Arcana's battle and encounter rules (Arcana (USA), our own disassembly notes)

In a dungeon a step can start a random battle, about 1 step in 11.6 on one class of tile (computed: a draw out of 12 coming up 0, 22 of the 256 values the random byte can take) and 1 step in 4 on a second class (computed: a draw out of 4, 64 of 256); every other tile starts none, except a boss tile. The tile also picks which of the chapter's monster groups appears. A boss tile with an entry in the chapter's boss list starts that entry's battle for certain while the entry's flag is clear; once the flag is set the tile starts nothing. A chest tile opens its chest instead. arcana/encounter-roll

About: Arcana Random encounter

Value: 0x10-0x1 F 22/256 · 0x20-0x2 F 64/256

The tile's encounter value comes from a 32 by 32 grid at $7F:FC00 and is kept in $16F5. Values $10-$1F are the first class, $20-$2F the second; below $10 and from $30 up no battle starts; $70 is a boss tile (the boss list at $18:9101, its flags in $18DF) and $80 a chest. The low four bits pick the group. Data Crystal's rate byte at $18:8044 is the start of the instruction that loads it; the 12 itself is at $18:8045 and governs only the first class, while the second uses the 4 at $18:804A. The counts assume every byte value can come up; which values the generator reaches was not measured.

Cited by: Random encounter

A random battle draws its opening out of 32: 0 gives a first strike and 1 an ambush under the banner "Yikes!!" (8 of the 256 random values each, computed), 2 to 31 an ordinary start under "COMBAT". In an ambush the party sits out the first round: party members are skipped until the first monster acts. A first strike is meant to skip the monsters, but it is cancelled on the first party member taken, before any monster comes up, so those battles play as ordinary ones and show "COMBAT". A scripted battle opens ordinarily, or as an ambush in one scripted case. arcana/battle-opening

About: Arcana Random encounter

Value: first strike 8/256 · ambush 8/256

The opening is kept in $1579 (1 first strike, 2 ambush, 3 ordinary). The round builder at $00:C905 takes the party's slots before the monsters'; opening 1 skips monster slots, opening 2 party slots, and any fighter taken sets the opening to 3. A scripted battle is one with $1901 set; the ambush case is $18FF = $19. The banner tables for openings 1 and 3 both point at "COMBAT".

Each round the game draws who acts next, with chances in proportion to a quarter of each fighter's Alertness (at least 1): it sums those weights over the fighters taking part, takes a random number modulo the sum, and picks the first fighter whose running total is above it. arcana/turn-order

About: Arcana Turn order

A weapon attack hits when a random number modulo 255 is below the hit chance: the attacker's accuracy less the target's evasion plus 245, capped at 255 (a result below 0 also reads as 255). Accuracy is 3 x (Alertness / 8 + 1) plus 64; one of its two flag bits sets the 64 to 127 and the other to 0. Evasion is the same figure from the target's evasion flags, plus 26 when bit 8 of the target's state is set and a row term of 0, 0, 26, 51 or 76. The same random number makes the hit a critical when it is also below the weapon's critical byte. arcana/hit-chance

About: Arcana Hit chance

Which flag bit is a raise and which a lowering wasn't read: the handlers of the Impair and Increase spells (effect classes 5 and 8) are unread.

A weapon hit's damage, with each product rounded down to a whole number: attack = power x (Strength figure x share / 256 + buff + 205 - share) / 256, where the Strength figure is Strength x 191 / 256 + 64, share is the weapon's strength-share byte, power is the weapon's power word (10 bare-handed), and buff is 25, or 50 with one of the attack flag bits set and 0 with the other. Defense = armor x 102 / 256 + shield x 51 / 256 + (Endurance x 223 / 256 + 32) x 51 / 256, plus 25, 50 or 0 by its own two flag bits. The damage is attack x (255 - defense) / 256, times the row factor (1, 1, or about 0.88, 0.76 or 0.64), the element factor, the race factor, 0.75 when bit 8 of the target's state is set and 2 on a critical, before the random spread. arcana/physical-damage

About: Arcana Damage formula

The row factors are $100, $100, $E1, $C3 and $A3 in 256ths (0.879, 0.762, 0.637). Which flag bit raises and which lowers wasn't read. A monster's share is 205 and its armor 16 (arcana-tables/monster-equipment), so its attack is power x (Strength figure x 205 / 256 + buff) / 256 and its defense starts from 6.

