Metroid (nmikstas's disassembly and the US ROM)

Wikidata lists Nintendo Research & Development 1 as the developer of Metroid and Nintendo as its publisher, and dates its release on the Famicom Disk System in Japan 6 August 1986. metroid/release

Cited by: Invincibility frames, Metroid

nmikstas's disassembly of Metroid, assembled with Ophis, equals the PRG ROM of Metroid (USA) byte for byte. metroid/build

The repo's README says the work "started off as a disassembly of the game engine by Kent Hansen many years ago". Much of bank 7's crash detection keeps raw labels such as LF282.

Cited by: Metroid

Samus's health is binary-coded decimal in tenths of a unit; HealthHi holds the full energy tanks in its high nibble and the tens digit in its low nibble, and HealthLo holds the ones digit in its high nibble. metroid/health-format

About: Metroid Health upgrade

$50 in HealthLo is 5 units, so its low nibble is a tenths digit the display leaves out.

Cited by: Health upgrade, Metroid

Samus starts with 30 units of energy and no energy tanks. metroid/start-health

About: Metroid Health upgrade

Value: 30 units

Cited by: Health upgrade, Metroid

When an enemy touches Samus, the game charges her the base damage of the current area, two bytes in that area's bank at $95CE, whatever the enemy; Brinstar's is 8 units, Norfair's 10, Kraid's hideout's 20, Ridley's hideout's 24, and Tourian's 30. metroid/area-damage

About: Metroid Damage formula

{
 "brinstar": 8,
 "norfair": 10,
 "kraid": 20,
 "ridley": 24,
 "tourian": 30
}

Two other routines in bank 7 charge a flat 5 units (`lda

Cited by: Metroid

With the Varia suit, SubtractHealth halves every amount it takes from Samus. metroid/varia

About: Metroid Damage formula

The halving works on decimal digits, so the routine adds $4F back when a bit carries out of HealthHiChange ($01 halves to 5.0 units).

Cited by: Metroid

A hit sets SamusBlink to $32, which is 50, gives Samus an upward speed of $FD with the hit gravity $38, and takes the push direction from the hit; the comment says "Set blink for 32 frames". metroid/blink-set

About: Metroid Invincibility frames

Value: 50

Cited by: Invincibility frames, Knockback, Metroid

Each frame, the crash detection skips the contact check between Samus and the enemies while SamusBlink is above 0. metroid/blink-guard

About: Metroid Invincibility frames

The same guard covers the Memus at the top of the routine and the enemy slots from $60 on.

Cited by: Invincibility frames, Metroid

CheckHealthStatus, which Samus's routines call once per frame, lowers SamusBlink by 1 each time, so the blink lasts 50 frames of game logic. metroid/blink-countdown

About: Metroid Invincibility frames

On the frame of a hit, the routine sets 50 and then counts it to 49.

Cited by: Invincibility frames, Metroid

On each frame of the blink, CheckHealthStatus sets Samus's horizontal speed in the push direction to the blink counter divided by 16, so the push ends when the counter falls below 16; below 48, it skips the push on any frame Samus has horizontal acceleration. metroid/push

About: Metroid Knockback

The probe reads ObjHorzSpeed as 1 throughout the push, so later movement code limits the stored 3 and 2; how is open. Lava sets DmgPushDir to $FF, no push.

Cited by: Knockback, Metroid

Measured in the ROM, a crawler held against Samus in Brinstar's first room takes 8 units with each hit, and the next hit lands 50 game frames later; one of the two gaps also held 2 lag frames, for 52 frames of video. metroid/blink-measured

About: Metroid Invincibility frames

{
 "probe": {
  "gameGaps": [
   50,
   50
  ],
  "videoGaps": [
   52,
   50
  ],
  "health": [
   220,
   140,
   60
  ]
 }
}

Health is in tenths of a unit in the probe's values.

Cited by: Invincibility frames, Metroid

Measured the same way, with the crawler moved off Samus during each blink, the three pushes carried her 37, 45, and 44 px away from it; two lifted her 19 px and one 2 px. metroid/push-measured

About: Metroid Knockback

{
 "probe": {
  "moved": [
   37,
   45,
   44
  ],
  "rise": [
   19,
   2,
   19
  ]
 }
}

The second hit's flags were $38 where the others' were $28; why it lifted Samus only 2 px is open.

Cited by: Knockback, Metroid

In lava, Samus's blink is set to 50 on every frame, so enemies can't touch her, while the lava takes 0.7 units twice every 8 frames without a push; the Varia suit cuts that to once, and SubtractHealth halves it to 0.3 units. metroid/lava

About: Metroid Invincibility frames

Cited by: Invincibility frames, Knockback, Metroid

The main loop calls RandomNumbers once per frame, after the frame's work; it adds 5 to RandomNumber1 five times, 25 in all, so the byte steps through all 256 values every 256 frames. metroid/rng

About: Metroid Random number generator

The comment's #$19 is 25. Whether an enemy drops anything therefore depends on the frame it dies.

