For as long as I can remember, when I first obtained a 1200XL and the 1200XL OS Manual Technical Reference Notes supplement.... I was always curious about this:
3.8 OPTION JUMPERS
The 1200XL is provided with a set of four hardware jumpers which are designed to tell the operating system how the system is configured As of the date of this writing, only one of the four jumpers has been assigned. specifically Jl. This is specified in the table below. During the power-on sequence, the 1200XL operating system reads the state of these jumpers and stores this state in the OS database variable JMPERS, location 030E.
The bit assignments for each of the four jumpers is as specified below. The bits are all active low, meaning that if a line reads a digital zero. the jumper is installed.
BIT FUNCTION HARDWARE NAME
o Self test enable (will run self test if low) J1 (pot 4) 1-3 Reserved for future use