Maths (Geometry): TAS4

       Name: TAS4                                                    [Show more]
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: Calculate the dot product of XX15 and one of the space station's
             orientation vectors
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DOCKIT calls TAS4


 Calculate the dot product of the vector in XX15 and one of the space station's
 orientation vectors, as determined by the value of Y. If vect is the space
 station orientation vector, we calculate this:

   (A X) = vect . XX15
         = vect_x * XX15 + vect_y * XX15+1 + vect_z * XX15+2

 Technically speaking, this routine can also calculate the dot product between
 XX15 and the sun's orientation vectors, as the sun and space station share the
 same ship data slot (the second ship data block at K%). However, the sun
 doesn't have orientation vectors, so this only gets called when that slot is
 being used for the space station.


 Arguments:

   Y                    The space station's orientation vector:

                          * If Y = 10, calculate nosev . XX15

                          * If Y = 16, calculate roofv . XX15

                          * If Y = 22, calculate sidev . XX15


 Returns:

   (A X)                The result of the dot product


.TAS4

 LDX K%+NI%,Y           ; Set Q = the Y-th byte of K%+NI%, i.e. vect_x from the
 STX Q                  ; second ship data block at K%

 LDA XX15               ; Set A = XX15

 JSR MULT12             ; Set (S R) = Q * A
                        ;           = vect_x * XX15

 LDX K%+NI%+2,Y         ; Set Q = the Y+2-th byte of K%+NI%, i.e. vect_y
 STX Q

 LDA XX15+1             ; Set A = XX15+1

 JSR MAD                ; Set (A X) = Q * A + (S R)
                        ;           = vect_y * XX15+1 + vect_x * XX15

 STA S                  ; Set (S R) = (A X)
 STX R

 LDX K%+NI%+4,Y         ; Set Q = the Y+2-th byte of K%+NI%, i.e. vect_z
 STX Q

 LDA XX15+2             ; Set A = XX15+2

 JMP MAD                ; Set:
                        ;
                        ;   (A X) = Q * A + (S R)
                        ;           = vect_z * XX15+2 + vect_y * XX15+1 +
                        ;             vect_x * XX15
                        ;
                        ; and return from the subroutine using a tail call