UDF Permanently undefined This instruction generates an Undefined Instruction exception (ESR_ELx.EC = 0b000000). The encodings for UDF used in this section are defined as permanently UNDEFINED. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 UDF #<imm> // The imm16 field is ignored by hardware. UNDEFINED; <imm> is a 16-bit unsigned immediate, in the range 0 to 65535, encoded in the "imm16" field. The PE ignores the value of this constant. // No operation.