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Elite on the BBC Micro

Version analysis of TT128

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

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

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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.

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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

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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
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