Moving: MVEIT (Part 1 of 9)

       Name: MVEIT (Part 1 of 9)                                     [Show more]
       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
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * BRIEF calls MVEIT
             * PAS1 calls MVEIT
             * TITLE calls MVEIT


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

   * Tidy the orientation vectors for one of the ship slots


 Arguments:

   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


.MVEIT

 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

.MV3

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