BooleanBinary and Hexadecimal numbering systems, and how to convert them to/from decimal numbers.
Although you won't generally need to work with raw BooleanBinary numbers, they are frequently used for Flag variables, which require boolean logic and bit-shifting.
Hex isn't quite as important (unless you want to process 0xDEADBEEF), but at a minimum, you'd want to recognize when they're being used, and why.