CS : Clear destination register (or set it to Imm16) if source register's sign bit is Set (number is negative)

If the source register (SRC1)'s MSB is set (the number is negative), then either :

Short version : the destination register (SRC2) is cleared

Long version : the destination register (SRC2) is set to the value of Imm16 (sign-extended).

This is one of the conditional instructions of the CMOV group, see also : CLR, CZ/CNZ, CO/CNE/CNO/CE, CNS.