BSR — Bit Scan Reverse

Opcode	Instruction	Op/En	64-bit Mode	Compat/Leg Mode	Description
0F BD /r	BSR r16, r/m16	RM	Valid	Valid	Bit scan reverse on r/m16.
0F BD /r	BSR r32, r/m32	RM	Valid	Valid	Bit scan reverse on r/m32.
REX.W + 0F BD /r	BSR r64, r/m64	RM	Valid	N.E.	Bit scan reverse on r/m64.

Instruction Operand Encoding ¶

Op/En	Operand 1	Operand 2	Operand 3	Operand 4
RM	ModRM:reg (w)	ModRM:r/m (r)	NA	NA

Description ¶

Searches the source operand (second operand) for the most significant set bit (1 bit). If a most significant 1 bit is found, its bit index is stored in the destination operand (first operand). The source operand can be a register or a memory location; the destination operand is a register. The bit index is an unsigned offset from bit 0 of the source operand. If the content source operand is 0, the content of the destination operand is undefined.

In 64-bit mode, the instruction’s default operation size is 32 bits. Using a REX prefix in the form of REX.R permits access to additional registers (R8-R15). Using a REX prefix in the form of REX.W promotes operation to 64 bits. See the summary chart at the beginning of this section for encoding data and limits.

Operation ¶

IF SRC = 0
    THEN
        ZF ← 1;
        DEST is undefined;
    ELSE
        ZF ← 0;
        temp ← OperandSize – 1;
        WHILE Bit(SRC, temp) = 0
        DO
            temp ← temp - 1;
        OD;
        DEST ← temp;
FI;

Flags Affected ¶

The ZF flag is set to 1 if the source operand is 0; otherwise, the ZF flag is cleared. The CF, OF, SF, AF, and PF flags are undefined.

Protected Mode Exceptions ¶

#GP(0)	If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#GP(0)	If the DS, ES, FS, or GS register contains a NULL segment selector.
#SS(0)	If a memory operand effective address is outside the SS segment limit.
#PF(fault-code)	If a page fault occurs.
#AC(0)	If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#UD	If the LOCK prefix is used.

Real-Address Mode Exceptions ¶

#GP	If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#SS	If a memory operand effective address is outside the SS segment limit.
#UD	If the LOCK prefix is used.

Virtual-8086 Mode Exceptions ¶

#GP(0)	If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#SS(0)	If a memory operand effective address is outside the SS segment limit.
#PF(fault-code)	If a page fault occurs.
#AC(0)	If alignment checking is enabled and an unaligned memory reference is made.
#UD	If the LOCK prefix is used.

Compatibility Mode Exceptions ¶

Same exceptions as in protected mode.

64-Bit Mode Exceptions ¶

#SS(0)	If a memory address referencing the SS segment is in a non-canonical form.
#GP(0)	If the memory address is in a non-canonical form.
#PF(fault-code)	If a page fault occurs.
#AC(0)	If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#UD	If the LOCK prefix is used.