Interpreting the position of the proportional controller normally requires
some preliminary work during the  vertical blanking  interval.

During  vertical blanking , you write a value into an address called
 potgo . for a standard x-y joystick, this value is hex 0001. writing to
this register starts the operation of some special hardware that reads the
potentiometer values and sets the values contained in the  pot registers 
(described below) to zero.

The read circuitry stays in a reset state for the first seven or eight
horizontal video scan lines. Following the reset interval, the circuit
allows a charge to begin building up on a timing capacitor whose charge
rate will be controlled by the position of the external controller
resistance. For each horizontal scan line thereafter, the circuit compares
the charge on the timing capacitor to a preset value. If the charge is
below the preset, the  pot counter  is incremented. if the charge is above
the preset, the counter value will be held until the next  potgo  is
issued.

     figure 8-5: effects of resistance on charging rate 

You normally issue  potgo  at the beginning of a video screen, then read
the values in the  pot registers  during the next  vertical blanking 
period, just before issuing  potgo  again.

Nothing in the system prevents the counters from overflowing (wrapping
past a count of 255). However, the system is designed to insure that the
counter cannot overflow within the span of a single screen. This allows
you to know for certain whether an overflow is indicated by the controller.