1. A resource access security system for controlling access to a data processing system comprising: a memory in which are stored one or more first tables, each respective first table being associated with a respectively different job/user privilege level and containing a list of domain entries of data processing resources, each domain entry having a respective domain clearance level, the number of domain entries in any first table being independent of the number of domain entries in any other first table, and one or more second tables, each respective second table being associated with a respectively different domain clearance level and containing a respective list of data processing resource entries, the number of data processing resource entries in any second table being independent of the number of entries in any other second table, and wherein each domain entry in said first table contains a pointer code which points to a second table associated with the domain clearance level of said each domain entry; and a data processing system access controller which is operative to enable access to said data processing system in accordance with a prescribed relationship between entries of said first and second tables and a data processing system access code supplied thereto, said data processing access code containing a first component representative of a domain of data processing resources, and a second component representative of a data processing resource to be employed in the course of execution of a data processing task.