Routines
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CC27: Check and change object state after positive events match/collision
Used by the routines at CB00, CB5B, CB96 and CBA8.
'Interactions' in Avalon are based on matching event values. For example:
  • using a (particular) key on a (particular) chest
  • using a spell on a room object, e.g. OPEN/UNLOCK
  • using an object on another object (e.g. flashing star on the floor marking in room 2)
Items that can be affected by other actions/items have extra data in the room item table (736F). This additional data indicates the next 'state' of the item (e.g. closed chest > open chest)
Some items are hidden until another item is used (in the aforementioned case of chests holding objects).
A successful value match between the acting item, and the item acted upon, advances the state of the object to the next one.
CC27 LD A,($EB8F) Event number ID to check for this item (acting on another item, e.g. a key opening its corresponding chest)
CC2A CP $00
CC2C JR Z,$CC36 If zero, there's nothing to check
CC2E CP $89
CC30 JP Z,$DA88 Special check for event/collision #137 - Maroc using CALIBURN on AVELACH. This ends the game.
CC33 CALL $E2B0 Copy room item event data into working buffer
Event/collision checking:
CC36 LD A,($EB8E) Event number for items acting on this item (e.g. for a chest, which is opened by a key)
CC39 LD C,A
CC3A CP $0F Event 15 means an item is collectible (Maroc can pick it up)
CC3C JP Z,$CC72 If so, check if Maroc has picked up the item, either by moving over it or given it by the servant
CC3F CALL $DFCD For all other events, run an event check for this item
CC42 CP $01 Has a collision/event occurred?
CC44 JP NZ,$CC72 If not, skip to the next routine
EVENT/COLLISION HAS TAKEN PLACE
For the object acted upon, advance its 'state' by one, to the next state:
CC47 LD A,($EB8C) Grab the byte containing the number of 'states' for the object
CC4A LD C,A
CC4B LD A,($EB89) Current object state number (starts at 1 for default object state)
CC4E INC A Increment object state by one
CC4F CP C
CC50 JR C,$CC56
CC52 JR Z,$CC56 Object state <= max states for this item, skip the next instruction
CC54 LD A,$01 Object state > max states for this item, so reset the object state to 1
CC56 LD HL,($EB44) Address pointer to start of item's data set at 736F
CC59 INC HL
CC5A INC HL
CC5B INC HL
CC5C INC HL Move to the 5th byte of set, containing the item's state number
CC5D LD (HL),A ...and store the new state number.
CC5E CALL $DADC Fetch object state and prepare graphics for printing
Set properties for the new object state
CC61 LD A,($EB8D) Get first byte of object state data
CC64 AND $F8 Filter out bits 0-2 (values 0-7)
CC66 LD C,A
CC67 LD A,($EB4E) Get the current item properties byte
CC6A AND $1F Keep bits 0-4...
CC6C OR $05 Set bits 0 (item is visible) and 2 (item is moving/will need erasing)
CC6E OR C ...append the rest of the bits from the new-state-object...
CC6F LD ($EB4E),A ...and re-store.
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