.LL72 LDA XX1+31 ; If bit 5 of the ship's byte #31 is clear, then the AND #%00100000 ; ship is not currently exploding, so jump down to EE31 BEQ EE31 LDA XX1+31 ; The ship is exploding, so set bit 3 of the ship's byte ORA #%00001000 ; #31 to denote that we are drawing something on-screen STA XX1+31 ; for this ship JMP DOEXP ; Jump to DOEXP to display the explosion cloud, ; returning from the subroutine using a tail call .EE31 LDA #%00001000 ; If bit 3 of the ship's byte #31 is clear, then there BIT XX1+31 ; is nothing already being shown for this ship, so skip BEQ LL74 ; to LL74 as we don't need to erase anything from the ; screen LDA #%00001000 ; Set bit 3 of A so the next instruction sets bit 3 of ; the ship's byte #31 to denote that we are drawing ; something on-screen for this ship .LL74 ORA XX1+31 ; Apply bit 3 of A to the ship's byte #31, so if there STA XX1+31 ; was no ship already on screen, the bit is clear, ; otherwise it is set LDY #9 ; Fetch byte #9 of the ship's blueprint, which is the LDA (XX0),Y ; number of edges, and store it in XX20 STA XX20 LDY #0 ; We are about to step through all the edges, using Y ; as a counter STY U ; Set U = 0 (though we increment it to 1 below) STY XX17 ; Set XX17 = 0, which we are going to use as a counter ; for stepping through the ship's edges INC U ; We are going to start calculating the lines we need to ; draw for this ship, and will store them in the ship ; line heap, using U to point to the end of the heap, so ; we start by setting U = 1 BIT XX1+31 ; If bit 6 of the ship's byte #31 is clear, then the BVC LL170 ; ship is not firing its lasers, so jump to LL170 to ; skip the drawing of laser lines ; The ship is firing its laser at us, so we need to draw ; the laser lines LDA XX1+31 ; Clear bit 6 of the ship's byte #31 so the ship doesn't AND #%10111111 ; keep firing endlessly STA XX1+31 LDY #6 ; Fetch byte #6 of the ship's blueprint, which is the LDA (XX0),Y ; number * 4 of the vertex where the ship has its lasers TAY ; Put the vertex number into Y, where it can act as an ; index into list of vertex screen coordinates we added ; to the XX3 heap LDX XX3,Y ; Fetch the x_lo coordinate of the laser vertex from the STX XX15 ; XX3 heap into XX15 INX ; If X = 255 then the laser vertex is not visible, as BEQ LL170 ; the value we stored in part 2 wasn't overwritten by ; the vertex calculation in part 6 and 7, so jump to ; LL170 to skip drawing the laser lines ; We now build a laser beam from the ship's laser vertex ; towards our ship, as follows: ; ; XX15(1 0) = laser vertex x-coordinate ; ; XX15(3 2) = laser vertex y-coordinate ; ; XX15(5 4) = x-coordinate of the end of the beam ; ; XX12(1 0) = y-coordinate of the end of the beam ; ; The end of the laser beam will be positioned to look ; good, rather than being directly aimed at us, as ; otherwise we would only see a flashing point of light ; as they unleashed their attack LDX XX3+1,Y ; Fetch the x_hi coordinate of the laser vertex from the STX XX15+1 ; XX3 heap into XX15+1 INX ; If X = 255 then the laser vertex is not visible, as BEQ LL170 ; the value we stored in part 2 wasn't overwritten by ; a vertex calculation in part 6 and 7, so jump to LL170 ; to skip drawing the laser beam LDX XX3+2,Y ; Fetch the y_lo coordinate of the laser vertex from the STX XX15+2 ; XX3 heap into XX15+2 LDX XX3+3,Y ; Fetch the y_hi coordinate of the laser vertex from the STX XX15+3 ; XX3 heap into XX15+3 LDA #0 ; Set XX15(5 4) = 0, so their laser beam fires to the STA XX15+4 ; left edge of the screen STA XX15+5 STA XX12+1 ; Set XX12(1 0) = the ship's z_lo coordinate, which will LDA XX1+6 ; effectively make the vertical position of the end of STA XX12 ; the laser beam move around as the ship moves in space LDA XX1+2 ; If the ship's x_sign is positive, skip the next BPL P%+4 ; instruction DEC XX15+4 ; The ship's x_sign is negative (i.e. it's on the left ; side of the screen), so switch the laser beam so it ; goes to the right edge of the screen by decrementing ; XX15(5 4) to 255 JSR CLIP ; Call CLIP to see if the laser beam needs to be ; clipped to fit on-screen, returning the clipped line's ; end-points in (X1, Y1) and (X2, Y2) BCS LL170 ; If the C flag is set then the line is not visible on ; screen, so jump to LL170 so we don't draw this line LDY U ; This instruction is left over from the other versions ; of Elite and has no effect ; ; It would fetch the ship line heap pointer from U, but ; the NES version does not have a ship line heap as the ; screen is redrawn for every frame JSR LOIN ; Draw the laser lineName: LL9 (Part 9 of 12) [Show more] Type: Subroutine Category: Drawing ships Summary: Draw ship: Draw laser beams if the ship is firing its laser at us Deep dive: Drawing shipsContext: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
This part sets things up so we can loop through the edges in the next part. It also draws a laser line if the ship is firing at us. When we get here, the heap at XX3 contains all the visible vertex screen coordinates.
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Entry point CLIP in subroutine LL145 (Part 1 of 4) (category: Drawing lines)
Another name for LL145 CLIP2 Don't initialise the values in SWAP or A
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Subroutine DOEXP (category: Drawing ships)
Draw an exploding ship
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Label EE31 is local to this routine
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Label LL170 in subroutine LL9 (Part 10 of 12)
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Label LL74 is local to this routine
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Subroutine LOIN (Part 1 of 7) (category: Drawing lines)
Draw a line: Calculate the line gradient in the form of deltas
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Temporary storage, used to store the address of a ship blueprint. For example, it is used when we add a new ship to the local bubble in routine NWSHP, and it contains the address of the current ship's blueprint as we loop through all the nearby ships in the main flight loop
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Workspace XX3 (category: Workspaces)
Temporary storage space for complex calculations