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

CP15
register
Virtual Memory System Architecture (VMSA)
A Banked CP15 register can contain a mixture of:
fields that are banked
fields that are read-only in Non-secure privileged modes but read/write in the Secure state.
The System Control Register SCTLR is an example of a register of that contains this mixture of fields.
The Secure copies of the Banked CP15 registers are sometimes referred to as the Secure Banked CP15
registers. The Non-secure copies of the Banked CP15 registers are sometimes referred to as the Non-secure
Banked CP15 registers.
Restricted access CP15 registers
Table B3-15 Banked CP15 registers (continued)
Banked register Permitted accessesa
c13 FCSEIDR, FCSE PID Register d Read/write in privileged modes only
CONTEXTIDR, Context ID Register Read/write in privileged modes only
TPIDRURW, User Read/Write Thread ID Read/write in unprivileged and privileged
modes
TPIDRURO, User Read-only Thread ID Read-only in User mode
Read/write in privileged modes
TPIDRPRW, Privileged Only Thread ID Read/write in privileged modes only
a. Any attempt to execute an access that is not permitted results in an Undefined Instruction exception.
b. Some bits are common to the Secure and the Non-secure register, see c1, System Control Register (SCTLR) on
page B3-96.
c. Register is IMPLEMENTATION DEFINED.
d. Banked only if the FCSE is implemented. The FCSE PID Register is RAZ/WI if the FCSE is not implemented.
When the Security Extensions are implemented, some CP15 registers are present only in the Secure security
state. These are called Restricted access registers, and their read/write access permissions are:
Restricted access CP15 registers cannot be modified in Non-secure state.
The NSACR can be read in Non-secure privileged modes, but not in Non-secure User mode. This
enables software running in a Non-secure privileged mode to read the access permissions for CP15
registers that have configurable access.
Apart from the NSACR, Restricted access CP15 registers cannot be read in Non-secure state.
Table B3-16 on page B3-74 shows the Restricted access CP15 registers when the Security Extensions are
implemented:
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. B3-73