QDSUB Saturating Double and Subtract 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. For more information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors. It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 ) . != 1111 0 0 0 1 0 1 1 0 (0) (0) (0) (0) 0 1 0 1 QDSUB{<c>}{<q>} {<Rd>,} <Rm>, <Rn> d = UInt(Rd); n = UInt(Rn); m = UInt(Rm); if d == 15 || n == 15 || m == 15 then UNPREDICTABLE; 1 1 1 1 1 0 1 0 1 0 0 0 1 1 1 1 1 0 1 1 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 <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. if ConditionPassed() then EncodingSpecificOperations(); (doubled, sat1) = SignedSatQ(2 * SInt(R[n]), 32); boolean sat2; (R[d], sat2) = SignedSatQ(SInt(R[m]) - SInt(doubled), 32); if sat1 || sat2 then PSTATE.Q = '1';