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Elite on the BBC Micro

Version analysis of SP2

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

Code variations between these versions are shown below.

Name: SP2 Type: Subroutine Category: Dashboard Summary: Draw a dot on the compass, given the planet/station vector
Draw a dot on the compass to represent the planet or station, whose normalised vector is in XX15. XX15 to XX15+2 The normalised vector to the planet or space station, stored as x in XX15, y in XX15+1 and z in XX15+2
.SP2 LDA XX15 \ Set A to the x-coordinate of the planet or station to \ show on the compass, which will be in the range -96 to \ +96 as the vector has been normalised JSR SPS2 \ Set (Y X) = A / 10, so X will be from -9 to +9, which \ is the x-offset from the centre of the compass of the \ dot we want to draw. Returns with the C flag clear

Code variation 1 of 5Related to the screen mode

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Cassette, Flight, 6502SP, Master

Electron

TXA \ Set COMX = 195 + X, as 186 is the pixel x-coordinate ADC #195 \ of the leftmost dot possible on the compass, and X can STA COMX \ be -9, which would be 195 - 9 = 186. This also means \ that the highest value for COMX is 195 + 9 = 204, \ which is the pixel x-coordinate of the rightmost dot \ in the compass... but the compass dot is actually two \ pixels wide, so the compass dot can overlap the right \ edge of the compass, but not the left edge
TXA \ Set COMX = 193 + X, as 184 is the pixel x-coordinate ADC #193 \ of the leftmost edge of the compass, and X can be -9, STA COMX \ which would be 193 - 9 = 184. This also means that the \ highest value for COMX is 193 + 9 = 202, and given \ that the compass dot is two pixels wide, this means \ the compass dot can overlap the left edge of the \ compass, but not the right edge
 LDA XX15+1             \ Set A to the y-coordinate of the planet or station to
                        \ show on the compass, which will be in the range -96 to
                        \ +96 as the vector has been normalised

 JSR SPS2               \ Set (Y X) = A / 10, so X will be from -9 to +9, which
                        \ is the y-offset from the centre of the compass of the
                        \ dot we want to draw. Returns with the C flag clear

 STX T                  \ Set COMY = 204 - X, as 203 is the pixel y-coordinate
 LDA #204               \ of the centre of the compass, the C flag is clear,
 SBC T                  \ and the y-axis needs to be flipped around (because
 STA COMY               \ when the planet or station is above us, and the
                        \ vector is therefore positive, we want to show the dot
                        \ higher up on the compass, which has a smaller pixel
                        \ y-coordinate). So this calculation does this:
                        \
                        \   COMY = 204 - X - (1 - 0) = 203 - X

Code variation 2 of 5Related to the 6502SP version

The compass on the cassette, disc and Master version uses yellow when the target is in front of us, while the 6502SP version uses white (and so does the Electron version, but only because it's monochrome).

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

Cassette, Flight

Master

6502SP

Electron

LDA #&F0 \ Set A to a 4-pixel mode 5 byte row in colour 2 \ (yellow/white), the colour for when the planet or \ station in the compass is in front of us
LDA #YELLOW2 \ Set A to yellow, the colour for when the planet or \ station in the compass is in front of us
LDA #WHITE2 \ Set A to white, the colour for when the planet or \ station in the compass is in front of us
LDA #&F0 \ Set A to &F0, the value we pass to DOT for drawing a \ two-pixel high dot, for when the planet or station \ in the compass is in front of us
 LDX XX15+2             \ If the z-coordinate of the XX15 vector is positive,
 BPL P%+4               \ skip the following instruction

Code variation 3 of 5Related to the screen mode

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

Cassette, Flight

6502SP, Master

Electron

LDA #&FF \ The z-coordinate of XX15 is negative, so the planet or \ station is behind us and the compass dot should be in \ green/cyan, so set A to a 4-pixel mode 5 byte row in \ colour 3
LDA #GREEN2 \ The z-coordinate of XX15 is negative, so the planet or \ station is behind us and the compass dot should be in \ green, so set A accordingly
LDA #&FF \ The z-coordinate of XX15 is negative, so the planet or \ station is behind us and the compass dot should be a \ single-height dash, so set A to &FF for the call to \ DOT below

Code variation 4 of 5A variation in the comments only

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Cassette, Flight, 6502SP, Master

Electron

STA COMC \ Store the compass colour in COMC
STA COMC \ Store the compass shape in COMC

Code variation 5 of 5Specific to an individual platform

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Cassette, Flight, 6502SP, Electron

Master

\ Fall through into DOT to draw the dot on the compass
JMP DOT \ Jump to DOT to draw the dot on the compass and return \ from the subroutine using a tail call
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