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TTBCR, Translation Table Base Control Register

The TTBCR characteristics are:

Purpose

The control register for stage 1 of the PL1&0 translation regime. Its controls include:

  • Where the VA range is split between addresses translated using TTBR0 and addresses translated using TTBR1.
  • The translation table format used by this stage of translation.

In Armv8.2, when the value of TTBCR.{EAE, T2E} is {1, 1}, TTBCR is used with TTBCR2.

Configuration

AArch32 System register TTBCR bits [31:0] are architecturally mapped to AArch64 System register TCR_EL1[31:0] .

The current translation table format determines which format of the register is used.

When EL3 is using AArch32, write access to TTBCR(S) is disabled when the CP15SDISABLE signal is asserted HIGH.

Some RW fields of this register have defined reset values. These apply only if the PE resets into an Exception level that is using AArch32. If the PE resets into EL3 using AArch32 then:

  • The EAE bit resets to 0 in both the Secure and the Non-secure instances of the register.
  • Other reset values apply only to the Secure instance of the register.

Attributes

TTBCR is a 32-bit register.

Field descriptions

The TTBCR bit assignments are:

When TTBCR.EAE == 0:
313029282726252423222120191817161514131211109876543210
EAE0000000000000000000000000PD1PD00N

EAE, bit [31]

Extended Address Enable. The meanings of the possible values of this bit are:

EAEMeaning
0b0

Use the VMSAv8-32 translation system with the Short-descriptor translation table format.

This field resets to 0.

Bits [30:6]

Reserved, RES0.

PD1, bit [5]

Translation table walk disable for translations using TTBR1. This bit controls whether a translation table walk is performed on a TLB miss, for an address that is translated using TTBR1. The encoding of this bit is:

PD1Meaning
0b0

Perform translation table walks using TTBR1.

0b1

A TLB miss on an address that is translated using TTBR1 generates a Translation fault. No translation table walk is performed.

This field resets to 0.

PD0, bit [4]

Translation table walk disable for translations using TTBR0. This bit controls whether a translation table walk is performed on a TLB miss for an address that is translated using TTBR0. The encoding of this bit is:

PD0Meaning
0b0

Perform translation table walks using TTBR0.

0b1

A TLB miss on an address that is translated using TTBR0 generates a Translation fault. No translation table walk is performed.

This field resets to 0.

Bit [3]

Reserved, RES0.

N, bits [2:0]

Indicate the width of the base address held in TTBR0. In TTBR0, the base address field is bits[31:14-N]. The value of N also determines:

  • Whether TTBR0 or TTBR1 is used as the base address for translation table walks.
  • The size of the translation table pointed to by TTBR0.

N can take any value from 0 to 7, that is, from 0b000 to 0b111.

When N has its reset value of 0, the translation table base is compatible with Armv5 and Armv6.

This field resets to 0.

When TTBCR.EAE == 1:
313029282726252423222120191817161514131211109876543210
EAEIMPLEMENTATION DEFINEDSH1ORGN1IRGN1EPD1A1000T1SZ00SH0ORGN0IRGN0EPD0T2E000T0SZ

EAE, bit [31]

Extended Address Enable. The meanings of the possible values of this bit are:

EAEMeaning
0b1

Use the VMSAv8-32 translation system with the Long-descriptor translation table format.

This field resets to 0.

IMPLEMENTATION DEFINED, bit [30]

IMPLEMENTATION DEFINED.

This field resets to 0.

SH1, bits [29:28]

Shareability attribute for memory associated with translation table walks using TTBR1. Defined values are:

SH1Meaning
0b00

Non-shareable.

0b10

Outer Shareable.

0b11

Inner Shareable.

Other values are reserved. The effect of programming this field to a Reserved value is that behavior is CONSTRAINED UNPREDICTABLE, as described in 'Unallocated values in fields of AArch32 System registers and translation table entries' in the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile, section K1.1.11.

This field resets to 0.

ORGN1, bits [27:26]

Outer cacheability attribute for memory associated with translation table walks using TTBR1.

ORGN1Meaning
0b00

Normal memory, Outer Non-cacheable.

0b01

Normal memory, Outer Write-Back Read-Allocate Write-Allocate Cacheable.

0b10

Normal memory, Outer Write-Through Read-Allocate No Write-Allocate Cacheable.

0b11

Normal memory, Outer Write-Back Read-Allocate No Write-Allocate Cacheable.

This field resets to 0.

IRGN1, bits [25:24]

Inner cacheability attribute for memory associated with translation table walks using TTBR1.

IRGN1Meaning
0b00

Normal memory, Inner Non-cacheable.

0b01

Normal memory, Inner Write-Back Read-Allocate Write-Allocate Cacheable.

0b10

Normal memory, Inner Write-Through Read-Allocate No Write-Allocate Cacheable.

0b11

Normal memory, Inner Write-Back Read-Allocate No Write-Allocate Cacheable.

This field resets to 0.

EPD1, bit [23]

Translation table walk disable for translations using TTBR1. This bit controls whether a translation table walk is performed on a TLB miss, for an address that is translated using TTBR1. The encoding of this bit is:

EPD1Meaning
0b0

Perform translation table walks using TTBR1.

0b1

A TLB miss on an address that is translated using TTBR1 generates a Translation fault. No translation table walk is performed.

This field resets to 0.

A1, bit [22]

Selects whether TTBR0 or TTBR1 defines the ASID. The encoding of this bit is:

A1Meaning
0b0

TTBR0.ASID defines the ASID.

0b1

TTBR1.ASID defines the ASID.

This field resets to 0.

Bits [21:19]

Reserved, RES0.

