1.2 Memory and Storage
1.2.1 Primary storage (Memory) - click to expand
This section covers:
The difference between RAM and ROM
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Other sections in this topic:
This page looks at the "main memory" of a computer system, known as primary storage. This is computer system storage that is used when the computer system is actually being used, it is temporary and must be high performance because it has a direct impact on how quickly the computer system works.
The Need for Primary Storage
A computer system uses primary storage to hold data and programs that are currently being used and processed. Most primary storage is volatile, which means that its contents are lost when the power is turned off (an exception to this is ROM - see below)
For your exam, you need to remember the first sentence above.
Random Access Memory (RAM)
Random Access Memory (RAM) is a used for a temporary store of instructions and data in use by the CPU for currently executing programs. Programs and data are loaded from the hard disk/solid state storage to RAM for processing. RAM is volatile, which means that the contents are lost when the power is turned off. RAM is read and write capable, which means that you can store new data on it and also read data from it.
Read Only Memory (ROM)
Read Only Memory (ROM) is a form of memory in a computer system that holds the instructions that are executed when the computer is first turned on, also known as the ‘bootstrap loader’. It is non volatile, which means that the contents remains when the power is turned off. ROM is read only, which means that it can only be read from and the contents are not changed.
The main purpose of ROM that you will need to learn for the exam is that it holds the boot-up program, but it does also have other uses. It can be used to store programs in special cartridges on portable games consoles and embedded systems sometimes hold their main program in ROM.
The Difference Between RAM and ROM
In your exam, you will need to be able to describe both RAM and ROM and explain what they are used for in a computer system. You will also need to be able to state the differences between the two.
This table summarises the main differences, and it is well worth remembering.
The first two rows are the most important!
Virtual Memory
Computer systems can sometimes run short of RAM, which will mean that the computer will not have any spare primary storage space and will be unable to continue (it will freeze). Modern operating systems make use of a special technique called virtual memory that can overcome this limitation. Click on the play buttons of the following presentation to watch an animation of how it works:
In the exam, you are likely to get a question about virtual memory. These questions will ask you to describe how virtual memory works, and will go on to ask you to explain how this can slow down the performance of the computer. You should clearly state that virtual memory acts as RAM when RAM is full.
When you add more RAM the computer is less reliant on virtual memory, and the computer's performance will increase (it will be faster).
Virtual memory has a slower access speed for two reasons, one is that the hard drive or other primary storage device (which is where the Virtual Memory is created), has a slower access speed than RAM. Secondly, all data that is in Virtual Memory needs to be loaded into RAM first before it can be processed by the CPU, this page swapping takes time and the whole process takes more time than reading from RAM directly.
Your answer may also need to discuss the advantages of using virtual memory. One advantage is that it allows more programs to run even when RAM is full, and another is that it can allow very large programs to run by loading only one section of the program at a time.
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