Bubble Bobble

01 · The cast

The eight kinds of enemy

Each round's list gives every enemy a kind from 0 to 7, and the kind picks a picture and a routine.

An enemy record is three bytes: its kind and column, its row and heading, and how long it waits before it moves. The six sprites not used by the players limit a round to six enemies.

The walkers share one routine ($EA08). An enemy on the same level as its player just walks. One below its player keeps a timer, and each time the timer runs out the enemy changes its mind about whether to climb; while it wants to, it jumps for any platform it finds overhead. Three of the walkers also shoot along their row when they face a player who is level with them ($EDCD).

Enemies and players never use the video chip's collision registers. Every test is a comparison of coordinates: a player dies within 12 pixels of an enemy ($1D32), and a bubble catches an enemy within 16 ($105B).

02 · Enemy movement

How an enemy decides, frame by frame

Each enemy's routine runs on every other frame. Below, the ported game runs one round at a time; step it and read what each enemy's routine did and why.

A walker ($EA08) falls when nothing is under it; on a floor it follows its heading bits, and turns at a wall. Whether it may climb is decided by its chase routine ($EE62). Level with its player it walks. Higher than its player it never climbs. Lower, it keeps a timer ($8660), reloaded each time from the kind's period plus a random 0-31, and each time the timer runs out it changes its mind about climbing ($8700). While it wants to climb it jumps for a platform it finds overhead, or across a gap at the end of its floor, fifteen steps up the arc. The shooter, the bottle thrower and the boulder roller are walkers with a shot added: one call in four, at random, a walker standing on a floor with no shot out and facing a player who is level with it readies one ($EDCD), and never when the player is lower. The hopper never walks: with the climb flag set and a platform within four rows overhead it jumps straight up, otherwise now and then it turns to face its player and hops the way it faces ($1F2E). The two flyers move every call, two pixels across and one or two up or down, reversing at walls, floors and ceilings ($EFC0, $EEB2). The dropper walks four pixels a call and, about once in twenty calls while its reload timer is out and a floor is under it, lets a bolt fall ($1E9F).

Once the round timer has run out every enemy is marked angry: its routine gets a second call on every second run of the actor loop, so it moves half as fast again ($16E4). An enemy that escapes a bubble is angry from then on.