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System Control Register

The SCTLR characteristics are:

Purpose
Provides the top-level control of the system, including its memory system.
Usage constraints

The accessibility to the SCTLR by Exception level is:

EL0 EL1(NS) EL1(S) EL2 EL3(SCR.NS = 1) EL3(SCR.NS = 0)
- RW RW RW RW RW

Control bits in the SCTLR that are not applicable to a VMSA implementation read as the value that most closely reflects that implementation, and ignore writes.

Some bits in the register are read-only. These bits relate to non-configurable features of an implementation, and are provided for compatibility with other versions of the architecture.

Write access to the Secure copy of SCTLR is disabled when the CP15SDISABLE signal is HIGH.

Configurations

The SCTLR is Banked for Secure and Non-secure states.

The architectural mapping of the SCTLR is:

  • The Non-secure SCTLR is mapped to the AArch64 SCTLR_EL1.
  • The Secure SCTLR is mapped to the AArch64 SCTLR_EL3.
Attributes
See the register summary in Table 4-85 c1 register summary.

The following figure shows the SCTLR bit assignments.

Figure 4-82 SCTLR bit assignments


The following table shows the SCTLR bit assignments.

Table 4-118 SCTLR bit assignments

Bits Name Access Function
[31] - - Reserved, RES0.
[30] TE Banked

Thumb Exception enable. This bit controls whether exceptions are taken in ARM or Thumb state:

0Exceptions, including reset, taken in ARM state.
1Exceptions, including reset, taken in Thumb state.

The primary input CFGTE defines the reset value of the TE bit of the Secure Banked register.
[29] AFE Banked

Access flag enable. This bit enables use of the AP[0] bit in the translation table descriptors as the Access flag. It also restricts access permissions in the translation table descriptors to the simplified model as described in the ARM® Architecture Reference Manual ARMv8. In the translation table descriptors, AP[0] is:

0An access permissions bit. The full range of access permissions is supported. No access flag is implemented. This is the reset value.
1The Access flag. Only the simplified model for access permissions is supported.

When TTBCR.EAE is set to 1, to enable use of the Long-descriptor translation table format, this bit is UNK/RES1.

This bit is permitted to be cached in a TLB.
[28] TRE Banked

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 that describes the memory region attributes in the VMSA. The possible values are:

0TEX remap disabled. TEX[2:0] are used, with the C and B bits, to describe the memory region attributes. This is the reset value.
1TEX remap enabled. TEX[2:1] are reassigned for use as bits managed by the operating system. The TEX[0], C and B bits describe the memory region attributes, with the MMU remap registers.

When TTBCR.EAE is set to 1, to enable use of the Long-descriptor translation table format, this bit is UNK/RES1.

This bit is permitted to be cached in a TLB.
[27:26] - - Reserved, RES0.
[25] EE Banked

Exception Endianness. 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. The values are:

0Little endian.
1Big endian.

The primary input CFGEND defines the reset value of the EE bit of the Secure Banked register.
[24] - - Reserved, RES0.
[23:22] - - Reserved, RES1.
[21] - - Reserved, RES0.
[20] UWXN Banked

Unprivileged write permission implies EL1 Execute Never (XN). You can use this bit to require all memory regions with unprivileged write permissions are treated as XN for accesses from software executing at EL1. Regions with unprivileged write permission are:

0Not forced to be XN. This is the reset value.
1Forced to be XN for accesses from software executing at EL1.

This bit is permitted to be cached in a TLB.
[19] WXN Banked

Write permission implies Execute Never (XN). You can use this bit to require all memory regions with write permissions are treated as XN. Regions with write permission are:

0Not forced to be XN. This is the reset value.
1Forced to be XN.

This bit is permitted to be cached in a TLB.
[18] nTWE Banked

WFE trap. The values are:

0A WFE instruction executed at EL0 that causes suspended execution as if the event register is not set and there is no pending WFE wake-up event. It is treated as UNDEFINED.
1WFE instructions executed as normal. This is the reset value.
[17] - - Reserved, RES0.
[16] nTWI Banked

WFI trap. The values are:

0A WFI instruction executed at EL0 that causes suspended execution as if there is not a pending WFI wake-up event. It is treated as UNDEFINED.
1WFE instructions executed as normal. This is the reset value.
[15:14] - - Reserved, RES0.
[13] V Banked

Vectors bit. This bit selects the base address of the exception vectors:

0Normal exception vectors, base address 0x00000000. This base address can be remapped.
1High exception vectors, base address 0xFFFF0000. This base address is never remapped.

The primary input VINITHI defines the reset value of the V bit of the Secure Banked register.
[12] I Banked

Instruction Cache enable. This is a global enable bit for Instruction Caches:

0Instruction Caches disabled. This is the reset value.
1Instruction Caches enabled.
[11] - - Reserved, RES1.
[10:9] - - Reserved, RES0.
[8] SED Banked

SETEND instruction disable. The values are:

0SETEND instruction is enabled. This is the reset value.
1SETEND instruction is UNALLOCATED.
[7] ITD Banked

IT instruction disable. The values are:

0IT instruction functionality is enabled. This is the reset value.
1

All encodings of the IT instruction are UNDEFINED when either:

  • hw[3:0] are not equal to 0b1000.
  • IT instructions with a subsequent 32-bit instruction.
  • Subsequent PC reading or writing instruction.
[6] THEE Banked

ThumbEE enable. This value is:

0ThumbEE is not implemented.
[5] CP15BEN Banked

AArch32 CP15 barrier enable. The values are:

0CP15 barrier operations disabled. Their encodings are UNDEFINED.
1CP15 barrier operations enabled. This is the reset value.
[4:3] - - Reserved, RES1.
[2] C Banked

Cache enable. This is a global enable bit for data and unified caches:

0Data and unified caches disabled. This is the reset value.
1Data and unified caches enabled.
[1] A Banked

Alignment check enable. This is the enable bit for Alignment fault checking:

0Alignment fault checking disabled. This is the reset value.
1Alignment fault checking enabled.
[0] M Banked

MMU enable. This is a global enable bit for the EL1 and EL0 stage 1 MMU:

0EL1 and EL0 stage 1 MMU disabled. This is the reset value.
1EL1 and EL0 stage 1 MMU enabled.

To access the SCTLR in AArch32 state, read or write the CP15 register with:

MRC p15, 0, <Rt>, c1, c0, 0; Read System Control Register
MCR p15, 0, <Rt>, c1, c0, 0; Write System Control Register

To access the SCTLR_EL1 in AArch64 state, read or write the register with:

MRS <Xt>, SCTLR_EL1; Read System Control Register
MSR SCTLR_EL1, <Xt>; Write System Control Register

To access the SCTLR_EL3 in AArch64 state, read or write the register with:

MRS <Xt>, SCTLR_EL3; Read System Control Register
MSR SCTLR_EL3, <Xt>; Write System Control Register
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