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Elite on the BBC Micro

Version analysis of DOT

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

Code variations between these versions are shown below.

Name: DOT Type: Subroutine Category: Dashboard

Code variation 1 of 6A variation in the comments only

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Cassette, Flight, Master, Electron

6502SP

Summary: Draw a dash on the compass
Summary: Implement the #DOdot command (draw a dash on the compass)

Arguments:

Code variation 2 of 6A variation in the comments only

This variation is blank in the 6502 Second Processor version.

Cassette, Flight, Master, Electron

COMX The screen pixel x-coordinate of the dash COMY The screen pixel y-coordinate of the dash

Code variation 3 of 6A variation in the comments only

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Cassette, Flight, Master

6502SP

Electron

COMC The colour and thickness of the dash: * &F0 = a double-height dash in yellow/white, for when the object in the compass is in front of us * &FF = a single-height dash in green/cyan, for when the object in the compass is behind us
OSSC(1 0) A parameter block as follows: * Byte #2 = The screen pixel x-coordinate of the dash * Byte #3 = The screen pixel x-coordinate of the dash * Byte #4 = The colour of the dash
COMC The thickness of the dash: * &F0 = a double-height dash in white, for when the object in the compass is in front of us * &FF = a single-height dash in white, for when the object in the compass is behind us
.DOT

Code variation 4 of 6Specific to an individual platform

This variation is blank in the Cassette, Disc (flight), 6502 Second Processor and Electron versions.

Master

LDA #%00001111 \ Set bits 1 and 2 of the Access Control Register at STA VIA+&34 \ SHEILA &34 to switch screen memory into &3000-&7FFF

Code variation 5 of 6Related to Elite's use of the Tube

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Cassette, Flight

Master

6502SP

Electron

LDA COMY \ Set Y1 = COMY, the y-coordinate of the dash STA Y1 LDA COMX \ Set X1 = COMX, the x-coordinate of the dash STA X1 LDA COMC \ Set COL = COMC, the mode 5 colour byte for the dash STA COL
LDA COMX \ Set X1 = COMX, the x-coordinate of the dash STA X1 LDX COMC \ Set COL = COMC, the mode 2 colour byte for the dash STX COL LDA COMY \ Set Y1 = COMY, the y-coordinate of the dash
LDY #2 \ Fetch byte #2 from the parameter block (the dash's LDA (OSSC),Y \ x-coordinate) and store it in X1 STA X1 INY \ Fetch byte #3 from the parameter block (the dash's LDA (OSSC),Y \ y-coordinate) and store it in X1 STA Y1 INY \ Fetch byte #3 from the parameter block (the dash's LDA (OSSC),Y \ colour) and store it in COL STA COL
LDA COMY \ Set Y1 = COMY, the y-coordinate of the dash STA Y1 LDA COMX \ Set X1 = COMX, the x-coordinate of the dash STA X1 LDA COMC \ Set A = COMC, the thickness of the dash

Code variation 6 of 6Related to the screen mode

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Cassette, Flight

Master

6502SP

Electron

CMP #&F0 \ If COL is &F0 then the planet/station is in front of BNE CPIX2 \ us and we want to draw a double-height dash, so if it \ isn't &F0 jump to CPIX2 to draw a single-height dash \ Otherwise fall through into CPIX4 to draw a double- \ height dash
CPX #YELLOW2 \ If the colour in X is yellow, then the planet/station BNE P%+8 \ is behind us, so skip the following three instructions \ so we only draw a single-height dash JSR CPIX2 \ Call CPIX2 to draw a single-height dash, i.e. the top \ row of a double-height dash LDA Y1 \ Fetch the y-coordinate of the row we just drew and DEC A \ decrement it, ready to draw the bottom row JSR CPIX2 \ Call CPIX2 to draw a single-height dash LDA #%00001001 \ Clear bits 1 and 2 of the Access Control Register at STA VIA+&34 \ SHEILA &34 to switch main memory back into &3000-&7FFF RTS \ Return from the subroutine
CMP #WHITE2 \ If the dash's colour is not white, jump to CPIX2 to BNE CPIX2 \ draw a single-height dash in the compass, as it is \ showing that the planet or station is behind us \ Otherwise the dash is white, which is in front of us, \ so fall through into CPIX4 to draw a double-height \ dash in the compass
CMP #&F0 \ If COMC is &F0 then the planet/station is in front of BNE CPIX2 \ us and we want to draw a double-height dash, so if it \ isn't &F0 jump to CPIX2 to draw a single-height dash \ Otherwise fall through into CPIX4 to draw a double- \ height dash
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