Appendix A explained "Coding Schemes and Number of Systems", which contained a very clear guideline to how computers code information. However, they also tried to explain the concept of a "Parity Bit", which I did not understand at all, even after reading it several times.
Thus I searched the Internet for a clearer explanation. I found many pages elaborating on this concept, but none of them contained such a good description as Wikipedia and wisegeek.com did. With the help of these two webpages and the book, let me explain what a "Parity Bit" is:
Every computer stores data in his memory. A computer does so either in even- or in odd-parity, meaning that for a computer with even-parity the total number of ones in bits in the byte must be an even number. (The same applies to odd-parity!)
So far this was, what I understood from the book. Now it comes to the muddy explanation of the book, which I ill explain by using the information gathered via Wikipedia and wisegeek.com (the following table from wikipedia.com helps in understanding the concept):
| 7 bits of data (number of 1s) | 8 bits including parity | |
|---|---|---|
| even | odd | |
| 0000000 (0) | 00000000 | 10000000 |
| 1010001 (3) | 11010001 | 01010001 |
| 1101001 (4) | 01101001 | 11101001 |
| 1111111 (7) | 11111111 | 01111111 |
Any byte consists of 8 bits of data, 7 bits plus the parity bit. The 7 bits contain the data to be stored whereas bit number 8 exists to check for errors. Bits are binary, which means that they consist of only "1"s and "0"s. So whenever data is transmitted to a computer, this machine has to make sure that the data does not contain any error, in example that means in a computer with even-parity, the 7 bits have to include an even number of "1"s within every byte. If they do not, the parity bit is set to "1" in order to assure that the byte contains an even number of "1"s. Otherwise, if the byte has an even number of "1"s the parity bit is simply not "switched on" and shows a "0". This way the computer ensures that the data, which is being stored in its memory or even only received by it, is accurate and free of errors.
I hope this explanation makes the concept of "Parity Bit" clear to all readers as it is to me now!
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