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ID_MMFR2, Memory Model Feature Register 2

The ID_MMFR2 characteristics are:

Purpose

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

Must be interpreted with ID_MMFR0, ID_MMFR1, ID_MMFR3, and ID_MMFR4.

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 G7.1.3.

Configuration

AArch32 System register ID_MMFR2 bits [31:0] are architecturally mapped to AArch64 System register ID_MMFR2_EL1[31:0] .

Attributes

ID_MMFR2 is a 32-bit register.

Field descriptions

The ID_MMFR2 bit assignments are:

313029282726252423222120191817161514131211109876543210
HWAccFlgWFIStallMemBarrUniTLBHvdTLBL1HvdRngL1HvdBGL1HvdFG

HWAccFlg, bits [31:28]

Hardware Access Flag. In earlier versions of the Arm Architecture, this field indicates support for a Hardware Access flag, as part of the VMSAv7 implementation. Defined values are:

HWAccFlgMeaning
0b0000

Not supported.

0b0001

Support for VMSAv7 Access flag, updated in hardware.

All other values are reserved.

From Armv8 the only permitted value is 0b000.

WFIStall, bits [27:24]

Wait For Interrupt Stall. Indicates the support for Wait For Interrupt (WFI) stalling. Defined values are:

WFIStallMeaning
0b0000

Not supported.

0b0001

Support for WFI stalling.

All other values are reserved.

From Armv8 the permitted values are 0b000 and 0b001.

MemBarr, bits [23:20]

Memory Barrier. Indicates the supported memory barrier System instructions in the (coproc==1111) encoding space:

MemBarrMeaning
0b0000

None supported.

0b0001

Supported memory barrier System instructions are:

  • Data Synchronization Barrier (DSB).
0b0010

As for 0b001, and adds:

  • Instruction Synchronization Barrier (ISB).
  • Data Memory Barrier (DMB).

All other values are reserved.

From Armv8 the only permitted value is 0b010.

Arm deprecates the use of these operations. ID_ISAR4.Barrier_instrs indicates the level of support for the preferred barrier instructions.

UniTLB, bits [19:16]

Unified TLB. Indicates the supported TLB maintenance operations, for a unified TLB implementation. Defined values are:

UniTLBMeaning
0b0000

Not supported.

0b0001

Supported unified TLB maintenance operations are:

  • Invalidate all entries in the TLB.
  • Invalidate TLB entry by VA.
0b0010

As for 0b001, and adds:

  • Invalidate TLB entries by ASID match.
0b0011

As for 0b010, and adds:

  • Invalidate instruction TLB and data TLB entries by VA All ASID. This is a shared unified TLB operation
0b0100

As for 0b011, and adds:

  • Invalidate Hyp mode unified TLB entry by VA.
  • Invalidate entire Non-secure PL1&0 unified TLB.
  • Invalidate entire Hyp mode unified TLB.
0b0101

As for 0b100, and adds the following operations: TLBIMVALIS, TLBIMVAALIS, TLBIMVALHIS, TLBIMVAL, TLBIMVAAL,TLBIMVALH.

0b0110

As for 0b101, and adds the following operations: TLBIIPAS2IS, TLBIIPAS2LIS, TLBIIPAS2, TLBIIPAS2L.

All other values are reserved.

In Armv8-A the only permitted value is 0b110.

HvdTLB, bits [15:12]

If the Unified TLB field (UniTLB, bits [19:16]) is not 0000, then the meaning of this field is IMPLEMENTATION DEFINED. Arm deprecates the use of this field by software.

L1HvdRng, bits [11:8]

Level 1 Harvard cache Range. Indicates the supported Level 1 cache maintenance range operations, for a Harvard cache implementation. Defined values are:

L1HvdRngMeaning
0b0000

Not supported.

0b0001

Supported Level 1 Harvard cache maintenance range operations are:

  • Invalidate data cache range by VA.
  • Invalidate instruction cache range by VA.
  • Clean data cache range by VA.
  • Clean and invalidate data cache range by VA.

All other values are reserved.

From Armv8 the only permitted value is 0b0000.

L1HvdBG, bits [7:4]

Level 1 Harvard cache Background fetch. Indicates the supported Level 1 cache background fetch operations, for a Harvard cache implementation. When supported, background fetch operations are non-blocking operations. Defined values are:

L1HvdBGMeaning
0b0000

Not supported.

0b0001

Supported Level 1 Harvard cache background fetch operations are:

  • Fetch instruction cache range by VA.
  • Fetch data cache range by VA.

All other values are reserved.

From Armv8 the only permitted value is 0b0000.

L1HvdFG, bits [3:0]

Level 1 Harvard cache Foreground fetch. Indicates the supported Level 1 cache foreground fetch operations, for a Harvard cache implementation. When supported, foreground fetch operations are blocking operations. Defined values are:

L1HvdFGMeaning
0b0000

Not supported.

0b0001

Supported Level 1 Harvard cache foreground fetch operations are:

  • Fetch instruction cache range by VA.
  • Fetch data cache range by VA.

All other values are reserved.

From Armv8 the only permitted value is 0b0000.

Accessing the ID_MMFR2

Accesses to this register use the following encodings:

MRC{<c>}{<q>} <coproc>, {#}<opc1>, <Rt>, <CRn>, <CRm>{, {#}<opc2>}

coprocopc1CRnCRmopc2
0b11110b0000b00000b00010b110
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HSTR_EL2.T0 == '1' then
        AArch64.AArch32SystemAccessTrap(EL2, 0x03);
    elsif EL2Enabled() && ELUsingAArch32(EL2) && HSTR.T0 == '1' then
        AArch32.TakeHypTrapException(0x03);
    elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.TID3 == '1' then
        AArch64.AArch32SystemAccessTrap(EL2, 0x03);
    elsif EL2Enabled() && ELUsingAArch32(EL2) && HCR.TID3 == '1' then
        AArch32.TakeHypTrapException(0x03);
    else
        return ID_MMFR2;
elsif PSTATE.EL == EL2 then
    return ID_MMFR2;
elsif PSTATE.EL == EL3 then
    return ID_MMFR2;


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