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ID_ISAR1_EL1, AArch32 Instruction Set Attribute Register 1

The ID_ISAR1_EL1 characteristics are:

Purpose

Provides information about the instruction sets implemented by the PE in AArch32 state.

Must be interpreted with ID_ISAR0_EL1, ID_ISAR2_EL1, ID_ISAR3_EL1, ID_ISAR4_EL1, and ID_ISAR5_EL1.

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 D7.1.3.

Configuration

AArch64 System register ID_ISAR1_EL1 bits [31:0] are architecturally mapped to AArch32 System register ID_ISAR1[31:0] .

In an implementation that supports only AArch64 state, this register is UNKNOWN.

Attributes

ID_ISAR1_EL1 is a 64-bit register.

Field descriptions

The ID_ISAR1_EL1 bit assignments are:

6362616059585756555453525150494847464544434241403938373635343332
RES0
JazelleInterworkImmediateIfThenExtendExcept_ARExceptEndian
313029282726252423222120191817161514131211109876543210

Bits [63:32]

Reserved, RES0.

Jazelle, bits [31:28]

Indicates the implemented Jazelle extension instructions. Defined values are:

JazelleMeaning
0b0000

No support for Jazelle.

0b0001

Adds the BXJ instruction and the J bit in the PSR. This setting might indicate a trivial implementation of the Jazelle extension.

All other values are reserved.

In Armv8-A, the only permitted value is 0b0001.

Interwork, bits [27:24]

Indicates the implemented Interworking instructions. Defined values are:

InterworkMeaning
0b0000

None implemented.

0b0001

Adds the BX instruction, and the T bit in the PSR.

0b0010

As for 0b0001, and adds the BLX instruction. PC loads have BX-like behavior.

0b0011

As for 0b0010, and guarantees that data-processing instructions in the A32 instruction set with the PC as the destination and the S bit clear have BX-like behavior.

All other values are reserved.

In Armv8-A, the only permitted value is 0b0011.

Immediate, bits [23:20]

Indicates the implemented data-processing instructions with long immediates. Defined values are:

ImmediateMeaning
0b0000

None implemented.

0b0001

Adds:

  • The MOVT instruction.
  • The MOV instruction encodings with zero-extended 16-bit immediates.
  • The T32 ADD and SUB instruction encodings with zero-extended 12-bit immediates, and the other ADD, ADR, and SUB encodings cross-referenced by the pseudocode for those encodings.

All other values are reserved.

In Armv8-A, the only permitted value is 0b0001.

IfThen, bits [19:16]

Indicates the implemented If-Then instructions in the T32 instruction set. Defined values are:

IfThenMeaning
0b0000

None implemented.

0b0001

Adds the IT instructions, and the IT bits in the PSRs.

All other values are reserved.

In Armv8-A, the only permitted value is 0b0001.

Extend, bits [15:12]

Indicates the implemented Extend instructions. Defined values are:

ExtendMeaning
0b0000

No scalar sign-extend or zero-extend instructions are implemented, where scalar instructions means non-Advanced SIMD instructions.

0b0001

Adds the SXTB, SXTH, UXTB, and UXTH instructions.

0b0010

As for 0b0001, and adds the SXTB16, SXTAB, SXTAB16, SXTAH, UXTB16, UXTAB, UXTAB16, and UXTAH instructions.

All other values are reserved.

In Armv8-A, the only permitted value is 0b0010.

Except_AR, bits [11:8]

Indicates the implemented A and R profile exception-handling instructions. Defined values are:

Except_ARMeaning
0b0000

None implemented.

0b0001

Adds the SRS and RFE instructions, and the A and R profile forms of the CPS instruction.

All other values are reserved.

In Armv8-A, the only permitted value is 0b0001.

Except, bits [7:4]

Indicates the implemented exception-handling instructions in the A32 instruction set. Defined values are:

ExceptMeaning
0b0000

Not implemented. This indicates that the User bank and Exception return forms of the LDM and STM instructions are not implemented.

0b0001

Adds the LDM (exception return), LDM (user registers), and STM (user registers) instruction versions.

All other values are reserved.

In Armv8-A, the only permitted value is 0b0001.

Endian, bits [3:0]

Indicates the implemented Endian instructions. Defined values are:

EndianMeaning
0b0000

None implemented.

0b0001

Adds the SETEND instruction, and the E bit in the PSRs.

All other values are reserved.

In Armv8-A, the permitted values are 0b0000 and 0b0001.

Accessing the ID_ISAR1_EL1

Accesses to this register use the following encodings:

MRS <Xt>, ID_ISAR1_EL1

op0op1CRnCRmop2
0b110b0000b00000b00100b001
if PSTATE.EL == EL0 then
    if IsFeatureImplemented("ARMv8.4-IDST") then
        if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.TGE == '1' then
            AArch64.SystemAccessTrap(EL2, 0x18);
        else
            AArch64.SystemAccessTrap(EL1, 0x18);
    else
        UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.TID3 == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    else
        return ID_ISAR1_EL1;
elsif PSTATE.EL == EL2 then
    return ID_ISAR1_EL1;
elsif PSTATE.EL == EL3 then
    return ID_ISAR1_EL1;
              


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