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13.84 QDSUB

Signed saturating Double and Subtract.


QDSUB{cond} {Rd}, Rm, Rn



is an optional condition code.


is the destination register.

Rm, Rn

are the registers holding the operands.


QDSUB calculates SAT(Rm - SAT(Rn * 2)). It saturates the result to the signed range –231x ≤ 231–1. 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.


All values are treated as two’s complement signed integers by this instruction.

Register restrictions

You cannot use PC for any operand.

You can use SP in A32 instructions but this is deprecated. You cannot use SP in T32 instructions.

Q flag

If saturation occurs, this instruction sets the Q flag. Toread the state of the Q flag, use an MRS instruction.


The 32-bit instruction is available in A32 and T32.

There is no 16-bit version of this instruction in T32.


    QDSUBLT r9, r0, r1
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