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# MECID_A0_EL2, Alternate MECID for EL2 and EL2&0 translation regimes

The MECID_A0_EL2 characteristics are:

## Purpose

Alternate MECID for EL2 and EL2&0 accesses translated by TTBR0_EL2.

## Configuration

This register is present only when FEAT_MEC is implemented. Otherwise, direct accesses to MECID_A0_EL2 are UNDEFINED.

If EL2 is not implemented, this register is RES0 from EL3.

## Attributes

MECID_A0_EL2 is a 64-bit register.

## Field descriptions

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
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
RES0
RES0 | MECID

#### Bits [63:16]

Reserved, RES0.

#### MECID, bits [15:0]

If MECIDWidth is less than 16 bits, bits[15:MECIDWidth] are RES0.

Note

MECIDWidth is defined in MECIDR_EL2.MECIDWidthm1.

The reset behavior of this field is:

- On a Warm reset, this field resets to an architecturally UNKNOWN value.

## Accessing MECID_A0_EL2

Accesses to this register use the following encodings in the System register encoding space:

#### MRS <Xt>, MECID_A0_EL2

op0 | op1 | CRn | CRm | op2
--- | --- | --- | --- | ---
0b11 | 0b100 | 0b1010 | 0b1000 | 0b001

if PSTATE.EL == EL0 then UNDEFINED; elsif PSTATE.EL == EL1 then UNDEFINED; elsif PSTATE.EL == EL2 then if !IsCurrentSecurityState(SS_Realm) then UNDEFINED; elsif HaveEL(EL3) && EL3SDDUndefPriority() && SCR_EL3.MECEn == '0' then UNDEFINED; elsif HaveEL(EL3) && SCR_EL3.MECEn == '0' then if EL3SDDUndef() then UNDEFINED; else AArch64.SystemAccessTrap(EL3, 0x18); else X[t, 64] = MECID_A0_EL2; elsif PSTATE.EL == EL3 then X[t, 64] = MECID_A0_EL2;

#### MSR MECID_A0_EL2, <Xt>

op0 | op1 | CRn | CRm | op2
--- | --- | --- | --- | ---
0b11 | 0b100 | 0b1010 | 0b1000 | 0b001

if PSTATE.EL == EL0 then UNDEFINED; elsif PSTATE.EL == EL1 then UNDEFINED; elsif PSTATE.EL == EL2 then if !IsCurrentSecurityState(SS_Realm) then UNDEFINED; elsif HaveEL(EL3) && EL3SDDUndefPriority() && SCR_EL3.MECEn == '0' then UNDEFINED; elsif HaveEL(EL3) && SCR_EL3.MECEn == '0' then if EL3SDDUndef() then UNDEFINED; else AArch64.SystemAccessTrap(EL3, 0x18); else MECID_A0_EL2 = X[t, 64]; elsif PSTATE.EL == EL3 then MECID_A0_EL2 = X[t, 64];

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26/03/2024 09:49; 67c0ae5282a7629ba0ea0ba7267b43cd4f7939f6

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