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POP instruction r64 ambiguity

POP r64 should contain REX Prefix. However, nasm doesn't output the prefix for some r64 registers.

Example:

  1. pop rax: quantr = 40 58, nasm = 58
  2. pop r10: quantr = 41 5a, nasm = 41 5a

40, 41 in the above example are the REX prefix.