HP 15c - Manual (Page 100)

100 Section 8: Program Branching and Controls
Flag 9. An overflow condition (described on page 61) automatically sets
flag 9. Flag 9 causes the display to blink or, if a program is running, waits
until execution is complete and then starts blinking the display.
Flag 9 may be cleared in three ways:

Press | " 9 (the common procedure for clearing flags).

Press −. This will only clear flag 9 and stop the blinking—it will
not clear the display.

Turn the calculator off. (Flag 9 is not cleared if the calculator turns
itself off.)
If you set flag 9 manually (F 9), it causes the display to blink irrespective
of the overflow status of the calculator. As usual, a program will run to
completion before the display starts blinking. Therefore, flag 9 can be used
as a programming tool to provide a visual signal for a selected condition.