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ID_PFR0_EL1, AArch32 Processor Feature Register 0

The ID_PFR0_EL1 characteristics are:

Purpose

Gives top-level information about the instruction sets supported by the PE in AArch32 state.

Must be interpreted with ID_PFR1_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 D10.4.1.

Configuration

AArch64 System register ID_PFR0_EL1 bits [31:0] are architecturally mapped to AArch32 System register ID_PFR0[31:0] .

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

Attributes

ID_PFR0_EL1 is a 64-bit register.

Field descriptions

The ID_PFR0_EL1 bit assignments are:

6362616059585756555453525150494847464544434241403938373635343332
RES0
RASDITAMUCSV2State3State2State1State0
313029282726252423222120191817161514131211109876543210

Bits [63:32]

Reserved, RES0.

RAS, bits [31:28]

RAS Extension version.

RASMeaning
0b0000

No RAS Extension.

0b0001

RAS Extension present.

0b0010

ARMv8.4-RAS present. As 0b0001, and adds support for additional ERXMISC<m> System registers.

Error records accessed through System registers conform to RAS System Architecture v1.1, which includes simplifications to ERR<n>STATUS, and support for the optional RAS Timestamp Extension.

All other values are reserved.

From Armv8.4, when ARMv8.4-DFE is not implemented, and ERRIDR.NUM is zero, the permitted values are IMPLEMENTATION DEFINED 0b0001 or 0b0010. Otherwise from Armv8.4 the only permitted value is 0b0010.

ARMv8.4-RAS implements the functionality identified by the value 0b0010.

In Armv8.2, the only permitted value is 0b0001.

In Armv8.1 and Armv8.0, the permitted values are 0b0000 and 0b0001.

DIT, bits [27:24]

From Armv8.4:

Data Independent Timing. Defined values are:

DITMeaning
0b0000

AArch32 does not guarantee constant execution time of any instructions.

0b0001

AArch32 provides the CPSR.DIT mechanism to guarantee constant execution time of certain instructions.

All other values are reserved.

ARMv8.4-DIT implements the functionality identified by the value 0b0001.

From Armv8.4, the only permitted value is 0b0001.


Otherwise:

Reserved, RES0.

AMU, bits [23:20]

From Armv8.4:

Activity Monitors Extension. Defined values are:

AMUMeaning
0b0000

Activity Monitors Extension is not implemented.

0b0001

Activity Monitors Extension Version 1 is implemented.

All other values are reserved.

AMUv1 implements the functionality identified by the value 0b0001.

In Armv8.0, Armv8.1, Armv8.2, and Armv8.3, the only permitted value is 0b0000.

From Armv8.4, the permitted values are 0b0000 and 0b0001.


Otherwise:

Reserved, RES0.

CSV2, bits [19:16]

From Armv8.5:

Speculative use of out of context branch targets. Defined values are:

CSV2Meaning
0b0000

This Device does not disclose whether branch targets trained in one hardware described context can affect speculative execution in a different hardware described context.

0b0001

Branch targets trained in one hardware described context can only affect speculative execution in a different hardware described context in a hard-to-determine way.

From Armv8.5 the only permitted value is 0b0001.

All other values are reserved.


Otherwise:

Reserved, RES0.

State3, bits [15:12]

T32EE instruction set support. Defined values are:

State3Meaning
0b0000

Not implemented.

0b0001

T32EE instruction set implemented.

All other values are reserved.

In Armv8-A the only permitted value is 0b0000.

State2, bits [11:8]

Jazelle extension support. Defined values are:

State2Meaning
0b0000

Not implemented.

0b0001

Jazelle extension implemented, without clearing of JOSCR.CV on exception entry.

0b0010

Jazelle extension implemented, with clearing of JOSCR.CV on exception entry.

All other values are reserved.

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

State1, bits [7:4]

T32 instruction set support. Defined values are:

State1Meaning
0b0000

T32 instruction set not implemented.

0b0001

T32 encodings before the introduction of Thumb-2 technology implemented:

  • All instructions are 16-bit.
  • A BL or BLX is a pair of 16-bit instructions.
  • 32-bit instructions other than BL and BLX cannot be encoded.
0b0011

T32 encodings after the introduction of Thumb-2 technology implemented, for all 16-bit and 32-bit T32 basic instructions.

All other values are reserved.

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

State0, bits [3:0]

A32 instruction set support. Defined values are:

State0Meaning
0b0000

A32 instruction set not implemented.

0b0001

A32 instruction set implemented.

All other values are reserved.

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

Accessing the ID_PFR0_EL1

Accesses to this register use the following encodings:

MRS <Xt>, ID_PFR0_EL1

op0op1CRnCRmop2
0b110b0000b00000b00010b000
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_PFR0_EL1;
elsif PSTATE.EL == EL2 then
    return ID_PFR0_EL1;
elsif PSTATE.EL == EL3 then
    return ID_PFR0_EL1;
              


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