Data copied to $A16D+.

Usually 1. Oscillates 0/1 when the road forks. Adjusts horizontal position of the untaken road.

Table of spawned vehicles and/or objects

A hazard is anything on the road: a moving object like the perp or an NPC car, or a fixed one like a barrier or a tumbleweed. Six slots of 20 bytes each. Slot 0 is always the perp car; spawn_cars and spawn_hazards allocate from slots 1 to 5 by scanning for the first with +0 clear. The Z80 always reaches these fields as (IX+n), never by absolute address, which is why the layout is given as byte offsets.

+0 (byte) Slot allocation flag: $FF when used, $00 when free. advance_hazard clears it once the object has passed the camera.

+1 (byte) Approach counter, running 21 down to 0 as the object nears. Advanced by the whole-units part of +13 each frame. advance_hazard retires the slot once it reaches 23 and only draws the object below 20. It doubles as the low byte of the road-buffer column while the object is drawn, and perp_behaviour reads the perp's copy as a road-buffer offset when comparing hazard positions.

+2 (byte) Horizontal screen position: the low byte of the computed road X.

+3 (byte) Horizontal clip flag: the high byte of that same X. Zero means on screen, negative clipped off the left edge, positive clipped off the right. check_collision rejects any hazard whose clip byte is non-zero, and the plot routine picks its entry point from the sign.

+4 (byte) Fractional part of the approach distance. Decremented each frame by the low byte of +13, and each borrow carries into +1. It is also the multiplicand for the perspective column at +6, so it doubles as the sub-row depth within the current distance step.

+5 (byte) Position across the road, 0 to 255 left to right (observed values 5..216). hazard_handler slides it five units a frame toward the lane position at +18, which is what makes a traffic car drift between lanes rather than jump.

+6 (byte) Perspective-scaled column, (+4 * height delta) >> 8, recomputed each frame by advance_hazard. It selects the sprite's column and the road-edge row the object is drawn against, less the wobble at +16.

+7 (byte) Hit sequence timer. Zero when nothing is happening, positive while a vehicle hit is in progress, negative for the perp's post-hit cooldown: perp_behaviour sets it to $FC (-4) and counts back up to zero, ignoring input meanwhile.

+8 (byte) Collision box width, used for the bounding-box overlap test in check_collision.

+9 (word) Address of the object's bitmap table (the LOD). spawn_cars overwrites it with a random vehicle from the stage's table.

+11 (word) Address of the hit handler called once per frame by advance_hazard after the object is drawn: perp_behaviour for the perp, hazard_handler for traffic and hazard_hit for barriers and tumbleweeds.

+13 (word) Fixed-point approach rate. The high byte is whole distance units per frame, added to +1; the low byte is fractional units per frame, subtracted from +4, where each borrow carries a further unit into +1. hazard_hit decays it by 1/32 a frame while an object wobbles away from a collision.

+15 (byte) What kind of object this is, and how far through a hit it is. $80 marks a spawned vehicle and $FF the perp, so bit 7 set means "is a vehicle", which is how spawn_cars counts the traffic already on screen. For a static hazard it is the hit state machine instead: 0 untouched, 2 wobbling, 1 finished.

+16 (byte) Horizontal wobble offset during a hit, indexed out of table_acdb by hazard_hit and subtracted from +6 as the object is drawn, so a struck barrier shudders and settles.

+17 (byte) Lane index, or for the perp the high byte of +1. For a traffic car it is the lane currently occupied, which perp_behaviour compares against its own lane to decide whether to swerve. While a static hazard is being hit it is reused again, as the running index into table_acdb.

+18 (byte) Target lane, or the wobble countdown. hazard_handler slides +5 toward this lane's position until the two agree; for a hazard mid-wobble it is instead the frame countdown, and the effect ends when it reaches zero.

+19 (byte) Sprite plot mode: 0 normal, 1 inverted. hazard_hit toggles it every frame of the wobble, flipping the sprite vertically to fake a barrier tumbling.

Padding: 19 bytes, one short of another 20-byte hazard slot, before the AY register soft copy at ay_chan_a_pitch

AY registers 0..11 [128K]

End of AY registers

Affects collision detection on the left hand side.

0 => no helicopter 1 => moves to left 2 => bobs around in the air 3/4 => moves from left 5 => bobs around in the air

These seem to get altered even when no sound is being produced.