UAO, User Access Override
The UAO characteristics are:
Purpose
Allows access to the User Access Override bit.
Configuration
This register is present only when FEAT_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)'.
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 |