1. An electronic computing system having a hierarchical security mechanism incorporated therein, said system including a memory for storing both instructions and data, an instruction execution unit for extracting series of instructions from the memory and executing same, and a processing unit for performing the data operations required by said instructions, said hierarchical security mechanism being dynamically actuable by application programs and including means actuable by an application programmer for assigning a hierarchical protection level for each program instruction sequence included in an overall running program, wherein successive hierarchical protection levels have successively reduced privelege to access storage locations in said memory, means for assigning a unique protection field to each such instruction sequence, means for storing the particular protection field assigned to each instruction sequence in every storage location in said memory to which it is intended that access by said instruction sequence is to be permitted, and means actuable during the running of a program for ascertaining that the protection field of each memory access matches the protection field of the instruction sequence which initiated said access, which comprises means for comparing the protection field of a requesting instruction with the stored protection field of the accessed location in memory and predicating access on a successful comparison, said comparison being a function of the hierarchical protection level of the requesting instruction.