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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

313029282726252423222120191817161514131211109876543210
!= 111100010110RnRd(0)(0)(0)(0)0101Rm
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

15141312111098765432101514131211109876543210
111110101000Rn1111Rd1011Rm

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

<c>

See Standard assembler syntax fields.

<q>

See Standard assembler syntax fields.

<Rd>

Is the general-purpose destination register, encoded in the "Rd" field.

<Rm>

Is the first general-purpose source register, encoded in the "Rm" field.

<Rn>

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';
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