Robotron: 2084: Routine at F7EE

Used by the routine at F4EC.

Iterates over the 18 three-byte entries in the sprite table at F558 (Y pixel row at +00 X byte column at +01 attribute byte at +02) to animate the border march effect on the intro screen. For each entry, erases the sprite at its current position (calls XorBlitSprite with bit 7 of E set in shadow to suppress attribute writes), advances the position by one step along a rectangular path (08 pixel rows vertically or 01 byte column horizontally: right across Y=00 down the right side at X=1C left across Y=A0 up the left side at X=00), then redraws the sprite at the new position (calls XorBlitSprite with bit 7 clear to write the attribute). Called once per frame by the attract mode loop at F4EC.

Used by the routine at F4EC. Iterates over the 18 three-byte entries in the sprite table at F558 (Y pixel row at +00 X byte column at +01 attribute byte at +02) to animate the border march effect on the intro screen. For each entry, erases the sprite at its current position (calls XorBlitSprite with bit 7 of E set in shadow to suppress attribute writes), advances the position by one step along a rectangular path (08 pixel rows vertically or 01 byte column horizontally: right across Y=00 down the right side at X=1C left across Y=A0 up the left side at X=00), then redraws the sprite at the new position (calls XorBlitSprite with bit 7 clear to write the attribute). Called once per frame by the attract mode loop at F4EC.
AnimateIntroSprites	F7EE	LD B,$18	B=18 (loop counter: 24 sprite entries).
	F7F0	LD HL,$F558	HL=F558 (attribute byte position of the first entry).
AnimateIntroSprites_Loop	F7F3	LD A,(HL)	A=attribute byte from the current entry.
	F7F4	EXX	Switch to shadow; E=attribute byte; SET bit 7 (erase-mode flag for XorBlitSprite); switch back.
	F7F5	LD E,A
	F7F6	SET 7,E
	F7F8	EXX
	F7F9	DEC HL	Back HL two bytes to the Y pixel row (start of entry).
	F7FA	DEC HL	Back HL two bytes to the Y pixel row (start of entry).
	F7FB	PUSH HL	Stash HL (entry start) and BC on the stack.
	F7FC	PUSH BC	Stash HL (entry start) and BC on the stack.
	F7FD	CALL XorBlitSprite	Call XorBlitSprite to erase the sprite at its current position.
	F800	POP BC	Restore BC and HL (entry start).
	F801	POP HL	Restore BC and HL (entry start).
Advance the sprite position by one step along the rectangular border path. Y pixel row is at (HL), X byte column at (HL+01). Steps are 08 pixel rows vertically or 01 byte column horizontally. Wraps at X=1C (right boundary, drop down a row) and Y=A0 (bottom boundary, scroll left).
	F802	XOR A	A=00.
	F803	CP (HL)	If Y position (*HL) = 00 jump to AnimateIntroSprites_ScrollX (top row: scroll X instead of Y).
	F804	JR Z,AnimateIntroSprites_ScrollX
	F806	INC HL	Read X position (*(HL+01)) and compare with 00; restore HL.
	F807	CP (HL)
	F808	DEC HL
	F809	JR NZ,AnimateIntroSprites_CheckBoundary	If X!=00 jump to AnimateIntroSprites_CheckBoundary.
	F80B	LD A,(HL)	Y −= 08 (on left side, scroll up); jump to AnimateIntroSprites_Draw.
	F80C	SUB $08
	F80E	LD (HL),A
	F80F	JR AnimateIntroSprites_Draw
AnimateIntroSprites_CheckBoundary	F811	LD A,$A0	A=A0; compare with Y position.
	F813	CP (HL)	A=A0; compare with Y position.
	F814	JR NZ,AnimateIntroSprites_ScrollDown	If Y!=A0 jump to AnimateIntroSprites_ScrollDown (not at bottom boundary, scroll down).
	F816	INC HL	X−− (at bottom boundary, scroll left); jump to AnimateIntroSprites_RestoreHL.
	F817	DEC (HL)
	F818	JR AnimateIntroSprites_RestoreHL
AnimateIntroSprites_DropRow	F81A	DEC HL	Back HL to the Y byte.
AnimateIntroSprites_ScrollDown	F81B	LD A,(HL)	Y += 08 (scroll down); jump to AnimateIntroSprites_Draw.
	F81C	ADD A,$08
	F81E	LD (HL),A
	F81F	JR AnimateIntroSprites_Draw
Top row (Y=00): if X has reached the right boundary (1C), drop a row (Y+=08); otherwise scroll right (X++).
AnimateIntroSprites_ScrollX	F821	INC HL	Read X (*(HL+01)); if X=1C jump to AnimateIntroSprites_DropRow (drop a row); otherwise X++.
	F822	LD A,(HL)
	F823	CP $1C
	F825	JR Z,AnimateIntroSprites_DropRow
	F827	INC (HL)	X++ (scroll right).
AnimateIntroSprites_RestoreHL	F828	DEC HL	Restore HL to the Y byte position.
AnimateIntroSprites_Draw	F829	PUSH HL	Stash HL (entry start) and BC on the stack.
	F82A	PUSH BC	Stash HL (entry start) and BC on the stack.
	F82B	EXX	Switch to shadow; RES bit 7 of E (clear erase-mode flag for draw pass); switch back.
	F82C	RES 7,E
	F82E	EXX
	F82F	CALL XorBlitSprite	Call XorBlitSprite to draw the sprite at the updated position.
	F832	POP BC	Restore BC and HL (entry start).
	F833	POP HL	Restore BC and HL (entry start).
	F834	INC HL	Advance HL by 05 to the attribute byte of the next entry.
	F835	INC HL
	F836	INC HL
	F837	INC HL
	F838	INC HL
	F839	DJNZ AnimateIntroSprites_Loop	Decrease counter and loop back to AnimateIntroSprites_Loop.
	F83B	RET	Return.