74  Section 5: Additional Operating Features
If a calculation results in a number whose magnitude is less than 10
-99
, the 
calculation is not halted, but the value 0 is used for that number in subsequent 
calculations.
Errors. If you attempt an improper operation – such as division by zero – the 
calculator will display the word Error followed by a digit (0 through 9). To clear 
the Error display, press any key. This does not execute that key’s function, but 
does restore the calculator to its condition before the improper operation was 
attempted. Refer to Appendix D for a list of error conditions.
Pr Error. If power to the calculator is interrupted, the calculator will display Pr 
Error when next turned on. This indicates that Continuous Memory – which 
contains all data, program, and status information – has been reset.
The ~ Key
Suppose you need to subtract $25.83 from $144.25, and you (mistakenly) key in 
25.83, press \, then key in 144.25. But then you realize that when written 
down on paper, the desired calculation reads 144.25 – 25.83, so that you have 
unfortunately keyed in the second number first. To correct this mistake, merely 
exchange the first and second numbers by pressing ~, the exchange key.
The ~ key is also useful for checking the first number entered to make sure 
you keyed it in correctly. Before pressing the operation key, however, you should 
press ~ again to return the second number entered to the display. Regardless 
of how many times you press ~, the calculator considers the number in the 
display to be the second number entered.
The F Key
Occasionally you may want to recall to the display the number that was there 
before an operation was performed. (This is useful for doing arithmetic 
calculations with constants and for recovering from errors in keying in numbers.) 
To do so, press gF (last x).
Keystrokes (RPN mode) Display
25.83\144.25
144.25
Oops! You mistakenly keyed in the 
second number first.
~
25.83
Exchanges the first and second 
numbers. The first number keyed in 
is now in the display.
-
118.42
The answer is obtained by pressing 
the operation key.