Maths (Arithmetic): DV42

       Name: DV42                                                    [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Calculate (P R) = 256 * DELTA / z_hi
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * STARS1 calls DV42
             * STARS6 calls DV42


 Calculate the following division and remainder:

   P = DELTA / (the Y-th stardust particle's z_hi coordinate)

   R = remainder as a fraction of A, where 1.0 = 255

 Another way of saying the above is this:

   (P R) = 256 * DELTA / z_hi

 DELTA is a value between 1 and 40, and the minimum z_hi is 16 (dust particles
 are removed at lower values than this), so this means P is between 0 and 2
 (as 40 / 16 = 2.5, so the maximum result is P = 2 and R = 128.

 This uses the same shift-and-subtract algorithm as TIS2, but this time we
 keep the remainder.


 Arguments:

   Y                    The number of the stardust particle to process


 Returns:

   C flag               The C flag is cleared


.DV42

 LDA SZ,Y               ; Fetch the Y-th dust particle's z_hi coordinate into A

                        ; Fall through into DV41 to do:
                        ;
                        ;   (P R) = 256 * DELTA / A
                        ;         = 256 * DELTA / Y-th stardust particle's z_hi