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Elite on the BBC Micro

Dashboard: SP2 [Electron version]

Name: SP2 [Show more] Type: Subroutine Category: Dashboard Summary: Draw a dot on the compass, given the planet/station vector
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * COMPAS calls SP2

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 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 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 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 STA COMC \ Store the compass shape in COMC \ Fall through into DOT to draw the dot on the compass