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⌂ ∞ ▽ Elite on the 6502

Drawing circles: TT128

[NES version, Bank 0]

       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
 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

 LDX #2                 ; Set STP = 2, the step size for the circle
 STX STP

 LDX #1                 ; Set the high byte of the pattern buffer address
 JSR SetPatternBuffer   ; variables to that of pattern buffer 1, so the circle
                        ; gets drawn into bitplane 1 only, giving a circle of
                        ; colour %10 (2), which is green

 JMP CIRCLE2_b1         ; 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