SENSE LIGHT 1
IN the original Fortran there is the command SENSE LIGHT 1 to 5.
So if you are looking for the flow of the program these lights provide a simple method. Of course now we debug or use write statements.
When did the SENSE LIGHT commands leave Fortran?
It was a pretty good idea, you could get your program to output morse code on the lights and follow without all those blasted write Statements.
Bring back the sense.
A monochrome sense light represents a single bit, so three sense lights gave you the equivalent of a WRITE with an I/O list that could be no longer than three bits.
Since there was no timesharing on those old monsters, a normal user (not the operator) would never see a sense light that was on for a few moments several hours earlier.
Similarly for sense switches as input devices to control execution.
Early versions of Fortran sometimes had odd, and sometimes machine specific, keywords.
Fortran on the 1960's vintage IBM 1620, had device specific I/O statements like ACCEPT for the keyboard, PUNCH to write to a card on the card reader/punch. The machine included 4 toggle switches whose status you could test in a Fortran II program using a special IF statement that included SENSE SWITCH in one way or another. Seems like we used it to come up with a seed for random number generation by having a loop that incremented a variable until the user flipped one of the toggle switches.