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MVFR0_EL1, AArch32 Media and VFP Feature Register 0

The MVFR0_EL1 characteristics are:

Purpose

Describes the features provided by the AArch32 Advanced SIMD and Floating-point implementation.

Must be interpreted with MVFR1_EL1 and MVFR2_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 MVFR0_EL1 bits [31:0] are architecturally mapped to AArch32 System register MVFR0[31:0] .

In an implementation where at least one Exception level supports execution in AArch32 state, but there is no support for Advanced SIMD and floating-point operation, this register is RAZ.

In an AArch64 only implementation, this register is UNKNOWN.

Attributes

MVFR0_EL1 is a 64-bit register.

Field descriptions

The MVFR0_EL1 bit assignments are:

6362616059585756555453525150494847464544434241403938373635343332
00000000000000000000000000000000
FPRoundFPShVecFPSqrtFPDivideFPTrapFPDPFPSPSIMDReg
313029282726252423222120191817161514131211109876543210

Bits [63:32]

Reserved, RES0.

FPRound, bits [31:28]

Floating-Point Rounding modes. Indicates whether the floating-point implementation provides support for rounding modes. Defined values are:

FPRoundMeaning
0b0000

Not implemented, or only Round to Nearest mode supported, except that Round towards Zero mode is supported for VCVT instructions that always use that rounding mode regardless of the FPSCR setting.

0b0001

All rounding modes supported.

All other values are reserved.

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

FPShVec, bits [27:24]

Short Vectors. Indicates whether the floating-point implementation provides support for the use of short vectors. Defined values are:

FPShVecMeaning
0b0000

Short vectors not supported.

0b0001

Short vector operation supported.

All other values are reserved.

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

FPSqrt, bits [23:20]

Square Root. Indicates whether the floating-point implementation provides support for the ARMv6 VFP square root operations. Defined values are:

FPSqrtMeaning
0b0000

Not supported in hardware.

0b0001

Supported.

All other values are reserved.

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

The VSQRT.F32 instruction also requires the single-precision floating-point attribute, bits [7:4], and the VSQRT.F64 instruction also requires the double-precision floating-point attribute, bits [11:8].

FPDivide, bits [19:16]

Indicates whether the floating-point implementation provides support for VFP divide operations. Defined values are:

FPDivideMeaning
0b0000

Not supported in hardware.

0b0001

Supported.

All other values are reserved.

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

The VDIV.F32 instruction also requires the single-precision floating-point attribute, bits [7:4], and the VDIV.F64 instruction also requires the double-precision floating-point attribute, bits [11:8].

FPTrap, bits [15:12]

Floating Point Exception Trapping. Indicates whether the floating-point implementation provides support for exception trapping. Defined values are:

FPTrapMeaning
0b0000

Not supported.

0b0001

Supported.

All other values are reserved.

A value of 0b0001 indicates that, when the corresponding trap is enabled, a floating-point exception generates an exception.

FPDP, bits [11:8]

Double Precision. Indicates whether the floating-point implementation provides support for double-precision operations. Defined values are:

FPDPMeaning
0b0000

Not supported in hardware.

0b0001

Supported, VFPv2.

0b0010

Supported, VFPv3, VFPv4, or Armv8. VFPv3 and Armv8 add an instruction to load a double-precision floating-point constant, and conversions between double-precision and fixed-point values.

All other values are reserved.

In Armv8-A the permitted values are 0b0000 and 0b0010.

A value of 0b0001 or 0b0010 indicates support for all VFP double-precision instructions in the supported version of VFP, except that, in addition to this field being nonzero:

  • VSQRT.F64 is only available if the Square root field is 0b0001.
  • VDIV.F64 is only available if the Divide field is 0b0001.
  • Conversion between double-precision and single-precision is only available if the single-precision field is nonzero.

FPSP, bits [7:4]

Single Precision. Indicates whether the floating-point implementation provides support for single-precision operations. Defined values are:

FPSPMeaning
0b0000

Not supported in hardware.

0b0001

Supported, VFPv2.

0b0010

Supported, VFPv3 or VFPv4. VFPv3 adds an instruction to load a single-precision floating-point constant, and conversions between single-precision and fixed-point values.

All other values are reserved.

In Armv8-A the permitted values are 0b0000 and 0b0010.

A value of 0b0001 or 0b0010 indicates support for all VFP single-precision instructions in the supported version of VFP, except that, in addition to this field being nonzero:

  • VSQRT.F32 is only available if the Square root field is 0b0001.
  • VDIV.F32 is only available if the Divide field is 0b0001.
  • Conversion between double-precision and single-precision is only available if the double-precision field is nonzero.

SIMDReg, bits [3:0]

Advanced SIMD registers. Indicates whether the Advanced SIMD and floating-point implementation provides support for the Advanced SIMD and floating-point register bank. Defined values are:

SIMDRegMeaning
0b0000

The implementation has no Advanced SIMD and floating-point support.

0b0001

The implementation includes floating-point support with 16 x 64-bit registers.

0b0010

The implementation includes Advanced SIMD and floating-point support with 32 x 64-bit registers.

All other values are reserved.

In Armv8-A the permitted values are 0b0000 and 0b0010.

Accessing the MVFR0_EL1

Accesses to this register use the following encodings:

MRS <Xt>, MVFR0_EL1

op0CRnop1op2CRm
0b110b00000b0000b0000b0011
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 MVFR0_EL1;
elsif PSTATE.EL == EL2 then
    return MVFR0_EL1;
elsif PSTATE.EL == EL3 then
    return MVFR0_EL1;
              


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