Robotron: 2084: Routine at C6DA

Used by the routines at Primary_Enemy_Entity_Handler, Find_Colliding_Entity, Handle_Homing_Enemy, Spawn_Primary_Wave_Enemies, Generate_Random_Byte, Process_Entity_Record_Fast, Handle_Horizontal_Enemy_Entity, Find_Colliding_Wave_Enemy, Pick_Random_Spawn_Position_Y, Process_Tracked_Enemy_Entity, Process_Cluster_Entity, Process_Player_Tracking_Enemy, Process_Wall_Bouncing_Enemy and Process_Final_Phase_Enemy_Entity.

Reads four little-endian words at HL (08 bytes). The first two load into main BC and DE; EXX swaps to the shadow set so the next two words load into shadow BC and DE while HL walks the same block. After swapping back, four ADD/SUB pairs (with further EXX) each end in RET C if the subtraction borrows (no hit). If none borrow, falls through to RET with carry clear (hit). HL ends just past the eighth byte. Used throughout the gameplay object loop for collisions (among others CCDA, CD25, D149, D22D, E03D, EB1C).

Used by the routines at Primary_Enemy_Entity_Handler, Find_Colliding_Entity, Handle_Homing_Enemy, Spawn_Primary_Wave_Enemies, Generate_Random_Byte, Process_Entity_Record_Fast, Handle_Horizontal_Enemy_Entity, Find_Colliding_Wave_Enemy, Pick_Random_Spawn_Position_Y, Process_Tracked_Enemy_Entity, Process_Cluster_Entity, Process_Player_Tracking_Enemy, Process_Wall_Bouncing_Enemy and Process_Final_Phase_Enemy_Entity. Reads four little-endian words at HL (08 bytes). The first two load into main BC and DE; EXX swaps to the shadow set so the next two words load into shadow BC and DE while HL walks the same block. After swapping back, four ADD/SUB pairs (with further EXX) each end in RET C if the subtraction borrows (no hit). If none borrow, falls through to RET with carry clear (hit). HL ends just past the eighth byte. Used throughout the gameplay object loop for collisions (among others CCDA, CD25, D149, D22D, E03D, EB1C).
HitTestBounds8AtHL	C6DA	LD B,(HL)	B=*HL (first word, low byte).
	C6DB	INC HL	Advance HL.
	C6DC	LD C,(HL)	C=*HL (first word, high byte).
	C6DD	INC HL	Advance HL.
	C6DE	LD D,(HL)	D=*HL (second word, low byte).
	C6DF	INC HL	Advance HL.
	C6E0	LD E,(HL)	E=*HL (second word, high byte).
	C6E1	EXX	EXX (shadow HL continues the load; shadow BC/DE hold first pair).
	C6E2	LD B,(HL)	B=*HL (third word, low byte).
	C6E3	INC HL	Advance HL.
	C6E4	LD C,(HL)	C=*HL (third word, high byte).
	C6E5	INC HL	Advance HL.
	C6E6	LD D,(HL)	D=*HL (fourth word, low byte).
	C6E7	INC HL	Advance HL.
	C6E8	LD E,(HL)	E=*HL (fourth word, high byte).
	C6E9	EXX	EXX (main holds first pair; shadow second pair; HL past all 08 bytes).
Each test adds two bytes from one pair, switches to the shadow pair, subtracts a byte from the other rectangle, and RET C on borrow (no overlap on that check).
	C6EA	LD A,B	A=B.
	C6EB	ADD A,D	A+=D (high bytes from main BC/DE).
	C6EC	EXX	EXX.
	C6ED	SUB B	Subtract shadow B from A.
	C6EE	EXX	EXX.
	C6EF	RET C	RET C if no hit (carry from SUB).
	C6F0	EXX	EXX.
	C6F1	LD A,B	A=B.
	C6F2	ADD A,D	A+=D (second high-byte sum).
	C6F3	EXX	EXX.
	C6F4	SUB B	Subtract shadow B from A.
	C6F5	RET C	RET C if no hit.
	C6F6	LD A,C	A=C.
	C6F7	ADD A,E	A+=E (low-byte sum).
	C6F8	EXX	EXX.
	C6F9	SUB C	Subtract shadow C from A.
	C6FA	EXX	EXX.
	C6FB	RET C	RET C if no hit.
	C6FC	EXX	EXX.
	C6FD	LD A,C	A=C.
	C6FE	ADD A,E	A+=E (final low-byte sum).
	C6FF	EXX	EXX.
	C700	SUB C	Subtract shadow C from A (last test).
	C701	RET	RET (carry clear if overlapping).