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BBC Micro Elite

Drawing pixels: PIXEL2

Name: PIXEL2 [View in context] Type: Subroutine Category: Drawing pixels Summary: Draw a stardust particle relative to the screen centre
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)
.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 negate ADC #1 \ it to a positive number, i.e. A is now |X1| .PX1 EOR #%10000000 \ Set X = -|A| TAX \ = -|X1| 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 - A LDA #97 \ = 97 - |Y1| SBC T \ \ so if Y is positive we display the point up from the \ centre, while a negative Y means down from the centre \ Fall through into PIXEL to draw the stardust at the \ screen coordinates in (X, A) }