\ The following loop iterates CNT3(1 0) times, i.e. &500 \ or 1280 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 A = A + YY \ = r7 + r6 TAX \ Set X = A \ = r6 + r7 JSR SQUA2 \ Set (A P) = A * A \ = (r6 + r7)^2 TAY \ Set Y = A \ = (r6 + r7)^2 / 256 ADC ZP+1 \ Set A = A + ZP+1 \ = (r6 + r7)^2 / 256 + (r5^2 + r6^2) / 256 \ = ((r6 + r7)^2 + r5^2 + r6^2) / 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 \ = (r6 + r7)^2 / 256 + r6^2 / 256 \ = ((r6 + r7)^2 + r6^2) / 256 CMP #16 \ If A >= 16, skip to PL1 to plot the pixel BCS PL1 LDA ZP \ If ZP is positive (i.e. r5 < 128), jump down to PLC3 BPL PLC3 \ to skip to the next pixel .PL1 \ If we get here then the following is true: \ \ 32 <= ((r6 + r7)^2 + r5^2 + r6^2) / 256 < 80 \ \ and either this is true: \ \ ((r6 + r7)^2 + r6^2) / 256 >= 16 \ \ or both these are true: \ \ ((r6 + r7)^2 + r6^2) / 256 < 16 \ r5 >= 128 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 = r6 + r7 \ y = r6 \ \ 32 <= ((r6 + r7)^2 + r5^2 + r6^2) / 256 < 80 \ \ Either: ((r6 + r7)^2 + r6^2) / 256 >= 16 \ \ Or: ((r6 + r7)^2 + r6^2) / 256 < 16 \ r5 >= 128 \ \ 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 (Part 3 of 3) [Show more] Type: Subroutine Category: Drawing planets Summary: Draw Saturn on the loading screen (draw the rings) 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: No direct references to this subroutine in this source file
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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: 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 PL1 is local to this routine
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Label PLC3 is local to this routine
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Label PLL3 is local to this routine
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Subroutine SQUA2 (category: Maths (Arithmetic))
Calculate (A P) = A * A
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Workspace ZP (category: Workspaces)
Important variables used by the loader