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Elite on the BBC Micro

Version analysis of PL9 (Part 1 of 3)

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

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

Code variation 1 of 2Related to Elite's use of the Tube

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, Master

6502SP

JSR WPLS2 \ Call WPLS2 to remove the planet from the screen
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

.PL20

Code variation 2 of 2Related to Elite's use of the Tube

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, Master

6502SP

RTS \ The planet doesn't fit on-screen, so return from the \ subroutine
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

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