Since we require all registers saved on input, read R0 from ENV instead
of passing it manually. Recognize the specification exception when R0
contains incorrect data. Keep high bits of result registers unmodified
when in 31 or 24-bit mode.
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
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