Maths (Geometry): SubtractCoords

       Name: SubtractCoords                                          [Show more]
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: Subtract from a specified coordinate variable
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyWingBalance calls SubtractCoords


 This routine subtracts (U T) from the specified 16-bit variable.

 Specifically, it calculates:

   variableY = variableY - (U T) * abs(A)

 where A is the last variable to be loaded before the subroutine call. So if
 the call follows an LDA instruction, for example, the following is calculated
 if A is positive:

   variableY = variableY - (U T)

 and the following is calculated if A is negative:

   variableY = variableY + (U T)


 Arguments:

   N flag               Determines the action:

                          * If positive, subtract (U T)

                          * If negative, add (U T)


.SubtractCoords

 BMI AddCoords          \ If A is negative, jump to AddCoords to calculate:
                        \
                        \   variableY = variableY + (U T)

 JSR Negate16Bit+2      \ Set (A T) = -(U T)

 STA U                  \ Set (U T) = (A T)
                        \           = -(U T)

                        \ Fall through into AddCoords to calculate:
                        \
                        \   variableY = variableY - (U T)