Memory access control using redundant and non-redundant encoding

摘要

A data processing system 2 including processing circuitry 4 operating in either a first mode or a second mode. Page table data 30 including access control bits 40, 42, is used to control permissions for memory access to memory pages. In the first mode, the access control bits include at least one instance of a redundant encoding. In the second mode, the redundant encoding is removed to provide more efficient use of the access control bit encoding space.

主权项

1. Apparatus for processing data comprising:
processing circuitry responsive to program instructions to perform data processing operations and configured to operate in a plurality of modes; and memory management circuitry coupled to said processing circuitry and to a memory and configured to control access permissions to data values stored within said memory in dependence upon page table data; wherein said memory has a memory address space divided in to a plurality of memory pages and said page table data comprises a plurality of access control bits for each of said plurality of memory pages defining access permissions for a respective memory page; when said processing circuitry is in a first of said plurality of modes using a predetermined number of access control bits to encode access permissions, said memory management circuitry is responsive to at least one instance of a redundant encoding in which a plurality of different combinations of said access control bits provides a same set of access permissions; and when said processing circuitry is in a second of said plurality of modes using said predetermined number of access control bits to encode access permissions, said memory management circuitry is responsive to said plurality of different combinations of access control bits to provide a plurality of different sets of access permissions in a non-redundant encoding.