Main loop: Main flight loop (Part 11 of 16)

       Name: Main flight loop (Part 11 of 16)                        [Show more]
       Type: Subroutine
   Category: Main loop
    Summary: For each nearby ship: Process missile lock and firing our laser
  Deep dive: Program flow of the main game loop
             Flipping axes between space views
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


 The main flight loop covers most of the flight-specific aspects of Elite. This
 section covers the following:

   * Continue looping through all the ships in the local bubble, and for each
     one:

     * If this is not the front space view, flip the axes of the ship's
        coordinates in INWK

     * Process missile lock

     * Process our laser firing


.MA26

 LDA QQ11               ; If this is not a space view, jump to MA15 to skip
 BEQ P%+5               ; missile and laser locking
 JMP MA15

 JSR PLUT               ; Call PLUT to update the geometric axes in INWK to
                        ; match the view (front, rear, left, right)

 LDA LAS                ; If we are firing our laser then LAS will contain the
 BNE main37             ; laser power (which will be non-zero), so jump to
                        ; main37 to check if the current ship is in the
                        ; crosshairs, to see if we are hitting it

 LDA MSAR               ; If the active missile is not looking for a target then
 BEQ main38             ; MSAR will be zero, so jump to main38 to skip the
                        ; following, as we can't target lock an unarmed missile

 LDA MSTG               ; If MSTG is positive (i.e. it does not have bit 7 set),
 BPL main38             ; then it indicates we already have a missile locked on
                        ; a target (in which case MSTG contains the ship number
                        ; of the target), so jump to main38 to skip targeting
                        ;
                        ; Or to put it another way, if MSTG = $FF, which means
                        ; there is no current target lock, then keep going

.main37

 JSR HITCH              ; Call HITCH to see if this ship is in the crosshairs,
 BCS main39             ; in which case the C flag will be set and we jump to
                        ; main39 (so if there is no missile or laser lock, we
                        ; jump to MA8 to skip the following)

.main38

 JMP MA8                ; Jump to MA8 to skip the following

.main39

 LDA MSAR               ; We have missile lock, so check whether the leftmost
 BEQ MA47               ; missile is currently armed, and if not, jump to MA47
                        ; to process laser fire, as we can't lock an unarmed
                        ; missile

 LDA MSTG               ; If MSTG is positive (i.e. it does not have bit 7 set),
 BPL MA47               ; then it indicates we already have a missile locked on
                        ; a target (in which case MSTG contains the ship number
                        ; of the target), so jump to MA47 to skip setting the
                        ; target and beeping

 JSR BEEP_b7            ; We have missile lock and an armed missile, so call
                        ; the BEEP subroutine to make a short, high beep

 LDX XSAV               ; Call ABORT2 to store the details of this missile
 LDY #109               ; lock, with the targeted ship's slot number in X
 JSR ABORT2             ; (which we stored in XSAV at the start of this ship's
                        ; loop at MAL1), and set the colour of the missile
                        ; indicator to the pattern number in Y (red
                        ; indicator = pattern 109)

.MA47

                        ; If we get here then the ship is in our sights, but
                        ; we didn't lock a missile, so let's see if we're
                        ; firing the laser

 LDA LAS                ; If we are firing the laser then LAS will contain the
 BEQ MA8                ; laser power (which we set in MA68 above), so if this
                        ; is zero, jump down to MA8 to skip the following

 LDX #15                ; We are firing our laser and the ship in INWK is in
 JSR EXNO               ; the crosshairs, so call EXNO to make the sound of
                        ; us making a laser strike on another ship

 LDA TYPE               ; Did we just hit the space station? If so, jump to
 CMP #SST               ; MA14+2 to make the station hostile, skipping the
 BEQ MA14+2             ; following as we can't destroy a space station

 CMP #SPL               ; Did we just hit a splinter? If not jump to main40 to
 BNE main40             ; skip the following

