Memory

The types of memory you need to know about are:

  • Main Memory (RAM)

  • Read Only Memory (ROM)

Random Access Memory (RAM)

Random Access Memory is the most commonly used. RAM is the computer’s short term memory. If a machine has 16 GB of RAM then every program (including the Operating System) is loaded into the system’s RAM.

RAM is known as volatile memory, this means that when it has no power it loses its contents. This is why if you have not saved a document to a hard disk etc. and the machine resets then because there is no power the contents of RAM are lost.

RAM is very fast compared to backing storage, to give you some idea DDR3 RAM can transfer approximately 12,800MB/s versus a Hard Disks 550MB/s.

Read Only Memory (ROM)

Unlike RAM, ROM does not need a constant supply of power to hold its data. It does however have one disadvantage, in that once its contents are written they cannot be changed.

ROM chips can be used in certain devices to hold their start-up instructions, or for example to hold the instructions for an embedded system like the control board of a washing machine.

Memory Addresses

The computer has to able to look at a particular place in memory, for example when loading a program etc. To do this each location in memory has a unique address. This is known as the concept of addressability.

These are usually referenced using a hexadecimal system (base 16) – this uses the numbers 0-9 then the letters A-F.