Comment by dreamcompiler

There's always some stack frame sitting there; typically the one that called the main loop routine in the first place. At some point a RET instruction will be executed, and control will return to that stack frame's return point. The idea of tail calls is to avoid creating a new stack frame when it doesn't add any value, but stack frames still exist--they have to exist when the last thing that happens in a routine is ordinary instruction processing rather than a CALL.