HP (Hewlett-Packard) HP 33s ユーザーズマニュアル

occurs when the magnitude of a number is smaller than the calculator 

can represent, so it substitutes zero. This can affect SOLVE results. For example, 
consider the equation 

2

1

whose root is infinite in value. Because of underflow, SOLVE returns a very large 
value as a root. (The calculator cannot represent infinity, anyway.)