11.82 QDSUB

Signed saturating Double and Subtract.

Syntax

QDSUB{cond} {Rd}, Rm, Rn

where:

cond

is an optional condition code.

Rd

is the destination register.

Rm, Rn

are the registers holding the operands.

Operation

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.

Note:

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 ARM instructions but this is deprecated in ARMv6T2 and above. You cannot use SP in Thumb instructions.

Q flag

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

Architectures

This ARM instruction is available in ARMv6 and above, and E variants of ARMv5T.

This 32-bit Thumb instruction is available in ARMv6T2 and above. For the ARMv7-M architecture, it is only available in an ARMv7E-M implementation.

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

Example

 QDSUBLT r9, r0, r1
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