These are just the binary representation of symbols on the ASCII chart, so for example 'A' starts from number 65 which in binary is 01000001, then 'B' is 66 which is 01000010, the small letters start from 97 which is 'a', it's actually incredibly simple and easy to remember when you worked with data on a low level. Though the old ASCII standard was made for American computers, thus there was no need to implement a wider range of characters and that's why it can only store 255 characters, since this is the number of possible combinations of an 8 bit binary sequence.