The SPS1 (Flight) subroutine - Elite on the BBC Micro and NES

       Name: SPS1                                                    [Show more]
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: Calculate the vector to the planet, sun or station and store it in
             XX15
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * COMPAS calls SPS1
             * Main flight loop (Part 9 of 16) calls SPS1
             * TACTICS (Part 3 of 7) calls SPS1


 Arguments:

   Y                    Determines the object whose vector we are calculating:

                          * 0 = calculate the vector to the planet

                          * NI% = calculate the vector to the sun/space station

 Other entry points:

   SPS1+1               A BRK instruction


.SPS1

                        \ --- Mod: Code removed for Elite-A: ------------------>

\ LDX #0                \ Copy the two high bytes of the planet's x-coordinate
\ JSR SPS3              \ into K3(2 1 0), separating out the sign bit into K3+2
\
\ LDX #3                \ Copy the two high bytes of the planet's y-coordinate
\ JSR SPS3              \ into K3(5 4 3), separating out the sign bit into K3+5
\
\ LDX #6                \ Copy the two high bytes of the planet's z-coordinate
\ JSR SPS3              \ into K3(8 7 6), separating out the sign bit into K3+8

                        \ --- And replaced by: -------------------------------->

 LDX #0                 \ Copy the two high bytes of the planet/sun/station's
 JSR SPS3               \ x-coordinate into K3(2 1 0), separating out the sign
                        \ bit into K3+2

 JSR SPS3               \ Copy the two high bytes of the planet/sun/station's
                        \ y-coordinate into K3(5 4 3), separating out the sign
                        \ bit into K3+5

 JSR SPS3               \ Copy the two high bytes of the planet/sun/station's
                        \ z-coordinate into K3(8 7 6), separating out the sign
                        \ bit into K3+8

                        \ --- End of replacement ------------------------------>

                        \ Fall through into TAS2 to build XX15 from K3
Maths (Geometry): SPS1

[Elite-A, Flight]
