The Hobbit: Routine at 34667

34667: Scroll Line

Used by the routine at PrintWriteACharacterToTheScreen. Scrolls the text window up by one line.
ScrollLine	34667	PUSH AF	Save AF, BC, HL and DE, and point HL at screen line 16416.
	34668	PUSH BC
	34669	PUSH HL
	34670	PUSH DE
Set up the initial values.
	34671	LD HL,16416
	34674	LD DE,16384	Point DE at the display file.
	34677	LD A,17	Load 17 for the scroll.
	34679	LD B,0	Start the pass count at zero.
ScrollLine_Loop	34681	PUSH HL	Save the screen pointers and count 8 pixel rows.
	34682	PUSH DE
	34683	LD C,8
ScrollLine_Line	34685	PUSH HL	Save the row addresses and the pixel count, and set the byte count to 32.
	34686	PUSH DE
	34687	PUSH BC
	34688	LD C,32
	34690	LDIR	Copy the row up one.
	34692	POP BC	Increment the MSB of HL and DE by one to point to the next line down.
	34693	POP DE
	34694	POP HL
	34695	INC H
	34696	INC D
	34697	DEC C	Drop one pixel row.
	34698	JR NZ,ScrollLine_Line	Jump back to ScrollLine_Line unless the whole line is moved (i.e. C is now zero).
	34700	POP DE	Restore the saved screen pointers from the stack and set BC to 32, one screen row.
	34701	POP HL
	34702	LD C,32
	34704	ADD HL,BC	Step one screen row, 32 bytes.
	34705	EX DE,HL	Swap the two screen pointers.
	34706	ADD HL,BC	Step the other pointer one screen row, 32 bytes.
	34707	EX DE,HL	Swap the two screen pointers.
	34708	PUSH AF	Save AF and use D as the next byte.
	34709	LD A,D	Save AF and use D as the next byte.
	34710	AND %00000111	Keep the selected bits.
	34712	JR Z,ScrollLine_SkipDE	Skip to ScrollLine_SkipDE if zero.
Handle changing the target between each screen buffer area.
	34714	LD A,D	Take the high byte of DE.
	34715	ADD A,7	Add 7 onto it.
	34717	LD D,A	Store the row address past the screen-third gap.
ScrollLine_SkipDE	34718	LD A,H	Take the high byte of the screen address.
	34719	AND %00000111	Keep the selected bits.
	34721	JR Z,ScrollLine_SkipHL	Skip to ScrollLine_SkipHL if zero.
Handle changing the source between each screen buffer area.
	34723	LD A,H	Take the high byte of the screen address.
	34724	ADD A,7	Add 7 onto it.
	34726	LD H,A	Store the other row address past the screen-third gap.
ScrollLine_SkipHL	34727	POP AF	Restore the line counter and drop one line.
	34728	DEC A	Restore the line counter and drop one line.
	34729	JR NZ,ScrollLine_Loop	Loop back to ScrollLine_Loop until the line counter is zero.
Handle the attributes.
	34731	LD HL,22560	Move the attributes up one character block.
	34734	LD DE,22528
	34737	LD BC,544
	34740	LDIR
This creates the indent.
	34742	LD B,42	Set a counter for 042 spaces.
	34744	LD HL,20512	Set the target screen location to 20512.
	34747	LD C,1	One column of the scrolled line.
	34749	LD A,32	Use the character ' '.
ScrollLine_Spacing	34751	CALL Print_ProportionalCharacter	Call Print_ProportionalCharacter to draw this character with the game's font.
	34754	DJNZ ScrollLine_Spacing	Decrease counter by one and loop back to ScrollLine_Spacing until counter is zero.
Housekeeping and return.
	34756	POP DE	Restore DE, HL, BC and AF from the stack.
	34757	POP HL
	34758	POP BC
	34759	POP AF
	34760	RET	Return.