Flight model: ProcessLanding (Part 1 of 7)

       Name: ProcessLanding (Part 1 of 7)                            [Show more]
       Type: Subroutine
   Category: Flight model
    Summary: If this is an emergency landing, make the landing a bumpy one
  Deep dive: Take-offs and landings
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyFlightModel (Part 6 of 7) calls ProcessLanding


 This part checks whether the plane is on the ground but not on the runway, and
 if this is the case, it implements a bumpy ride by randomly changing the
 plane's height and roll, with larger changes at bigger speeds.


 Arguments:

   L                    The current value of onGround:

                          * 0 = we are not on the ground

                          * 1 = we are on the ground


.ProcessLanding

 LDA L                  \ Fetch the current value of onGround

 BEQ clan1              \ If onGround is zero then we are not on the ground, so
                        \ jump to clan1 to skip the bumpy ride

 JSR CheckPlaneOnRunway \ Check whether the plane is over the runway

 BCC clan1              \ If the plane is on the runway, then jump to clan1 to
                        \ skip the bumpy ride

                        \ If we get here then we are on the ground but we aren't
                        \ on the runway, so this must be an emergency landing
                        \ outside the airport, and we apply a suitably bumpy
                        \ ride, depending on the plane's velocity

 ASL landingStatus      \ Shift landingStatus left and set bit 0

 LDX #&EE               \ Set X so the call to ApplyBumpyRide updates the yPlane
                        \ variable, so it makes the plane bump up by a random
                        \ amount

 CLC                    \ Clear the C flag so the call to ApplyBumpyRide
                        \ adds a random amount to yPlane (rather than taking it
                        \ away)

 LDY #8                 \ Set the scale factor in Y so the random amount we bump
                        \ the plane up by is in the range 0 to zVelocityP >> 9,
                        \ so if we try to land at 100 mph, when zVelocityP is
                        \ (9 64) = 2368, then this would bump the plane up
                        \ by a random value in the range 0 to 4 (as 2368 >> 9
                        \ is 4)

 JSR ApplyBumpyRide     \ Bump the plane up by a random height in the range 0 to
                        \ zVelocityP >> 9, so the bump height is higher with
                        \ higher forward velocity (so the faster we are trying
                        \ to land, the more the plane bounces up when it hits
                        \ the runway)

 LDX #&EC               \ Set X so the call to ApplyBumpyRide updates the
                        \ zRotation variable, so it makes the plane roll by a
                        \ random amount

 LDY #4                 \ Set the scale factor in Y so the random amount we roll
                        \ the plane by is in the range 0 to zVelocityP >> 5,
                        \ so if we try to land at 100 mph, when zVelocityP is
                        \ (9 64) = 2368, then this would roll the plane by a
                        \ random value in the range 0 to 74 (as 2368 >> 5 is 74)

 LDA VIA+&64            \ Read the 6522 User VIA T1C-L timer 1 low-order
                        \ counter (SHEILA &64), which decrements one million
                        \ times a second and will therefore be pretty random

 LSR A                  \ Set the C flag randomly, so the call to ApplyBumpyRide
                        \ randomly adds or subtracts the random roll amount

 JSR ApplyBumpyRide     \ Roll the plane up by a random angle in the range 0 to
                        \ zVelocityP >> 5, in a random direction, so the roll
                        \ amount is higher with higher forward velocity (so the
                        \ faster we are trying to land, the more the plane rolls
                        \ when it hits the runway)