Routines
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A16D: Multiply Words
Used by the routine at GetRoomPointer.
Multiplies the word in HL by the word in DE. GetRoomPointer uses it to multiply the current room number by 06 to find that room's record in Table_RoomMap.
Input
HL Multiplicand
DE Multiplier
Output
HL Product
MultiplyWords A16D PUSH BC Stash BC on the stack.
A16E LD B,$10 B=10.
A170 LD A,H A=H.
A171 LD C,L C=L.
A172 LD HL,$0000 HL=0000.
MultiplyWords_0 A175 ADD HL,HL HL+=HL.
For each bit of the multiplier, shift it into the carry and add the multiplicand when that bit is set.
A176 AND A Fold the next multiplier bit into the product.
A177 RL C
A179 RLA
A17A JR NC,MultiplyWords_1
A17C ADD HL,DE
MultiplyWords_1 A17D DJNZ MultiplyWords_0
A17F POP BC Restore BC from the stack, leaving the product in HL.
A180 RET Return.
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