Maths (Geometry): SPS4

       Name: SPS4                                                    [Show more]
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: Calculate the vector to the space station
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * SP1 calls SPS4


 Calculate the vector between our ship and the space station and store it in
 XX15.


.SPS4

 LDX #8                 ; First we need to copy the space station's coordinates
                        ; into K3, so set a counter to copy the first 9 bytes
                        ; (the 3-byte x, y and z coordinates) from the station's
                        ; data block at K% + NI% into K3

.SPL1

 LDA K%+NI%,X           ; Copy the X-th byte from the station's data block at
 STA K3,X               ; K% + NI% to the X-th byte of K3

 DEX                    ; Decrement the loop counter

 BPL SPL1               ; Loop back to SPL1 until we have copied all 9 bytes

 JMP TAS2               ; Call TAS2 to build XX15 from K3, returning from the
                        ; subroutine using a tail call