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Elite on the BBC Micro and NES

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

[BBC Micro cassette version]

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 Variations: See code variations for this subroutine in the different versions 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 BNE MA15 \ missile and laser locking JSR PLUT \ Call PLUT to update the geometric axes in INWK to \ match the view (front, rear, left, right) JSR HITCH \ Call HITCH to see if this ship is in the crosshairs, BCC MA8 \ in which case the C flag will be set (so if there is \ no missile or laser lock, we jump to MA8 to skip the \ following) 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 JSR BEEP \ 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 #&0E \ 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 colour in Y (red = &0E) .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 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 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 LDA INWK+31 \ Set bit 7 of the enemy ship's byte #31, to indicate ORA #%10000000 \ that it has been killed STA INWK+31 BCS MA8 \ If the enemy ship type is >= SST (i.e. missile, \ asteroid, canister, Thargon or escape pod) then \ jump down to MA8 JSR DORND \ Fetch a random number, and jump to oh if it is BPL oh \ positive (50% chance) LDY #0 \ Fetch the first byte of the hit ship's blueprint, AND (XX0),Y \ which determines the maximum number of bits of \ debris shown when the ship is destroyed, and AND \ with the random number we just fetched STA CNT \ Store the result in CNT, so CNT contains a random \ number between 0 and the maximum number of bits of \ debris that this ship will release when destroyed .um BEQ oh \ We're going to go round a loop using CNT as a counter \ so this checks whether the counter is zero and jumps \ to oh when it gets there (which might be straight \ away) LDX #OIL \ Call SFS1 to spawn a cargo canister from the now LDA #0 \ deceased parent ship, giving the spawned canister an JSR SFS1 \ AI flag of 0 (no AI, no E.C.M., non-hostile) DEC CNT \ Decrease the loop counter BPL um \ Jump back up to um (this BPL is effectively a JMP as \ CNT will never be negative) .oh 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