OEL TTX2000 Issue 2.3: Routine at 08B0

08B0: clear_screen: Clear display bitmap and attributes.

Used by the routines at clear_page_buffer, save_routine and warm_start_routine.
clear_screen	08B0	PUSH BC	Preserve registers.
	08B1	PUSH DE
	08B2	PUSH HL
	08B3	LD HL,$4000	Clear bitmap and attributes to zero.
	08B6	LD DE,$4001
	08B9	LD BC,$1AFF
	08BC	LD (HL),$00
	08BE	LDIR
	08C0	POP HL	Restore registers and return.
	08C1	POP DE
	08C2	POP BC
	08C3	RET