We have made reference to computer memory several times in this series, history of the computer, now it is time to look at how it has developed, and is still developing. A look at a block diagram of a modern PC will show how the memory is central to the way a computer operates. Everything passes through it. Many would say the processor, or CPU is the most important part, but you can see that everything depends on the memory. The easiest way to improve the performance of a computer is to add more memory. The layout of the PC is much the same as the original digital computers. They didn't have monitors, or optical disks, but they had control panels, a teletype, for operator communication, and paper tapes, punch cards, magnetic tapes and drums for data storage. The invention of the core memory inthe late 1940s really made the computer a practical, general purpose, commercial reality. What do we mean by memory? An obvious answer would be a component that remembers something to be used later. Does this mean like a hard drive? This is certainly something you can use to remember what you 'told' it. However, in computer talk, this would be called 'mass storage'. By this we imply a very large capacity in comparison to the 'core' memory, which is the basis of the computer. Also mass storage is normally much slower to access, as it is primarily mechanical. So what we mean by 'computer memory' is a storage medium which can be accessed very quickly, and at any particular location, or address. This is the meaning of a term you have probably heard - 'RAM' - which is an abbreviation of Random Access Memory. Another term you may have heard, I used in the previous paragraph, 'core' memory. Core has a very appropriate meaning of the 'center' of the computer, like, for example, an apple core or the earth's core (as featured in many a science fiction movie!). However the origin of 'core' is a reference to the type of memory used in early digital computers. The cores were rings of ferrite material, similar to the built-in antenna used in your AM radio. The ring was about the size of a small shirt collar button, and shaped like a donut! Each core represented one bit, so you would have needed 16 of these to form 1 byte. We will be looking at 6 bit characters. At the time 6, 7, or 8 bit characters were the standard, and normally included a 'Parity' bit. Parity is a form of error checking, we will be looking at that in another article. For the moment we will just say that 'Odd Parity' means that the 6 bits in a character, plus the parity bit, must add up to an odd number of '1' bits. If there are an even number of 1 bits, as in the binary 000101 (decimal 5), we must make the parity bit=1 (1000101). If there is an odd number of bits, as in binary 000111 (decimal 7), we make the parity bit=0 (0000111). Now when we write a '7' to memory we also write a parity bit of 0. When we read it back, we check for an odd number of bits in the data we read out. If the parity check fails, we have a problem. In Part 2 we will look at the physical layout of the core memory. |
