Main loop: Main flight loop (Part 14 of 16)

       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
             The space station safe zone
    Context: See this subroutine in context in the source code
 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 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

 JSR SpawnSpaceStation  ; If we are close enough, add a new space station to our
                        ; local bubble of universe

 BCS MA23S              ; If we spawned the space station, jump to MA23S to skip
                        ; the following

                        ; We didn't manage to spawn the space station, so now
                        ; we rotate the planet around the x-axis and then the
                        ; z-axis, and then have another go
                        ;
                        ; We rotate the planet by swapping the orientation
                        ; vectors as follows (in parallel):
                        ;
                        ;   * Set nosev = roofv
                        ;
                        ;   * Set roofv = sidev
                        ;
                        ;   * Set sidev = nosev
                        ;
                        ; First we need to set up a ship data block for the
                        ; station in INWK again, which we do by copying the
                        ; first nine bytes from the planet data block in K% to
                        ; INWK (we only need the coordinates and the orientation
                        ; vectors for the calculation, and we'll do the latter
                        ; in a minute)

 LDX #8                 ; Set a counter in X to copy 9 bytes from K%+0 to K%+8

.main44

 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 main44             ; Loop back for the next byte until we have copied the
                        ; all 9 bytes of coordinate data from K% to INWK

                        ; Next we rotate the orientation vectors

 LDX #5                 ; Set a counter in X to swap each of the six bytes of
                        ; the orientation vectors (the comments below are for
                        ; when X = 0, in which we swap the x_lo bytes)

.main45

 LDY INWK+9,X           ; Set Y to nosev_x_lo

 LDA INWK+15,X          ; Set nosev_x_lo = roofv_x_lo
 STA INWK+9,X

 LDA INWK+21,X          ; Set roofv_x_lo = sidev_x_lo
 STA INWK+15,X

 STY INWK+21,X          ; Set sidev_x_lo = Y = nosev_x_lo

 DEX                    ; Decrement the loop counter

 BPL main45             ; Loop back until we have swapped all six bytes of each
                        ; orientation vector

 JSR SpawnSpaceStation  ; Now we check to see if we are close enough to the
                        ; rotated planet, and if so, 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