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No Operation does nothing, other than advance the value of the program counter by 4. This instruction can be used for instruction alignment purposes.
The timing effects of including a NOP instruction in a program are not guaranteed. It can increase execution time, leave it unchanged, or even reduce it. Therefore, NOP instructions are not suitable for timing loops.
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 |
1 | 1 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 |
CRm | op2 |
SystemHintOp op;
case CRm:op2 of
when '0000 000' op = SystemHintOp_NOP;
when '0000 001' op = SystemHintOp_YIELD;
when '0000 010' op = SystemHintOp_WFE;
when '0000 011' op = SystemHintOp_WFI;
when '0000 100' op = SystemHintOp_SEV;
when '0000 101' op = SystemHintOp_SEVL;
when '0000 110'
if !HaveDGHExt() then EndOfInstruction(); // Instruction executes as NOP
op = SystemHintOp_DGH;
when '0000 111' SEE "XPACLRI";
when '0001 xxx'
case op2 of
when '000' SEE "PACIA1716";
when '010' SEE "PACIB1716";
when '100' SEE "AUTIA1716";
when '110' SEE "AUTIB1716";
otherwise EndOfInstruction(); // Instruction executes as NOP
when '0010 000'
if !HaveRASExt() then EndOfInstruction(); // Instruction executes as NOP
op = SystemHintOp_ESB;
when '0010 001'
if !HaveStatisticalProfiling() then EndOfInstruction(); // Instruction executes as NOP
op = SystemHintOp_PSB;
when '0010 010'
if !HaveSelfHostedTrace() then EndOfInstruction(); // Instruction executes as NOP
op = SystemHintOp_TSB;
when '0010 100'
op = SystemHintOp_CSDB;
when '0011 xxx'
case op2 of
when '000' SEE "PACIAZ";
when '001' SEE "PACIASP";
when '010' SEE "PACIBZ";
when '011' SEE "PACIBSP";
when '100' SEE "AUTIAZ";
when '101' SEE "AUTHASP";
when '110' SEE "AUTIBZ";
when '111' SEE "AUTIBSP";
when '0100 xx0'
op = SystemHintOp_BTI;
// Check branch target compatibility between BTI instruction and PSTATE.BTYPE
SetBTypeCompatible(BTypeCompatible_BTI(op2<2:1>));
otherwise EndOfInstruction// Empty.(); // Instruction executes as NOP
// do nothingcase op of
whenSystemHintOp_YIELDHint_Yield();
when SystemHintOp_DGHHint_DGH();
when SystemHintOp_WFE
integer localtimeout = -1; // No local timeout event is generated
Hint_WFE(localtimeout, WFxType_WFE);
when SystemHintOp_WFI
integer localtimeout = -1; // No local timeout event is generated
Hint_WFI(localtimeout, WFxType_WFI);
when SystemHintOp_SEVSendEvent();
when SystemHintOp_SEVLSendEventLocal();
when SystemHintOp_ESB
if HaveTME() && TSTATE.depth > 0 then
FailTransaction(TMFailure_ERR, FALSE);
SynchronizeErrors();
AArch64.ESBOperation();
if PSTATE.EL IN {EL0, EL1} && EL2Enabled() then AArch64.vESBOperation();
TakeUnmaskedSErrorInterrupts();
when SystemHintOp_PSBProfilingSynchronizationBarrier();
when SystemHintOp_TSBTraceSynchronizationBarrier();
when SystemHintOp_CSDBConsumptionOfSpeculativeDataBarrier();
when SystemHintOp_BTISetBTypeNext('00');
otherwise // do nothing
If PSTATE.DIT is 1:
Internal version only: isa v32.13, AdvSIMD v29.05, pseudocode v2020-12, sve v2020-12
; Build timestamp: 2020-12-16T162020-12-16T14:5341
Copyright © 2010-2020 Arm Limited or its affiliates. All rights reserved. This document is Non-Confidential.
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