Routines
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87F6: Find Free Slot
Used by the routines at SpawnSpecialEnemy, UpdateEnemySpawning and FireShot.
Searches for a free enemy slot (one with type FF) by stepping IX through the slots, which are 18 bytes long. No slot is returned if too many enemies are alive or if the player's ship is exploding.
Input
A Number of slots to search
IX Address of the first slot to try
Output
IX Address of the free slot
F The zero flag is set if a free slot was found
FindFreeSlot 87F6 LD B,A Store the number of slots in B.
87F7 LD A,($8E2B) Fetch the number of enemies that are alive from 8E2B.
87FA CP $07 Jump to FindFreeSlot_None if it is 07 or more.
87FC JR NC,FindFreeSlot_None
87FE LD A,($8F53) Fetch the death counter from 8F53.
8801 AND A Test it.
8802 JR NZ,FindFreeSlot_None Jump to FindFreeSlot_None if the ship is exploding.
8804 LD DE,$0018 Set the size of a slot in DE.
FindFreeSlot_Loop 8807 LD A,(IX+$00) Fetch the type of the object in the slot.
880A INC A Increment it.
880B RET Z Return with the zero flag set if the type was FF.
880C ADD IX,DE Move on to the next slot.
880E DJNZ FindFreeSlot_Loop Decrease the slot counter by one and loop back to FindFreeSlot_Loop until none remain.
FindFreeSlot_None 8810 OR %00000001 Clear the zero flag.
8812 RET Return.
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