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Chapter 17 -- 80386 Instruction Set
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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
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
up:
Chapter 17 -- 80386 Instruction Set
prev: BOUND Check Array Index Against Bounds
next: BSR Bit Scan Reverse