The two program counters were first introduced in the National Bureau of Standards' SWAC computer. The idea was that you would use the second program counter fo

The two program counters were first introduced in the National Bureau of Standards' SWAC computer. The idea was that you would use the second program counter for a subroutine call, and then switch back to the first program counter to return to your original code. I don't think anyone really used the backward counting on the program counter; as far as I can tell, that was a side-effect of all the registers being able to auto-increment or auto-decrement.