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⌂ ∞ ▽ Elite on the 6502

Drawing planets: PLL1 (Part 2 of 3)

[6502 Second Processor version, Loader 1]

       Name: PLL1 (Part 2 of 3)                                      [Show more]
       Type: Subroutine
   Category: Drawing planets
    Summary: Draw Saturn on the loading screen (draw the stars)
  Deep dive: Drawing Saturn on the loading screen

    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: No direct references to this subroutine in this source file




                        \ The following loop iterates CNT2(1 0) times, i.e. &1DD
                        \ or 477 times, and draws the background stars on the
                        \ loading screen

.PLL2

 JSR DORND              \ Set A and X to signed random numbers between -128 and
                        \ 127, say A = r3

 TAX                    \ Set X = A
                        \       = r3

 JSR SQUA2              \ Set (A P) = A * A
                        \           = r3^2

 STA ZP+1               \ Set ZP+1 = A
                        \          = r3^2 / 256

 JSR DORND              \ Set A and X to signed random numbers between -128 and
                        \ 127, say A = r4

 STA YY                 \ Set YY = r4

 JSR SQUA2              \ Set (A P) = A * A
                        \           = r4^2

 ADC ZP+1               \ Set A = A + r3^2 / 256
                        \       = r4^2 / 256 + r3^2 / 256
                        \       = (r3^2 + r4^2) / 256

 CMP #&11               \ If A < 17, jump down to PLC2 to skip to the next pixel
 BCC PLC2

 LDA YY                 \ Set A = r4

 JSR PIX                \ Draw a pixel at screen coordinate (X + 128, A + 128),
                        \ so that's at:
                        \
                        \   (r3 + 128, r4 + 128)
                        \
                        \ where:
                        \
                        \   (r3^2 + r4^2) / 256 >= 17

.PLC2

 DEC CNT2               \ Decrement the counter in CNT2 (the low byte)

 BNE PLL2               \ Loop back to PLL2 until CNT2 = 0

 DEC CNT2+1             \ Decrement the counter in CNT2+1 (the high byte)

 BNE PLL2               \ Loop back to PLL2 until CNT2+1 = 0