CPACR, Architectural Feature Access Control Register

The CPACR characteristics are:

Purpose

Controls access to trace, and to Advanced SIMD and floating-point functionality from EL0, EL1, and EL3.

In an implementation that includes EL2, the CPACR has no effect on instructions executed at EL2.

Configuration

AArch32 System register CPACR bits [31:0] are architecturally mapped to AArch64 System register CPACR_EL1[31:0] .

Bits in the NSACR control Non-secure access to the CPACR fields. See the field descriptions for more information.

Note

In the register field descriptions, controls are described as applying at specified Privilege levels. This is because, in Secure state, a PL1 control:

See 'Security state, Exception levels, and AArch32 execution privilege' in the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile, section G1.7.

Some or all RW fields of this register have defined reset values. These apply only if the PE resets into an Exception level that is using AArch32. Otherwise, RW fields in this register reset to architecturally UNKNOWN values.

Attributes

CPACR is a 32-bit register.

Field descriptions

The CPACR bit assignments are:

313029282726252423222120191817161514131211109876543210
ASEDIS00TRCDIS0000cp11cp1000000000000000000000

ASEDIS, bit [31]

Disables PL0 and PL1 execution of Advanced SIMD instructions.

ASEDISMeaning
0b0

This control permits execution of Advanced SIMD instructions at PL0 and PL1.

0b1

All instruction encodings that are Advanced SIMD instruction encodings, but are not also floating-point instruction encodings, are UNDEFINED at PL0 and PL1.

If the implementation does not include Advanced SIMD and floating-point functionality, this field is RES0. Otherwise, it is IMPLEMENTATION DEFINED whether this field is implemented as a RW field. If it is not implemented as a RW field, it is RAZ/WI.

If EL3 is implemented and is using AArch32, and the value of NSACR.NSASEDIS is 1, this field behaves as RAO/WI in Non-secure state, regardless of its actual value. This applies even if the field is implemented as RAZ/WI.

For the list of instructions affected by this field, see 'Controls of Advanced SIMD operation that do not apply to floating-point operation' in the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile, section E1.

See the description of CPACR.cp10 for a list of other controls that can disable or trap execution of Advanced SIMD instructions in AArch32 state.

This field resets to 0.

Bits [30:29]

Reserved, RES0.

TRCDIS, bit [28]

Traps PL0 and PL1 System register accesses to all implemented trace registers to Undefined mode.

TRCDISMeaning
0b0

This control has no effect on PL0 and PL1 System register accesses to trace registers.

0b1

PL0 and PL1 System register accesses to all implemented trace registers are trapped to Undefined mode.

If the implementation does not include a PE trace unit, or does not include a System register interface to the PE trace unit registers, this field is RES0. Otherwise, it is IMPLEMENTATION DEFINED whether this field is implemented as a RW field. If it is not implemented as a RW field, it is RAZ/WI.

If EL3 is implemented and is using AArch32, and the value of NSACR.NSTRCDIS is 1, this field behaves as RAO/WI in Non-secure state, regardless of its actual value. This applies even if the field is implemented as RAZ/WI.

Note

System register accesses to the trace registers can have side-effects. When a System register access is trapped, any side-effects that are normally associated with the access do not occur before the exception is taken.

This field resets to an architecturally UNKNOWN value.

Bits [27:24]

Reserved, RES0.

cp11, bits [23:22]

The value of this field is ignored. If this field is programmed with a different value to the cp10 field then this field is UNKNOWN on a direct read of the CPACR.

If the implementation does not include Advanced SIMD and floating-point functionality, this field is RES0.

In Non-secure state, if EL3 is implemented and is using AArch32, when the value of NSACR.cp10 is 0, this field behaves as RAZ/WI, regardless of its actual value.

This field resets to 0.

cp10, bits [21:20]

Defines the access rights for the floating-point and Advanced SIMD functionality. Possible values of the field are:

cp10Meaning
0b00

PL0 and PL1 accesses to floating-point and Advanced SIMD registers or instructions are UNDEFINED.

0b01

PL0 accesses to floating-point and Advanced SIMD registers or instructions are UNDEFINED.

