Ok, so the most flexible solution that I see, that should fit for both
i386 and x86_64 would be :
1 byte : "=Q" : Any register accessible as rh: a, b, c, and d.
2, 4 bytes : "=R" : Legacy registerâthe eight integer registers available
on all i386 processors (a, b, c, d, si, di, bp, sp). 8 bytes : (only for x86_64)
"=r" : A register operand is allowed provided that it is in a
general register.
That should make sure x86_64 won't try to use REX prefixed opcodes for
1, 2 and 4 bytes values.
Does it make sense ?