1. A synchronous central processor for a general purpose digital data processing system, said central processor being synchronized by clock pulses produced by clock means and having a given repertoire of instructions, said processor comprising: cache means adapted to store machine words, said machine words including instructions and operands; a plurality of instruction execution units, each of which unit executes a different set of the instructions of the repertoire independently of and in parallel with one another, each instruction requiring a predetermined number of clock periods to complete and each execution unit having a results stack into which the result of the execution of each instruction is stored; instruction fetch means for fetching instructions from the cache means and for queuing instructions awaiting execution; central unit means for obtaining instructions of a given program from the instruction fetch means in program order, for decoding the instructions, for preparing the address of an operand to be operated upon by the instruction, for checking to determine if the addressed operand is stored in the cache means, for accessing the addressed operand from the cache and for dispatching the instruction and its operand to the one of said plurality of instruction execution units for executing it, each of said instruction execution units executing the instructions dispatched to it in the order received to produce a result and storing the result of the execution of each instruction in its results stack in the order of execution, the number of clock periods required to execute each instruction differing so that the results of the execution of instructions of a given program by the instruction execution units are not produced in program order; and collector means for obtaining in program order the results of the execution of the instructions of the program from the results stacks of the instruction execution units and for writing into the cache means the results of the execution of instructions of the program which are to be written into memory in program order.