Free42 NSTK ENTER behavior

[ijabbott]

To me, the biggest limitation of the 4STK model is the silent errors introduced into the calculation if you are not aware that you have gone over the limit. You really need to be on your toes when dealing with a deeply nested expressions, planning in advance how to tackle it, and determining whether 4 stack registers is enough. Cheap Casio scientific calculators have a similar (though more generous) limit, but are gracious enough to signal an error when the limit is exceeded. That is impossible with the 4STK model because losing numbers off the top of the stack is part of the normal operation.