The SHLD instruction shifts the first operand provided by the r/m field to the left as many bits as specified by the count operand. The second operand (r16 or r32) provides the bits to shift in from the right (starting with bit 0). The result is stored back into the r/m operand. The register remains unaltered.
The count operand is provided by either an immediate byte or the contents of the CL register. These operands are taken MODULO 32 to provide a number between 0 and 31 by which to shift. Because the bits to shift are provided by the specified registers, the operation is useful for multi-precision shifts (64 bits or more). The SF, ZF and PF flags are set according to the value of the result. The CF flag is set to the value of the last bit shifted out. The OF and AF flags are left undefined.