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⌂ ∞ Revs on the BBC Micro

Driving model: ApplySteeringForce

       Name: ApplySteeringForce                                      [Show more]
       Type: Subroutine
   Category: Driving model
    Summary: Apply steering to xTyreForceNose and zTyreForceNose
  Deep dive: The core driving model
             Matching the code to the driving model

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyDrivingModel calls ApplySteeringForce




 Calculate the following in parallel:

   xTyreForceNose = xTyreForceNose + zTyreForceNose * steering

   zTyreForceNose = zTyreForceNose - xTyreForceNose * steering



.ApplySteeringForce

 LDX #2                 \ Set X = 2, so in the call to MultiplyCoords we use
                        \ (steeringHi steeringLo) as the 16-bit sign-magnitude
                        \ value to multiply

 LDY #12                \ Set Y = 12, so in the call to MultiplyCoords we use
                        \ zTyreForceNose as the 16-bit signed number

 LDA #%00000000         \ Clear bits 6 and 7 of H, to store the result rather
 STA H                  \ than adding, and leave the sign of the result alone in
                        \ the call to MultiplyCoords+7

 LDA #14                \ Set A = 14, so in the call to MultiplyCoords+7, we
                        \ store the result in xSteeringForce

 JSR MultiplyCoords+7   \ Set:
                        \
                        \   variableA = variableY * variableX
                        \
                        \ so:
                        \
                        \   xSteeringForce = zTyreForceNose * steering

 LDX #2                 \ Set X = 2, so in the call to MultiplyCoords we use
                        \ (steeringHi steeringLo) as the 16-bit sign-magnitude
                        \ value to multiply

 LDY #10                \ Set Y = 10, so in the call to MultiplyCoords we use
                        \ xTyreForceNose as the 16-bit signed number

 LDA #%11000000         \ Set bits 6 and 7 of H, to add the result rather than
 STA H                  \ replacing, and negate the result in the call to
                        \ MultiplyCoords+7

 LDA #12                \ Set A = 12, so in the call to MultiplyCoords+7, we
                        \ store the result in zTyreForceNose

 JSR MultiplyCoords+7   \ Set:
                        \
                        \   variableA = variableA - variableY * variableX
                        \
                        \ so:
                        \
                        \   zTyreForceNose =   zTyreForceNose
                        \                    - xTyreForceNose * steering

 LDX #10                \ Set X = 8, so the call to AddSteeringForce adds
                        \ xSteeringForce to xTyreForceNose

 JSR AddSteeringForce   \ Set xTyreForceNose = xTyreForceNose + xSteeringForce
                        \           = xTyreForceNose + zTyreForceNose * steering

 RTS                    \ Return from the subroutine