Epic,
It looks like you are going to have an easy task to make the conversion. THe FIFW appears to be very compatible to the RSLogix FFL (FIFO Load), and the FIFR is similar to the RSLogix FFU (FIFO Unload).
Melsoft FIFW parameter (S) = RSLogix FFL "Source", stores the value to be entered next into the FIFO stack.
Melsoft FIFW parameter (D) = RSLogix FFL "FIFO", is the address of the stack. The same address is programmed for the FFL and FFU instructions.
Melsoft FIFW parameter "Data storage position" = head address of data table + content of pointer, where RSLogix FFL and FFU "Position" word is the next available location where the instruction loads data into the stack. This value changes after each Load or Unload operation.
The RSLogix FFL and FFU also have additional parameters, such as Length (total number of stack words), and the Control file address, where the status bits, stack length, and the position value are stored.