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⌂ ∞ Elite on the BBC Micro and NES

Moving: MVEIT (Part 1 of 9)

[NES version, Bank 0]

       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
             * ESCAPE calls MVEIT
             * Main flight loop (Part 6 of 16) calls MVEIT
             * PAS1 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

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 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 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_b1            ; 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