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BBC Micro Elite

Drawing circles: TT128

Name: TT128 [View in context] Type: Subroutine Category: Drawing circles Summary: Draw a circle on a chart
Draw a circle with the centre at (QQ19, QQ19+1) and radius K. Arguments: QQ19 The x-coordinate of the centre of the circle QQ19+1 The y-coordinate of the centre of the circle K The radius of the circle
.TT128 { LDA QQ19 \ Set K3 = the x-coordinate of the centre STA K3 LDA QQ19+1 \ Set K4 = the y-coordinate of the centre STA K4 LDX #0 \ Set the high bytes of K3(1 0) and K4(1 0) to 0 STX K4+1 STX K3+1 \STX LSX \ This instruction is commented out in the original \ source INX \ Set LSP = 1 to reset the ball line heap STX LSP LDX #2 \ Set STP = 2, the step size for the circle STX STP JSR CIRCLE2 \ Call CIRCLE2 to draw a circle with the centre at \ (K3(1 0), K4(1 0)) and radius K \LDA #&FF \ These instructions are commented out in the original \STA LSX \ source RTS \ Return from the subroutine }