Avalon: Routine at D4B2

D4B2: Draw Maroc's shadow and flickering barrier for SHIELD, PROTECT and DEFENCE spells

Follows previous section (D49D).
Although Maroc's vertical position varies depending on his movement speed, his shadow remains at a fixed point on the screen (vertical pixel position 95) When the SHIELD, PROTECT and DEFENCE spells are active, a flickering shield appears around Maroc Draw Maroc's shadow:
D4B2	LD HL,($EAC4)	Graphic address pointer for Maroc's shadow (98B7)
D4B5	LD A,$5F	Vertical screen position - 95 pixels from top of playing area
D4B7	LD ($EAD2),A
D4BA	CALL $DB98	Draw shadow
Check for spells that generate a flickering effect
D4BD	LD A,($EAE9)	Check currently active effect spell (a positive match for any of them jumps to D4D8)
D4C0	CP $02	SHIELD spell active
D4C2	JR Z,$D4D8	SHIELD spell active
D4C4	CP $0B	PROTECT spell active
D4C6	JR Z,$D4D8	PROTECT spell active
D4C8	CP $12	DEFENCE spell active
D4CA	JR Z,$D4D8	DEFENCE spell active
If no matches to the above none of the spells is active. The next check is to see if any of the spells have just been deactivated: If the player deactivates any of the above 3 spells, the 'currently active effect spell' byte at EAE9 is set to 255 This tells this routine to erase any 'barrier' graphics that might be on screen
D4CC	CP $FF
D4CE	JR NZ,$D4FC	If no 255 match, there's no effect spell graphics to draw/erase. Jump out of this routine to D4FC.
SHIELD, PROTECT or DEFENCE have been deactivated, so make sure any on-screen barrier graphics are removed
D4D0	LD A,$00
D4D2	LD ($EAE9),A	Set currently active effect spell to 0 (no spell active)
D4D5	LD ($EACF),A	Set draw/erase flag to 0 (ERASE)
Draw/erase the flickering barrier graphics
D4D8	LD A,($EAC1)	Get Maroc's vertical screen position, from top of playing area, in pixels
D4DB	LD ($EAD2),A	...and copy into working graphic data buffer
D4DE	LD HL,($EC96)	Graphics address pointer for shield barrier graphics (9893/A9EC)
Work out which graphics frame to use for barrier graphic. Uses the game counter at EAAA to alternate frame graphics.
D4E1	LD A,($EAAA)	Get the game counter byte
D4E4	AND $01	Check the lowest bit (bit 0) of the game counter (0 = even, 1 = odd)
D4E6	ADD A,A	Multiply by 2 If bit 0 = 0, offset will be 2 x 0 = 0, so frame 1 is used. If bit 0 = 1, offset will be 2 x 1 = 2, so a +2 offset to the graphics address which gives frame 2.
D4E7	LD E,A	Put offset into DE register pair
D4E8	LD D,$00	Put offset into DE register pair
D4EA	ADD HL,DE	...and add to the base graphic pointer address (9893)
D4EB	LD ($EB51),HL	Store graphics address pointer to the frame that needs drawing
D4EE	CALL $DB98	Draw the barrier graphic on screen
The barrier graphic is only drawn briefly - in fact, the very next routine immediately erases it. This is what creates the flicker effect.
D4F1	LD A,$00
D4F3	LD ($EACF),A	Set draw/erase flag to 0 (ERASE)
D4F6	LD HL,($EB51)	Retrieve address for barrier graphics frame
D4F9	CALL $DB98	...And erase the graphic