Underwurlde: Routine at 9588

9588: Reset World Map

Used by the routine at Select_Game_Options. Writes 00 to the second byte of each of the first 0240 entries of the World_Map world map. The second byte of an entry holds the data of a rope (see Set_Up_Rope), which changes during the game. The counters are set up so that the loop runs 40 times on the first pass and 0100 times on each of the next two passes. The last 0100 entries of the map (the last sixteen rows of rooms) are not reset.
Reset_World_Map	9588	LD HL,$7680	Point at the end of the world map (Start_Game_Selection).
	958B	LD DE,$7000	Point at the start of the world map (World_Map).
	958E	AND A	Subtract the start from the end of the map to get the size of the map in bytes.
	958F	SBC HL,DE
	9591	EX DE,HL	Exchange the registers so that DE holds the size and HL the address.
	9592	INC HL	Move to the second byte of the first entry.
	9593	SRL D	Halve the size of the map to get the number of entries.
	9595	RR E	Halve the size of the map to get the number of entries.
	9597	LD C,D	Copy the number of entries into the loop counters (the high byte into C and the low byte into B).
	9598	LD B,E
Reset_World_Map_Loop	9599	LD (HL),$00	Write 00 to the second byte of the entry.
	959B	INC HL	Move on to the second byte of the next entry.
	959C	INC HL	Move on to the second byte of the next entry.
	959D	DJNZ Reset_World_Map_Loop	Decrease the entry counter (low byte) and loop back until it is zero.
	959F	DEC C	Decrease the high byte of the entry counter and loop back until the second byte of every entry has been cleared.
	95A0	JR NZ,Reset_World_Map_Loop
	95A2	RET	Return.