X86_64 Architecture, how does it handle instructions larger than 64 bits?
Being a 64-bit architecture does not mean there is a single 64-bit instruction register where everything must fit. X86_64 instructions can be up to 15 bytes in length. Each opcode is decoded for execution. In your example, there would be an opcode for moving data that needs another opcode byte to know what to do, and then it knows that 64-bit immediate address follows that must be read and which register will be the target.