1. A data processing system in which instructions are transferred in blocks called instruction lines from a relatively low speed memory to a relatively high speed cache memory and from which cache memory instruction lines are fetched for execution and are deleted a line at a time, said system including an instruction compounding unit in which instructions are processed in order to generate tag information that indicates instructions that can be executed in parallel, said data processing system comprising in combination: means to address a first instruction line in said cache memory in order to transfer said first instruction line from said cache memory to an instruction fetch unit; means to generate a miss signal if said first instruction line is not resident in said high speed cache memory; means responsive to said miss signal for transferring said first instruction line from said relatively low speed memory to said instruction compounding unit; means responsive to said miss signal for determining an address of a second instruction line that is next in succession to said first instruction line for transfer to said instruction fetch unit; means for transferring said second instruction line to said instruction compounding unit if said second instruction line resides in said cache memory; said instruction compounding unit processing instructions from said first instruction line and said second instruction line in order to generate tag information indicating an instruction in said first instruction line that can be executed in parallel with an instruction in said second instruction line means for determining if an instruction line in said cache memory has been deleted and replaced prior to its execution; means to determine if said deleted and replaced instruction line contains instructions that have been compounded with another instruction line in said cache memory; and means to delete tag information that indicates an instruction in said another instruction line can be compounded with an instruction in said instruction line that has been deleted and replaced.