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ARM® Cortex®-A57 MPCore Processor Technical Reference ManualRevision r1p2
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4.3.12 AArch32 Instruction Set Attribute Register 0, EL1

The ID_ISAR0_EL1 characteristics are:
Purpose
Provides information about the instruction set that the processor supports in AArch32.
Usage constraints
The ID_ISAR0_EL1 must be interpreted with:
  • ID_ISAR1_EL1.
  • ID_ISAR2_EL1.
  • ID_ISAR3_EL1.
  • ID_ISAR4_EL1.
  • ID_ISAR5_EL1.
The accessibility to the ID_ISAR0_EL1 by Exception level is:
EL0 EL1(NS) EL1(S) EL2 EL3(SCR.NS = 1) EL3(SCR.NS = 0)
- RO RO RO RO RO
Configurations
The ID_ISAR0_EL1 is:
  • Common to Secure and Non-secure states.
  • Architecturally mapped to the AArch32 ID_ISAR0 register.
Attributes
See the register summary in Table 4-1 AArch64 identification registers.
The following figure shows the ID_ISAR0_EL1 bit assignments.
Figure 4-11 ID_ISAR0_EL1 bit assignments
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The following table shows the ID_ISAR0_EL1 bit assignments.

Table 4-26 ID_ISAR0_EL1 bit assignments

Bits Name Function
[31:28] - Reserved, RES0.
[27:24] Divide
Returns 0x2 to indicate the processor implements the following divide instructions:
  • SDIV and UDIV in the T32 instruction set.
  • SDIV and UDIV in the A32 instruction set.
[23:20] Debug Returns 0x1 to indicate the processor implements the BKPT debug instruction.
[19:16] Coproc Returns 0x0 to indicate the processor implements no coprocessor instructions, except for separately attributed architectures including CP15, CP14, and Advanced SIMD and FP.
[15:12] CmpBranch Returns 0x1 to indicate the processor implements the CBNZ and CBZ, Compare and Branch, instructions in the T32 instruction set.
[11:8] Bitfield Returns 0x1 to indicate the processor implements the BFC, BFI, SBFX, and UBFX, bit field instructions.
[7:4] BitCount Returns 0x1 to indicate the processor implements the CLZ bit counting instruction.
[3:0] Swap Returns 0x0 to indicate the processor implements no swap instructions in the A32 instruction set.
To access the ID_ISAR0_EL1 in AArch64 state, read the register with:
MRS <Xt>, ID_ISAR0_EL1; Read AArch32 Instruction Set Attribute Register 0
To access the ID_ISAR0 in AArch32 state, read the CP15 register with:
MRC p15, 0, <Rt>, c0, c2, 0; Read AArch32 Instruction Set Attribute Register 0
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