Moving: SFS2

       Name: SFS2                                                    [Show more]
       Type: Subroutine
   Category: Moving
    Summary: Move a ship in space along one of the coordinate axes
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * SFS1 calls SFS2


 Move a ship's coordinates by a certain amount in the direction of one of the
 axes, where X determines the axis. Mathematically speaking, this routine
 translates the ship along a single axis by a signed delta.


 Arguments:

   A                    The amount of movement, i.e. the signed delta

   X                    Determines which coordinate axis of INWK to move:

                          * X = 0 moves the ship along the x-axis

                          * X = 3 moves the ship along the y-axis

                          * X = 6 moves the ship along the z-axis


.SFS2

 ASL A                  ; Set R = |A * 2|, with the C flag set to bit 7 of A
 STA R

 LDA #0                 ; Set bit 7 of A to the C flag, i.e. the sign bit from
 ROR A                  ; the original argument in A

 JMP MVT1               ; Add the delta R with sign A to (x_lo, x_hi, x_sign)
                        ; (or y or z, depending on the value in X) and return
                        ; from the subroutine using a tail call