HP 9100B Operating & Programming Manual page 31

THE KEYBOARD
SWITCHES
ILLEGAL OPERATIONS LIGHT, to the left of the CRT, indicates
that a mathematically illegal operation has been performed,
either from the keyboard or from the program.
An illegal operation during a program will not stop the program;
however, the light will remain set. Press any key on the key-
board to reset the light (see Page 16 for examples and appar-
ent exceptions to this).
Illegal operations are:
Division by zero
{X where x < 0
ln x where x
<
0; log x where x
<
0
arc sin x where jxj
>
1; arc cos x where lx l
>
1
arc cosh x where x
<
1; arc tanh x where !xl
>
1
21