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Elite on the BBC Micro

Moving: MVEIT (Part 1 of 9) [Elite-A, Docked]

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)