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Elite on the BBC Micro

Drawing circles: TT128 (6502SP version)

Name: TT128 [View in context] Type: Subroutine [Compare versions] Category: Drawing circles Summary: Draw a circle on a chart Deep dive: Drawing circles
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 LDA #RED \ Send a #SETCOL RED command to the I/O processor to JSR DOCOL \ switch to colour 2, which is red in the chart view JMP CIRCLE2 \ Jump to CIRCLE2 to draw a circle with the centre at \ (K3(1 0), K4(1 0)) and radius K, returning from the \ subroutine using a tail call