Drawing pixels: PIXEL2, Removed

       Name: PIXEL2, Removed                                         [Show more]
       Type: Subroutine
   Category: Drawing pixels
    Summary: Draw a stardust particle relative to the screen centre
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


 Draw a point (X1, Y1) from the middle of the screen with a size determined by
 a distance value. Used to draw stardust particles.


 Arguments:

   X1                   The x-coordinate offset

   Y1                   The y-coordinate offset (positive means up the screen
                        from the centre, negative means down the screen)

   ZZ                   The distance of the point (further away = smaller point)


                        \ --- Mod: Code removed for Elite-A: ------------------>

\.PIXEL2
\
\LDA X1                 \ Fetch the x-coordinate offset into A
\
\BPL PX1                \ If the x-coordinate offset is positive, jump to PX1
\                       \ to skip the following negation
\
\EOR #%01111111         \ The x-coordinate offset is negative, so flip all the
\CLC                    \ bits apart from the sign bit and add 1, to convert it
\ADC #1                 \ from a sign-magnitude number to a signed number
\
\.PX1
\
\EOR #%10000000         \ Set X = X1 + 128
\TAX                    \
\                       \ So X is now the offset converted to an x-coordinate,
\                       \ centred on x-coordinate 128
\
\LDA Y1                 \ Fetch the y-coordinate offset into A and clear the
\AND #%01111111         \ sign bit, so A = |Y1|
\
\CMP #96                \ If |Y1| >= 96 then it's off the screen (as 96 is half
\BCS PX4                \ the screen height), so return from the subroutine (as
\                       \ PX4 contains an RTS)
\
\LDA Y1                 \ Fetch the y-coordinate offset into A
\
\BPL PX2                \ If the y-coordinate offset is positive, jump to PX2
\                       \ to skip the following negation
\
\EOR #%01111111         \ The y-coordinate offset is negative, so flip all the
\ADC #1                 \ bits apart from the sign bit and subtract 1, to negate
\                       \ it to a positive number, i.e. A is now |Y1|
\
\.PX2
\
\STA T                  \ Set A = #Y + 1 - Y1
\LDA #Y+1               \
\SBC T                  \ So if Y1 is positive we display the point up from the
\                       \ centre at y-coordinate 97, while a negative Y1 means
\                       \ down from the centre
\
\                       \ Fall through into PIXEL to draw the stardust at the
\                       \ screen coordinates in (X, A)

                        \ --- End of removed code ----------------------------->