Routines
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B8E2: Keep Object In Room In X
Used by the routine at BlockMovement.
Brings the speed along x of an object closer to zero until the object stays within the walls of the room. Nothing is done while the object is entering the room, or while it is near an arch, as it can then go out through the wall.
Input
C Speed along x
IX The address of the object
Output
C The new speed along x
Return while entering or near an arch.
KeepObjectInRoomInX B8E2 LD A,(IX+$0C) Fetch the collision flags.
B8E5 AND %11110000 Keep only the countdown from entering the room.
B8E7 RET NZ Return if the object has only just entered the room.
B8E8 BIT 0,(IX+$07) Return if the object ignores the walls.
B8EC RET NZ
B8ED LD A,($A71D) Fetch RoomSizeX.
B8F0 LD B,A Copy it to B.
Slow x until the object stays in the room.
KeepObjectInRoomInX_Test B8F1 LD A,(IX+$01) Fetch the x position of the object.
B8F4 ADD A,C Add C.
B8F5 SUB $80 Subtract 80.
B8F7 JR NC,KeepObjectInRoomInX_Compare Jump to KeepObjectInRoomInX_Compare if that is not negative.
B8F9 NEG Negate A.
KeepObjectInRoomInX_Compare B8FB ADD A,(IX+$04) Add the half size of the object, to get the distance of its edge from the middle.
B8FE CP B Return if the edge is inside the limit.
B8FF JR C,KeepObjectInRoomInX_Done
B901 SET 0,(IX+$0C) Flag that the object has been stopped along x.
B905 LD A,C Copy C to A.
B906 CALL BringSpeedCloserToZero Call BringSpeedCloserToZero.
B909 LD C,A Copy it to C.
B90A JR NZ,KeepObjectInRoomInX_Test Jump to KeepObjectInRoomInX_Test to test again, unless the distance is zero.
KeepObjectInRoomInX_Done B90C RET Return.
View the equivalent code in;
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