You copied the Doc URL to your clipboard.

ID_MMFR4_EL1, AArch32 Memory Model Feature Register 4

The ID_MMFR4_EL1 characteristics are:

Purpose

Provides information about the implemented memory model and memory management support in AArch32 state.

Must be interpreted with ID_MMFR0_EL1, ID_MMFR1_EL1, ID_MMFR2_EL1, and ID_MMFR3_EL1.

For general information about the interpretation of the ID registers see 'Principles of the ID scheme for fields in ID registers' in the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile, section D10.4.1.

Configuration

AArch64 System register ID_MMFR4_EL1 bits [31:0] are architecturally mapped to AArch32 System register ID_MMFR4[31:0] .

In an implementation that supports only AArch64 state, this register is UNKNOWN.

Attributes

ID_MMFR4_EL1 is a 64-bit register.

Field descriptions

The ID_MMFR4_EL1 bit assignments are:

6362616059585756555453525150494847464544434241403938373635343332
RES0
EVTCCIDXLSMHPDSCnPXNXAC2SpecSEI
313029282726252423222120191817161514131211109876543210

Bits [63:32]

Reserved, RES0.

EVT, bits [31:28]

When ARMv8.2-EVT is implemented:

Enhanced Virtualization Traps. If EL2 is implemented, indicates support for the HCR2.{TTLBIS, TOCU, TICAB, TID4} traps. Defined values are:

EVTMeaning
0b0000

HCR2.{TTLBIS, TOCU, TICAB, TID4} traps are not supported.

0b0001

HCR2.{TOCU, TICAB, TID4} traps are supported. HCR2.TTLBIS trap is not supported.

0b0010

HCR2.{TTLBIS, TOCU, TICAB, TID4} traps are supported.

All other values are reserved.

In Armv8.0, the only permitted value is 0b0000.

From Armv8.1, the permitted values are 0b0000, 0b0001, and 0b0010.

From Armv8.5 the permitted values are:

  • 0b0000 when EL2 is not implemented.
  • 0b0001 when EL2 is implemented. This feature is identified as ARMv8.2-EVT.


Otherwise:

Reserved, RES0.

CCIDX, bits [27:24]

From Armv8.3:

Support for use of the revised CCSIDR format and the presence of the CCSIDR2 is indicated. Defined values are:

CCIDXMeaning
0b0000

32-bit format implemented for all levels of the CCSIDR, and the CCSIDR2 register is not implemented.

0b0001

64-bit format implemented for all levels of the CCSIDR, and the CCSIDR2 register is implemented.

All other values are reserved.

From Armv8.3, the permitted values are 0b0000 and 0b0001. This feature is identified as ARMv8.3-CCIDX.


Otherwise:

Reserved, RAZ.

LSM, bits [23:20]

From Armv8.2:

Indicates support for LSMAOE and nTLSMD bits in HSCTLR and SCTLR. Defined values are:

LSMMeaning
0b0000

LSMAOE and nTLSMD bits not supported.

0b0001

LSMAOE and nTLSMD bits supported.

All other values are reserved.

ARMv8.2-LSMAOC implements the functionality identified by the value 0b0001.


Otherwise:

Reserved, RAZ.

HPDS, bits [19:16]

From Armv8.2:

Hierarchical permission disables bits in translation tables. Defined values are:

HPDSMeaning
0b0000

Disabling of hierarchical controls not supported.

0b0001

Supports disabling of hierarchical controls using the TTBCR2.HPD0, TTBCR2.HPD1, and HTCR.HPD bits.

0b0010

As for value 0b0001, and adds possible hardware allocation of bits[62:59] of the translation table descriptors from the final lookup level for IMPLEMENTATION DEFINED use.

All other values are reserved.

ARMv8.2-AA32HPD implements the functionality identified by the value 0b0001.

ARMv8.2-TTPBHA implements the functionality added by the value 0b0010.

Note

The value 0b0000 implies that the encoding for TTBCR2 is unallocated.


Otherwise:

Reserved, RAZ.

CnP, bits [15:12]

From Armv8.2:

Common not Private translations. Defined values are:

CnPMeaning
0b0000

Common not Private translations not supported.

0b0001

Common not Private translations supported.

All other values are reserved.

ARMv8.2-TTCNP implements the functionality identified by the value 0b0001.

From Armv8.2 the only permitted value is 0b0001.


Otherwise:

Reserved, RAZ.

XNX, bits [11:8]

From Armv8.2:

Support for execute-never control distinction by Exception level at stage 2. Defined values are:

XNXMeaning
0b0000

Distinction between EL0 and EL1 execute-never control at stage 2 not supported.

0b0001

Distinction between EL0 and EL1 execute-never control at stage 2 supported.

All other values are reserved.

ARMv8.2-TTS2UXN implements the functionality identified by the value 0b0001.

When ARMv8.2-TTS2UXN is implemented:

  • If all of the following conditions are true it is IMPLEMENTATION DEFINED whether the value of ID_MMFR4_EL1.XNX is 0b0000 or 0b0001:
  • If EL2 can use AArch32 then the only permitted value is 0b0001.


Otherwise:

Reserved, RAZ.

AC2, bits [7:4]

Indicates the extension of the ACTLR and HACTLR registers using ACTLR2 and HACTLR2. Defined values are:

AC2Meaning
0b0000

ACTLR2 and HACTLR2 are not implemented.

0b0001

ACTLR2 and HACTLR2 are implemented.

All other values are reserved.

In Armv8.0 and Armv8.1 the permitted values are 0b0000 and 0b0001.

From Armv8.2, the only permitted value is 0b0001.

SpecSEI, bits [3:0]

When RAS is implemented:

Describes whether the PE can generate SError interrupt exceptions from speculative reads of memory, including speculative instruction fetches. The defined values of this field are:

SpecSEIMeaning
0b0000

The PE never generates an SError interrupt due to an External abort on a speculative read.

0b0001

The PE might generate an SError interrupt due to an External abort on a speculative read.

All other values are reserved.


Otherwise:

Reserved, RES0. This provides no information about whether the PE generates a speculative SError interrupt.

Accessing the ID_MMFR4_EL1

Accesses to this register use the following encodings:

MRS <Xt>, ID_MMFR4_EL1

op0op1CRnCRmop2
0b110b0000b00000b00100b110
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && (!IsZero(ID_MMFR4_EL1) || boolean IMPLEMENTATION_DEFINED "ID_MMFR4_EL1 trapped by HCR_EL2.TID3 and ID_MMFR4 trapped by HCR_EL2.TID3 and HCR.TID3") && HCR_EL2.TID3 == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    else
        return ID_MMFR4_EL1;
elsif PSTATE.EL == EL2 then
    return ID_MMFR4_EL1;
elsif PSTATE.EL == EL3 then
    return ID_MMFR4_EL1;
              


Was this page helpful? Yes No