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80386 Programmer's Reference Manual -- Opcode BSF

BSF -- Bit Scan Forward

Opcode	Instruction	Clocks	Description	Example
0F BC	bsfw r/m16,r16	10+3n	Bit scan forward on r/m word	`bsfw %cx,%bx bsfw (%ebx,1),%bx bsfw (%ebx,2),%bx bsfw (%ebx,%ebp,1),%bx`
0F BC	bsfl r/m32,r32	10+3n	Bit scan forward on r/m dword	`bsfl %ecx,%ebx bsfl (%ebx,2),%ebx bsfl (%ebx,4),%ebx bsfl (%ebx,%ebp,1),%ebx`

Notes

is the number of leading zero bits.

Operation




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;

Description

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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