1. A data processing apparatus comprising: processing logic for executing software routines including a system control register setting routine; a plurality of system control registers for storing access control information for a plurality of system resources available to the processing logic when executing at least a subset of said software routines; at least one write control register containing one or more fields, each field being associated with one or more of the system control registers; and disable control logic for generating a disable signal, when the disable signal is clear the system control register setting routine being operable to write access control information in said system control registers, and for each of said one or more fields to selectively provide write restriction data in that field; when the disable control logic sets the disable signal, the at least one write control register only being readable by any software routines of said subset, and for each field that has write restriction data therein those associated one or more system control registers indicated by the write restriction data also only being readable by any software routines of said subset.