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# SCXTNUM_EL0, EL0 Read/Write Software Context Number

The SCXTNUM_EL0 characteristics are:

## Purpose

Provides a number that can be used to separate out different context numbers with the EL0 exception level, for the purpose of protecting against side-channels using branch prediction and similar resources.

## Configuration

This register is present only when ARMv8.0-CSV2 is implemented. Otherwise, direct accesses to SCXTNUM_EL0 are UNDEFINED.

RW fields in this register reset to architecturally UNKNOWN values.

## Attributes

SCXTNUM_EL0 is a 64-bit register.

## Field descriptions

The SCXTNUM_EL0 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
Software Context Number
Software Context Number
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 [63:0]

Software Context Number. A number to identify the context within the EL0 exception level.

This field resets to an architecturally UNKNOWN value.

## Accessing the SCXTNUM_EL0

Accesses to this register use the following encodings:

#### MRS <Xt>, SCXTNUM_EL0

op0 | op1 | CRn | CRm | op2
--- | --- | --- | --- | ---
0b11 | 0b011 | 0b1101 | 0b0000 | 0b111

if PSTATE.EL == EL0 then if !ELUsingAArch32(EL1) && !(EL2Enabled() && HCR_EL2.<E2H,TGE> == '11') && SCTLR_EL1.TSCXT == '1' then if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.TGE == '1' then AArch64.SystemAccessTrap(EL2, 0x18); else AArch64.SystemAccessTrap(EL1, 0x18); elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.<E2H,TGE> != '11' && HCR_EL2.EnSCXT == '0' then AArch64.SystemAccessTrap(EL2, 0x18); elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.<E2H,TGE> == '11' && SCTLR_EL2.TSCXT == '1' then AArch64.SystemAccessTrap(EL2, 0x18); elsif HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then AArch64.SystemAccessTrap(EL3, 0x18); else return SCXTNUM_EL0; elsif PSTATE.EL == EL1 then if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.EnSCXT == '0' then AArch64.SystemAccessTrap(EL2, 0x18); elsif HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then AArch64.SystemAccessTrap(EL3, 0x18); else return SCXTNUM_EL0; elsif PSTATE.EL == EL2 then if HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then AArch64.SystemAccessTrap(EL3, 0x18); else return SCXTNUM_EL0; elsif PSTATE.EL == EL3 then return SCXTNUM_EL0;

#### MSR SCXTNUM_EL0, <Xt>

op0 | op1 | CRn | CRm | op2
--- | --- | --- | --- | ---
0b11 | 0b011 | 0b1101 | 0b0000 | 0b111

if PSTATE.EL == EL0 then if !ELUsingAArch32(EL1) && !(EL2Enabled() && HCR_EL2.<E2H,TGE> == '11') && SCTLR_EL1.TSCXT == '1' then if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.TGE == '1' then AArch64.SystemAccessTrap(EL2, 0x18); else AArch64.SystemAccessTrap(EL1, 0x18); elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.<E2H,TGE> != '11' && HCR_EL2.EnSCXT == '0' then AArch64.SystemAccessTrap(EL2, 0x18); elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.<E2H,TGE> == '11' && SCTLR_EL2.TSCXT == '1' then AArch64.SystemAccessTrap(EL2, 0x18); elsif HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then AArch64.SystemAccessTrap(EL3, 0x18); else SCXTNUM_EL0 = X[t]; elsif PSTATE.EL == EL1 then if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.EnSCXT == '0' then AArch64.SystemAccessTrap(EL2, 0x18); elsif HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then AArch64.SystemAccessTrap(EL3, 0x18); else SCXTNUM_EL0 = X[t]; elsif PSTATE.EL == EL2 then if HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then AArch64.SystemAccessTrap(EL3, 0x18); else SCXTNUM_EL0 = X[t]; elsif PSTATE.EL == EL3 then SCXTNUM_EL0 = X[t];

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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