Memory in a computer system refers to the components that store (or remember!) things. There are different types of memory with different purposes. Some are very fast and expensive such as the cache memory discussed earlier, and some are cheaper and slower such as the memory sticks you probably use at school. These different types of memory have different characteristics and can be compared in terms of their access speed (how quickly you can read from them or write to them), their price and the whether they can store data when the power is turned off (volatility). Let’s consider some common types of memory in computer systems:

Random Access Memory (RAM)

RAM is the type of memory used in the computer’s main memory. Many people say “RAM” when they mean main memory. Nothing in a computer is really “random” so random access just refers to the fact that you can write anywhere in that memory space at any time, you don’t have to put the next thing straight after the last one like you do on a magnetic tape, for example.

When a program is running it has to be loaded from the hard disk into the main memory so the processor can access the instructions. Any data needed for that program to run is also loaded into main memory while the program is running. The main purpose of RAM is to act as temporary storage for programs and data, just for the duration of that program. Once the program has finished and is closed, it is no longer in main memory.

So why doesn’t the processor get the instructions straight from the disk? It is about access speed. Reading from and writing to a hard disk is very slow compared to the speed of the processor. Just as we used the high speed cache between the processor and main memory we need main memory to store the programs currently in use or the computer would be really slow.

As every program in use should, ideally, be in main memory while it runs, the amount of main memory also affects the performance of the computer. If a computer system has lots of programs running at the same time it needs lots of main memory.

DDR stands for Double Data Rate

Common Types of RAM: