Avalon: Routine at 53403

53403: Missile fired by Maroc or a creature - check for active shield/barrier spells

Used by the routine at 51359.
...There's a missile/bolt flying around in the room..! This routine copies the missile's data from the table at 27868 into a working (missile) buffer at 60317. It also checks if Maroc has a counter defence spell active (SHIELD, DEFENCE etc.) Each missile type (at 27868) has up to THREE counter defensive barrier-type spells that will negate/repel it These are stored in bytes 5, 7 and 8 of the missile's 8-byte data set (AMULET spell repels all missiles) The following instructions check if each of these values in a missile's data set (stored in the current missile data buffer at 60321, 60323 and 60324) match the currently active defensive barrier spell type If there's a match, a special missile/barrier collision routine (57990) is CALLed
53403	LD A,(60238)	Missile properties byte
53406	AND 1	Check if it's is on-screen (bit 1 is set)
53408	JP Z,53569	...if not, skip out to a later instruction
53411	LD HL,(60228)	Get address pointer to missile type (in table at 27868)
53414	LD BC,8	Copy the 8 bytes of missile data into missile working buffer table at 60317
53417	LD DE,60317
53420	LDIR
53422	LD A,(60319)	Missile attribute (INK) colour
53425	LD (60072),A	Copy into item data buffer
53428	LD A,(60320)	Missile's event/collision type to check (stored as the 4th byte in the missile/bolt table at 27868)
53431	CP 0
53433	JR Z,53438	If it's zero (no event), don't copy any event data (shouldn't happen as there's a value for each missile)
53435	CALL 58032	...If it's not zero, copy item event data into working buffer
COUNTER SPELL #1 CHECK
53438	LD A,(60137)	Get currently active 'effect' spell (value = 0 if none currently in effect)
53441	LD C,A	Copy to C register
53442	LD A,(60321)	Defence counter spell #1 (the spell in this slot is always AMULET)
53445	CP 0
53447	JR Z,53452	Check for zero - if no counter spell in this slot (shouldn't happen as there's a value for every counter slot)
53449	CP C	Check counter spell vs current active spell number
53450	JR Z,53472	If there's a match, can skip the next two checks straight to 53472
COUNTER SPELL #2 CHECK
53452	LD A,(60323)	Defence counter spell #2
53455	CP 0	Check for zero - if no counter spell in this slot (shouldn't happen as there's a value for every counter slot)
53457	JR Z,53462
53459	CP C	Check counter spell vs current active spell number
53460	JR Z,53472	If there's a match, can skip the final check and jump straight to 53472
COUNTER SPELL #2 CHECK
53462	LD A,(60324)	Defence counter spell #3
53465	CP 0	heck for zero - if no counter spell in this slot (shouldn't happen as there's a value for every counter slot)
53467	JR Z,53526
53469	CP C	Check counter spell vs current active spell number
53470	JR NZ,53526	If there's NO match, jump out of the routine here
The following instructions are redundant (possibly duplicated), as CP C has already happened in a previous check
53472	CP C	-
53473	JR NZ,53526	-
There's a match between the missile/bolt fired and an active shielding spell around Maroc that will negate/deflect it The following instructions check for a collision against Maroc's flickering shielding/protective spell
53475	LD C,1	For the usual collision/event detection routine (57293) the C register is set an event/collision type. On this occasion loading the C register is unnecessary as it's not actually needed/used for this (special) missile routine.
53477	CALL 57990	Check for a collision between the fired bolt/missile and the shield/barrier around Maroc
53480	CP 0	The A register is returned with 0 if no collision, or 8 (this byte is used elsewhere for the accompanying sound effect) if there's a collision
53482	JR Z,53526	If no collision, jump out of this routine/check