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What the manual says

The relevant paragraph is short enough to quote in full. "In order to combine the moves, the attacks must occur in succession (i.e. for the Blizzard Attack, Chaz's Zan must immediately follow Rune's Wat). Since characters attack in a descending order of power (most powerful attacks come first), you must assign the other characters less powerful attacks. As soon as you discover a Combination Attack, write it down! You can even put it into a macro later." Then the dare.

Two of those sentences are right, one is wrong, and one is half right. Succession is the whole mechanism. Macros are the practical way to get it. The order of turns has nothing to do with the power of the attacks. And for Blizzard specifically, Zan does not have to follow Wat; the two only have to be adjacent, in either order. The code for all of this is below.

The tables

The disassembly by lory1990 rebuilds the US ROM byte for byte and names the data. Three tables and one loop are involved.

ComboData, at $285424, is fifteen rows of eight bytes. Each row says what the combo does: effect (damage or instant death), targeting (one enemy or all), the success threshold for the death combos, which enemy stat resists it, and its element. Row zero is all zeros and is the value a fighter carries when no combo is pending; the engine indexes the table with the combo number and number zero has to mean "none". Rows one to fourteen are Paradin Blow to Destruction.

ComboSetupData, at $28549C, is the trigger table: for each of the fourteen combos, how many participants it needs, whether their order matters, and a list of accepted technique and skill combinations terminated by $FF. There are 67 accepted lists in total, most of them for Firestorm, which takes any of the three Foi techniques or Flaeli on one side and any Zan or Hewn on the other.

ComboNames holds fourteen names in the window font. And the loop that walks ComboSetupData, in the routine at $27DE7C, is written as moveq #$D,d7 followed by a dbf: fourteen iterations, combo number computed as fourteen minus the loop counter. There is no fifteenth entry to reach, no flag that would unlock one, and no orphaned row anywhere near.

Where did fifteen come from? The manual was written from development material, and a data table with fifteen rows, one of them blank, is the kind of thing that gets counted by someone who is not reading the code. That is a guess. What is not a guess is the other candidate the community has offered over the years: Feeve, a technique that exists in the technique table (it raises elemental resistance for the party, costs five TP, and nobody learns it) and appears in no combo trigger list at all. It is unused, but it is not a combination.

How a combo is detected

Everything starts with the turn order. At the beginning of a round the game lists every fighter that is not dead, asleep or paralysed, takes each one's battle agility, and adds a random number between zero and half of the highest agility on the field. Then it sorts the list, highest first, with a bubble sort. That is the order of the round. A fast character usually goes first, but "usually" is doing real work: with Rika at 60 and Rune at 50, Rune goes first about one round in five.

When a character's turn comes up, before their command is executed, the routine at $27DE7C runs. It reads the turn order from the current position forward, at most four entries, and for each one checks a list of conditions. The entry has to be a party member, not an enemy. The fighter must be free of paralysis, sleep and death. The queued command must be a technique or a skill, not an attack, an item or a defend. A technique needs enough TP, and the caster must not be sealed; a skill needs a use left. The first entry that fails any of these ends the scan, and everything up to it is the candidate window.

The window is then compared against the fourteen trigger lists in table order, Paradin Blow first, Destruction last, and the first combo that matches wins. For an unordered two-piece combo the game tries the pair as it stands and then swapped; for an unordered three-piece combo it tries all six permutations; for an ordered combo it tries only the sequences written in the table. A match with N participants marks the current actor's action as "combo", stores the combo number, and advances the turn counter past the other N−1 participants, whose own turns are consumed. Each participant still pays: TP for a technique, one use for a skill.

Two consequences follow that guides tend to leave out. First, order in the trigger table is a priority. If four consecutive actors queue Foi, Zan, Wat and Tsu, the game sees Foi and Zan, matches Firestorm (combo 2), and never gets to consider Triblaster (combo 8); Wat and Tsu then resolve on their own. Second, the scan is anchored to the first participant's turn. A character whose turn has already passed cannot be part of a combo with someone who acts later, even if both queued the right pieces, which is why a combo you have set up carefully can vanish when the round's agility roll reshuffles the order.

