My one and only classic HP is an HP-80, like many with the 1247... serial number but the ROM chip shows a date of 1/73.

It exhibits a strage characteristic when in exponential display mode with a number less than 1, and CHS is pressed. Instead of only inserting a negative sign in the left-most position, it also changes the exponential value to -99.

Toggling CHS leaves the exponential value at -99 but toggles the left-most negative sign as expected. However, pressing SAVE resets the exponential to the original value, switching back to decimal display shows the correct value and calculations are correct even when the exponential value shows -99.

For an example keystrokes and display:

Power on " 0.00 "

.1 SAVE " 0.10 "

Shift 7 " 1. -01"

CHS "-1. -99"

Shift 2 "-0.10 "

So it seems the CHS induced exponential -99 display is just a display curiosity, but I cannot find any reference to it anywhere. Is this something other people have seen on the HP-80?

Kind regards, MAX