river-raid: Routine at 7158

7158: Fighter operation routine

Operates fighter jets that fly horizontally across the screen. Fighters move 4 pixels per frame and wrap around when reaching screen edges. Uses XOR blending mode for rendering.

Moves 4 pixels per frame (faster than ships/helicopters)
Left-facing: decrements X, wraps at $00 to $E8
Right-facing: increments X, wraps at $E8 to $04
Sets COLLISION_MODE_FIGHTER for collision detection

Input
B	Y position
C	X position
D	Object definition byte

operate_fighter

7158

LD ($8B0A),BC

Store current position to previous_object_coordinates for rendering.

715C

BIT 6,D

Check orientation: if bit 6 clear (right-facing), jump to fighter_right_advance.

715E

JP Z,fighter_right_advance

fighter_left_advance

7161

DEC C

Move left 4 pixels (DEC C × 4). If C == 0, reset via fighter_left_reset.

7162

DEC C

7163

DEC C

7164

DEC C

7165

LD A,C

7166

CP $00

7168

CALL Z,fighter_left_reset