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Main loop: Main game loop (Part 4 of 6)

[BBC Micro disc version, Flight]

Name: Main game loop (Part 4 of 6) [Show more] Type: Subroutine Category: Main loop Summary: Potentially spawn a lone bounty hunter or up to four pirates Deep dive: Program flow of the main game loop Ship data blocks Fixing ship positions
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

This section covers the following: * Potentially spawn (47% chance) either a lone bounty hunter (a Cobra Mk III, Asp Mk II, Python or Fer-de-lance), or a group of up to 4 pirates (a mix of Sidewinders, Mambas, Kraits, Adders, Geckos, Cobras Mk I and III, and Worms) * Also potentially spawn a Constrictor if this is the mission 1 endgame, or Thargoids if mission 2 is in progress
DEC EV \ Decrement EV, the extra vessels spawning delay, and if BPL MLOOPS \ it is still positive, jump to MLOOPS to stop spawning, \ so we only do the following when the EV counter runs \ down INC EV \ EV is negative, so bump it up again, setting it back \ to 0 LDA TP \ Fetch bits 2 and 3 of TP, which contain the status of AND #%00001100 \ mission 2 CMP #%00001000 \ If bit 3 is set and bit 2 is clear, keep going to BNE nopl \ spawn a Thargoid as we are transporting the plans in \ mission 2 and the Thargoids are trying to stop us, \ otherwise jump to nopl to skip spawning a Thargoid JSR DORND \ Set A and X to random numbers CMP #200 \ If the random number in A < 200 (78% chance), jump to BCC nopl \ nopl to skip spawning a Thargoid .fothg2 JSR GTHG \ Call GTHG to spawn a Thargoid ship and a Thargon \ companion .nopl JSR DORND \ Set A and X to random numbers LDY gov \ If the government of this system is 0 (anarchy), jump BEQ LABEL_2 \ straight to LABEL_2 to start spawning pirates or a \ lone bounty hunter CMP #120 \ If the random number in A >= 120 (53% chance), jump to BCS MLOOPS \ MLOOPS to stop spawning (so there's a 47% chance of \ spawning pirates or a lone bounty hunter) AND #7 \ Reduce the random number in A to the range 0-7, and CMP gov \ if A is less than government of this system, jump BCC MLOOPS \ to MLOOPS to stop spawning (so safer governments with \ larger gov numbers have a greater chance of jumping \ out, which is another way of saying that more \ dangerous systems spawn pirates and bounty hunters \ more often) .LABEL_2 \ Now to spawn a lone bounty hunter or a group of \ pirates JSR Ze \ Call Ze to initialise INWK to a potentially hostile \ ship, and set A and X to random values \ \ Note that because Ze uses the value of X returned by \ DORND, and X contains the value of A returned by the \ previous call to DORND, this does not set the new ship \ to a totally random location. See the deep dive on \ "Fixing ship positions" for details CMP #100 \ If the random number in A >= 100 (61% chance), jump BCS mt1 \ to mt1 to spawn pirates, otherwise keep going to \ spawn a lone bounty hunter INC EV \ Increase the extra vessels spawning counter, to \ prevent the next attempt to spawn extra vessels AND #3 \ Set A = random number in the range 0-3, which we \ will now use to determine the type of ship ADC #CYL2 \ Add A to #CYL2 (we know the C flag is clear as we \ passed through the BCS above), so A is now one of the \ lone bounty hunter ships, i.e. Cobra Mk III (pirate), \ Asp Mk II, Python (pirate) or Fer-de-lance \ \ Interestingly, this logic means that the Moray, which \ is the ship after the Fer-de-lance in the XX21 table, \ never spawns, as the above logic chooses a blueprint \ number in the range CYL2 to CYL2+3 (i.e. 24 to 27), \ and the Moray is blueprint 28 \ \ No other code spawns the ship with blueprint 28, so \ this means the Moray is never seen in Elite \ \ This is presumably a bug, which could be very easily \ fixed by inserting one of the following instructions \ before the ADC #CYL2 instruction above: \ \ * SEC would change the range to 25 to 28, which \ would cover the Asp Mk II, Python (pirate), \ Fer-de-lance and Moray \ \ * LSR A would set the C flag to a random number to \ give a range of 24 to 28, which would cover the \ Cobra Mk III (pirate), Asp Mk II, Python (pirate), \ Fer-de-lance and Moray \ \ It's hard to know what the authors' original intent \ was, but the second approach makes the Moray and Cobra \ Mk III the rarest choices, with the Asp Mk II, Python \ and Fer-de-Lance being more likely, and as the Moray \ is described in the literature as a rare ship, and the \ Cobra can already be spawned as part of a group of \ pirates (see mt1 below), I tend to favour the LSR A \ solution over the SEC approach TAY \ Copy the new ship type to Y JSR THERE \ Call THERE to see if we are in the Constrictor's \ system in mission 1 BCC NOCON \ If the C flag is clear then we are not in the \ Constrictor's system, so skip to NOCON LDA #%11111001 \ Set the AI flag of this ship so that it has E.C.M., STA INWK+32 \ has a very high aggression level of 28 out of 31, is \ hostile, and has AI enabled - nasty stuff! LDA TP \ Fetch bits 0 and 1 of TP, which contain the status of AND #%00000011 \ mission 1 LSR A \ Shift bit 0 into the C flag BCC NOCON \ If bit 0 is clear, skip to NOCON as mission 1 is not \ in progress ORA MANY+CON \ Bit 0 of A now contains bit 1 of TP, so this will be \ set if we have already completed mission 1, so this OR \ will be non-zero if we have either completed mission \ 1, or there is already a Constrictor in our local \ bubble of universe (in which case MANY+CON will be \ non-zero) BEQ YESCON \ If A = 0 then mission 1 is in progress, we haven't \ completed it yet, and there is no Constrictor in the \ vicinity, so jump to YESCON to spawn the Constrictor .NOCON TYA \ Set A to the new ship type in Y EQUB &2C \ Skip the next instruction by turning it into \ &2C &A9 &1F, or BIT &1FA9, which does nothing apart \ from affect the flags .YESCON LDA #CON \ If we jump straight here, we are in the mission 1 \ endgame and it's time to spawn the Constrictor, so \ set A to the Constrictor's type .focoug JSR NWSHP \ Spawn the new ship, whether it's a pirate or \ Constrictor .mj1 JMP MLOOP \ Jump down to MLOOP, as we are done spawning ships .mt1 AND #3 \ It's time to spawn a group of pirates, so set A to a \ random number in the range 0-3, which will be the \ loop counter for spawning pirates below (so we will \ spawn 1-4 pirates) STA EV \ Delay further spawnings by this number STA XX13 \ Store the number in XX13, the pirate counter .mt3 JSR DORND \ Set A and X to random numbers STA T \ Set T to a random number JSR DORND \ Set A and X to random numbers AND T \ Set A to the AND of two random numbers, so each bit \ has 25% chance of being set which makes the chances \ of a smaller number higher AND #7 \ Reduce A to a random number in the range 0-7, though \ with a bigger chance of a smaller number in this range STA CPIR \ Set CPIR to this random number in the range 0-7 .more LDA CPIR \ Set A to the ship type in CPIR ADC #PACK \ #PACK is set to #SH3, the ship type for a Sidewinder, \ so this sets our new ship type to one of the pack \ hunters, namely a Sidewinder, Mamba, Krait, Adder, \ Gecko, Cobra Mk I, Worm or Cobra Mk III (pirate) JSR NWSHP \ Try adding a new ship of type A to the local bubble BCS P%+7 \ If the ship was successfully added, skip the following \ two instructions DEC CPIR \ The ship wasn't added, which might be because the ship \ blueprint for this ship type isn't in the currently \ loaded ship blueprints file, so decrement CPIR to \ point to the previous ship type, so we can try \ spawning that type of pirate instead BPL more \ Loop back to more to have another go at spawning this \ pirate, until we have tried spawning a Sidewinder when \ CPIR is 0, in which case give up and move on to the \ next pirate to spawn DEC XX13 \ Decrement the pirate counter BPL mt3 \ If we need more pirates, loop back up to mt3, \ otherwise we are done spawning, so fall through into \ the end of the main loop at MLOOP