WRFFR

Write the first-fault register

Read the source predicate register and place in the first-fault register (FFR). This instruction is intended to restore a saved FFR and is not recommended for general use by applications.

This instruction requires that the source predicate contains a monotonic predicate value, in which starting from bit 0 there are zero or more 1 bits, followed only by 0 bits in any remaining bit positions. If the source is not a monotonic predicate value, then the resulting value in the FFR will be UNPREDICTABLE. It is not possible to generate a non-monotonic value in FFR when using SETFFR followed by first-fault or non-fault loads.

This instruction is illegal when executed in Streaming SVE mode, unless FEAT_SME_FA64 is implemented and enabled.

313029282726252423222120191817161514131211109876543210
00100101001010001001000Pn00000

WRFFR <Pn>.B

if !IsFeatureImplemented(FEAT_SVE) then UNDEFINED; constant integer n = UInt(Pn);

Assembler Symbols

<Pn>

Is the name of the source scalable predicate register, encoded in the "Pn" field.

Operation

CheckNonStreamingSVEEnabled(); constant integer VL = CurrentVL; constant integer PL = VL DIV 8; constant bits(PL) operand = P[n, PL]; constant integer hsb = HighestSetBit(operand); if hsb < 0 || IsOnes(operand<hsb:0>) then FFR[PL] = operand; else // not a monotonic predicate FFR[PL] = bits(PL) UNKNOWN;

Operational information

If FEAT_SVE2 is implemented or FEAT_SME is implemented, then if PSTATE.DIT is 1:


Internal version only: aarchmrs v2024-03_relA, pseudocode v2024-03_rel, sve v2024-03_rel ; Build timestamp: 2024-03-26T09:45

Copyright © 2010-2024 Arm Limited or its affiliates. All rights reserved. This document is Non-Confidential.