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The Q flag in AArch64 state

In AArch64 state, you cannot read or write to the Q flag because in A64 there are no saturating arithmetic instructions that operate on the general purpose registers.

The Advanced SIMD saturating arithmetic instructions set the QC bit in the floating-point status register (FPSR) to indicate that saturation has occurred. You can identify such instructions by the Q mnemonic modifier, for example SQADD.