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Drawing circles: TT128

[Acorn Electron version]

Name: TT128 [Show more] Type: Subroutine Category: Drawing circles Summary: Draw a circle on a chart Deep dive: Drawing circles
Context: 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: * TT14 calls TT128

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 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 RTS \ Return from the subroutine