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Elite on the BBC Micro

Drawing circles: CIRCLE (6502SP version)

Name: CIRCLE [View in context] Type: Subroutine [Compare versions] Category: Drawing circles Summary: Draw a circle for the planet Deep dive: Drawing circles
Draw a circle with the centre at (K3, K4) and radius K. Used to draw the planet's main outline. Arguments: K The planet's radius K3(1 0) Pixel x-coordinate of the centre of the planet K4(1 0) Pixel y-coordinate of the centre of the planet
.CIRCLE JSR CHKON \ Call CHKON to check whether the circle fits on-screen BCS RTS2 \ If CHKON set the C flag then the circle does not fit \ on-screen, so return from the subroutine (as RTS2 \ contains an RTS) LDX K \ Set X = K = radius LDA #8 \ Set A = 8 CPX #4 \ If the radius < 4, skip to PL89 BCC PL89 LSR A \ Halve A so A = 4 CPX #50 \ If the radius < 50, skip to PL89 BCC PL89 LSR A \ Halve A so A = 2 .PL89 STA STP \ Set STP = A. STP is the step size for the circle, so \ the above sets a smaller step size for bigger circles \ Fall through into CIRCLE3 to draw the circle with the \ correct step size