Signed Double and Subtract, saturating the result to the signed
range -231 ≤
QDSUB instruction calculates
SAT(. Saturation can
occur on the doubling operation, on the subtraction, or on both.
If saturation occurs on the doubling but not on the subtraction,
the Q flag is set but the final result is unsaturated.
Rn * 2))
All values are treated as two’s complement signed integers by this instruction.