If your instructor has been programming for years then you heard them wrong. in Assembly language it's in decimal unless it has a (h) (H) as a suffice. If the first ASCII character in the hex is A,B,C,D,E,F then it needs to be preceeded with a 0 so as not to be confused with a label!
1234h is okay
0FACEh is okay
FACEh is NOT okay! It's a label!
We have no idea what is in ch because it is loaded from a byte in memory at location
and we have no idea what is in memory at that location as there is no provided block of memory to observe!
BUT, ch can not have 1DEF as ch is an 8-bit register therefore 00h - FFh