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HTTBR, Hyp Translation Table Base Register

The HTTBR characteristics are:

Purpose

Holds the base address of the translation table for the initial lookup for stage 1 of an address translation in the EL2 translation regime, and other information for this translation regime.

Configuration

AArch32 System register HTTBR bits [47:1] are architecturally mapped to AArch64 System register TTBR0_EL2[47:1] .

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

RW fields in this register reset to architecturally UNKNOWN values.

Attributes

HTTBR is a 64-bit register.

Field descriptions

The HTTBR bit assignments are:

6362616059585756555453525150494847464544434241403938373635343332
RES0BADDR
BADDRCnP
313029282726252423222120191817161514131211109876543210

Bits [63:48]

Reserved, RES0.

BADDR, bits [47:1]

Translation table base address, bits[47:x], Bits [x-1:1] are RES0, with the additional requirement that if bits[x-1:3] are not all zero, this is a misaligned translation table base address, with effects that are CONSTRAINED UNPREDICTABLE, and must be one of the following:

  • Register bits [x-1:3] are treated as if all the bits are zero. The value read back from these bits is either the value written or zero.
  • The result of the calculation of an address for a translation table walk using this register can be corrupted in those bits that are nonzero.

x is determined from the value of HTCR.T0SZ as follows:

  • If HTCR.T0SZ is 0 or 1, x = 5 - HTCR.T0SZ.
  • If HTCR.T0SZ is greater than 1, x = 14 - HTCR.T0SZ.

If bits[47:40] of the translation table base address are not zero, an Address size fault is generated.

This field resets to an architecturally UNKNOWN value.

CnP, bit [0]

When ARMv8.2-TTCNP is implemented:

Common not Private. This bit indicates whether each entry that is pointed to by HTTBR is a member of a common set that can be used by every PE in the Inner Shareable domain for which the value of HTTBR.CnP is 1.

CnPMeaning
0b0

The translation table entries pointed to by HTTBR are permitted to differ from corresponding entries for HTTBR for other PEs in the Inner Shareable domain. This is not affected by the value of HTTBR.CnP on those other PEs.

0b1

The translation table entries pointed to by HTTBR are the same as the translation table entries pointed to by HTTBR on every other PE in the Inner Shareable domain for which the value of HTTBR.CnP is 1.

Note

If the value of the HTTBR.CnP bit is 1 on multiple PEs in the same Inner Shareable domain and those HTTBRs do not point to the same translation table entries when the other conditions specified for the case when the value of CnP is 1 apply, then the results of translations are CONSTRAINED UNPREDICTABLE, see 'CONSTRAINED UNPREDICTABLE behaviors due to caching of control or data values' in the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile.

This field resets to an architecturally UNKNOWN value.


Otherwise:

Reserved, RES0.

Accessing the HTTBR

Accesses to this register use the following encodings:

MRRC{<c>}{<q>} <coproc>, {#}<opc1>, <Rt>, <Rt2>, <CRm>

coprocCRmopc1
0b11110b00100b0100
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HSTR_EL2.T2 == '1' then
        AArch64.AArch32SystemAccessTrap(EL2, 0x04);
    elsif EL2Enabled() && ELUsingAArch32(EL2) && HSTR.T2 == '1' then
        AArch32.TakeHypTrapException(0x04);
    else
        UNDEFINED;
elsif PSTATE.EL == EL2 then
    return HTTBR;
elsif PSTATE.EL == EL3 then
    if SCR.NS == '0' then
        UNDEFINED;
    else
        return HTTBR;
              

MCRR{<c>}{<q>} <coproc>, {#}<opc1>, <Rt>, <Rt2>, <CRm>

coprocCRmopc1
0b11110b00100b0100
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HSTR_EL2.T2 == '1' then
        AArch64.AArch32SystemAccessTrap(EL2, 0x04);
    elsif EL2Enabled() && ELUsingAArch32(EL2) && HSTR.T2 == '1' then
        AArch32.TakeHypTrapException(0x04);
    else
        UNDEFINED;
elsif PSTATE.EL == EL2 then
    HTTBR = R[t2]:R[t];
elsif PSTATE.EL == EL3 then
    if SCR.NS == '0' then
        UNDEFINED;
    else
        HTTBR = R[t2]:R[t];
              


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