| We have made reference to computer memory | | | | and at any particular location, or address. This is the |
| several times in this series, history of the computer, | | | | meaning of a term you have probably heard - 'RAM' - |
| now it is time to look at how it has developed, and is | | | | which is an abbreviation of Random Access Memory. |
| still developing. A look at a block diagram of a | | | | Another term you may have heard, I used in the |
| modern PC will show how the memory is central to | | | | previous paragraph, 'core' memory. Core has a very |
| the way a computer operates. Everything passes | | | | appropriate meaning of the 'center' of the computer, |
| through it. Many would say the processor, or CPU is | | | | like, for example, an apple core or the earth's core |
| the most important part, but you can see that | | | | (as featured in many a science fiction movie!). |
| everything depends on the memory. The easiest | | | | However the origin of 'core' is a reference to the |
| way to improve the performance of a computer is | | | | type of memory used in early digital computers. The |
| to add more memory. | | | | cores were rings of ferrite material, similar to the |
| The layout of the PC is much the same as the | | | | built-in antenna used in your AM radio. The ring was |
| original digital computers. They didn't have monitors, | | | | about the size of a small shirt collar button, and |
| or optical disks, but they had control panels, a | | | | shaped like a donut! Each core represented one bit, |
| teletype, for operator communication, and paper | | | | so you would have needed 16 of these to form 1 |
| tapes, punch cards, magnetic tapes and drums for | | | | byte. We will be looking at 6 bit characters. At the |
| data storage. The invention of the core memory | | | | time 6, 7, or 8 bit characters were the standard, and |
| inthe late 1940s really made the computer a practical, | | | | normally included a 'Parity' bit. |
| general purpose, commercial reality. | | | | Parity is a form of error checking, we will be looking |
| What do we mean by memory? An obvious answer | | | | at that in another article. For the moment we will just |
| would be a component that remembers something | | | | say that 'Odd Parity' means that the 6 bits in a |
| to be used later. Does this mean like a hard drive? | | | | character, plus the parity bit, must add up to an odd |
| This is certainly something you can use to remember | | | | number of '1' bits. If there are an even number of 1 |
| what you 'told' it. However, in computer talk, this | | | | bits, as in the binary 000101 (decimal 5), we must |
| would be called 'mass storage'. By this we imply a | | | | make the parity bit=1 (1000101). If there is an odd |
| very large capacity in comparison to the 'core' | | | | number of bits, as in binary 000111 (decimal 7), we |
| memory, which is the basis of the computer. Also | | | | make the parity bit=0 (0000111). Now when we write |
| mass storage is normally much slower to access, as | | | | a '7' to memory we also write a parity bit of 0. |
| it is primarily mechanical. | | | | When we read it back, we check for an odd number |
| So what we mean by 'computer memory' is a | | | | of bits in the data we read out. If the parity check |
| storage medium which can be accessed very quickly, | | | | fails, we have a problem. |