T1SZ, bits [18:16]

See 'Selecting between TTBR0 and TTBR1, VMSAv8-32 Long-descriptor translation table format' in the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile for how TTBCR.{T1SZ, T0SZ} determine the input address ranges and memory region sizes translated using TTBR0 and TTBR1.

This field resets to 0.

Bits [15:14]

Reserved, RES0.

SH0, bits [13:12]

Shareability attribute for memory associated with translation table walks using TTBR0.

SH0Meaning
0b00

Non-shareable

0b10

Outer Shareable

0b11

Inner Shareable

Other values are reserved. The effect of programming this field to a Reserved value is that behavior is CONSTRAINED UNPREDICTABLE, as described in 'Unallocated values in fields of AArch32 System registers and translation table entries' in the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile, section K1.1.11.

This field resets to 0.

ORGN0, bits [11:10]

Outer cacheability attribute for memory associated with translation table walks using TTBR0.

ORGN0Meaning
0b00

Normal memory, Outer Non-cacheable.

0b01

Normal memory, Outer Write-Back Read-Allocate Write-Allocate Cacheable.

0b10

Normal memory, Outer Write-Through Read-Allocate No Write-Allocate Cacheable.

0b11

Normal memory, Outer Write-Back Read-Allocate No Write-Allocate Cacheable.

This field resets to 0.

IRGN0, bits [9:8]

Inner cacheability attribute for memory associated with translation table walks using TTBR0.

IRGN0Meaning
0b00

Normal memory, Inner Non-cacheable.

0b01

Normal memory, Inner Write-Back Read-Allocate Write-Allocate Cacheable.

0b10

Normal memory, Inner Write-Through Read-Allocate No Write-Allocate Cacheable.

0b11

Normal memory, Inner Write-Back Read-Allocate No Write-Allocate Cacheable.

This field resets to 0.

EPD0, bit [7]

Translation table walk disable for translations using TTBR0. This bit controls whether a translation table walk is performed on a TLB miss, for an address that is translated using TTBR0. The encoding of this bit is:

EPD0Meaning
0b0

Perform translation table walks using TTBR0.

0b1

A TLB miss on an address that is translated using TTBR0 generates a Translation fault. No translation table walk is performed.

This field resets to 0.

T2E, bit [6]

When ARMv8.2-AA32HPD is implemented:

TTBCR2 Enable.

T2EMeaning
0b0

TTBCR2 is disabled. The contents of TTBCR2 are treated as 0 for all purposes other than reading or writing the register.

0b1

TTBCR2 is enabled.

If TTBCR.EAE==0, then the behavior is as if the bit is 0.


Otherwise:

Reserved, RES0.

Bits [5:3]

Reserved, RES0.

T0SZ, bits [2:0]

See 'Selecting between TTBR0 and TTBR1, VMSAv8-32 Long-descriptor translation table format' in the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile for how TTBCR.{T1SZ, T0SZ} determine the input address ranges and memory region sizes translated using TTBR0 and TTBR1.

This field resets to 0.

Accessing the TTBCR

Accesses to this register use the following encodings:

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

opc1opc2CRncoprocCRm
0b0000b0100b00100b11110b0000
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HSTR_EL2.T2 == '1' then
        AArch64.AArch32SystemAccessTrap(EL2, 0x03);
    elsif EL2Enabled() && ELUsingAArch32(EL2) && HSTR.T2 == '1' then
        AArch32.TakeHypTrapException(0x03);
    elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.TRVM == '1' then
        AArch64.AArch32SystemAccessTrap(EL2, 0x03);
    elsif EL2Enabled() && ELUsingAArch32(EL2) && HCR.TRVM == '1' then
        AArch32.TakeHypTrapException(0x03);
    elsif HaveEL(EL3) && ELUsingAArch32(EL3) then
        if SCR.NS == '0' then
            return TTBCR_S;
        else
            return TTBCR_NS;
    else
        return TTBCR;
elsif PSTATE.EL == EL2 then
    if HaveEL(EL3) && ELUsingAArch32(EL3) then
        return TTBCR_NS;
    else
        return TTBCR;
elsif PSTATE.EL == EL3 then
    if SCR.NS == '0' then
        return TTBCR_S;
    else
        return TTBCR_NS;
              

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

opc1opc2CRncoprocCRm
0b0000b0100b00100b11110b0000
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HSTR_EL2.T2 == '1' then
        AArch64.AArch32SystemAccessTrap(EL2, 0x03);
    elsif EL2Enabled() && ELUsingAArch32(EL2) && HSTR.T2 == '1' then
        AArch32.TakeHypTrapException(0x03);
    elsif HaveEL(EL3) && ELUsingAArch32(EL3) && SCR.NS == '0' && CP15SDISABLE == HIGH then
        UNDEFINED;
    elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.TVM == '1' then
        AArch64.AArch32SystemAccessTrap(EL2, 0x03);
    elsif EL2Enabled() && ELUsingAArch32(EL2) && HCR.TVM == '1' then
        AArch32.TakeHypTrapException(0x03);
    elsif HaveEL(EL3) && ELUsingAArch32(EL3) then
        if SCR.NS == '0' then
            TTBCR_S = R[t];
        else
            TTBCR_NS = R[t];
    else
        TTBCR = R[t];
elsif PSTATE.EL == EL2 then
    if HaveEL(EL3) && ELUsingAArch32(EL3) then
        TTBCR_NS = R[t];
    else
        TTBCR = R[t];
elsif PSTATE.EL == EL3 then
    if SCR.NS == '0' && CP15SDISABLE == HIGH then
        UNDEFINED;
    else
        if SCR.NS == '0' then
            TTBCR_S = R[t];
        else
            TTBCR_NS = R[t];
              


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