Spectrum ROM: Routine at 03582

Now enter the main scrolling loop. The B register holds the number of the top line to be scrolled, the HL register pair the starting address in the display area of this line and the C register the pixel line counter.

Used by the routine at PO_SCR. The number of lines of the display that are to be scrolled has to be held on entry to the main subroutine in the B register.
CL_SC_ALL	03582	LD B,23	The entry point after 'scroll?'
This entry point is used by the routine at PO_SCR. The main entry point - from above and when scrolling for INPUT...AT.
CL_SCROLL	03584	CALL CL_ADDR	Find the starting address of the line.
	03587	LD C,8	There are eight pixel lines to a complete line.
Now enter the main scrolling loop. The B register holds the number of the top line to be scrolled, the HL register pair the starting address in the display area of this line and the C register the pixel line counter.
CL_SCR_1	03589	PUSH BC	Save both counters.
	03590	PUSH HL	Save the starting address.
	03591	LD A,B	Jump forward unless dealing at the present moment with a 'third' of the display.
	03592	AND 7
	03594	LD A,B
	03595	JR NZ,CL_SCR_3
The pixel lines of the top lines of the 'thirds' of the display have to be moved across the 2K boundaries. (Each 'third' is 2K.)
CL_SCR_2	03597	EX DE,HL	The result of this manipulation is to leave HL unchanged and DE pointing to the required destination.
	03598	LD HL,63712
	03601	ADD HL,DE
	03602	EX DE,HL
	03603	LD BC,32	There are 32 characters.
	03606	DEC A	Decrease the counter as one line is being dealt with.
	03607	LDIR	Now move the thirty two bytes.
The pixel lines within the 'thirds' can now be scrolled. The A register holds, on the first pass, 1 to 7, 9 to 15, or 17 to 23.
CL_SCR_3	03609	EX DE,HL	Again DE is made to point to the required destination, this time only thirty two locations away.
	03610	LD HL,65504
	03613	ADD HL,DE
	03614	EX DE,HL
	03615	LD B,A	Save the line number in B.
	03616	AND 7	Now find how many characters there are remaining in the 'third'.
	03618	RRCA
	03619	RRCA
	03620	RRCA
	03621	LD C,A	Pass the 'character total' to the C register.
	03622	LD A,B	Fetch the line number.
	03623	LD B,0	BC holds the 'character total' and a pixel line from each of the characters is 'scrolled'.
	03625	LDIR
	03627	LD B,7	Now prepare to increment the address to jump across a 'third' boundary.
	03629	ADD HL,BC	Increase HL by 1792.
	03630	AND 248	Jump back if there are any 'thirds' left to consider.
	03632	JR NZ,CL_SCR_2	Jump back if there are any 'thirds' left to consider.
Now find if the loop has been used eight times - once for each pixel line.
	03634	POP HL	Fetch the original address.
	03635	INC H	Address the next pixel line.
	03636	POP BC	Fetch the counters.
	03637	DEC C	Decrease the pixel line counter and jump back unless eight lines have been moved.
	03638	JR NZ,CL_SCR_1
Next the attribute bytes are scrolled. Note that the B register still holds the number of lines to be scrolled and the C register holds zero.
	03640	CALL CL_ATTR	The required address in the attribute area and the number of characters in B lines are found.
	03643	LD HL,65504	The displacement for all the attribute bytes is thirty two locations away.
	03646	ADD HL,DE
	03647	EX DE,HL
	03648	LDIR	The attribute bytes are 'scrolled'.
It remains now to clear the bottom line of the display.
	03650	LD B,1	The B register is loaded with 1 and CL_LINE is entered.