1. A process for extracting the source operand data for a floating point calculation from program code stored in tangible media for a computer system, comprising the steps of: invoking said program code, containing a plurality of instructions including millicoded instructions, said program code including a millicode program instruction for passing instruction text from said program code to cause a millicode entry, entering from said millicode entry control information into a millicode routine and for setting register tags in a floating point register control unit, executing said millicode routine and passing therein to an extract floating point register indirect (EXFDI) instruction for extracting for a floating point calculation through register indirect addressing contents of a floating point register array and for writing said contents of the floating point register array to a millicode working register, and performing with said EXFDI instruction a register indirect access operation by passing instruction text to an instruction decoder, and determining with said instruction decoder which selected one of the register tags in said floating point register control unit is specified by the instruction text of said EXFDI instruction, and selecting the specified register tag with a multiplexor of the floating point register control unit.