Actually, I'm lucky. My sons do not have nicknames, unless you count Dave for David, which he has only tolerated since he became an adult. The opcode is the part of the instruction that says what action is to be taken. Examples are ADD, Compare, Move, Copy, Load, Store, etc. Could you please explain further the differences between kernal and user modes for the ISA level.
Some operations are dangerous if misused.
To prevent accidental or deliberate misuse, those operations are restricted to use by the operating system or other trusted software and are not available for user programs. The protected instructions prevent one user program from interfering with another, for example. They also prevent a user program from accessing the part of memory where the system software is loaded. They restrict the user access to peripheral devices, so there will not accidently be a reset signal or something that would put the device into a confused and unproductive state. Page 336 talks about self modifying programs.