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⌂ ∞ ▽ Elite on the 6502

Drawing ships: LL9 (Part 1 of 12)

[NES version, Bank 1]

       Name: LL9 (Part 1 of 12)                                      [Show more]
       Type: Subroutine
   Category: Drawing ships
    Summary: Draw ship: Check if ship is exploding, check if ship is in front
  Deep dive: Drawing ships

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * HAS1 calls LL9
             * LL9_b1 calls LL9




 This routine draws the current ship on the screen. This part checks to see if
 the ship is exploding, or if it should start exploding, and if it does it sets
 things up accordingly.

 In this code, XX1 is used to point to the current ship's data block at INWK
 (the two labels are interchangeable).



 Arguments:

   XX1                  XX1 shares its location with INWK, which contains the
                        zero-page copy of the data block for this ship from the
                        K% workspace

   INF                  The address of the data block for this ship in workspace
                        K%

   XX0                  The address of the blueprint for this ship



 Other entry points:

   EE51                 Remove the current ship from the screen, called from
                        SHPPT before drawing the ship as a point



.LL25

 JMP PLANET             ; Jump to the PLANET routine, returning from the
                        ; subroutine using a tail call

.LL9

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDA TYPE               ; If the ship type is negative then this indicates a
 BMI LL25               ; planet or sun, so jump to PLANET via LL25 above

 LDA #31                ; Set XX4 = 31 to store the ship's distance for later
 STA XX4                ; comparison with the visibility distance. We will
                        ; update this value below with the actual ship's
                        ; distance if it turns out to be visible on-screen

 LDA NEWB               ; If bit 7 of the ship's NEWB flags is set, then the
 BMI EE51               ; ship has been scooped or has docked, so jump down to
                        ; EE51 to skip drawing the ship so it doesn't appear
                        ; on-screen

 LDA #%00100000         ; If bit 5 of the ship's byte #31 is set, then the ship
 BIT XX1+31             ; is currently exploding, so jump down to EE28
 BNE EE28

 BPL EE28               ; If bit 7 of the ship's byte #31 is clear then the ship
                        ; has not just been killed, so jump down to EE28

                        ; Otherwise bit 5 is clear and bit 7 is set, so the ship
                        ; is not yet exploding but it has been killed, so we
                        ; need to start an explosion

 ORA XX1+31             ; Clear bits 6 and 7 of the ship's byte #31, to stop the
 AND #%00111111         ; ship from firing its laser and to mark it as no longer
 STA XX1+31             ; having just been killed

 LDA #0                 ; Set the ship's acceleration in byte #31 to 0, updating
 LDY #28                ; the byte in the workspace K% data block so we don't
 STA (INF),Y            ; have to copy it back from INWK later

 LDY #30                ; Set the ship's pitch counter in byte #30 to 0, to stop
 STA (INF),Y            ; the ship from pitching

 JSR HideShip           ; Update the ship so it is no longer shown on the
                        ; scanner

 LDA #18                ; Set the explosion cloud counter in INWK+34 to 18 so we
 STA INWK+34            ; can use it in DOEXP when drawing the explosion cloud

 LDY #37                ; Set byte #37 of the ship's data block to a random
 JSR DORND              ; number to use as a random number seed value for
 STA (INF),Y            ; generating the explosion cloud

 INY                    ; Set byte #38 of the ship's data block to a random
 JSR DORND              ; number to use as a random number seed value for
 STA (INF),Y            ; generating the explosion cloud

 INY                    ; Set byte #39 of the ship's data block to a random
 JSR DORND              ; number to use as a random number seed value for
 STA (INF),Y            ; generating the explosion cloud

 INY                    ; Set byte #40 of the ship's data block to a random
 JSR DORND              ; number to use as a random number seed value for
 STA (INF),Y            ; generating the explosion cloud

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

.EE28

 LDA XX1+8              ; Set A = z_sign

.EE49

 BPL LL10               ; If A is positive, i.e. the ship is in front of us,
                        ; jump down to LL10

.LL14

                        ; If we get here then we do not draw the ship on-screen,
                        ; for example when the ship is no longer on-screen, or
                        ; is too far away to be fully drawn, and so on

 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 EE51
 BEQ EE51               ; to skip drawing the ship

 LDA XX1+31             ; The ship is exploding, so clear bit 3 of the ship's
 AND #%11110111         ; byte #31 to denote that the ship is no longer being
 STA XX1+31             ; drawn on-screen

 JMP DOEXP              ; Jump to DOEXP to remove the explosion burst sprites
                        ; from the screen (if they are visible), returning from
                        ; the subroutine using a tail call

.EE51

 LDA XX1+31             ; Clear bits 3 and 6 in the ship's byte #31, which stops
 AND #%10110111         ; drawing the ship on-screen (bit 3), and denotes that
 STA XX1+31             ; the explosion has not been drawn and there are no
                        ; lasers firing (bit 6)

 RTS                    ; Return from the subroutine