Routines
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A16D: Multiply Words
Used by the routine at GetRoomPointer.
HL=HL*DE. GetRoomPointer uses it to turn the current room number into an offset of 06 bytes per room in Table_RoomMap.
Input
HL First number
DE Second number
Output
HL HL*DE
MultiplyWords A16D PUSH BC Stash BC on the stack.
A16E LD B,$10 Count 10 bits.
A170 LD A,H Hold the first number in A and C.
A171 LD C,L
A172 LD HL,$0000 HL=0000.
MultiplyWords_Loop A175 ADD HL,HL Shift the running total left by one bit.
Test the next bit of the first number. When it is set, add the second number into the total.
A176 AND A Clear the carry, then shift the next bit of the first number into it. Jump to MultiplyWords_Next when that bit is clear.
A177 RL C
A179 RLA
A17A JR NC,MultiplyWords_Next
A17C ADD HL,DE Add the second number into the total.
MultiplyWords_Next A17D DJNZ MultiplyWords_Loop Decrease the bit counter by one and loop back to MultiplyWords_Loop until the counter is zero.
A17F POP BC Restore BC from the stack.
A180 RET Return.
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