Bit Scan Reverse

Encoding

Description

The BSR instruction searches the source operand for the most significant set bit. The zero-based index of this bit is stored in the destination operand.

ZF is set (due to the source being zero), the result in the destination operand is undefined.

Operation

public void BSR(ref U16 dest, U16 src)
{
    if (src == 0)
        return; // `dest` is undefined

    U16 idx = 15;
    while (src.Bit[idx] == 0)
        idx--;
    dest = idx;
}

public void BSR(ref U32 dest, U32 src)
{
    if (src == 0)
        return; // `dest` is undefined

    U32 idx = 31;
    while (src.Bit[idx] == 0)
        idx--;
    dest = idx;
}

public void BSR(ref U64 dest, U64 src)
{
    if (src == 0)
        return; // `dest` is undefined

    U64 idx = 63;
    while (src.Bit[idx] == 0)
        idx--;
    dest = idx;
}

Flags Affected

CF (carry flag)

Undefined.

PF (parity flag)

Undefined.

AF (auxiliary flag)

Undefined.

ZF (zero flag)

Set according to the source.

SF (sign flag)

Undefined.

OF (overflow flag)

Undefined.

Exceptions

Real-Address Mode

Virtual-8086 Mode

Protected Mode

Compatibility Mode

Long Mode