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ID_ISAR4, Instruction Set Attribute Register 4

The ID_ISAR4 characteristics are:

Purpose

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

Must be interpreted with ID_ISAR0, ID_ISAR1, ID_ISAR2, ID_ISAR3, and ID_ISAR5.

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 G7.1.3.

Configuration

AArch32 System register ID_ISAR4 bits [31:0] are architecturally mapped to AArch64 System register ID_ISAR4_EL1[31:0] .

Attributes

ID_ISAR4 is a 32-bit register.

Field descriptions

The ID_ISAR4 bit assignments are:

313029282726252423222120191817161514131211109876543210
SWP_fracPSR_MSynchPrim_fracBarrierSMCWritebackWithShiftsUnpriv

SWP_frac, bits [31:28]

Indicates support for the memory system locking the bus for SWP or SWPB instructions. Defined values are:

SWP_fracMeaning
0b0000

SWP or SWPB instructions not implemented.

0b0001

SWP or SWPB implemented but only in a uniprocessor context. SWP and SWPB do not guarantee whether memory accesses from other masters can come between the load memory access and the store memory access of the SWP or SWPB.

All other values are reserved. This field is valid only if the ID_ISAR0.Swap_instrs field is 0b0000.

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

PSR_M, bits [27:24]

Indicates the implemented M profile instructions to modify the PSRs. Defined values are:

PSR_MMeaning
0b0000

None implemented.

0b0001

Adds the M profile forms of the CPS, MRS, and MSR instructions.

All other values are reserved.

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

SynchPrim_frac, bits [23:20]

Used in conjunction with ID_ISAR3.SynchPrim to indicate the implemented Synchronization Primitive instructions. Possible values are:

SynchPrim_fracMeaning
0b0000

If SynchPrim == 0b0000, no Synchronization Primitives implemented. If SynchPrim == 0b0001, adds the LDREX and STREX instructions. If SynchPrim == 0b0010, also adds the CLREX, LDREXB, LDREXH, STREXB, STREXH, LDREXD, and STREXD instructions.

0b0011

If SynchPrim == 0b0001, adds the LDREX, STREX, CLREX, LDREXB, LDREXH, STREXB, and STREXH instructions.

All other combinations of SynchPrim and SynchPrim_frac are reserved.

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

Barrier, bits [19:16]

Indicates the implemented Barrier instructions in the A32 and T32 instruction sets. Defined values are:

BarrierMeaning
0b0000

None implemented. Barrier operations are provided only as System instructions in the (coproc==0b1111) encoding space.

0b0001

Adds the DMB, DSB, and ISB barrier instructions.

All other values are reserved.

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

SMC, bits [15:12]

Indicates the implemented SMC instructions. Defined values are:

SMCMeaning
0b0000

None implemented.

0b0001

Adds the SMC instruction.

All other values are reserved.

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

Writeback, bits [11:8]

Indicates the support for Writeback addressing modes. Defined values are:

WritebackMeaning
0b0000

Basic support. Only the LDM, STM, PUSH, POP, SRS, and RFE instructions support writeback addressing modes. These instructions support all of their writeback addressing modes.

0b0001

Adds support for all of the writeback addressing modes.

All other values are reserved.

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

WithShifts, bits [7:4]

Indicates the support for instructions with shifts. Defined values are:

WithShiftsMeaning
0b0000

Nonzero shifts supported only in MOV and shift instructions.

0b0001

Adds support for shifts of loads and stores over the range LSL 0-3.

0b0011

As for 0b0001, and adds support for other constant shift options, both on load/store and other instructions.

0b0100

As for 0b0011, and adds support for register-controlled shift options.

All other values are reserved.

In Armv8-A the only permitted value is 0b0100.

Unpriv, bits [3:0]

Indicates the implemented unprivileged instructions. Defined values are:

UnprivMeaning
0b0000

None implemented. No T variant instructions are implemented.

0b0001

Adds the LDRBT, LDRT, STRBT, and STRT instructions.

0b0010

As for 0b0001, and adds the LDRHT, LDRSBT, LDRSHT, and STRHT instructions.

All other values are reserved.

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

Accessing the ID_ISAR4

Accesses to this register use the following encodings:

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

opc1opc2CRncoprocCRm
0b0000b1000b00000b11110b0010
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HSTR_EL2.T0 == '1' then
        AArch64.AArch32SystemAccessTrap(EL2, 0x03);
    elsif EL2Enabled() && ELUsingAArch32(EL2) && HSTR.T0 == '1' then
        AArch32.TakeHypTrapException(0x03);
    elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.TID3 == '1' then
        AArch64.AArch32SystemAccessTrap(EL2, 0x03);
    elsif EL2Enabled() && ELUsingAArch32(EL2) && HCR.TID3 == '1' then
        AArch32.TakeHypTrapException(0x03);
    else
        return ID_ISAR4;
elsif PSTATE.EL == EL2 then
    return ID_ISAR4;
elsif PSTATE.EL == EL3 then
    return ID_ISAR4;
              


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