Today, when I turned my 15C on (having been off for a couple of weeks), the display was flashing, showing the contents of the X-register. When I hit the Clx key (back arrow), it stopped flashing. The self test and the keyboard test ran fine, and it seems to be working fine. Anybody have any idea why the display would flash like that?

Thanks and best regards, Hal

There are two documented ways to make the display blink as you mention:

1. An overflow condition, where the result of some computation exceeds the available range (i.e., > 9.999999999E+99). A running program which generates an overflow will continue running, but when it eventually stops, the display will be left blinking (unless the program clears flag 9, as discussed next).

2. Manually setting flag 9
   

You can stop the display blinking by pressing the backarrow key, executing CF 9, or turning off the machine. If it turns itself off, flag 9 won't be cleared.

Best regards from V.