Wow! There are so many things to memorize! Did my previous teacher remember the "area code table" thoroughly?
But, according to what you said, if he remembered that table, the four characters "operating system" would be very simple and could be calculated in less than a minute. But of course I saw him calculate for more than half an hour. After that, he gave me this answer. Then I asked him how he calculated it, and he said it was very difficult and didn't tell me. After he left, I picked up his draft paper and wrote many groups of hexadecimal and binary numbers, as well as decimal ones, in a messy state~~~(sweat~~~)
If he remembered the area code table, he wouldn't have calculated for so long. If he didn't remember that table, I really don't know what he was up to~~~~