Cited by: Item drop table, Metroid

When an enemy dies, a RandomNumber1 below 16 drops nothing; otherwise its low 3 bits pick from ItemDropTbl, which holds a missile at 0 and 3, energy at 1, 4, and 6, and $89 at 2, 5, and 7. metroid/drop-roll

About: Metroid Item drop table

Cited by: Item drop table, Metroid, Metroid Fusion

Item $89 drops only for an enemy whose byte in the area's table at $968B has bit 0 set, and Samus collects it as 20 units of energy; the table's comment calls it "No item". metroid/drop-89

About: Metroid Item drop table

Disputed: The table's comment calls `$89` "No item"; it drops nothing only for enemies whose `$968B` byte has bit 0 clear. In Brinstar, data indexes 0, 1, 2, 6, and 8 have it set.

Cited by: Item drop table, Metroid

Each door re-rolls the room's caps from RandomNumber1, 0 to 15 missile drops and 64 to 94 energy drops, and zeroes the counts. A drop that would pass its cap is lost; one that passes the test adds to its count, even a $89 roll that the enemy then can't drop. metroid/drop-caps

About: Metroid Item drop table

Disputed: The comments give "16 max" missile drops and "128 max" energy drops; `AND #$0F` gives at most 15, and `ASL` then `ORA #$40` at most 94. The defines call the counts power-ups "collected by Samus", but the drop code counts each drop that passes its cap, collected or not, and even a `$89` it then withholds.

A missile drop also needs missile capacity (MaxMissiles above 0). A room whose missile cap rolls 0 drops no missiles.

Cited by: Item drop table, Metroid

Until the first door, the caps on missile and energy drops are 39 when the game starts in Brinstar, because SamusInit stores the mirroring value $27 into them, or 47 ($2F) elsewhere. metroid/start-caps

About: Metroid Item drop table

Cited by: Metroid

A small energy ball gives Samus 5 units and a $89 ball 20; a missile gives 2 missiles; an enemy whose data index carries the 30-unit flag gives 30 units or 30 missiles instead. metroid/pickup-amounts

About: Metroid Item drop table

Missiles stop at the capacity, MaxMissiles. No Brinstar data index has the flag; in Tourian, indexes 0 and 1 do ($50).

Cited by: Item drop table, Metroid

In Tourian, enemies with data index 2 or 4 drop nothing, and any other kill that would drop nothing resets both caps to 1 and both counts to 0 and rolls the same RandomNumber1 again, skipping the test for 16. metroid/tourian-drops

About: Metroid Item drop table

Each retry shifts the enemy's $968B byte right once more, so a $89 roll that fails one pass can succeed on the next. The atlas hasn't tied Tourian's data indexes to enemies.

Cited by: Item drop table, Metroid

Called in the ROM for all 256 values of RandomNumber1, the drop code gives a Brinstar enemy of data index 0 60 missiles, 90 small energy balls, 90 $89 balls, and 16 nothing; index 3, whose byte in the table LF74B reads ($968B + data index, kept in $00) has bit 0 clear, gives 106 nothing and no $89. metroid/drop-measured

About: Metroid Item drop table

{
 "probe": {
  "index0": {
   "missile": 60,
   "energy": 90,
   "big": 90,
   "none": 16
  },
  "index3": {
   "missile": 60,
   "energy": 90,
   "big": 0,
   "none": 106
  },
  "nomissiles": {
   "missile": 0,
   "energy": 90,
   "big": 90,
   "none": 76
  },
  "capped": {
   "missile": 0,
   "energy": 0,
   "big": 0,
   "none": 256
  }
 }
}

Without missile capacity, the 60 missile rolls drop nothing; at both caps, nothing drops.

Cited by: Item drop table, Metroid

An energy tank adds a tank, up to 6, and fills Samus's health to her new maximum. metroid/energy-tank

About: Metroid Health upgrade

Value: 6

Cited by: Health upgrade, Metroid

Samus's health can't exceed 99.9 units above her tanks, $100t+99.9$ units for $t$ tanks, which is 699.9 with all six. metroid/max-health

About: Metroid Health upgrade

The tanks' energy sits in the high nibble of HealthHi, the full tanks, and the display shows the rest as two digits.

Cited by: Health upgrade, Metroid

A missile tank raises Samus's missile capacity by 5. metroid/missile-tank

Cited by: Ammunition, Metroid