Track geometry: MultiplyHeight

       Name: MultiplyHeight                                          [Show more]
       Type: Subroutine
   Category: Track geometry
    Summary: Multiply the height above ground of a specified track segment by A
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyElevation (Part 3 of 5) calls MultiplyHeight
             * ApplyElevation (Part 5 of 5) calls MultiplyHeight


 For segment vector Y, calculate:

   A = A * yTrackSegmentI

 flipping the sign if we are facing backwards.


 Arguments:

   A                    The number to multiply the height by

   Y                    Index of the segment vector to multiply


 Returns:

   N flag               Set according to the result in A


 Other entry points:

   MultiplyHeight+11    Set A = A * U while retaining the sign in A (the sign of
                        A must be on the stack before calling this entry point)


.MultiplyHeight

 STA U                  \ Set U to the multiplication factor in A

 LDA yTrackSegmentI,Y   \ Store the sign of the height * directionFacing on
 EOR directionFacing    \ the stack
 PHP

 LDA yTrackSegmentI,Y   \ Set A to the track height in the Y-th yTrackSegmentI

 JSR Absolute8Bit       \ Set A = |A|
                        \       = |yTrackSegmentI|

 JSR Multiply8x8        \ Set (A T) = A * U
                        \           = U * |yTrackSegmentI|

 PLP                    \ Set the N flag to the sign of yTrackSegmentI *
                        \ directionFacing, so this will be set if the height
                        \ sign is different to bit 7 of directionFacing, clear
                        \ if the height sign matches bit 7 of directionFacing

 JSR Absolute8Bit       \ Give A the sign in the N flag

 RTS                    \ Return from the subroutine