Owner's handbook and programming guide, HP-41C

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Range Error and Error Ignore Flags

Two flags in the HP-41C can be used to control how the calculator reacts to range error (overflows and underflows) and all operating errors. Flag 24 is the range error ignore flag flag 25 is the error ignore flag. These flags provide excellent error detection and handling in your programs.

Flags 24 and 25 are both cleared each time you turn the calculator on.

Range Errors

Remember from part I of this handbook that any calculation that exceeds the computation or storage range of the calculator is an error (except in statistics calculations). Normally, when such a calculation is attempted, the HP-41C immediately displays OUT OF RANGE and the error-causing function is not executed. Flag 24 allows you to ignore these out-of-range errors.

If flag 24 is set, the the HP-41C places ±9.999999999 × 10⁹⁹ into the affected register and execution continues. Notice that the range error ignore flag is not cleared when the error occurs. Since flag 24 is cleared (automatically) only when you turn the calculator on, you only need to set it one time at the beginning of the program. All subsequent range errors will be ignored by the calculator.

Specifically, a range error is an overflow where a number is generated that exceeds ±9.999999999 × 10⁹⁹. Underflows (numbers closer to zero than ±1 × 10^–99) do not cause the OUT OF RANGE message to be displayed. Zeros are placed into the affected register. Other range errors that can be ignored by flag 24 are listed in appendix E.

For example, the following program demonstrates how flag 24 works. An infinite loop in the program begins with 1 × 10¹⁰ and alternately multiplies and divides that number by 1 × 10¹⁰. Each time through the loop, the result from the previous multiply is multiplied by 1 × 10¹⁰, and the result from the previous divide is divided by 1 × 10¹⁰. You can watch as the displayed numbers approach the overflow (9.999999999 × 10⁹⁹) and the underflow (0.000000000 × 10⁰⁰). Since flag 24 is set, the overflow error does not cause the program to stop.