The correct answer is: B. EXX instruction

The EXX instruction is used to switch the contents of the two 16-bit registers HL and DE. It is a two-byte instruction with the opcode 1110 1000. The first byte of the instruction specifies the register pair to be switched, and the second byte specifies the destination register.

The EXX instruction is used to switch the contents of the two 16-bit registers HL and DE for a variety of reasons. For example, it can be used to swap the values of two variables, to move data from one location to another, or to perform arithmetic operations on two values.

The EXX instruction is a powerful tool that can be used to perform a variety of tasks. However, it is important to use it carefully, as it can also be used to overwrite important data.

The other options are incorrect because:

  • A. Register instruction is a general term that refers to any instruction that operates on a register. It does not specifically refer to the EXX instruction.
  • C. Instruction opcodes are the codes that represent the instructions in a computer program. They do not specifically refer to the EXX instruction.
  • D. AXX instruction is not a valid instruction.