 LDX LAS                ; We just hit a splinter, so we now check whether we
 CPX #Mlas              ; have a mining laser fitted (in which case the laser
 BEQ MA14+2             ; power in LAS will be Mlas), and if so, jump to MA14+2
                        ; to skip the following, so we can't destroy splinters
                        ; with mining lasers (which is handy, as we probably
                        ; want to scoop them up instead)

.main40

 CMP #CON               ; If the ship we hit is less than #CON - i.e. it's not
 BCC BURN               ; a Constrictor, Cougar, Dodo station or the Elite logo,
                        ; jump to BURN to skip the following

 LDA LAS                ; Set A to the power of the laser we just used to hit
                        ; the ship (i.e. the laser in the current view)

 CMP #(Armlas AND 127)  ; If the laser is not a military laser, jump to MA14+2
 BNE MA14+2             ; to skip the following, as only military lasers have
                        ; any effect on the Constrictor or Cougar (or the Elite
                        ; logo, should you ever bump into one of those out there
                        ; in the black...)

 LSR LAS                ; Divide the laser power of the current view by 4, so
 LSR LAS                ; the damage inflicted on the super-ship is a quarter of
                        ; the damage our military lasers would inflict on a
                        ; normal ship

.BURN

 LDA INWK+35            ; Fetch the hit ship's energy from byte #35 and subtract
 SEC                    ; our current laser power, and if the result is greater
 SBC LAS                ; than zero, the other ship has survived the hit, so
 BCS MA14               ; jump down to MA14 to make it angry

 ASL INWK+31            ; Set bit 7 of the ship byte #31 to indicate that it has
 SEC                    ; now been killed
 ROR INWK+31

 JSR HideShip_b1        ; Update the ship so it is no longer shown on the
                        ; scanner

 LDA LAS                ; Are we using mining lasers? If not, jump to nosp to
 CMP #Mlas              ; spawn canisters, otherwise keep going
 BNE nosp

                        ; If we get here then we are using mining lasers

 LDA TYPE               ; Did we just kill an asteroid? If so, jump to main41
 CMP #AST               ; to break the asteroid up into splinters
 BEQ main41

 CMP #6                 ; Did we just kill a boulder? If not jump to nosp to
 BNE nosp               ; spawn canisters, otherwise keep going

                        ; If we get here then we are using mining lasers and we
                        ; just blasted a boulder

 JSR DORND              ; Set A and X to random numbers

 BPL main43             ; If bit 7 of A is clear (50% chance), jump to main43 to
                        ; skip spawning any splinters

 LDA #1                 ; Otherwise set A = 1 so we spawn one splinter below

 BNE main42             ; Jump to main42 to spawn one splinter (this BNE is
                        ; effectively a JMP as A is never zero)

.main41

                        ; If we get here then we are using mining lasers and we
                        ; just blasted an asteroid

 JSR DORND              ; Set A and X to random numbers

 ORA #1                 ; Reduce the random number in A to the range 1-3
 AND #3

.main42

 LDX #SPL               ; Set X to the ship type for a splinter

 JSR SPIN2              ; Call SPIN2 to spawn A items of type X (so we spawn
                        ; 1-3 splinters if we just destroyed an asteroid, or we
                        ; spawn one splinter 50% of the time if we just
                        ; destroyed a boulder)

 JMP main43             ; Jump to main43 to skip spawning canisters

.nosp

 LDY #PLT               ; Randomly spawn some alloy plates
 JSR SPIN

 LDY #OIL               ; Randomly spawn some cargo canisters
 JSR SPIN

.main43

 LDX TYPE               ; Set X to the type of the ship that was killed so the
                        ; following call to EXNO2 can award us the correct
                        ; number of fractional kill points

 JSR EXNO2              ; Call EXNO2 to process the fact that we have killed a
                        ; ship (so increase the kill tally, make an explosion
                        ; sound and so on)

.MA14

 STA INWK+35            ; Store the hit ship's updated energy in ship byte #35

 LDA TYPE               ; Call ANGRY to make this ship hostile, now that we
 JSR ANGRY              ; have hit it