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

Virtual Memory System Architecture (VMSA)
The following subsections give more information about UNPREDICTABLE and UNDEFINED behavior for
CP15:
Unallocated CP15 encodings
Rules for MCR and MRC accesses to CP15 registers
Effects of the Security Extensions on page B3-70.
Unallocated CP15 encodings
When MCR and MRC instructions perform CP15 operations, the CRn value for the instruction is the major
register specifier for the CP15 space. Accesses to unallocated major registers are UNDEFINED. For the
ARMv7-A Architecture, this means that:
for an implementation that includes the Security Extensions, accesses with = {c4, c14} are
UNDEFINED
for an implementation that does not include the Security Extensions, accesses with
= {c4, c12, c14} are UNDEFINED
In an allocated CP15 major register specifier, MCR and MRC accesses to all unallocated encodings are
UNPREDICTABLE for privileged accesses. For the ARMv7-A architecture this means that:
if the Security Extensions are implemented, any privileged MCR or MRC access with != {c4, c14}
and a combination of , and values not shown in Figure B3-10 on page B3-65 is
UNPREDICTABLE.
if the Security Extensions are not implemented, any privileged MCR or MRC access with
!= {c4, c12, c14} and a combination of , and values not shown in
Figure B3-10 on page B3-65 is UNPREDICTABLE.
Note
As shown in Figure B3-10 on page B3-65, accesses to unallocated principal ID registers map onto MIDR.
These are accesses with = c0, =0, = c0, and = {4, 6, 7}.
Rules for MCR and MRC accesses to CP15 registers
All MCR operations from the PC are UNPREDICTABLE for all coprocessors, including for CP15.
All MRC operations to APSR_nzcv are UNPREDICTABLE for CP15.
The following accesses are UNPREDICTABLE:
an MCR access to an encoding for which no write behavior is defined in any circumstances
an MRC access to an encoding for which no read behavior is defined in any circumstances.
Except for CP15 encoding that are accessible in User mode, all MCR and MRC accesses from User mode are
UNDEFINED. This applies to all User mode accesses to unallocated CP15 encodings. Individual register
descriptions, and the summaries of the CP15 major registers, show the CP15 encodings that are accessible
in User mode.
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. B3-69