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GCSCRE0_EL1

GCSCRE0_EL1, Guarded Control Stack Control Register (EL0)

The GCSCRE0_EL1 characteristics are:

Purpose

Controls the Guarded Control Stack at EL0.

Configuration

This register is present only when FEAT_GCS is implemented. Otherwise, direct accesses to GCSCRE0_EL1 are UNDEFINED.

Attributes

GCSCRE0_EL1 is a 64-bit register.

Field descriptions

63	62	61	60	59	58	57	56	55	54	53	52	51	50	49	48	47	46	45	44	43	42	41	40	39	38	37	36	35	34	33	32
31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
RES0
RES0																					nTR	STREn	PUSHMEn	RES0		RVCHKEN	RES0				PCRSEL

Bits [63:11]

Reserved, RES0.

nTR, bit [10]

Trap GCS register accesses from EL0.

nTR	Meaning
0b0	Read accesses to GCSPR_EL0 at EL0 cause a Trap exception.
0b1	This control does not cause any instructions to be trapped.

The reset behavior of this field is:

On a Warm reset, this field resets to '0'.

STREn, bit [9]

Execution of the following instructions are trapped:

GCSSTR.
GCSSTTR.

STREn	Meaning
0b0	Execution of any of the specified instructions at EL0 cause a GCS exception.
0b1	This control does not cause any instructions to be trapped.

The reset behavior of this field is:

On a Warm reset, this field resets to '0'.

PUSHMEn, bit [8]

Trap GCSPUSHM instruction.

PUSHMEn	Meaning
0b0	Execution of a GCSPUSHM instruction at EL0 causes a Trap exception.
0b1	This control does not cause any instructions to be trapped.

The reset behavior of this field is:

On a Warm reset, this field resets to '0'.

Bits [7:6]

Reserved, RES0.

RVCHKEN, bit [5]

Return value check enable.

RVCHKEN	Meaning
0b0	Return value checking disabled at EL0.
0b1	Return value checking enabled at EL0.

The reset behavior of this field is:

On a Warm reset, this field resets to an architecturally UNKNOWN value.

Bits [4:1]

Reserved, RES0.

PCRSEL, bit [0]

Guarded Control Stack procedure call return enable selection.

PCRSEL	Meaning
0b0	Guarded Control Stack at EL0 is not PCR Selected.
0b1	Guarded Control Stack at EL0 is PCR Selected.

The reset behavior of this field is:

On a Warm reset, this field resets to '0'.

Accessing GCSCRE0_EL1

Accesses to this register use the following encodings in the System register encoding space:

MRS <Xt>, GCSCRE0_EL1

op0	op1	CRn	CRm	op2
0b11	0b000	0b0010	0b0101	0b010

if !IsFeatureImplemented(FEAT_GCS) then Undefined(); elsif PSTATE.EL == EL0 then Undefined(); elsif PSTATE.EL == EL1 then if HaveEL(EL3) && EL3SDDUndefPriority() && SCR_EL3().GCSEn == '0' then Undefined(); elsif EL2Enabled() && IsFeatureImplemented(FEAT_FGT) && (!HaveEL(EL3) || SCR_EL3().FGTEn == '1') && HFGRTR_EL2().nGCS_EL0 == '0' then AArch64_SystemAccessTrap(EL2, 0x18); elsif HaveEL(EL3) && SCR_EL3().GCSEn == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x18); end; else X{64}(t) = GCSCRE0_EL1(); end; elsif PSTATE.EL == EL2 then if HaveEL(EL3) && EL3SDDUndefPriority() && SCR_EL3().GCSEn == '0' then Undefined(); elsif HaveEL(EL3) && SCR_EL3().GCSEn == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x18); end; else X{64}(t) = GCSCRE0_EL1(); end; elsif PSTATE.EL == EL3 then X{64}(t) = GCSCRE0_EL1(); end;

MSR GCSCRE0_EL1, <Xt>

op0	op1	CRn	CRm	op2
0b11	0b000	0b0010	0b0101	0b010

if !IsFeatureImplemented(FEAT_GCS) then Undefined(); elsif PSTATE.EL == EL0 then Undefined(); elsif PSTATE.EL == EL1 then if HaveEL(EL3) && EL3SDDUndefPriority() && SCR_EL3().GCSEn == '0' then Undefined(); elsif EL2Enabled() && IsFeatureImplemented(FEAT_FGT) && (!HaveEL(EL3) || SCR_EL3().FGTEn == '1') && HFGWTR_EL2().nGCS_EL0 == '0' then AArch64_SystemAccessTrap(EL2, 0x18); elsif HaveEL(EL3) && SCR_EL3().GCSEn == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x18); end; else GCSCRE0_EL1() = X{64}(t); end; elsif PSTATE.EL == EL2 then if HaveEL(EL3) && EL3SDDUndefPriority() && SCR_EL3().GCSEn == '0' then Undefined(); elsif HaveEL(EL3) && SCR_EL3().GCSEn == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x18); end; else GCSCRE0_EL1() = X{64}(t); end; elsif PSTATE.EL == EL3 then GCSCRE0_EL1() = X{64}(t); end;

2026-03-12 12:23:09, 2025-09_rel_asl1

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