Everything in a Fairlight room -- the knight, the creatures, the things lying about, the doors, the furniture, the floor and the walls -- is a box, and moving is trying a box in a new place against every other box. There is no separate map of the room for collisions: the drawing is only a picture, and what the knight stands on, walks into, climbs and pushes are the object records. This page is how a move is made, from the direction byte to the gravity that is added to it, the test against the boxes, what happens when it fails, and what that gives: walls, floors, stairs, pushing, riding, jumping and falling.

Directions and steps

A direction is a byte with a bit for each way, the same for every object (its +13 on the ground, +18 in the air, and the copies the move works with):

Bit	Means
0	turn back when stopped, rather than stop
1	rising: no gravity this pass
2	down +6 (H to ENTER; the stick left)
3	up +6 (Y to P; the stick right)
4	up
5	down: gravity
6	up +8 (Q to T; the stick up)
7	down +8 (A to G; the stick down)

The knight's keys (read in KNIGHT_CONTROLS) set one of the four floor directions; a later test wins, so he never walks diagonally, though other things do. A step (STEP_ALONG_X and the two after it) is two units along one floor axis, one along each of the two for a diagonal, and always two up or down. On the screen, up +6 goes up and to the right, up +8 up and to the left.

Boxes

A record's box is a corner and three sizes: from +6 for +9 along +6, from +8 for +11 along +8, and from its top, +7, down +10 (read in TEST_ONE_RECORD, the author's BB2). Two boxes meet when they overlap along all three axes, each test strict, so boxes that only touch do not. Room 2's records, their boxes drawn over the room:

FIND_OBSTACLE walks the records in use from the last down to the first and stops at the first whose box meets the one being tried, leaving out the mover's own and anything carried or gone (a door always counts). The first six records (FIXED_RECORDS) are the room's own box -- in room 2, read in the running game:

Record	What	Corner	Size	Set by the patch
1	the floor	50, 50, 50	255 by 10 by 255	its top, +7
2	the ceiling	50, 180, 50	255 by 10 by 255	its top, +7
3	a wall across +6	178, 255, 50	10 by 255 by 255	+6
4	the other	40, 255, 50	10 by 255 by 255	+6
5	a wall across +8	50, 255, 40	255 by 255 by 10	+8
6	the other	50, 255, 174	255 by 255 by 10	+8

So an object stands on the floor when its foot, +7 less +10, is at 50: the floor's box is ten high with its top there, and the step down that gravity asks for meets it.

Gravity

Every object's update ends in the same code (ANIMATE_AND_MOVE). There, unless the direction has bit 1 (rising), the up bit is taken out -- ANDed with $EF from $FF8B -- and down is added -- ORed with $20, the top six bits of $23 at $FF85. Both are constants among the variables: a new game restores them from the master copy, and a search of the code for every instruction that could store into either found none. So everything that is not rising tries to fall two units every pass, and a floor, a table or another object is what stops it. That is also what makes a thing lying on the floor cost time every pass (the pass traced): its fall is tried, and stopped, again and again.

A step down sets bit 7 of the state, in the air. While it is set the object's own case is not run at all: it goes on the way +18 says, and a fall keeps only the down bit there, so everything falls straight down, the knight at the end of a jump too. The air ends when the fall is stopped.

What a blocked move does

When the tried box meets another record, BUMPED deals with doors (for the knight, a way to another room) and OBJECTS_MEET with what the two objects do to each other (the creatures page); what is left comes to BLOCKED_MOVE (MEET_DECOY). It works out how much of the move that one object stops, trying from where the mover was:

• each moving axis alone, +6, the height and +8. One that collides is taken out of the move, and the mover's direction on it turned round if it bounces (bit 0) or cleared if not. So something walking diagonally into a wall slides along it;
• then the pairs: +6 with the height colliding takes the height out, +6 with +8 both, the height with +8 the height;
• if all three are still in after that, only the whole diagonal collides, and none of it is made.

Then what is left of the move is tried again, and may meet something else. Three of the collisions do more:

• Climbing. Running along +6 into an object of kind 3, or along +8 into one of kind 2, is a climb: the mover goes up for three passes. Those objects are the invisible steps of a staircase.
• Riding. Coming down onto an object that is moving -- by its own state, or coasting -- the mover rides it: its state's bit 4, and the other's direction copied into its own at the end of the move, so next pass it goes the same way.
• Pushing. Unless the mover landed on it, or it is a door or still, the other is pushed if its +16 is no more than 5 above the mover's: it coasts (bit 7 of its state, in the air) the way the mover asked to go, for the mover's +16 less its own, plus 6, passes. +16 is 16 plus the weight, and the knight's is 18, so he can push anything that can move at all; the heavier it is, the sooner it stops.

