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⌂ ∞ ▽ Revs on the BBC Micro

Driving model: ApplySteeringSpeed

       Name: ApplySteeringSpeed                                      [Show more]
       Type: Subroutine
   Category: Driving model
    Summary: Apply steering to the car's speed in xVelocity and zVelocity
  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 ApplySteeringSpeed




 Calculate the following in parallel:

   zVelocity = zVelocity + xVelocity * steering

   xVelocity = xVelocity - zVelocity * steering



.ApplySteeringSpeed

 LDX #2                 \ Set X = 2, so in the call to MultiplyCoords+7 we use
                        \ (steeringHi steeringLo) as the 16-bit sign-magnitude
                        \ value to multiply

 LDY #9                 \ Set Y = 9, so in the call to MultiplyCoords+7 we use
                        \ zVelocity as the 16-bit signed number

 LDA #%10000000         \ Clear bit 6 and set bit 7 of H, to store the result
 STA H                  \ rather than adding, and negate the result 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 = -zVelocity * steering

 LDX #2                 \ Set X = 2, so in the call to MultiplyCoords+7 we use
                        \ (steeringHi steeringLo) as the 16-bit sign-magnitude
                        \ value to multiply

 LDY #8                 \ Set Y = 8, so in the call to MultiplyCoords+7 we use
                        \ xVelocity as the 16-bit signed number

 LDA #%01000000         \ Set bit 6 and clear bit 7 of H, to add the result
 STA H                  \ rather than replacing, and leave the sign of the
                        \ result alone in the call to MultiplyCoords+7

 LDA #9                 \ Set A = 9, so in the call to MultiplyCoords+7, we
                        \ store the result in zVelocity

 JSR MultiplyCoords+7   \ Set:
                        \
                        \   variableA = variableA + variableY * variableX
                        \
                        \ so:
                        \
                        \   zVelocity = zVelocity + xVelocity * steering

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

 JSR AddSteeringForce   \ Set xVelocity = xVelocity + xSteeringForce
                        \               = xVelocity - zVelocity * steering

 RTS                    \ Return from the subroutine