ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

Application Level Memory Model
A3.6 Access rights
ARMv7 includes additional attributes for memory regions, that enable:
Data accesses to be restricted, based on the privilege of the access. See Privilege level access controls
for data accesses.
Instruction fetches to be restricted, based on the privilege of the process or thread making the fetch.
See Privilege level access controls for instruction accesses.
On a system that implements the Security Extensions, accesses to be restricted to memory accesses
with the Secure memory attribute. See Memory region security status on page A3-39.
A3.6.1 Privilege level access controls for data accesses
The memory attributes can define that a memory region is:
not accessible to any accesses
accessible only to Privileged accesses
accessible to Privileged and Unprivileged accesses.
The access privilege level is defined separately for explicit read and explicit write accesses. However, a
system that defines the memory attributes is not required to support all combinations of memory attributes
for read and write accesses.
A Privileged access is an access made during privileged execution, as a result of a load or store operation
other than LDRT, STRT, LDRBT, STRBT, LDRHT, STRHT, LDRSHT, or LDRSBT.
An Unprivileged access is an access made as a result of load or store operation performed in one of these
cases:
when the processor is in an unprivileged mode
when the processor is in any mode and the access is made as a result of a LDRT, STRT, LDRBT, STRBT,
LDRHT, STRHT, LDRSHT, or LDRSBT instruction.
A Data Abort exception is generated if the processor attempts a data access that the access rights do not
permit. For example, a Data Abort exception is generated if the processor is in unprivileged mode and
attempts to access a memory region that is marked as only accessible to Privileged accesses.
A3.6.2 Privilege level access controls for instruction accesses
Memory attributes can define that a memory region is:
Not accessible for execution
Accessible for execution by Privileged processes only
Accessible for execution by Privileged and Unprivileged processes.
To define the instruction access rights to a memory region, the memory attributes describe, separately, for
the region:
its read access rights, see Privilege level access controls for data accesses
A3-38 Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. ARM DDI 0406B