System Control
ARM DDI 0500D
Copyright © 2013-2014 ARM. All rights reserved.
4-193
ID021414
Non-Confidential
Table 4-193
shows the SCTLR bit assignments.
Table 4-193 SCTLR bit assignments
Bits
Name
Function
[31]
-
Reserved,
RES
0.
[30]
TE
T32 Exception enable. This bit controls whether exceptions are taken in A32 or T32 state:
0
Exceptions, including reset, taken in A32 state.
1
Exceptions, including reset, taken in T32 state.
The input
CFGTE
defines the reset value of the TE bit.
[29]
AFE
Access Flag Enable. This bit enables use of the AP[0] bit in the translation descriptors as the Access flag. It
also restricts access permissions in the translation descriptors to the simplified model:
0
In the translation table descriptors, AP[0] is an access permissions bit. The full range of access
permissions is supported. No Access flag is implemented. This is the reset value.
1
In the translation table descriptors, AP[0] is the Access flag. Only the simplified model for
access permissions is supported.
[28]
TRE
TEX remap enable. This bit enables remapping of the TEX[2:1] bits for use as two translation table bits that
can be managed by the operating system. Enabling this remapping also changes the scheme used to describe
the memory region attributes in the VMSA:
0
TEX remap disabled. TEX[2:0] are used, with the C and B bits, to describe the memory region
attributes. This is the reset value.
1
TEX remap enabled. TEX[2:1] are reassigned for use as bits managed by the operating system.
The TEX[0], C and B bits are used to describe the memory region attributes, with the MMU
remap registers.
[27:26]
-
Reserved,
RES
0.
[25]
EE
Exception Endianness bit. The value of this bit defines the value of the CPSR.E bit on entry to an exception
vector, including reset. This value also indicates the endianness of the translation table data for translation
table lookups:
0
Little endian.
1
Big endian.
The input
CFGEND
defines the reset value of the EE bit.
[24]
-
Reserved,
RES
0.
[23:22]
-
Reserved,
RES
1.
[21]
-
Reserved,
RES
0.
[20]
UWXN
Unprivileged write permission implies EL1
Execute Never
(XN). This bit can be used to require all memory
regions with unprivileged write permissions to be treated as XN for accesses from software executing at EL1.
0
Regions with unprivileged write permission are not forced to be XN, this is the reset value.
1
Regions with unprivileged write permission are forced to be XN for accesses from software
executing at EL1.
[19]
WXN
Write permission implies
Execute Never
(XN). This bit can be used to require all memory regions with write
permissions to be treated as XN.
0
Regions with write permission are not forced to be XN, this is the reset value.
1
Regions with write permissions are forced to be XN.
[18]
nTWE
Not trap WFE.
0
If a WFE instruction executed at EL0 would cause execution to be suspended, such as if the
event register is not set and there is not a pending WFE wakeup event, it is taken as an
exception to EL1 using the 0x1 ESR code.
1
WFE instructions are executed as normal.