We present separation results on genuinely (or strongly) time bounded sequential, parallel and nondeterministic complexity classes defined by RAMs with fixed set of arithmetic operations. In particular, we separate non-uniform polynomial time from non-uniform parallel polynomial time for the set of operations {+, −, *} (answering a question of [M 88]), and uniform deterministic polynomial time from uniform nondeterministic polynomial time for the set of operations {t+, −, DIV c }, where DIV c denotes a restricted integer division operation.