A-level Computing 2009/AQA/Computer Components, The Stored Program Concept and the Internet/Machine Level Architecture/Functional characteristics of a processor





Addressable memory
A computer must be able to access main memory for reading and writing, they do this by using addressable memory. Main memory is a little like a set of school lockers, each with a different number. Each locker contains a block of data and if you fill up one locker you can use the next locker to expand into. Looking at the example above you can see locker '0' contains '8975', whilst lockers 1 to 6 contain the sentence "The Cat sat on the dog!". Locker '7' is empty, locker '8' contains a boolean value and locker '9' contains the number 48. As you can see if we only used one character for the locker number then we could only ever have 10 lockers. If you limit the number of addresses you can use then you limit the amount of memory you can talk to. If you have a small address bus then you won't be able to have much main memory.

The way that data is stored in a computer is very similar:

Stored program concept
Building on the Von Neumann architecture we get the idea of how the stored program concept works. If you have ever loaded a game on a console you might notice that: This is the stored program concept in motion! Let's take apart what is happening:
 * 1) you need to insert a disc
 * 2) the disc spins
 * 3) the game says loading
 * 4) the game plays
 * 1) You insert an optical disk (secondary storage) with the code on
 * 2) The code is loaded into main memory
 * 3) The processor fetches, decodes and executes instructions from main memory to play game