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Moving: MVEIT (Part 1 of 9)

[BBC Micro disc version, Flight]

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 Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * ESCAPE calls MVEIT * Main flight loop (Part 6 of 16) 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 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 \ 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