Underwurlde: Routine at 38280

38280: Reset World Map

Used by the routine at Select_Game_Options. Writes 0 to the second byte of each of the first 576 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 64 times on the first pass and 256 times on each of the next two passes. The last 256 entries of the map (the last sixteen rows of rooms) are not reset.
Reset_World_Map	38280	LD HL,30336	Point at the end of the world map (Start_Game_Selection).
	38283	LD DE,28672	Point at the start of the world map (World_Map).
	38286	AND A	Subtract the start from the end of the map to get the size of the map in bytes.
	38287	SBC HL,DE
	38289	EX DE,HL	Exchange the registers so that DE holds the size and HL the address.
	38290	INC HL	Move to the second byte of the first entry.
	38291	SRL D	Halve the size of the map to get the number of entries.
	38293	RR E	Halve the size of the map to get the number of entries.
	38295	LD C,D	Copy the number of entries into the loop counters (the high byte into C and the low byte into B).
	38296	LD B,E
Reset_World_Map_Loop	38297	LD (HL),0	Write 0 to the second byte of the entry.
	38299	INC HL	Move on to the second byte of the next entry.
	38300	INC HL	Move on to the second byte of the next entry.
	38301	DJNZ Reset_World_Map_Loop	Decrease the entry counter (low byte) and loop back until it is zero.
	38303	DEC C	Decrease the high byte of the entry counter and loop back until the second byte of every entry has been cleared.
	38304	JR NZ,Reset_World_Map_Loop
	38306	RET	Return.