We describe and compare storage type by cost, speed of access to data, and size in megabytes. A megabyte (MB) is about a million characters of text or a high quality photo.
Main Store- ROM and RAM
There are different kinds of storage chip- those that can't hold data when the computer is switched off (volatile) and those that can. We use the latter to store data we always need (e.g. the software for starting a computer after switch on- operating system software). Of course this kind of data must not change- we mustn't be able to write data to such chips as we would over-write existing data. So these chips are read-only memory (ROM) (memory is another word for storage). But we can write to volatile chips, so they provide read and write memory, RAM (Random access memory). RAM holds the data and programs that you are working on at one time.
- Holds the processor's data (you don't have much to do with it)
- is part of the CPU
- is always on one or more chips
- is accessed much faster than backing store (because access to data on chips is faster)
- tends to be a few hundred megabytes (on desktop computers)
The backing store is classed as peripheral hardware, although it is sometimes inside the same box as the CPU. It normally takes the form of a hard disk and is used for storing your own data.
- holds your data
- is peripheral
- usually isn't on chips (except for storage stick)
- takes longer to access than the main store
- may be tens of thousands of megabytes (tens of gigabytes)