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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.

313029282726252423222120191817161514131211109876543210
00100101001010001001000Pn00000

WRFFR <Pn>.B

if !HaveSVE() then UNDEFINED;
integer n = UInt(Pn);

Assembler Symbols

<Pn>

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

Operation

CheckSVEEnabled();
bits(PL) operand = P[n];

hsb = HighestSetBit(operand);
if hsb < 0 || IsOnes(operand<hsb:0>) then  
    FFR[] = operand;
else // not a monotonic predicate
    FFR[] = bits(PL) UNKNOWN;

Operational information

If PSTATE.DIT is 1:

  • The execution time of this instruction is independent of:
    • The values of the data supplied in any of its registers.
    • The values of the NZCV flags.
  • The response of this instruction to asynchronous exceptions does not vary based on:
    • The values of the data supplied in any of its registers.
    • The values of the NZCV flags.