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Saturating instructions

The saturating instructions are:

  • QADD

  • QDADD

  • QDSUB

  • QSUB

  • SSAT

  • USAT.

Some of the parallel instructions are also saturating.

Saturating arithmetic

These operations are saturating (SAT). This means that, for some value of 2n that depends on the instruction:

  • for a signed saturating operation, if the full result would be less than -2n, the result returned is -2n

  • for an unsigned saturating operation, if the full result would be negative, the result returned is zero

  • if the full result would be greater than 2n - 1, the result returned is 2n - 1.

When any of these things occurs, it is called saturation. Some instructions set the Q flag when saturation occurs.

Note

Saturating instructions do not clear the Q flag when saturation does not occur. To clear the Q flag, use an MSR instruction.

The Q flag can also be set by two other instructions, but these instructions do not saturate.

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