Adding two 16-bit numbers is one of the first real programs every assembly language student writes — and for good reason. It touches every foundational concept at once: segment registers, data declarations, arithmetic instructions, result storage, and program termination. This post walks through a working implementation in three assembler environments: MASM/TASM (the classic DOS toolchain), emu8086 (the popular Windows emulator used in college labs), and NASM (the modern open-source assembler). All examples are tested and produce verifiable output.