Final answer:
The x86_64 instruction used to call an external function in languages like C, C++, and Fortran is the CALL instruction, which transfers control to the target function and saves the return address on the stack.
Step-by-step explanation:
The x86_64 instruction used to call an external function, whether it is written in C, C++, Fortran, or any other language that can interface with assembly code, is the CALL instruction. When you compile a program written in high-level languages like C or C++, the compiler translates your code into assembly language, where function calls are translated into the CALL instruction. This instruction takes a single argument which is the target function's starting address in memory. When executed, the CPU saves the address of the instruction following the CALL on the stack, which is used later to return to the calling function with the RET instruction. This process is part of the assembly language and is crucial for function calls to work correctly within compiled programs.