1. A method for executing a machine instruction in a central processing unit, the method comprising: obtaining, by a processor, a machine instruction for execution, the machine instruction being defined for computer execution according to a computer architecture, the machine instruction comprising an opcode, one register field, another register field, an index field, a base field, and a displacement value; performing a shift function on a significand of a decimal floating point datum as that function is defined by the opcode of the machine instruction, wherein the significand is stored in a location designated by the one register field, the shift function comprising shifting in one direction one or more decimal digits of the significand a number of positions specified by a plurality of second operand bits determined using the index field, the base field and displacement value of the machine instruction; and placing a result of the shift function in a location designated by the other register field.