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ID_AA64DFR0_EL1, AArch64 Debug Feature Register 0

The ID_AA64DFR0_EL1 characteristics are:

Purpose

Provides top level information about the debug system in AArch64 state.

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

The external register EDDFR gives information from this register.

Attributes

ID_AA64DFR0_EL1 is a 64-bit register.

Field descriptions

The ID_AA64DFR0_EL1 bit assignments are:

6362616059585756555453525150494847464544434241403938373635343332
RES0TraceBufferTraceFilt DoubleLockPMSVer
CTX_CMPsRES0WRPsRES0BRPsPMUVerTraceVerDebugVer
313029282726252423222120191817161514131211109876543210

Bits [63:48]

Reserved, RES0.

TraceBuffer, bits [47:44]

When TRBE is implemented:

Trace Buffer Extension version. Defined values are:

TraceBufferMeaning
0b0000

Trace Buffer Extension not implemented.

0b0001

Trace Buffer Extension implemented.

All other values are reserved.


Otherwise:

Reserved, RES0.

TraceFilt, bits [43:40]

From Armv8.4:

Armv8.4 Self-hosted Trace Extension version. Defined values are:

TraceFiltMeaning
0b0000

Armv8.4 Self-hosted Trace Extension not implemented.

0b0001

Armv8.4 Self-hosted Trace Extension implemented.

All other values are reserved.


Otherwise:

Reserved, RES0.

DoubleLock, bits [39:36]

OS Double Lock implemented. Defined values are:

DoubleLockMeaning
0b0000

OS Double Lock implemented. OSDLR_EL1 is read/write.

0b1111

OS Double Lock not implemented. OSDLR_EL1 is RAZ/WI.

ARMv8.0-DoubleLock implements the functionality added by the value 0b0000.

All other values are reserved.

PMSVer, bits [35:32]

From Armv8.2:

Statistical Profiling Extension version. Defined values are:

PMSVerMeaning
0b0000

Statistical Profiling Extension not implemented.

0b0001

Statistical Profiling Extension implemented.

0b0010

As 0b0001 and also includes support for:

  • The Event packet Alignment flag.
  • If SVE is implemented, the Scalable Vector extensions to Statistical Profiling.

SPE implements the functionality added by the value 0b0001.

ARMv8.3-SPE implements the functionality added by the value 0b0010. If ARMv8.3-SPE is implemented, then ID_AA64DFR0_EL1.PMSVer is not permitted to read as 0b0000 or 0b0001.

All other values are reserved.


Otherwise:

Reserved, RES0.

CTX_CMPs, bits [31:28]

Number of breakpoints that are context-aware, minus 1. These are the highest numbered breakpoints.

Bits [27:24]

Reserved, RES0.

WRPs, bits [23:20]

Number of watchpoints, minus 1. The value of 0b0000 is reserved.

Bits [19:16]

Reserved, RES0.

BRPs, bits [15:12]

Number of breakpoints, minus 1. The value of 0b0000 is reserved.

PMUVer, bits [11:8]

Performance Monitors Extension version.

This field does not follow the standard ID scheme, but uses the Alternative ID scheme described in 'Alternative ID scheme used for the Performance Monitors Extension version' in the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile.

Defined values are:

PMUVerMeaning
0b0000

Performance Monitors Extension not implemented.

0b0001

Performance Monitors Extension implemented, PMUv3.

0b0100

PMUv3 for Armv8.1. As 0b0001, and also includes support for:

0b0101

PMUv3 for Armv8.4. As 0b0100 and also includes support for the PMMIR_EL1 register.

0b0110

PMUv3 for Armv8.5. As 0b0101 and also includes support for:

  • 64-bit event counters.
  • If EL2 is implemented, the MDCR_EL2.HCCD control bit.
  • If EL3 is implemented, the MDCR_EL3.SCCD control bit.
0b1111

IMPLEMENTATION DEFINED form of performance monitors supported, PMUv3 not supported. Arm does not recommend this value in new implementations.

ARMv8.1-PMU implements the functionality added by the value 0b0100.

ARMv8.4-PMU implements the functionality added by the value 0b0101.

ARMv8.5-PMU implements the functionality added by the value 0b0110.

All other values are reserved.

From Armv8.1, the value 0b0001 is not permitted.

From Armv8.4, the value 0b0100 is not permitted.

From Armv8.5, the value 0b0101 is not permitted.

TraceVer, bits [7:4]

Trace support. Indicates whether System register interface to a PE trace unit is implemented. Defined values are:

TraceVerMeaning
0b0000

PE trace unit System registers not implemented.

0b0001

PE trace unit System registers implemented.

All other values are reserved.

A value of 0b0000 only indicates that no System register interface to a PE trace unit is implemented. A PE trace unit might nevertheless be implemented without a System register interface.

See the ETM Architecture Specification for more information.

DebugVer, bits [3:0]

Debug architecture version. Indicates presence of Armv8 debug architecture. Defined values are:

DebugVerMeaning
0b0110

Armv8 debug architecture.

0b0111

Armv8 debug architecture with Virtualization Host Extensions.

0b1000

Armv8.2 debug architecture

0b1001

Armv8.4 debug architecture

All other values are reserved.

ARMv8.2-Debug adds the functionality indicated by the value 0b1000.

  • If ARMv8.1-VHE is not implemented the only permitted value is 0b0110.
  • In an Armv8.0 implementation the value 0b1000 is not permitted.

Accessing the ID_AA64DFR0_EL1

Accesses to this register use the following encodings:

MRS <Xt>, ID_AA64DFR0_EL1

op0op1CRnCRmop2
0b110b0000b00000b01010b000
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.TID3 == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    else
        return ID_AA64DFR0_EL1;
elsif PSTATE.EL == EL2 then
    return ID_AA64DFR0_EL1;
elsif PSTATE.EL == EL3 then
    return ID_AA64DFR0_EL1;
              


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