0b10

Reserved. The effect of programming this field to this value is CONSTRAINED UNPREDICTABLE. See 'Unallocated values in fields of AArch32 System registers and translation table entries' in the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile, section J1.1.11.

0b11

This control permits full access to the floating-point and Advanced SIMD functionality from PL0 and PL1.

The floating-point and Advanced SIMD features controlled by these fields are:

Note

The CPACR has no effect on floating-point and Advanced SIMD accesses from PL2. These can be disabled by the HCPTR.TCP10 field.

If the implementation does not include Advanced SIMD and floating-point functionality, this field is RES0.

In Non-secure state, if EL3 is implemented and is using AArch32, when the value of NSACR.cp10 is 0, this field behaves as RAZ/WI, regardless of its actual value.

Execution of floating-point and Advanced SIMD instructions in AArch32 state can be disabled or trapped by the following controls:

See the descriptions of the controls for more information.

This field resets to 0.

Bits [19:0]

Reserved, RES0.

Accessing the CPACR

Accesses to this register use the following encodings:

MRC{<c>}{<q>} <coproc>, {#}<opc1>, <Rt>, <CRn>, <CRm>{, {#}<opc2>}

opc1opc2CRncoprocCRm
0b0000b0100b00010b11110b0000

if PSTATE.EL == EL0 then UNDEFINED; elsif PSTATE.EL == EL1 then if EL2Enabled() && !ELUsingAArch32(EL2) && HSTR_EL2.T1 == '1' then AArch64.AArch32SystemAccessTrap(EL2, 0x03); elsif EL2Enabled() && ELUsingAArch32(EL2) && HSTR.T1 == '1' then AArch32.TakeHypTrapException(0x03); elsif EL2Enabled() && !ELUsingAArch32(EL2) && CPTR_EL2.TCPAC == '1' then AArch64.AArch32SystemAccessTrap(EL2, 0x03); elsif EL2Enabled() && ELUsingAArch32(EL2) && HCPTR.TCPAC == '1' then AArch32.TakeHypTrapException(0x03); elsif HaveEL(EL3) && !ELUsingAArch32(EL3) && CPTR_EL3.TCPAC == '1' then AArch64.AArch32SystemAccessTrap(EL3, 0x03); else return CPACR; elsif PSTATE.EL == EL2 then if HaveEL(EL3) && !ELUsingAArch32(EL3) && CPTR_EL3.TCPAC == '1' then AArch64.AArch32SystemAccessTrap(EL3, 0x03); else return CPACR; elsif PSTATE.EL == EL3 then return CPACR;

MCR{<c>}{<q>} <coproc>, {#}<opc1>, <Rt>, <CRn>, <CRm>{, {#}<opc2>}

opc1opc2CRncoprocCRm
0b0000b0100b00010b11110b0000

if PSTATE.EL == EL0 then UNDEFINED; elsif PSTATE.EL == EL1 then if EL2Enabled() && !ELUsingAArch32(EL2) && HSTR_EL2.T1 == '1' then AArch64.AArch32SystemAccessTrap(EL2, 0x03); elsif EL2Enabled() && ELUsingAArch32(EL2) && HSTR.T1 == '1' then AArch32.TakeHypTrapException(0x03); elsif EL2Enabled() && !ELUsingAArch32(EL2) && CPTR_EL2.TCPAC == '1' then AArch64.AArch32SystemAccessTrap(EL2, 0x03); elsif EL2Enabled() && ELUsingAArch32(EL2) && HCPTR.TCPAC == '1' then AArch32.TakeHypTrapException(0x03); elsif HaveEL(EL3) && !ELUsingAArch32(EL3) && CPTR_EL3.TCPAC == '1' then AArch64.AArch32SystemAccessTrap(EL3, 0x03); else CPACR = R[t]; elsif PSTATE.EL == EL2 then if HaveEL(EL3) && !ELUsingAArch32(EL3) && CPTR_EL3.TCPAC == '1' then AArch64.AArch32SystemAccessTrap(EL3, 0x03); else CPACR = R[t]; elsif PSTATE.EL == EL3 then CPACR = R[t];




13/12/2018 16:42; 6379d01c197f1d40720d32d0f84c419c9187c009

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