Avalon: Routine at 54450

54450: Draw Maroc's shadow and flickering barrier for SHIELD, PROTECT and DEFENCE spells

Follows previous section (54429).
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:
54450	LD HL,(60100)	Graphic address pointer for Maroc's shadow (39095)
54453	LD A,95	Vertical screen position - 95 pixels from top of playing area
54455	LD (60114),A
54458	CALL 56216	Draw shadow
Check for spells that generate a flickering effect
54461	LD A,(60137)	Check currently active effect spell (a positive match for any of them jumps to 54488)
54464	CP 2	SHIELD spell active
54466	JR Z,54488	SHIELD spell active
54468	CP 11	PROTECT spell active
54470	JR Z,54488	PROTECT spell active
54472	CP 18	DEFENCE spell active
54474	JR Z,54488	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 60137 is set to 255 This tells this routine to erase any 'barrier' graphics that might be on screen
54476	CP 255
54478	JR NZ,54524	If no 255 match, there's no effect spell graphics to draw/erase. Jump out of this routine to 54524.
SHIELD, PROTECT or DEFENCE have been deactivated, so make sure any on-screen barrier graphics are removed
54480	LD A,0
54482	LD (60137),A	Set currently active effect spell to 0 (no spell active)
54485	LD (60111),A	Set draw/erase flag to 0 (ERASE)
Draw/erase the flickering barrier graphics
54488	LD A,(60097)	Get Maroc's vertical screen position, from top of playing area, in pixels
54491	LD (60114),A	...and copy into working graphic data buffer
54494	LD HL,(60566)	Graphics address pointer for shield barrier graphics (39059/43500)
Work out which graphics frame to use for barrier graphic. Uses the game counter at 60074 to alternate frame graphics.
54497	LD A,(60074)	Get the game counter byte
54500	AND 1	Check the lowest bit (bit 0) of the game counter (0 = even, 1 = odd)
54502	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.
54503	LD E,A	Put offset into DE register pair
54504	LD D,0	Put offset into DE register pair
54506	ADD HL,DE	...and add to the base graphic pointer address (39059)
54507	LD (60241),HL	Store graphics address pointer to the frame that needs drawing
54510	CALL 56216	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.
54513	LD A,0
54515	LD (60111),A	Set draw/erase flag to 0 (ERASE)
54518	LD HL,(60241)	Retrieve address for barrier graphics frame
54521	CALL 56216	...And erase the graphic