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⌂ ∞ Elite on the BBC Micro and NES

Main loop: Main flight loop (Part 15 of 16)

[NES version, Bank 0]

       Name: Main flight loop (Part 15 of 16)                        [Show more]
       Type: Subroutine
   Category: Main loop
    Summary: Perform altitude checks with the planet and sun and process fuel
             scooping if appropriate
  Deep dive: Program flow of the main game loop
             Scheduling tasks with the main loop counter
             The NES combat demo
             The Trumbles mission

    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:

   * Perform an altitude check with the planet (every 16 iterations of the main
     loop, on iterations 10 and 20 of each 32)

   * Perform an altitude check with the sun and process fuel scooping (every
     32 iterations of the main loop, on iteration 20 of each 32)



.MA22

 LDA MJ                 ; If we are in witchspace, jump down to MA23S to skip
 BNE MA23S              ; the following, as there are no planets or suns to
                        ; bump into in witchspace

.MA93

 LDA demoInProgress     ; If the demo is not in progress, jump to main46 to skip
 BEQ main46             ; the following

                        ; If we get here then the demo is in progress, so now we
                        ; check to see if we have destroyed all the demo ships

 LDA JUNK               ; Set Y to the number of pieces of space junk (in JUNK)
 CLC                    ; plus the number of missiles (in MANY+1)
 ADC MANY+1
 TAY

 LDA FRIN+2,Y           ; There are Y non-ship items in the bubble, so if slot
 BNE main46             ; Y+2 is not empty (given that the first two slots are
                        ; the planet and sun), then this means there is at least
                        ; one ship in the bubble along with the junk and
                        ; missiles, so jump to main46 to skip the following as
                        ; we haven't yet destroyed all the ships in the combat
                        ; practice demo

 LDA #1                 ; If we get here then we have destroyed all the ships in
 JMP ShowScrollText_b6  ; the demo, so jump to ShowScrollText with A = 1 to show
                        ; the results of combat practice, returning from the
                        ; subroutine using a tail call

.main46

 LDA MCNT               ; Fetch the main loop counter and calculate MCNT mod 32,
 AND #31                ; which tells us the position of this loop in each block
                        ; of 32 iterations

 CMP #10                ; If this is the tenth or twentieth iteration in this
 BEQ main47             ; block of 32, do the following, otherwise jump to MA29
 CMP #20                ; to skip the planet altitude check and move on to the
 BNE MA29               ; sun distance check

.main47

 LDA #80                ; If our energy bank status in ENERGY is >= 80, skip
 CMP ENERGY             ; printing the following message (so the message is
 BCC main48             ; only shown if our energy is low)

 LDA #100               ; Print recursive token 100 ("ENERGY LOW{beep}") as an
 JSR MESS               ; in-flight message

 LDY #7                 ; Call the NOISE routine with Y = 7 to make a beep to
 JSR NOISE              ; indicate low energy

.main48

 JSR CheckAltitude      ; Perform an altitude check with the planet, ending the
                        ; game if we hit the ground

 JMP MA23               ; Jump to MA23 to skip to the next section

.MA28

 JMP DEATH              ; If we get here then we just crashed into the planet
                        ; or got too close to the sun, so jump to DEATH to start
                        ; the funeral preparations and return from the main
                        ; flight loop using a tail call

.MA29

 CMP #15                ; If this is the 15th iteration in this block of 32,
 BNE MA33               ; do the following, otherwise jump to MA33 to skip the
                        ; docking computer manoeuvring

 LDA auto               ; If auto is zero, then the docking computer is not
 BEQ MA23               ; activated, so jump to MA23 to skip to the next
                        ; section

 LDA #123               ; Set A = 123 and jump down to MA34 to print token 123
 BNE MA34               ; ("DOCKING COMPUTERS ON") as an in-flight message

.MA33

 AND #15                ; If this is the 6th iteration in this block of 16,
 CMP #6                 ; do the following, otherwise jump to MA23 to skip the
 BNE MA23               ; sun altitude check

 LDA #30                ; Set CABTMP to 30, the cabin temperature in deep space
 STA CABTMP             ; (i.e. one notch on the dashboard bar)

 LDA SSPR               ; If we are inside the space station safe zone, jump to
 BNE MA23               ; MA23 to skip the following, as we can't have both the
                        ; sun and space station at the same time, so we clearly
                        ; can't be flying near the sun

