Intel and compatable CPU's Programming Information
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BSF -- Bit Scan Forward
Opcode Instruction Clocks Description 0F BC BSF r16,r/m16 10+3n Bit scan forward on r/m word 0F BC BSF r32,r/m32 10+3n Bit scan forward on r/m dword
Notesis the number of leading zero bits.
IF r/m = 0 THEN ZF := 1; register := UNDEFINED; ELSE temp := 0; ZF := 0; WHILE BIT[r/m, temp = 0] DO temp := temp + 1; register := temp; OD; FI;
DescriptionBSF scans the bits in the second word or doubleword operand starting with bit 0. The ZF flag is cleared if the bits are all 0; otherwise, the ZF flag is set and the destination register is loaded with the bit index of the first set bit.
Flags AffectedZF as described above
Protected Mode Exceptions#GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault
Real Address Mode ExceptionsInterrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH
Virtual 8086 Mode ExceptionsSame exceptions as in Real Address Mode; #PF(fault-code) for a page fault