.PLL1 \ The following loop iterates CNT(1 0) times, i.e. &300 \ or 768 times, and draws the planet part of the \ loading screen's Saturn LDA VIA+&44 \ Read the 6522 System VIA T1C-L timer 1 low-order STA RAND+1 \ counter (SHEILA &44), which increments 1000 times a \ second so this will be pretty random, and store it in \ RAND+1 among the hard-coded random seeds in RAND JSR DORND \ Set A and X to random numbers, say A = r1 JSR SQUA2 \ Set (A P) = A * A \ = r1^2 STA ZP+1 \ Set ZP(1 0) = (A P) LDA P \ = r1^2 STA ZP JSR DORND \ Set A and X to random numbers, say A = r2 STA YY \ Set YY = A \ = r2 JSR SQUA2 \ Set (A P) = A * A \ = r2^2 TAX \ Set (X P) = (A P) \ = r2^2 LDA P \ Set (A ZP) = (X P) + ZP(1 0) ADC ZP \ STA ZP \ first adding the low bytes TXA \ And then adding the high bytes ADC ZP+1 BCS PLC1 \ If the addition overflowed, jump down to PLC1 to skip \ to the next pixel STA ZP+1 \ Set ZP(1 0) = (A ZP) \ = r1^2 + r2^2 LDA #1 \ Set ZP(1 0) = &4001 - ZP(1 0) - (1 - C) SBC ZP \ = 128^2 - ZP(1 0) STA ZP \ \ (as the C flag is clear), first subtracting the low \ bytes LDA #&40 \ And then subtracting the high bytes SBC ZP+1 STA ZP+1 BCC PLC1 \ If the subtraction underflowed, jump down to PLC1 to \ skip to the next pixel \ If we get here, then both calculations fitted into \ 16 bits, and we have: \ \ ZP(1 0) = 128^2 - (r1^2 + r2^2) \ \ where ZP(1 0) >= 0 JSR ROOT \ Set ZP = SQRT(ZP(1 0)) LDA ZP \ Set X = ZP >> 1 LSR A \ = SQRT(128^2 - (a^2 + b^2)) / 2 TAX LDA YY \ Set A = YY \ = r2 CMP #128 \ If YY >= 128, set the C flag (so the C flag is now set \ to bit 7 of A) ROR A \ Rotate A and set the sign bit to the C flag, so bits \ 6 and 7 are now the same, i.e. A is a random number in \ one of these ranges: \ \ %00000000 - %00111111 = 0 to 63 (r2 = 0 - 127) \ %11000000 - %11111111 = 192 to 255 (r2 = 128 - 255) \ \ The PIX routine flips bit 7 of A before drawing, and \ that makes -A in these ranges: \ \ %10000000 - %10111111 = 128-191 \ %01000000 - %01111111 = 64-127 \ \ so that's in the range 64 to 191 JSR PIX \ Draw a pixel at screen coordinate (X, -A), i.e. at \ \ (ZP / 2, -A) \ \ where ZP = SQRT(128^2 - (r1^2 + r2^2)) \ \ So this is the same as plotting at (x, y) where: \ \ r1 = random number from 0 to 255 \ r1 = random number from 0 to 255 \ (r1^2 + r1^2) < 128^2 \ \ y = r2, squished into 64 to 191 by negation \ \ x = SQRT(128^2 - (r1^2 + r1^2)) / 2 \ \ which is what we want .PLC1 DEC CNT \ Decrement the counter in CNT (the low byte) BNE PLL1 \ Loop back to PLL1 until CNT = 0 DEC CNT+1 \ Decrement the counter in CNT+1 (the high byte) BNE PLL1 \ Loop back to PLL1 until CNT+1 = 0 \ 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 random numbers, 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 random numbers, 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, -A), i.e. at \ (r3, -r4), where (r3^2 + r4^2) / 256 >= 17 \ \ Negating a random number from 0 to 255 still gives a \ random number from 0 to 255, so this is the same as \ plotting at (x, y) where: \ \ x = random number from 0 to 255 \ y = random number from 0 to 255 \ (x^2 + y^2) div 256 >= 17 \ \ which is what we want .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 \ The following loop iterates CNT3(1 0) times, i.e. &333 \ or 819 times, and draws the rings around the loading \ screen's Saturn .PLL3 JSR DORND \ Set A and X to random numbers, say A = r5 STA ZP \ Set ZP = r5 JSR SQUA2 \ Set (A P) = A * A \ = r5^2 STA ZP+1 \ Set ZP+1 = A \ = r5^2 / 256 JSR DORND \ Set A and X to random numbers, say A = r6 STA YY \ Set YY = r6 JSR SQUA2 \ Set (A P) = A * A \ = r6^2 STA T \ Set T = A \ = r6^2 / 256 ADC ZP+1 \ Set ZP+1 = A + r5^2 / 256 STA ZP+1 \ = r6^2 / 256 + r5^2 / 256 \ = (r5^2 + r6^2) / 256 LDA ZP \ Set A = ZP \ = r5 CMP #128 \ If A >= 128, set the C flag (so the C flag is now set \ to bit 7 of ZP, i.e. bit 7 of A) ROR A \ Rotate A and set the sign bit to the C flag, so bits \ 6 and 7 are now the same CMP #128 \ If A >= 128, set the C flag (so again, the C flag is \ set to bit 7 of A) ROR A \ Rotate A and set the sign bit to the C flag, so bits \ 5-7 are now the same, i.e. A is a random number in one \ of these ranges: \ \ %00000000 - %00011111 = 0-31 \ %11100000 - %11111111 = 224-255 \ \ In terms of signed 8-bit integers, this is a random \ number from -32 to 31. Let's call it r7 ADC YY \ Set X = A + YY TAX \ = r7 + r6 JSR SQUA2 \ Set (A P) = r7 * r7 TAY \ Set Y = A \ = r7 * r7 / 256 ADC ZP+1 \ Set A = A + ZP+1 \ = r7^2 / 256 + (r5^2 + r6^2) / 256 \ = (r5^2 + r6^2 + r7^2) / 256 BCS PLC3 \ If the addition overflowed, jump down to PLC3 to skip \ to the next pixel CMP #80 \ If A >= 80, jump down to PLC3 to skip to the next BCS PLC3 \ pixel CMP #32 \ If A < 32, jump down to PLC3 to skip to the next pixel BCC PLC3 TYA \ Set A = Y + T ADC T \ = r7^2 / 256 + r6^2 / 256 \ = (r6^2 + r7^2) / 256 CMP #16 \ If A > 16, skip to PL1 to plot the pixel BCS PL1 LDA ZP \ If ZP is positive (50% chance), jump down to PLC3 to BPL PLC3 \ skip to the next pixel .PL1 LDA YY \ Set A = YY \ = r6 JSR PIX \ Draw a pixel at screen coordinate (X, -A), where: \ \ X = (random -32 to 31) + r6 \ A = r6 \ \ Negating a random number from 0 to 255 still gives a \ random number from 0 to 255, so this is the same as \ plotting at (x, y) where: \ \ r5 = random number from 0 to 255 \ r6 = random number from 0 to 255 \ r7 = r5, squashed into -32 to 31 \ \ x = r5 + r7 \ y = r5 \ \ 32 <= (r5^2 + r6^2 + r7^2) / 256 <= 79 \ Draw 50% fewer pixels when (r6^2 + r7^2) / 256 <= 16 \ \ which is what we want .PLC3 DEC CNT3 \ Decrement the counter in CNT3 (the low byte) BNE PLL3 \ Loop back to PLL3 until CNT3 = 0 DEC CNT3+1 \ Decrement the counter in CNT3+1 (the high byte) BNE PLL3 \ Loop back to PLL3 until CNT3+1 = 0Name: PLL1 [Show more] Type: Subroutine Category: Drawing planets Summary: Draw Saturn on the loading screen Deep dive: Drawing Saturn on the loading screenContext: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * Elite loader calls PLL1

