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Elite on the BBC Micro

Version analysis of PROJ

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

Code variations between these versions are shown below.

Name: PROJ Type: Subroutine Category: Drawing ships Summary: Project the current ship onto the screen Deep dive: Extended screen coordinates
Project the current ship's location onto the screen, either returning the screen coordinates of the projection (if it's on-screen), or returning an error via the C flag. In this context, "on-screen" means that the point is projected into the following range: centre of screen - 1024 < x < centre of screen + 1024 centre of screen - 1024 < y < centre of screen + 1024

Code variation 1 of 3A variation in the comments only

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

Cassette, Flight, 6502SP, Master

Electron

This is to cater for ships (and, more likely, planets and suns) whose centres are off-screen but whose edges may still be visible.
This is to cater for ships (and, more likely, planets) whose centres are off-screen but whose edges may still be visible.
The projection calculation is: K3(1 0) = #X + x / z K4(1 0) = #Y + y / z where #X and #Y are the pixel x-coordinate and y-coordinate of the centre of the screen. Arguments: INWK The ship data block for the ship to project on-screen Returns: K3(1 0) The x-coordinate of the ship's projection on-screen K4(1 0) The y-coordinate of the ship's projection on-screen C flag Set if the ship's projection doesn't fit on the screen, clear if it does project onto the screen A Contains K4+1, the high byte of the y-coordinate

Code variation 2 of 3A variation in the comments only

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

Electron

Other entry points: RTS2 Contains an RTS
.PROJ LDA INWK \ Set P(1 0) = (x_hi x_lo) STA P \ = x LDA INWK+1 STA P+1 LDA INWK+2 \ Set A = x_sign JSR PLS6 \ Call PLS6 to calculate: \ \ (X K) = (A P) / (z_sign z_hi z_lo) \ = (x_sign x_hi x_lo) / (z_sign z_hi z_lo) \ = x / z BCS PL2-1 \ If the C flag is set then the result overflowed and \ the coordinate doesn't fit on the screen, so return \ from the subroutine with the C flag set (as PL2-1 \ contains an RTS) LDA K \ Set K3(1 0) = (X K) + #X ADC #X \ = #X + x / z STA K3 \ \ first doing the low bytes TXA \ And then the high bytes. #X is the x-coordinate of ADC #0 \ the centre of the space view, so this converts the STA K3+1 \ space x-coordinate into a screen x-coordinate LDA INWK+3 \ Set P(1 0) = (y_hi y_lo) STA P LDA INWK+4 STA P+1 LDA INWK+5 \ Set A = -y_sign EOR #%10000000 JSR PLS6 \ Call PLS6 to calculate: \ \ (X K) = (A P) / (z_sign z_hi z_lo) \ = -(y_sign y_hi y_lo) / (z_sign z_hi z_lo) \ = -y / z BCS PL2-1 \ If the C flag is set then the result overflowed and \ the coordinate doesn't fit on the screen, so return \ from the subroutine with the C flag set (as PL2-1 \ contains an RTS) LDA K \ Set K4(1 0) = (X K) + #Y ADC #Y \ = #Y - y / z STA K4 \ \ first doing the low bytes TXA \ And then the high bytes. #Y is the y-coordinate of ADC #0 \ the centre of the space view, so this converts the STA K4+1 \ space x-coordinate into a screen y-coordinate CLC \ Clear the C flag to indicate success

Code variation 3 of 3A variation in the labels only

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

Electron

.RTS2
 RTS                    \ Return from the subroutine

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