What variables to allocate to Internal RAM and what to External RAM ?

[pest]

When in a project with both Internal and External RAM, and space available in both, how to choose what variables are to be allocated where ? Is there a general rule on when to allocate to External RAM or Internal RAM ?

Similarly how about code in External or Internal ROM ?

[kef]

Internal memory is faster and more reliable than any external memory. So you should try to keep everything in internal memory. In case you don't have enough RAM, you should use external RAM for less time and safety critical data.