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 = 97 - Y1
\ LDA #97               \
\ SBC T                 \ So if Y is positive we display the point up from the
\                       \ centre at y-coordinate 97, while a negative Y means
\                       \ down from the centre
\
\                       \ Fall through into PIXEL to draw the stardust at the
\                       \ screen coordinates in (X, A)

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