FD, ED, FA, EA define pre/post indexing and the up/down bit by reference to the form of stack required.
The F and E refer to a “full” or “empty” stack, i.e. whether a pre-index has to be done (full) before storing
to the stack. The A and D refer to whether the stack is ascending or descending. If ascending, a STM will go
up and LDM down, if descending, vice-versa.