Version analysis of TT128 - Elite on the 6502

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

Cassette, Disc (flight), Disc (docked), 6502 Second Processor, Master and Electron

Code variations between these versions are shown below.

       Name: TT128
       Type: Subroutine
   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

Code variation 1 of 3Minor and very low-impact

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

Cassette
Flight, Docked, 6502SP, Electron
Master
 LDX #0                 \ Set the high bytes of K3(1 0) and K4(1 0) to 0
 STX K4+1
 STX K3+1

\STX LSX                \ This instruction is commented out in the original
                        \ source

 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
 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
 STZ K4+1               \ Set the high bytes of K3(1 0) and K4(1 0) to 0
 STZ K3+1

 LDX #1                 \ Set LSP = 1 to reset the ball line heap
 STX LSP

 INX                    \ Set STP = 2, the step size for the circle
 STX STP

Code variation 2 of 3Related to the screen mode

This variation is blank in the Cassette, Disc (flight), Disc (docked) and Electron versions.

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

Master
6502SP
 LDA #RED               \ Switch to colour 2, which is red in the chart view
 STA COL
 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

Code variation 3 of 3Minor and very low-impact

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

Cassette
Docked, 6502SP, Master
Flight, Electron
 JSR CIRCLE2            \ Call CIRCLE2 to draw a circle with the centre at
                        \ (K3(1 0), K4(1 0)) and radius K

\LDA #&FF               \ These instructions are commented out in the original
\STA LSX                \ source

 RTS                    \ Return from the subroutine
 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
 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