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⌂ ∞ ▽ Elite on the BBC Micro and NES

Drawing planets: PL9 (Part 1 of 3)

[6502 Second Processor version]

       Name: PL9 (Part 1 of 3)                                       [Show more]
       Type: Subroutine
   Category: Drawing planets
    Summary: Draw the planet, with either an equator and meridian, or a crater

    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:
             * PLANET calls PL9




 Draw the planet with radius K at pixel coordinate (K3, K4), and with either an
 equator and meridian, or a crater.



 Arguments:

   K(1 0)               The planet's radius

   K3(1 0)              Pixel x-coordinate of the centre of the planet

   K4(1 0)              Pixel y-coordinate of the centre of the planet

   INWK                 The planet's ship data block



.PL9

 JSR LS2FL              \ Call LS2FL to send the ball line heap to the I/O
                        \ processor for drawing on-screen, which will erase the
                        \ planet from the screen

 STZ LSP                \ Reset the ball line heap by setting the ball line heap
                        \ pointer to 0

 JSR CIRCLE             \ Call CIRCLE to draw the planet's new circle

 BCS PL20               \ If the call to CIRCLE returned with the C flag set,
                        \ then the circle does not fit on-screen, so jump to
                        \ PL20 to return from the subroutine

 LDA K+1                \ If K+1 is zero, jump to PL25 as K(1 0) < 256, so the
 BEQ PL25               \ planet fits on the screen and we can draw meridians or
                        \ craters

.PL20

 JMP LS2FL              \ The planet doesn't fit on-screen, so jump to LS2FL to
                        \ send the ball line heap to the I/O processor for
                        \ drawing on-screen, returning from the subroutine using
                        \ a tail call

.PL25

 LDA TYPE               \ If the planet type is 128 then it has an equator and
 CMP #128               \ a meridian, so this jumps to PL26 if this is not a
 BNE PL26               \ planet with an equator - in other words, if it is a
                        \ planet with a crater

                        \ Otherwise this is a planet with an equator and
                        \ meridian, so fall through into the following to draw
                        \ them