Cited by: Damage formula

Healing spells skip the roll to land, and spells of effect class 8 and up (from Dodge All on, arcana-tables/spells) always land; Ruinous Mission and Flee face only the immunity check. Every other spell rolls to land: chance = its accuracy + the caster's magic figure - the target's, where the magic figure is (Intelligence - 55) x 66 / 256 and 0 below 55 Intelligence; Chaos Wind, Entomb and Destroy add 52 to the caster's side when the element factor is 1.6 and to the target's when it is 0.4. It lands when a random number modulo 255 is below the chance (capped at 255; below 0 it misses). A damage spell then deals power x (205 + (Intelligence - 55) x 393 / 256) / 256, times (255 - magic defense) / 256, then add 1, times the element factor, before the random spread; magic defense is the shield's byte plus (Intelligence - 55) x 99 / 256, capped at 255. Healing spells give their power plus a spread of 13/256. arcana/spell-damage

About: Arcana Damage formula

Damage gets a random spread on top: with S = base x 38 / 256 + 1 (at most 255), the result is the base plus a random number modulo S, then held between 1 and 999. A base of 100 comes out 100 to 114. Healing spells use 13 in place of 38. arcana/damage-spread

About: Arcana Damage modifier

Value: spread 38/256 · heal spread 13/256 · min 1 · max 999

Cited by: Damage formula

The four elements form a cycle: Wind is strong against Earth, Earth against Water, Water against Fire and Fire against Wind (1.6 times), each element is 0.8 times against itself, 1.2 against the element two steps on and 0.4 against the one that beats it. An attack with several elements takes the average of their factors plus 13/256 for each element past the first, so one with all four does about 1.15 times against any element. A target without an element takes 1 times, plus the same 13/256 for each attack element past the first. A target with several elements is judged by the first of Wind, Earth, Water; the attacker's element is its weapon's, else its own. arcana/elements

About: Arcana Elemental weakness

{
 "Wind": {
  "Wind": 0.8,
  "Earth": 1.6,
  "Water": 1.2,
  "Fire": 0.4
 },
 "Earth": {
  "Wind": 0.4,
  "Earth": 0.8,
  "Water": 1.6,
  "Fire": 1.2
 },
 "Water": {
  "Wind": 1.2,
  "Earth": 0.4,
  "Water": 0.8,
  "Fire": 1.6
 },
 "Fire": {
  "Wind": 1.6,
  "Earth": 1.2,
  "Water": 0.4,
  "Fire": 0.8
 }
}

The factor words are $CC (0.797), $19A (1.602), $134 (1.203) and $66 (0.398) in 256ths. Fire and no-element targets share column 6, but a target with no element never reaches the table.

Monsters carry race bits (bit 1 marks the undead), and a weapon hit scales by them: when neither side has race bits of its own the factor is 1; otherwise it is 1.2 when the attacker's race bits, its weapon's included, meet the target's, and 0.61 when they don't. A party member has none of its own, so it hits a monster with race bits at 0.61 unless its weapon carries a matching bit, and at 1.2 with one. Against a party member, the target's bits are the ones its armor and shield don't carry, so a monster with race bits hits at 1.2 unless the defender's armor or shield carries all of them. arcana/race-factor

About: Arcana Type effectiveness

Value: match 1.2 · miss 0.61 · neither 1

$133 is 1.199 and $9D 0.613 in 256ths.

Before a spell rolls, a check turns some away: a target with race bit $10, which all 16 boss monsters carry, ignores every checked spell whose element word lacks bit $10: Chaos Wind, Entomb and Destroy, the ten paralysis, petrify, stop, sleep and confusion spells, Requiem, Emancipation, Final Liberation, Ruinous Mission and Flee, and a spell with bit 1 (Requiem, Emancipation) affects only targets with race bit 1, the undead. arcana/spell-immunity

About: Arcana Boss fight

A boss in the battle stops running: if any fighter has race bit $10, which all 16 boss monsters carry, the run command prints "But there was no escape." What happens when no boss is present, and whether that path can still fail, was not read. arcana/run

About: Arcana Fleeing

A monster whose spell byte isn't 0 chooses its spell 1 time in 2 while its HP is above half its maximum, and 3 times in 4 at half or below. arcana/monster-spells

Value: above half 0.5 · at or below half 0.75

Reaching level 60 gives Rooks +100 HP, +30 MP and +10 in each stat, against +6, +2, +4, +2, +2, +2 at level 59: the level-up adds the row for the new level, and each character's table has rows only for levels 0 to 59, so level 60 adds the next character's starting row (Sylph's, for Rooks); for Axs it is the 12 bytes after the table. Level 60 is the cap, and its experience still needed is cleared. arcana/level-60

The spirit in the party has no experience of its own: its level is set to Rooks' whenever his goes up, the experience routine returns at once for its slot, and its equipment figures come from tables by a quarter of that level (arcana-tables/spirit-equipment). A spirit, Teefa or Axs joining the party takes Rooks' level. arcana/spirit-level

The spell list has two field spells meant, by their names, to reduce encounters: diminish Encounters (4 MP) and diminish Encounters 2 (8 MP). What they change is unread; the step routine's roll reads only the tile's value. arcana/diminish-encounters

About: Arcana Random encounter

Both are effect class 13, with Home and the two Way Point spells.

Cited by: Random encounter