FIT2D uses two main internal arrays to store image data and to perform almost all operations. The default array will be referred to as the ``current data array'' ; this is where data is stored when input, and where data to be displayed graphically must be situated. The second array will be referred to as the ``memory''. The ``memory'' is like the memory facility on many calculators except that the single memory may take up 50 Mbytes or more, and may store a full image.
To move data from the current data array to the memory and vice versa, most graphical menus contain the EXCHANGE button, and the keyboard interface contains the EXCHANGE command. The output of all operations, which produce output data, within the GUI is in the current data array. In many cases the original data is transferred to the memory, and thus may be recovered using the EXCHANGE button. In other cases the original data may be recovered using the inverse operation e.g. SUBTRACT after ADD.