An execution system for instructions having source and sink operand designations includes an arithmetic unit, execution means for holding an instruction for controlling the arithmetic unit and a plurality of operand registers, each having tag means associated therewith for indicating the nature of data stored therein. Instruction modification logic, on the basis of indications of availability of the operand registers, inserts modified source and sink operand register designations in instructions which are then stored in an instruction register stack. Interlock logic controls the transfer of instructions from the instruction register stack to the execution means as a function of the source and sink operand register designations in the modified instructions and the data tags associated with the operand registers.