Main loop: Main flight loop (Part 14 of 16) [6502SP version]

       Name: Main flight loop (Part 14 of 16)                        [Show more]
       Type: Subroutine
   Category: Main loop
    Summary: Spawn a space station if we are close enough to the planet
  Deep dive: Program flow of the main game loop
             Scheduling tasks with the main loop counter
             Ship data blocks
    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: No direct references to this subroutine in this source file


 The main flight loop covers most of the flight-specific aspects of Elite. This
 section covers the following:

   * Spawn a space station if we are close enough to the planet (every 32
     iterations of the main loop)


 LDA MJ                 \ If we are in witchspace, jump down to MA23S to skip
 BNE MA23S              \ the following, as there are no space stations in
                        \ witchspace

 LDA MCNT               \ Fetch the main loop counter and calculate MCNT mod 32,
 AND #31                \ jumping to MA93 if it is on-zero (so the following
 BNE MA93               \ code only runs every 32 iterations of the main loop

 LDA SSPR               \ If we are inside the space station safe zone, jump to
 BNE MA23S              \ MA23S to skip the following, as we already have a
                        \ space station and don't need another

 TAY                    \ Set Y = A = 0 (A is 0 as we didn't branch with the
                        \ previous BNE instruction)

 JSR MAS2               \ Call MAS2 to calculate the largest distance to the
 BNE MA23S              \ planet in any of the three axes, and if it's
                        \ non-zero, jump to MA23S to skip the following, as we
                        \ are too far from the planet to bump into a space
                        \ station

                        \ We now want to spawn a space station, so first we
                        \ need to set up a ship data block for the station in
                        \ INWK that we can then pass to NWSPS to add a new
                        \ station to our bubble of universe. We do this by
                        \ copying the planet data block from K% to INWK so we
                        \ can work on it, but we only need the first 29 bytes,
                        \ as we don't need to worry about bytes #29 to #35
                        \ for planets (as they don't have rotation counters,
                        \ AI, explosions, missiles, a ship line heap or energy
                        \ levels)

 LDX #28                \ So we set a counter in X to copy 29 bytes from K%+0
                        \ to K%+28

.MAL4

 LDA K%,X               \ Load the X-th byte of K% and store in the X-th byte
 STA INWK,X             \ of the INWK workspace

 DEX                    \ Decrement the loop counter

 BPL MAL4               \ Loop back for the next byte until we have copied the
                        \ first 28 bytes of K% to INWK

                        \ We now check the distance from our ship (at the
                        \ origin) towards the planet's surface, by adding the
                        \ planet's nosev vector to the planet's centre at
                        \ (x, y, z) and checking our distance to the end
                        \ point along the relevant axis

 INX                    \ Set X = 0 (as we ended the above loop with X as &FF)

 LDY #9                 \ Call MAS1 with X = 0, Y = 9 to do the following:
 JSR MAS1               \
                        \   (x_sign x_hi x_lo) += (nosev_x_hi nosev_x_lo) * 2
                        \
                        \   A = |x_hi|

 BNE MA23S              \ If A > 0, jump to MA23S to skip the following, as we
                        \ are too far from the planet in the x-direction to
                        \ bump into a space station

 LDX #3                 \ Call MAS1 with X = 3, Y = 11 to do the following:
 LDY #11                \
 JSR MAS1               \   (y_sign y_hi y_lo) += (nosev_y_hi nosev_y_lo) * 2
                        \
                        \   A = |y_hi|

 BNE MA23S              \ If A > 0, jump to MA23S to skip the following, as we
                        \ are too far from the planet in the y-direction to
                        \ bump into a space station

 LDX #6                 \ Call MAS1 with X = 6, Y = 13 to do the following:
 LDY #13                \
 JSR MAS1               \   (z_sign z_hi z_lo) += (nosev_z_hi nosev_z_lo) * 2
                        \
                        \   A = |z_hi|

 BNE MA23S              \ If A > 0, jump to MA23S to skip the following, as we
                        \ are too far from the planet in the z-direction to
                        \ bump into a space station

 LDA #192               \ Call FAROF2 to compare x_hi, y_hi and z_hi with 192,
 JSR FAROF2             \ which will set the C flag if all three are < 192, or
                        \ clear the C flag if any of them are >= 192

 BCC MA23S              \ Jump to MA23S if any one of x_hi, y_hi or z_hi are
                        \ >= 192 (i.e. they must all be < 192 for us to be near
                        \ enough to the planet to bump into a space station)

 JSR WPLS               \ Call WPLS to remove the sun from the screen, as we
                        \ can't have both the sun and the space station at the
                        \ same time

 JSR NWSPS              \ Add a new space station to our local bubble of
                        \ universe

.MA23S

 JMP MA23               \ Jump to MA23 to skip the following planet and sun
                        \ altitude checks