Version analysis of MVEIT (Part 1 of 9) - Elite on the BBC Micro and NES

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

Cassette, Disc (flight), Disc (docked), 6502 Second Processor, Master and Electron

Code variations between these versions are shown below.

       Name: MVEIT (Part 1 of 9)
       Type: Subroutine
   Category: Moving
    Summary: Move current ship: Tidy the orientation vectors
  Deep dive: Program flow of the ship-moving routine
             Scheduling tasks with the main loop counter


 This routine has multiple stages. This stage does the following:

   * Tidy the orientation vectors for one of the ship slots


 Arguments:

Code variation 1 of 3A variation in the comments only

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked, 6502SP, Master
Electron
   INWK                 The current ship/planet/sun's data block

   XSAV                 The slot number of the current ship/planet/sun

   TYPE                 The type of the current ship/planet/sun
   INWK                 The current ship/planet's data block

   XSAV                 The slot number of the current ship/planet

   TYPE                 The type of the current ship/planet


.MVEIT

Code variation 2 of 3Specific to an individual platform

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, 6502SP, Master, Electron
Docked
 LDA INWK+31            \ If bits 5 or 7 of ship byte #31 are set, jump to MV30
 AND #%10100000         \ as the ship is either exploding or has been killed, so
 BNE MV30               \ we don't need to tidy its orientation vectors or apply
                        \ tactics
 LDA INWK+31            \ If bit 5 of ship byte #31 is set, jump to MV3 as the
 AND #%00100000         \ ship is exploding, so we don't need to tidy its
 BNE MV3                \ orientation vectors

 LDA MCNT               \ Fetch the main loop counter

 EOR XSAV               \ Fetch the slot number of the ship we are moving, EOR
 AND #15                \ with the loop counter and apply mod 15 to the result.
 BNE MV3                \ The result will be zero when "counter mod 15" matches
                        \ the slot number, so this makes sure we call TIDY 12
                        \ times every 16 main loop iterations, like this:
                        \
                        \   Iteration 0, tidy the ship in slot 0
                        \   Iteration 1, tidy the ship in slot 1
                        \   Iteration 2, tidy the ship in slot 2
                        \     ...
                        \   Iteration 11, tidy the ship in slot 11
                        \   Iteration 12, do nothing
                        \   Iteration 13, do nothing
                        \   Iteration 14, do nothing
                        \   Iteration 15, do nothing
                        \   Iteration 16, tidy the ship in slot 0
                        \     ...
                        \
                        \ and so on

 JSR TIDY               \ Call TIDY to tidy up the orientation vectors, to
                        \ prevent the ship from getting elongated and out of
                        \ shape due to the imprecise nature of trigonometry
                        \ in assembly language

Code variation 3 of 3A variation in the labels only

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

Docked
.MV3

                        \ Fall through into part 7 (parts 2-6 are not required
                        \ when we are docked)