessed data and instructions.

Because it is so small, instructions and data stored here are quicker to access than those stored in RAM.

Why is secondary storage needed?

It provides long-term, non-volatile storage for data and programs, so that when the computer is turned off data and programs can continue to be stored.

It also offers a much larger storage capacity than RAM, however is not directly embedded on the motherboard, so is further away from the CPU thus is slower to access by the CPU.

Name the types of secondary storage and name the storage characteristics

• Optical

• Magnetic

• Solid state

• Capacity

• Durability

• Portability

• Access speed

• Cost

• Reliability

How do magnetic storage devices work?

Magnetic discs are read with a moving head inside the disk drive and uses the varying magnetic field of the platter to read/edit data.

Eg. floppy disks, magnetic disk drives (internal and external)

Has moving parts so has slower access speeds, are less durable and are less portable.

However, they have a large storage capacity and are cheaper per gigabyte than solid state.

How do optical storage devices work?

Optical storage uses a laser to read and write data onto an optical disc.

Eg. CD, DVD, Blu-ray

This means that optical storage has the slowest read/ write speeds.

How do solid state storage devices work?

Solid state stores data using flash memory which uses electronic circuits (transistors) to store data as electrical charges.

Because there are no moving parts, it is very robust and data can be accessed very quickly

Eg. SSDs, USB, memory card

Make a table for to compare the characteristics of the three types of secondary storage devices

Characteristic	Magnetic (HDD, magnetic tape)	Optical (CD, DVD, Blu-ray)	Solid State (SSD, USB, SD card)
Capacity	Very high (TBs)	Low–medium	Medium–high
Speed	Medium	Slow	Very fast
Portability	Low (internal HDDs not portable)	High (small and light)	Very high
Durability	Low – moving parts can be damaged	Medium – can scratch	High – no moving parts
Cost	Low cost per GB	Very low per disc	High cost per GB