Flight model: ScaleByAltitude

       Name: ScaleByAltitude                                         [Show more]
       Type: Subroutine
   Category: Flight model
    Summary: Multiply the high byte of the plane's altitude by a 16-bit number
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyAerodynamics (Part 1 of 3) calls ScaleByAltitude
             * ApplyTurnAndThrust (Part 2 of 2) calls ScaleByAltitude
             * ApplyBumpyRide calls via ScaleByAltitude-1


 Calculate:

   (Y X V) = (Y X) * ~yPlaneHi


 Arguments:

   (Y X)                A signed 16-bit number


 Returns:

   (Y X V)              The result, (Y X) * ~yPlaneHi


 Other entry points:

   ScaleByAltitude-1      Contains an RTS


.ScaleByAltitude

 STY Q                  \ Set (Q P) = (Y X)
 STX P

 LDA yPlaneHi           \ Set R = ~yPlaneHi
 EOR #&FF
 STA R

 JSR Multiply8x16       \ Set (G W V) = (Q P) * R
                        \             = (Y X) * ~yPlaneHi
                        \
                        \ and also set A to the high byte of the result

 TAY                    \ Set (Y X V) = (G W V)
 LDX W

 RTS                    \ Return from the subroutine