HP -01 Owner's Manual page 97

Look at the contents of these registers. This should give you some insight as to how the
automatic doubling and automatic squaring features of the HP-01 work. If you press @), the
squaring operation would be performed. If the arithmetic operator were €, pressing @ would
double the operand.
Key in the second operand, 3. This entry goes into the Y-register and overwrites the X-operand
placed there.
Press Registers Display
X F Y
0 3
If you press (@ at this point, the Y-register would be cleared. Your HP-01 would return to the
same state it was immediately after the operator key was pressed; you could change the
operator or key in another operand.
Operational Description 95