Skip to navigation

Elite on the BBC Micro

Version analysis of Main flight loop (Part 14 of 16)

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

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

Code variation 1 of 4Related to the Electron version

As the Electron version doesn't have witchspace, we always need to do the main loop check for possibly arriving in the station's safe zone.

This variation is blank in the Electron version.

Cassette, Flight, 6502SP, Master

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)

Code variation 2 of 4Specific to an individual platform

In the cassette version, we don't remove the sun from the screen if we are potentially looking at it, though this check is removed in other versions, so perhaps it isn't needed (the logic in PLANET seems to support this).

This variation is blank in the Disc (flight), 6502 Second Processor, Master and Electron versions.

Cassette

LDA QQ11 \ If the current view is not a space view, skip the BNE P%+5 \ following instruction (so we only remove the sun from \ the screen if we are potentially looking at it)

Code variation 3 of 4Related to the Electron version

As the Electron version doesn't have suns, we don't need to remove it from the screen when we enter the station's safe zone.

This variation is blank in the Electron version.

Cassette, Flight, 6502SP, Master

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

Code variation 4 of 4A variation in the comments only

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, 6502SP, Master

Electron

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