Routines
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41325: Multiply Words
Used by the routine at GetRoomPointer.
HL=HL*DE. GetRoomPointer uses it to turn the current room number into an offset of 6 bytes per room in Table_RoomMap.
Input
HL First number
DE Second number
Output
HL HL*DE
MultiplyWords 41325 PUSH BC Stash BC on the stack.
41326 LD B,16 Count 16 bits.
41328 LD A,H Hold the first number in A and C.
41329 LD C,L
41330 LD HL,0 HL=0000.
MultiplyWords_Loop 41333 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.
41334 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.
41335 RL C
41337 RLA
41338 JR NC,MultiplyWords_Next
41340 ADD HL,DE Add the second number into the total.
MultiplyWords_Next 41341 DJNZ MultiplyWords_Loop Decrease the bit counter by one and loop back to MultiplyWords_Loop until the counter is zero.
41343 POP BC Restore BC from the stack.
41344 RET Return.
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