Avalon: Routine at 53003

Creatures will move erratically and randomly, if the UNSEEN spell is active - except for the WRAITH and DEMON, who can sense the magic and use it to home in on Maroc's position.

guardians of chaos reacting to UNSEEN spell

You can see (in the animated image) that some creatures have a tendency to cluster towards the bottom right of the screen.

This is because the randomizer routine at 52815 generates:

a pseudo-random horizontal speed/direction between 28 and +32 - in multiples of 4 (negative = left, positive = right)
a pseudo-random vertical speed/direction between -7 and +8 (negative = up, positive = down)

These values are only very slightly skewed towards down & right, but the randomizer runs often, hence the visible result.

Check if the UNSEEN spell is active:

Used by the routines at 52695 and 53054.
Creatures will move erratically and randomly, if the UNSEEN spell is active - except for the WRAITH and DEMON, who can sense the magic and use it to home in on Maroc's position. You can see (in the animated image) that some creatures have a tendency to cluster towards the bottom right of the screen. This is because the randomizer routine at 52815 generates: a pseudo-random horizontal speed/direction between 28 and +32 - in multiples of 4 (negative = left, positive = right) a pseudo-random vertical speed/direction between -7 and +8 (negative = up, positive = down) These values are only very slightly skewed towards down & right, but the randomizer runs often, hence the visible result. Check if the UNSEEN spell is active:
53003	LD A,(60137)	Currently active spell
53006	CP 9	...Is it the UNSEEN spell?
53008	JR NZ,53021	...Is it the UNSEEN spell?
53010	LD A,(60074)	If so get the incremental game counter
53013	AND 15	Check if any of bits 0-3 set
53015	JP NZ,52815	If so, randomize the movement pattern
53018	JP 53323	Switch back to normal movement every 16 frames
Jump here from 52782 if creature is DEMON or WRAITH and Maroc's active spell number >=4 These two creatures can 'sniff out' powerful magic. If Maroc has UNSEEN active, they are able to home in on his position, making them potentially more dangerous, though DEMONs will not fire missiles which tend to be more difficult to avoid They will also move towards Maroc's screen position if he casts ANY 'powerful' spell (i.e. spell number >=4) Calculate vertical position in relation to Maroc:
53021	LD A,(60234)	Retrieve vertical position within current room, in pixels, from top of room playing area
53024	LD C,A	Store in C register
53025	LD A,(60097)	Maroc's vertical (Y-axis) screen position
53028	SUB C	Subtract graphic position to get vertical offset
53029	RLA	Shift bit 7 to determine if creature is higher or lower than Maroc on the screen
53030	SBC A,A	After subtracting, A register will either contain 255 (-1) = below Maroc, or 0 = above Maroc
53031	XOR 7	Value will now either be 7 or -7 (248)
53033	LD (60237),A	Set as creature's vertical movement speed/direction
Calculate horizontal position in relation to Maroc:
53036	LD A,(60232)	Retrieve horizontal position within current room, in 4-pixel/half-character steps
53039	LD C,A
53040	LD A,(60096)	Maroc's horizontal (X-axis) screen position
53043	SUB C	Subtract graphic position to get horizontal offset
53044	RLA	Precision used to calculate = 2 pixels whereas the difference is in 4-pixel steps, so double the value
53045	SBC A,A	After subtracting, A register will either contain 255 (-1) = to the right of Maroc, or 0 = to the left of Maroc
53046	XOR 7	Value will now either be 7 or -7 (248)
53048	LD (60236),A	Store as creature's horizontal movement speed/direction
53051	JP 53323	Store as creature's horizontal movement speed/direction