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# SCXTNUM_EL3, EL3 Read/Write Software Context Number

The SCXTNUM_EL3 characteristics are:

## Purpose

Provides a number that can be used to separate out different context numbers with the EL3 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_EL3 are UNDEFINED.

RW fields in this register reset to architecturally UNKNOWN values.

## Attributes

SCXTNUM_EL3 is a 64-bit register.

## Field descriptions

The SCXTNUM_EL3 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 EL3 exception level.

This field resets to an architecturally UNKNOWN value.

## Accessing the SCXTNUM_EL3

Accesses to this register use the following encodings:

#### MRS <Xt>, SCXTNUM_EL3

op0 | op1 | CRn | CRm | op2
--- | --- | --- | --- | ---
0b11 | 0b110 | 0b1101 | 0b0000 | 0b111

if PSTATE.EL == EL0 then UNDEFINED; elsif PSTATE.EL == EL1 then UNDEFINED; elsif PSTATE.EL == EL2 then UNDEFINED; elsif PSTATE.EL == EL3 then return SCXTNUM_EL3;

#### MSR SCXTNUM_EL3, <Xt>

op0 | op1 | CRn | CRm | op2
--- | --- | --- | --- | ---
0b11 | 0b110 | 0b1101 | 0b0000 | 0b111

if PSTATE.EL == EL0 then UNDEFINED; elsif PSTATE.EL == EL1 then UNDEFINED; elsif PSTATE.EL == EL2 then UNDEFINED; elsif PSTATE.EL == EL3 then SCXTNUM_EL3 = X[t];

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15/04/2019 16:47; 8c23526ba8a13a72aada4137daf3e3977b49036e

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