Flight model: GetMoments

       Name: GetMoments                                              [Show more]
       Type: Subroutine
   Category: Flight model
    Summary: Calculate the pitching, rolling and yawing moments due to the
             current pitch, roll and yaw rates
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyAerodynamics (Part 3 of 3) calls GetMoments
             * Multiply8x16 calls via GetMoments-1


 This routine calculates:

   xTemp3 = yLiftDrag - (xTurn * 250 / 256)

   yTemp3 = xLiftDrag - (yTurn * 250 / 256)

   zTemp3 = -zTurn * 2

 which contain the moments due to the plane's current movement. These then get
 scaled by altitude and put into (xMoments yMoments zMoments).


 Other entry points:

   GetMoments-1         Contains an RTS


.GetMoments

 LDX #1                 \ Set a counter in X to work through the x- and y-axes

.torq1

 LDA #125               \ Set R = 125
 STA R

 LDA xTurnHi,X          \ Set (Q P) = (xTurnTop xTurnHi)
 STA P                  \           = xTurn
 LDA xTurnTop,X
 STA Q

 ASL P                  \ Set (Q P) = (Q P) * 2
 ROL Q                  \           = xTurn * 2

 JSR Multiply8x16-2     \ Store X in VV and set:
                        \
                        \   (G W V) = (Q P) * R
                        \           = xTurn * 2 * 125
                        \           = xTurn * 250

 LDA VV                 \ Set A and X to the axis number that the above call
 TAX                    \ stored in VV, which is the same as the axis counter X
                        \ (so it's 0 for the x-axis and 1 for the y-axis)

 EOR #1                 \ Set Y to the opposite axis number to X, so when we are
 TAY                    \ calculating xTemp3 in the following and X = 0, Y = 1
                        \ so we use yLiftDrag, and when we are calculating
                        \ yTemp3, X = 1 and Y = 0, so we use xLiftDrag

 SEC                    \ Set xTemp3 = yLiftDrag - (G W)
 LDA xLiftDragLo,Y      \            = yLiftDrag - (xTurn * 250 / 256)
 SBC W
 STA xTemp3Lo,X
 LDA xLiftDragHi,Y
 SBC G
 STA xTemp3Hi,X

 DEX                    \ Decrement the loop counter to move to the next axis

 BPL torq1              \ Loop back until we have processed both axes

 SEC                    \ Set (A zTemp3Lo) = 0 - (zTurnTop zTurnHi)
 LDA #0                 \                   = -zTurn
 SBC zTurnHi            \
 STA zTemp3Lo           \ starting with the high bytes

 LDA #0                 \ And then the low bytes
 SBC zTurnTop

 ASL zTemp3Lo           \ Set (zTemp3Hi zTemp3Lo) = (A zTemp3Lo) << 1
 ROL A                  \                           = -zTurn * 2
 STA zTemp3Hi

 RTS                    \ Return from the subroutine