I'm probably wrong, but my intuition tells me (in terms of Theory of Computation) that Forth is a programming language for Push-Down-Automata, and not a full-blown Turing Machine.
"a Turing machine is also equivalent to a two-stack PDA with standard LIFO semantics, by using one stack to model the tape left of the head and the other stack for the tape to the right."
Forth is essentially a two-stack PDA as described.
