80386 Programmer's Reference Manual -- Opcode BSF

BSF -- Bit Scan Forward

OpcodeInstructionClocks DescriptionExample
0F BCbsfw r/m16,r1610+3n Bit scan forward on r/m wordbsfw %cx,%bx
bsfw (%ebx,1),%bx
bsfw (%ebx,2),%bx
bsfw (%ebx,%ebp,1),%bx
0F BCbsfl r/m32,r3210+3n Bit scan forward on r/m dwordbsfl %ecx,%ebx
bsfl (%ebx,2),%ebx
bsfl (%ebx,4),%ebx
bsfl (%ebx,%ebp,1),%ebx


is the number of leading zero bits.


IF r/m = 0
   ZF := 1;
   register := UNDEFINED;
   temp := 0;
   ZF := 0;
   WHILE BIT[r/m, temp = 0]
      temp := temp + 1;
      register := temp;


BSF 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 Affected

ZF 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 Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

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