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## QDSUB

Saturating Double and Subtract subtracts a doubled register value from another register value, and writes the result to the destination register. Both the doubling and the subtraction have their results saturated to the 32-bit signed integer range -231 <= x <= 231 - 1. If saturation occurs in either operation, it sets PSTATE.Q to 1.

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 ) .

### A1

 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 != 1111 0 0 0 1 0 1 1 0 Rn Rd (0) (0) (0) (0) 0 1 0 1 Rm cond

#### A1

QDSUB{<c>}{<q>} {<Rd>,} <Rm>, <Rn>

```d = UInt(Rd);  n = UInt(Rn);  m = UInt(Rm);
if d == 15 || n == 15 || m == 15 then UNPREDICTABLE;```

### T1

 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 1 1 1 1 0 1 0 1 0 0 0 Rn 1 1 1 1 Rd 1 0 1 1 Rm

#### T1

QDSUB{<c>}{<q>} {<Rd>,} <Rm>, <Rn>

```d = UInt(Rd);  n = UInt(Rn);  m = UInt(Rm);
if d == 15 || n == 15 || m == 15 then UNPREDICTABLE; // Armv8-A removes UNPREDICTABLE for R13```

For more information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors.

### Assembler Symbols



 Is the general-purpose destination register, encoded in the "Rd" field.
 Is the first general-purpose source register, encoded in the "Rm" field.
 Is the second general-purpose source register, encoded in the "Rn" field.

### Operation

```if ConditionPassed() then
EncodingSpecificOperations();
(doubled, sat1) = SignedSatQ(2 * SInt(R[n]), 32);
(R[d], sat2)  = SignedSatQ(SInt(R[m]) - SInt(doubled), 32);
if sat1 || sat2 then
PSTATE.Q = '1';```