Primary MECID for EL2&0 translation regimes
Primary MECID for EL2&0 accesses translated by TTBR1_EL2.
This register is present only when FEAT_MEC is implemented and FEAT_AA64 is implemented. Otherwise, direct accesses to MECID_P1_EL2 are UNDEFINED.
If EL2 is not implemented, this register is RES0 from EL3.
MECID_P1_EL2 is a 64-bit register.
| 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 | ||||||||||||||||||||||||||||||
Reserved, RES0.
If MECIDWidth is less than 16 bits, bits[15:MECIDWidth] are RES0.
MECIDWidth is defined in MECIDR_EL2.MECIDWidthm1.
The reset behavior of this field is:
Accesses to this register use the following encodings in the System register encoding space:
MRS <Xt>, MECID_P1_EL2
(op0 = 0b11, op1 = 0b100, CRn = 0b1010, CRm = 0b1000, op2 = 0b010)
if !(IsFeatureImplemented(FEAT_MEC) && IsFeatureImplemented(FEAT_AA64)) then Undefined(); elsif HaveEL(EL3) && PSTATE.EL == EL2 && EL3SDDUndefPriority() && SCR_EL3().MECEn == '0' then Undefined(); elsif 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) && SCR_EL3().MECEn == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x18); end; else X{64}(t) = MECID_P1_EL2(); end; elsif PSTATE.EL == EL3 then X{64}(t) = MECID_P1_EL2(); end;
MSR MECID_P1_EL2, <Xt>
(op0 = 0b11, op1 = 0b100, CRn = 0b1010, CRm = 0b1000, op2 = 0b010)
if !(IsFeatureImplemented(FEAT_MEC) && IsFeatureImplemented(FEAT_AA64)) then Undefined(); elsif HaveEL(EL3) && PSTATE.EL == EL2 && EL3SDDUndefPriority() && SCR_EL3().MECEn == '0' then Undefined(); elsif 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) && SCR_EL3().MECEn == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x18); end; else MECID_P1_EL2() = X{64}(t); end; elsif PSTATE.EL == EL3 then MECID_P1_EL2() = X{64}(t); end;
Version 2026.06 — Copyright © 2010-2026 Arm Limited or its affiliates.
This site is provided as a community resource and is NOT affiliated with nor endorsed by Arm Limited.