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Elite on the BBC Micro

Version analysis of WPLS

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

Code variations between these versions are shown below.

Name: WPLS Type: Subroutine Category: Drawing suns Summary: Remove the sun from the screen Deep dive: Drawing the sun
We do this by redrawing it using the lines stored in the sun line heap when the sun was originally drawn by the SUN routine. Arguments: SUNX(1 0) The x-coordinate of the vertical centre axis of the sun

Code variation 1 of 3A variation in the comments only

This variation is blank in the Disc (docked) and 6502 Second Processor versions.

Cassette, Flight, Master

Other entry points: WPLS-1 Contains an RTS
.WPLS

Code variation 2 of 3A variation in the labels only

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Cassette, Flight, Master

Docked

6502SP

LDA LSX \ If LSX < 0, the sun line heap is empty, so return from BMI WPLS-1 \ the subroutine (as WPLS-1 contains an RTS)
LDA LSX \ If LSX < 0, the sun line heap is empty, so return from BMI PL44 \ the subroutine (as PL44 contains a CLC and RTS)
LDA LSX \ If LSX < 0, the sun line heap is empty, so return from BMI WP1 \ the subroutine (as WP1 contains an RTS)
 LDA SUNX               \ Set YY(1 0) = SUNX(1 0), the x-coordinate of the
 STA YY                 \ vertical centre axis of the sun that's currently on
 LDA SUNX+1             \ screen
 STA YY+1

 LDY #2*Y-1             \ #Y is the y-coordinate of the centre of the space
                        \ view, so this sets Y as a counter for the number of
                        \ lines in the space view (i.e. 191), which is also the
                        \ number of lines in the LSO block

.WPL2

 LDA LSO,Y              \ Fetch the Y-th point from the sun line heap, which
                        \ gives us the half-width of the sun's line on this line
                        \ of the screen

 BEQ P%+5               \ If A = 0, skip the following call to HLOIN2 as there
                        \ is no sun line on this line of the screen

 JSR HLOIN2             \ Call HLOIN2 to draw a horizontal line on pixel line Y,
                        \ with centre point YY(1 0) and half-width A, and remove
                        \ the line from the sun line heap once done

 DEY                    \ Decrement the loop counter

 BNE WPL2               \ Loop back for the next line in the line heap until
                        \ we have gone through the entire heap

 DEY                    \ This sets Y to &FF, as we end the loop with Y = 0

 STY LSX                \ Set LSX to &FF to indicate the sun line heap is empty

Code variation 3 of 3Related 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, Docked, Master

6502SP

RTS \ Return from the subroutine
JMP HBFL \ Call HBFL to send the contents of the horizontal line \ buffer to the I/O processor for drawing on-screen, \ returning from the subroutine using a tail call
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