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Variable CNT (category: Drawing planets)

A counter for use in drawing Saturn's planetary body

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Variable CNT2 (category: Drawing planets)

A counter for use in drawing Saturn's background stars

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Variable CNT3 (category: Drawing planets)

A counter for use in drawing Saturn's rings

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Subroutine DORND (category: Utility routines)

Generate random numbers

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Subroutine PIX (category: Drawing pixels)

Draw a single pixel at a specific coordinate

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Label PL1 is local to this routine

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Label PLC1 is local to this routine

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Label PLC2 is local to this routine

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Label PLC3 is local to this routine

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Subroutine PLL1 (category: Drawing planets)

Draw Saturn on the loading screen

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Label PLL2 is local to this routine

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Label PLL3 is local to this routine

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Variable RAND (category: Drawing planets)

The random number seed used for drawing Saturn

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Subroutine ROOT (category: Maths (Arithmetic))

Calculate ZP = SQRT(ZP(1 0))

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Subroutine SQUA2 (category: Maths (Arithmetic))

Calculate (A P) = A * A

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Configuration variable: VIA = &FE00

Memory-mapped space for accessing internal hardware, such as the video ULA, 6845 CRTC and 6522 VIAs (also known as SHEILA)

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Workspace ZP (category: Workspaces)

Important variables used by the loader