Climbing room 2's stairs

The knight put at 120, 78, 160, at the foot of the stairs, with Y held (up +6), in the simulator when this page was built -- his +6 and his top, +7, after each pass:

Pass	+6	+7
1	122	78
2	124	78
3	126	78
4	128	78
5	130	78
6	132	78
7	132	78	in the air
8	132	80	in the air
9	132	82	in the air
10	132	84
11	134	82	in the air
12	136	82
13	138	82
14	138	82	in the air
15	138	84	in the air
16	138	86	in the air
17	138	88
18	140	86	in the air
19	140	84	in the air
20	142	84
21	144	84
22	144	84	in the air
23	144	86	in the air
24	144	88	in the air
25	144	90
26	146	88	in the air
27	148	88
28	150	88
29	150	88	in the air
30	150	90	in the air

Each time he walks into the next step he stops, goes up for three passes, falls back two onto the step and walks on: 12 units higher after 30 passes. The steps are boxes of kind 3, 10 high and each a little higher than the one before, laid over each other.

Pushing a stool

The knight put behind one of room 2's stools (record 14, +16 = 19: weight 3) and walked into it with Y. The rule gives it 18 less 19, plus 6: 5 passes of coasting a push. Measured, the knight's +6, the stool's, and its countdown (+15) while it coasts:

Pass	Knight's +6	Stool's +6		Its +15
1	118	130
2	120	130
3	122	130
4	122	132	coasting	4
5	124	132
6	124	134	coasting	4
7	126	136	coasting	3
8	128	138	coasting	2
9	130	138
10	130	140	coasting	4
11	132	142	coasting	3
12	134	144	coasting	2

The stool moves ahead of him two units a pass while it coasts, and he catches it up and pushes again.

Jumping

SPACE or SYMBOL SHIFT (KNIGHT_CONTROLS) makes the knight's direction his walking one with up and rising added, +15 = 8 and his state 5. For eight passes CHE3D's state-5 case moves him up two a pass without gravity, the way he was walking; then he is state 8 again and falls. SPACE for one pass in room 29, when this page was built:

Pass	State	In the air	+7 (his top)
1	5		80
2	5		82
3	5		84
4	5		86
5	5		88
6	5		90
7	5		92
8	5		94
9	8	yes	92
10	8	yes	90
11	8	yes	88
12	8	yes	86
13	8	yes	84
14	8	yes	82
15	8	yes	80
16	8	yes	78
17	8		78

Up 16 in 8 passes, and down again. A jump is how the knight goes up through a hole in a ceiling -- a door whose way is up -- and over low things.

Falling

The knight counts the passes he falls for ($FF94, at the commit in ANIMATE_AND_MOVE); when he lands, his update (KNIGHT_UPDATE) takes a LIFE point for every pass over 20 -- two units a pass, so a fall of more than 40. Dropped from above room 29's floor in the simulator when this page was built:

Height	Passes falling	LIFE by the rule	LIFE lost
30	15	0	0
40	20	0	0
50	25	5	5
60	30	10	10

The first pass in a room

On the first pass after a room is entered (bit 2 of GAME_FLAGS) nothing moves: each object's direction is cleared, but all three axes are marked as moving, so the test runs on a box one unit up +6, two down and one down +8 from where it is, and whatever that meets is dealt with as usual; then the object is only drawn. What the offset box is for has not been worked out.

Beside Krumlinde's reading

Ville Krumlinde's disassembly reads the sizes as half-widths about a centre; the box test compares against the corner and the corner plus the size, and the pick-up's search box, measured in stage 2, reaches exactly as far as a corner-and-size box would. He finds no code that makes things fall, and leaves open what holds them up: gravity is the down bit forced into every direction that is not rising, applied by the ordinary step, and floors are the fixed records and the invisible boxes. His assembler source writes the minus-20 step through the records as a load of the variable at $FFEC; it is the number, which only looks like the address.

Confirmed and inferred

• Run in the simulator when this page was built: room 2's boxes read; the falls, the jump, the climb and the push, each from a staged place with the creatures frozen.
• Searched when this page was built: no instruction stores into the two gravity constants.
• Read from the code: the direction bits, the order of the axis tests, riding, the push rule.
• Inferred: the box outlines' alignment with the picture. The slopes of the projection are ISO_MOVE's; its constants come from the knight's fixed point, and the outlines fit the sprites and the stairs by eye.