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Saturating instructions

Some ARM and Thumb instructions perform saturating arithmetic.

The saturating instructions are:

  • QADD.

  • QDADD.

  • QDSUB.

  • QSUB.

  • SSAT.

  • USAT.

Some of the parallel instructions are also saturating.

Saturating arithmetic

Saturation means that, for some value of 2 n that depends on the instruction:

  • For a signed saturating operation, if the full result would be less than –2 n , the result returned is –2 n .

  • 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 2 n – 1, the result returned is 2 n – 1.

When any of these 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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