 LDY #NIK%              ; Set Y to NIK%+4, which is the offset in K% for the
                        ; sun's data block, as the second block at K% is
                        ; reserved for the sun (or space station)

 JSR MAS2               ; Call MAS2 to calculate the largest distance to the
 BNE MA23               ; sun in any of the three axes, and if it's non-zero,
                        ; jump to MA23 to skip the following, as we are too far
                        ; from the sun for scooping or temperature changes

 JSR MAS3               ; Set A = x_hi^2 + y_hi^2 + z_hi^2, so using Pythagoras
                        ; we now know that A now contains the square of the
                        ; distance between our ship (at the origin) and the
                        ; heart of the sun at (x_hi, y_hi, z_hi)

 EOR #%11111111         ; Invert A, so A is now small if we are far from the
                        ; sun and large if we are close to the sun, in the
                        ; range 0 = far away to $FF = extremely close, ouch,
                        ; hot, hot, hot!

 ADC #30                ; Add the minimum cabin temperature of 30, so we get
                        ; one of the following:
                        ;
                        ;   * If the C flag is clear, A contains the cabin
                        ;     temperature, ranging from 30 to 255, that's hotter
                        ;     the closer we are to the sun
                        ;
                        ;   * If the C flag is set, the addition has rolled over
                        ;     and the cabin temperature is over 255

 STA CABTMP             ; Store the updated cabin temperature

 BCS MA28               ; If the C flag is set then jump to MA28 to die, as
                        ; our temperature is off the scale

 CMP #224               ; If the cabin temperature < 224 then jump to MA23 to
 BCC MA23               ; skip fuel scooping, as we aren't close enough

 CMP #240               ; If the cabin temperature < 240 then jump to nokilltr
 BCC nokilltr           ; as the heat isn't high enough to kill Trumbles

 LDA TRIBBLE+1          ; If TRIBBLE(1 0) = 0 then there are no Trumbles in the
 ORA TRIBBLE            ; hold, so jump to nokilltr to skip the following
 BEQ nokilltr

 LSR TRIBBLE+1          ; Halve the number of Trumbles in TRIBBLE(1 0) as the
 ROR TRIBBLE            ; cabin temperature is high enough to kill them off
                        ; (this will eventually bring the number down to zero)

 LDY #31                ; Call the NOISE routine with Y = 31 to make the sound
 JSR NOISE              ; of Trumbles being killed off by the heat of the sun

.nokilltr

 LDA BST                ; If we don't have fuel scoops fitted, jump to MA23 to
 BEQ MA23               ; skip fuel scooping, as we can't scoop without fuel
                        ; scoops

 LDA DELT4+1            ; We are now successfully fuel scooping, so it's time
 BEQ MA23               ; to work out how much fuel we're scooping. Fetch the
                        ; high byte of DELT4, which contains our current speed
                        ; divided by 4, and if it is zero, jump to BA23 to skip
                        ; skip fuel scooping, as we can't scoop fuel if we are
                        ; not moving

 LSR A                  ; If we are moving, halve A to get our current speed
                        ; divided by 8 (so it's now a value between 1 and 5, as
                        ; our speed is normally between 1 and 40). This gives
                        ; us the amount of fuel that's being scooped in A, so
                        ; the faster we go, the more fuel we scoop, and because
                        ; the fuel levels are stored as 10 * the fuel in light
                        ; years, that means we just scooped between 0.1 and 0.5
                        ; light years of free fuel

 ADC QQ14               ; Set A = A + the current fuel level * 10 (from QQ14)

 CMP #70                ; If A > 70 then set A = 70 (as 70 is the maximum fuel
 BCC P%+4               ; level, or 7.0 light years)
 LDA #70

 STA QQ14               ; Store the updated fuel level in QQ14

 BCS MA23               ; If A >= 70, jump to BA23 to skip fuel scooping, as the
                        ; fuel tanks are already full

 JSR MakeScoopSound     ; Make the sound of the fuel scoops working

 JSR SetSelectionFlags  ; Set the selected system flags for the new system and
                        ; update the icon bar if required

 LDA #160               ; Set A to token 160 ("FUEL SCOOPS ON")

.MA34

 JSR MESS               ; Print the token in A as an in-flight message