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Elite on the BBC Micro

Version analysis of SIGHT

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

Code variations between these versions are shown below.

Name: SIGHT Type: Subroutine Category: Flight Summary: Draw the laser crosshairs
.SIGHT LDY VIEW \ Fetch the laser power for our new view LDA LASER,Y

Code variation 1 of 4A variation in the labels only

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Cassette, Flight, 6502SP, Master, Electron

Docked

BEQ LO2 \ If it is zero (i.e. there is no laser fitted to this \ view), jump to LO2 to return from the subroutine (as \ LO2 contains an RTS)
BEQ BOL1-1 \ If it is zero (i.e. there is no laser fitted to this \ view), jump to BOL1-1 to return from the subroutine \ (as BOL1-1 contains &60, which is the opcode for an \ RTS)

Code variation 2 of 4Related to the Master version

In the Master version, the laser crosshairs are different colours for the different laser types.

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

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Master

6502SP

LDY #0 \ Set Y to 0, to represent a pulse laser CMP #POW \ If the laser power in A is equal to a pulse laser, BEQ SIGHT1 \ jump to SIGHT1 with Y = 0 INY \ Increment Y to 1, to represent a beam laser CMP #(POW+128) \ If the laser power in A is equal to a beam laser, BEQ SIGHT1 \ jump to SIGHT1 with Y = 1 INY \ Increment Y to 2, to represent a military laser CMP #Armlas \ If the laser power in A is equal to a military laser, BEQ SIGHT1 \ jump to SIGHT1 with Y = 2 INY \ Increment Y to 3, to represent a mining laser .SIGHT1 LDA SIGHTCOL,Y \ Set the colour from the SIGHTCOL table STA COL
LDA #YELLOW \ Send a #SETCOL YELLOW command to the I/O processor to JSR DOCOL \ switch to colour 1, which is yellow in the space view
 LDA #128               \ Set QQ19 to the x-coordinate of the centre of the
 STA QQ19               \ screen

 LDA #Y-24              \ Set QQ19+1 to the y-coordinate of the centre of the
 STA QQ19+1             \ screen, minus 24 (because TT15 will add 24 to the
                        \ coordinate when it draws the crosshairs)

 LDA #20                \ Set QQ19+2 to size 20 for the crosshairs size
 STA QQ19+2

Code variation 3 of 4Specific to an individual platform

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Cassette, Flight, Docked, Master, Electron

6502SP

JSR TT15 \ Call TT15 to draw crosshairs of size 20 just to the \ left of the middle of the screen
JSR TT15b \ Call TT15b to draw crosshairs of size 20 just to the \ left of the middle of the screen, in the current \ colour (yellow)
 LDA #10                \ Set QQ19+2 to size 10 for the crosshairs size
 STA QQ19+2

Code variation 4 of 4Specific to an individual platform

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, Docked, Master, Electron

6502SP

JMP TT15 \ Call TT15 to draw crosshairs of size 10 at the same \ location, which will remove the centre part from the \ laser crosshairs, leaving a gap in the middle, and \ return from the subroutine using a tail call
JMP TT15b \ Call TT15b to draw crosshairs of size 10 at the same \ location, which will remove the centre part from the \ laser crosshairs, leaving a gap in the middle, and \ return from the subroutine using a tail call
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