.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 decrements one million \ times a second and will therefore be pretty random, \ and store it in location RAND+1, which is among the \ main game code's random seeds in RAND (so this seeds \ the random number generator) 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 \ r2 = random number from 0 to 255 \ (r1^2 + r2^2) < 128^2 \ \ y = r2, squished into 64 to 191 by negation \ \ x = SQRT(128^2 - (r1^2 + r2^2)) / 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 = 0Name: PLL1 (Part 1 of 3) [Show more] Type: Subroutine Category: Drawing planets Summary: Draw Saturn on the loading screen (draw the planet) 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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Subroutine DORND (category: Maths (Arithmetic))
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 PLC1 is local to this routine
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Subroutine PLL1 (Part 1 of 3) (category: Drawing planets)
Draw Saturn on the loading screen (draw the planet)
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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