The fourteen, from the trigger table

Who can supply each piece comes from the level tables and the scripted skill grants, also in the disassembly: Chaz learns Zan at level 12 and Rayblade at 27, Alys Death at 13, Hahn Astral at 29 and Savol at 33, Rune Diem at 24, Tandle at 27, Efess at 29, Negatis at 32 and Legeon at 35, Rika Deban at 14, Kyra her Tandle at 39; Wren's Hijammer, Burstroc and Posibolt and Demi's Phonon are installed by story events.

Some things in that table only show up when you read the trigger lists entry by entry.

• The same person never pairs with themself. Rune has Flaeli and Hewn, Kyra has her own copies (the ROM stores them as separate skills, and hers hit harder: Hewn 64 against 112, Tandle 144 against 176). Firestorm lists Rune's Flaeli with Kyra's Hewn and Kyra's Flaeli with Rune's Hewn, and omits the two pairings a single character would need two turns for. Black Hole likewise lists the Gra family with Negatis, and since Negatis is Rune's, the Gra has to be Kyra's.
• Triblaster wants the level-one techniques. Foi, Wat and Tsu, not their Gi and Na upgrades. Any other three-way of the same families does nothing.
• Three combos are ordered. Conductthnd needs the Wat cast to come before Tandle; Shooting Star needs Burstroc before the fire; Destruction needs Rika's Deban to be the first of the four, after which Megid, Legeon and Posibolt may land in any order. The manual's example, Wat then Zan for Blizzard, is one of the unordered ones.
• Lethal Image has a window. Rika joins at the Bio Plant and Alys is struck down at Zio's fort a dungeon later; from then until her death she is in the party with the dead flag set, and the turn ordering skips her. Alys must be level 13 for Death, so the whole opportunity is the road from Zema to Zio's fort. It is the combo the manual's dare most plausibly cost people the longest.

What a combo is worth

A combo is not a fixed-power attack. The damage routine, entered at $2864 for combos, walks the participants and adds up two things: the casting stat of each one (mental for a technique, and for a skill the stat its own data names, strength for Phonon, attack power for Airslash) and the power byte of each technique or skill used. Those two sums, together with the combo's element and the enemy's resistance stat, go into the same formula every other attack uses:

with STAT the participants' stats added together and POWER their tech powers added together. Sixteen rolls of 0 to 7 average 56, so the first term is about STAT again: a combo does, on average, roughly twice the combined stat plus twice the combined power, scaled by how much the target minds the element, minus its resistance. Nafoi is worth 136 power points and Nazan 128; a Firestorm from those two is a single hit built on 264 power and two mental stats, against every enemy, when either alone would have been one stat and one power. The element factor comes from the enemy's property byte for that element (immune, resistant, normal, weak, very weak), so a fire combo against a fire-immune enemy is still a zero.

The five instant-death combos skip the damage formula. For each enemy the game rolls 0 to 63, adds the participants' combined casting stat minus the enemy's resisting stat, multiplies by the element factor, and the result must exceed 64 (the "hit" byte in ComboData) for that enemy to die. An enemy immune to the element multiplies the roll by zero and survives every time, which is why Circuit Breaker, keyed to the mechanical element and resisted by strength, is for machines and nothing else.

Method

All of this comes from the lory1990 disassembly of the US ROM, which assembles to the retail image, cross-checked against the ROM itself for the table addresses given above. The routines are the combo detector at $27DE7C (labelled by its address), Character_DoCombo at $AB98, the combo branch of the damage code at $2864, Battle_OrderTurns for the agility roll and sort, and Battle_CalculateDamage and Battle_CalculateChances for the two formulas. Who learns what is read from the per-character level tables and from the event handlers that grant android skills.

The decoded tables, with every accepted trigger list, are in psiv-combos.json, so the fourteen can be counted by anyone who would rather not take a manual's word for it, or mine.