Ah, debug. I used debug extensively for machine code programming on DOS. I couldn't afford MASM and certainly didn't have access to free assemblers. My first hello world program was almost exactly this.

However, I always ended my programs with INT 20 instead of RET, or INT 21 with AH = 0. RET worked both for standalone programs or in debug, but there was one exception: if you ran your code repeatedly in debug with G 100. It would work the first time, but IIRC the stack wasn't reinitialized to return to the correct place on subsequent G commands. I spent a lot of time typing, running and patching in a single session and INT 20 always worked correctly.