Most instructions have one or more opcode fields which specifies the basic instruction type ( such as arithmetic , logical , jump , etc. ) and the actual operation ( such as add or compare ) and other fields that may give the type of the operand ( s ) , the addressing mode ( s ) , the addressing offset ( s ) or index , or the actual value itself ( such constant operands contained in an instruction are called immediates ) .
Most instructions have one or more opcode fields .