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# UAO, User Access Override

The UAO characteristics are:

## Purpose

Allows access to the User Access Override bit.

## Configuration

This register is present only when ARMv8.2-UAO is implemented. Otherwise, direct accesses to UAO are UNDEFINED.

## Attributes

UAO is a 64-bit register.

## Field descriptions

The UAO bit assignments are:

63 | 62 | 61 | 60 | 59 | 58 | 57 | 56 | 55 | 54 | 53 | 52 | 51 | 50 | 49 | 48 | 47 | 46 | 45 | 44 | 43 | 42 | 41 | 40 | 39 | 38 | 37 | 36 | 35 | 34 | 33 | 32
RES0
RES0 | UAO | RES0
31 | 30 | 29 | 28 | 27 | 26 | 25 | 24 | 23 | 22 | 21 | 20 | 19 | 18 | 17 | 16 | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0

#### Bits [63:24]

Reserved, RES0.

#### UAO, bit [23]

User Access Override.

UAO | Meaning
--- | ---
0b0 | The behavior of LDTR* and STTR* instructions is as defined in the base Armv8 architecture.
0b1 | When executed at EL1, or at EL2 with HCR_EL2.{E2H, TGE} == {1, 1}, LDTR* and STTR* instructions behave as the equivalent LDR* and STR* instructions.

When executed at EL3, or at EL2 with HCR_EL2.E2H == 0 or HCR_EL2.TGE == 0, the LDTR* and STTR* instructions behave as the equivalent LDR* and STR* instructions, regardless of the setting of the PSTATE.UAO bit.

#### Bits [22:0]

Reserved, RES0.

## Accessing the UAO

For details on the operation of the MSR (immediate) accessor, see MSR (immediate)in the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile.

Accesses to this register use the following encodings:

#### MRS <Xt>, UAO

op0 | op1 | CRn | CRm | op2
--- | --- | --- | --- | ---
0b11 | 0b000 | 0b0100 | 0b0010 | 0b100

if PSTATE.EL == EL0 then UNDEFINED; elsif PSTATE.EL == EL1 then return Zeros(40):PSTATE.UAO:Zeros(23); elsif PSTATE.EL == EL2 then return Zeros(40):PSTATE.UAO:Zeros(23); elsif PSTATE.EL == EL3 then return Zeros(40):PSTATE.UAO:Zeros(23);

#### MSR UAO, <Xt>

op0 | op1 | CRn | CRm | op2
--- | --- | --- | --- | ---
0b11 | 0b000 | 0b0100 | 0b0010 | 0b100

if PSTATE.EL == EL0 then UNDEFINED; elsif PSTATE.EL == EL1 then PSTATE.UAO = X[t]<23>; elsif PSTATE.EL == EL2 then PSTATE.UAO = X[t]<23>; elsif PSTATE.EL == EL3 then PSTATE.UAO = X[t]<23>;

#### MSR UAO, #<imm>

op0 | op1 | CRn | op2
--- | --- | --- | ---
0b00 | 0b000 | 0b0100 | 0b011

AArch32 Registers | AArch64 Registers | AArch32 Instructions | AArch64 Instructions | Index by Encoding | External Registers | External Registers by Offset | Registers by Functional Group | Proprietary Notice

27/03/2019 21:59; e5e4db499bf9867a4b93324c4dbac985d3da9376

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