Driving model: ApplyDeltas

       Name: ApplyDeltas                                             [Show more]
       Type: Subroutine
   Category: Driving model
    Summary: Apply the deltas in the x-axis and z-axis to the player's
             coordinates
  Deep dive: The core driving model
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyDrivingModel calls ApplyDeltas


 Calculate the following:

   xPlayerCoord = xPlayerCoord + xPlayerSpeed * 2

   zPlayerCoord = zPlayerCoord + zPlayerSpeed * 2

   playerYawAngle = playerYawAngle + spinYawAngle


.ApplyDeltas

 LDX #1                 \ Set X = 1, to act as an axis counter for xPlayerSpeed
                        \ and zPlayerSpeed
                        \
                        \ The comments below are for when X = 1

 LDY #2                 \ Set Y = 2, to act as a variable counter in the
                        \ following loop, iterating through values 0 and 2, as
                        \ Y is decremented twice at the end of the loop
                        \
                        \ The comments below are for when Y = 2

.delt1

 LDA #0                 \ Set V = 0, to use as the sign bit for (V A T)
 STA V

 LDA xPlayerSpeedHi,X   \ Set (V A T) = zPlayerSpeed
 STA T
 LDA xPlayerSpeedTop,X

 BPL delt2              \ If A is positive, jump to delt2 to skip the following
                        \ instruction

 DEC V                  \ Set V = &FF, so it contains the correct sign bit for
                        \ (V A T)

.delt2

 ASL T                  \ Set (U A T) = (V A T) << 1
 ROL A
 ROL V
 STA U

 LDA xPlayerCoordLo,Y   \ Set zPlayerCoord = zPlayerCoord + (U A T)
 ADC T                  \                  = zPlayerCoord + zPlayerSpeed * 2
 STA xPlayerCoordLo,Y   \
                        \ starting with the low bytes

 LDA xPlayerCoordHi,Y   \ Then the high bytes
 ADC U
 STA xPlayerCoordHi,Y

 LDA xPlayerCoordTop,Y  \ And then the top bytes
 ADC V
 STA xPlayerCoordTop,Y

 DEY                    \ Decrement Y twice so the next iteration adds to the
 DEY                    \ xPlayerCoord variable

 DEX                    \ Decrement X so the next iteration sets (V A T) to
                        \ xPlayerSpeed

 BPL delt1              \ Loop back until we have updated both xPlayerCoord and
                        \ zPlayerCoord

 LDA playerYawAngleLo   \ Set playerYawAngle = playerYawAngle + spinYawAngle
 CLC                    \
 ADC spinYawAngleHi     \ starting with the low bytes
 STA playerYawAngleLo

 LDA playerYawAngleHi   \ And then the high bytes
 ADC spinYawAngleTop
 STA playerYawAngleHi

 RTS                    \ Return from the subroutine