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⌂ ∞ ▽ Elite on the 6502

Version analysis of CIRCLE

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

       Name: CIRCLE
       Type: Subroutine
   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

Code variation 1 of 5Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron
 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)

Code variation 2 of 5Specific to an individual platform

This variation is blank in the 6502 Second Processor version.

Cassette, Flight, Docked, Master, Electron
 LDA #0                 \ Set LSX2 = 0 to indicate that the ball line heap is
 STA LSX2               \ not empty, as we are about to fill it

 LDX K                  \ Set X = K = radius

 LDA #8                 \ Set A = 8

Code variation 3 of 5Related to a standard feature

The cassette, disc and Master versions define a circle as being small when it has a radius of less than 8, while in the 6502SP version, a circle is small if it has a radius of less than 4, and in the Electron version, small circles have a radius of less than 9. Small circles are drawn with a step size of 8.

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked, Master
6502SP
Electron
 CPX #8                 \ If the radius < 8, skip to PL89
 BCC PL89

 CPX #4                 \ If the radius < 4, skip to PL89
 BCC PL89

 CPX #9                 \ If the radius < 9, skip to PL89
 BCC PL89

 LSR A                  \ Halve A so A = 4

Code variation 4 of 5Related to a standard feature

The cassette, disc and Master versions define a circle as being medium when it has a radius of less than 60, while in the 6502SP version, a circle is medium if it has a radius of less than 50. Medium circles are drawn with a step size of 4, and large circles are drawn with a step size of 2. The Electron version, meanwhile, only has small and medium circles (no large), with medium circles having a radius of 9 or more.

This variation is blank in the Electron version.

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked, Master
6502SP
 CPX #60                \ If the radius < 60, skip to PL89
 BCC PL89

 LSR A                  \ Halve A so A = 2

 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

Code variation 5 of 5A variation in the comments only

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked, Master, Electron
6502SP
                        \ Fall through into CIRCLE2 to draw the circle with the
                        \ correct step size

                        \ Fall through into CIRCLE3 to draw the circle with the
                        \ correct step size