1. An apparatus for determining a length of an instruction being processed by a computer system in which instructions vary in length and appear sequentially in an instruction stream without differentiation from one another, the apparatus comprising: means for providing a boundary marker for each instruction to indicate a boundary between that instruction and another instruction in the instruction stream; means, coupled to the boundary marker providing means, for setting each boundary marker to indicate a predesignated boundary; a first channel for processing a first instruction in sequence; a second channel for processing an instruction next following the first instruction in sequence; means, coupled to the first channel, for determining an actual boundary of the first instruction as it is processed; means, coupled to the boundary marker providing means and the actual boundary determining means, for comparing the boundary marker and the actual boundary of the first instruction to determine whether they match; means, responsive to the comparing means, for updating the boundary marker of the first instruction to the actual boundary of the first instruction when the boundary value and the actual boundary of the first instruction do not match; means, coupled to the comparing means and the second channel, for aborting processing of the next instruction when the actual boundary and the boundary marker of the first instruction do not match; and means, coupled to the boundary providing means, for indicating a boundary between the first instruction and the next instruction from the stream of instructions based on the boundary marker of the first instruction.