B1.115 TTBR0 with Long-descriptor translation table format

TTBR0 has a specific format when using the Long-descriptor translation table format. TTBCR.EAE determines which format of the register is in use.

The following figure shows the TTBR0 bit assignments when TTBCR.EAE is 1.

Figure B1-64 TTBR0 bit assignments, TTBCR.EAE is 1
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Reserved, res0.
ASID, [55:48]

An ASID for the translation table base address. The TTBCR.A1 field selects either TTBR0.ASID or TTBR1.ASID.

BADDR[47:x], [47:0]

Translation table base address, bits[47:x]. Bits [x-1:0] are res0.

x is based on the value of TTBCR.T0SZ, and is calculated as follows:

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

The value of x determines the required alignment of the translation table, that must be aligned to 2x bytes.

If bits [x-1:3] are not all zero, this is a misaligned Translation Table Base Address. Its effects are constrained unpredictable, where bits [x-1:0] are treated as if all the bits are zero. The value read back from those bits is the value written.

To access the TTBR0 when TTBCR.EAE==1:

MRRC p15,0,<Rt>,<Rt2>,c2 ; Read 64-bit TTBR0 into Rt (low word) and Rt2 (high word)
MCRR p15,0,<Rt>,<Rt2>,c2 ; Write Rt (low word) and Rt2 (high word) to 64-bit TTBR0

Register access is encoded as follows:

Table B1-95 TTBR0 access encoding

coproc opc1 CRm
1111 0000 0010
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