Release notes for the A64 Instruction Set Architecture for Armv8.7-A 2020-12

Product status

The information in this release covers multiple versions of the architecture. The content relating to different versions is given different quality ratings.

The information in this release relating to v8.7 of the architecture and the features introduced in this release is at Alpha quality. Alpha quality means that most major features of the specification are included, features and details might be missing.

The information in this release relating to versions of the architecture before v8.7 and features introduced in previous releases is at Beta quality. Beta quality means that all major features of the specification are included, some details might be missing.

Change history

The following general changes are made:

  • The following features are added to the MPAM system architecture: Error MSI, Monitor Overflow MSI, and Monitor Overflow Status.
  • The optimized pseudocode for the double-precision variants of the FADDP, FMAXNMP, FMAXP, FMINNMP, and FMINP instructions is corrected to add missing checks.
  • The FEAT_MTE feature is split into FEAT_MTE, Instruction only, and FEAT_MTE2, Full MTE, features.
  • The STGP operational pseudocode is updated to check access is aligned to a tag granule.
  • The execute code for CSEL, CSINC, CSINV, CSNEG, CCMP, and CCMN is updated to create a dependency only via the register which is selected, not both of its input registers.
  • The ST64BV0 operational pseudocode is updated to correctly store the content of ACCDATA_EL1 in case of a Big Endian access.

The following changes are made to the instruction definitions:

  • None.

The following changes are made to the Shared Pseudocode:

  • Removed unused function ReservedValue().
  • The decoding of Load/Store Multiple instructions inside FirstStageTranslation is replaced by a new memory accessor type AccType_ATOMICSTREAM.
  • Removed unused function UnallocatedEncoding().
  • Permission checks for Data/Instruction cache maintenance instructions corrected.
  • The pseudocode function AArch64.ExceptionReturn() moves the call to SynchroniseContext() following the checks for error synchronization.
  • The pseudocode functions AArch64.SecondStageTranslate() and AArch32.SecondStageTranslate() are updated to not generate an alignment fault to a memory location marked as Device for stage 2 for stage 1 walks.
  • The pseudocode function AArch64.SecondStageTranslate() and AArch64.SecondStageTranslate() is updated to not generate an alignment fault to a memory location marked as Device for stage 2 for stage 1 walks.
  • The pseudocode function ProfilingBufferOwner() is updated to account for Secure EL2.
  • The pseudocode function AArch64.WatchpointMatch() is updated to check for check for the atomic Read-Write memory access with the DBGWCR.LSC configuration.
  • The pseudocode function CollectTimeStamp() is adjusted to include the missing reserved value checks.
  • The pseudocode function CombineS1S2AttrHints() is corrected to take into account Device memory when S2FWB is enabled.
  • The pseudocode function AArch64.SoftwareStepException() is updated to include the IFSC of Debug Exception.
  • The pseudocode functions AArch64.CheckPermission() and AArch32.CheckPermission() are updated to initialise priv_xn and user_xn prior to PAN checks that re determines the value of priv_w.
  • The STGP pre-index and post-index decode pseudocode is updated to check for the presence of FEAT_MTE.
  • The pseudocode function TraceTimeStamp() is updated to not treat the value '10' for TRFCR_EL1.TS and TRFCR_EL2.TS as UNPREDICTABLE in all cases.
  • In pseudocode function AArch64.CheckWatchpoint() WnR bit to be set as 'read' in Atomic read-write operations if DBGWCR.LSC has a match on read.
  • The FEAT_MTE feature has been split into FEAT_MTE, Instruction only, and FEAT_MTE2, Full MTE. The pseudocode is updated accordingly for A64 base instructions and functions to use HaveMTE2Ext() instead of HaveMTEExt().
  • The pseudocode function IsSErrorEdgeTriggered() takes a new parameter indicating the target Exception level. This function now checks the Execution state of the target Exception level to determine the correct syndrome checks. In AArch64 state syndrome<24> is now checked to establish if the SError is IMPLEMENTATION DEFINED.
  • The decode pseudocode for SUBP(S) and STG pre-index/post-index MTE instructions is updated to check for the presence of FEAT_MTE.
  • The pseudocode functions AArch64.FirstStageTranslate() and AArch64.SecondStageTranslate() are updated with a check for data abort when the memory attribute of the data location accessed is not one of Normal Inner Non-cacheable Outer Non-cacheable, Device-GRE, Device-nGRE, Device-nGnRE, Device-nGnRnE.
  • The access type AccType_DC_UNPRIV is now qualified with PSTATE.EL and AccType_DC. DC_UNPRIV usage is removed from the pseudocode.
  • The pseudocode function AArch64.WFxTrap() now correctly updates the Instruction Specific Syndrome if execution of WFET or WFIT is trapped.
  • The pseudocode function AArch64.FaultSyndrome() now correctly sets the Load/Store Type field within the ISS when an LD64B, ST64B, ST64BV, or ST64BV0 instruction generates a Data Abort for a Translation fault, Access flag fault, or Permission fault. AArch64.ExclusiveFault() takes two additional boolean parameters: secondstage and s1fs1walk.
  • Enumeration literal Unpredictable_PMUEventCount is added for Constrained Unpredictable access to Performance Monitor Event Count register access.
  • Enumeration literal Unpredictable_IGNORETRAPINDEBUG is added for register access trap ignore in Debug state.
  • The pseudocode functions CheckLDST64BEnabled(), CheckST64BVEnabled(), and CheckST64BV0Enabled() now check if the bits in HCRX_EL2 behave as 0 for indirect reads.
  • The pseudocode for FEAT_XS is partially included in this release, including the addition of the pseudocode for the Arch64 DSBnXS instruction and the updates related to FEAT_XS in the DSB instructions.

Known issues

  • Clarification required to MRS/MSR instruction behavior with the SYS instruction following relaxation.
  • The description of DSB encoding when FEAT_SSBS implemented will be clarified by making SSBB and PSSBB architectural aliases of DSB, and point to SSBB when CRm is 0b0000, and PSSBB when CRm is 0b0100.
  • The pseudocode for FEAT_LPA2 is not available in this release.
  • The pseudocode for FEAT_XS is not available in full. Effect of FEAT_XS on memory attributes and on A32 TLBI instructions is not present.
  • In the browsable XML, hyperlinking with some pseudocode functions do not link correctly.
  • Some architectural features have limited or no descriptions in pseudocode and are not fully covered by the functional testing. Affected features are listed below:
    • Address translation system instructions.
    • Ordering of memory accesses.
    • Self-hosted debug and behavior of PE when in Debug state.
    • Trace architecture, including self-hosted trace and external trace.
    • RAS architecture.
    • Statistical Profiling Extension.
    • Performance Monitors Extension.
    • Activity Monitors Extension.
    • Generic Timers.
    • Generic Interrupt Controller.
    • Multi-processing.
  • The LDP, LDNP and STP instruction pseudocode does not show single-copy atomicity for 16 byte accesses, if the overall memory access is aligned to 16 bytes, or for access fewer than 16 bytes, where all accessed bytes are within a single 16-byte quantity aligned to 16 bytes, to Normal Inner-WB, Outer-WB Cacheable memory.

Potential upcoming changes

  • The Shared pseudocode for VMSA will be refactored.
  • The pseudocode for FEAT_XS and FEAT_LPA2 will be added.
  • The pseudocode for system instructions will be added.