Notes

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 o Even though the documentation is clear (1970, 1973, and 1982 manuals) that shifts and
   rotates move modulo 2^8, that is no more than 255, the diagnostics make it clear that a
   rotate of -256 should move 256 positions to the right.

 o SOS and SOJ do not subtract one, they add -1.  This makes the arithmetic flags work out
   right.

 o The documentation differs between the manuals but it appears that none of them are
   correct with regards to detecting overflow in the DIV instruction.  I don't know yet
   what the correct algorithm is.
 
   The best I have is this.  Use the description in the 1983 manual to detect overflow
   before you start but then at the end, check that the sign bits for the dividend,
   divisor, and quotient all match.  If not, signal overflow except if the quotient is 0.
   This doesn't pass the diagnostics but I think it's correct.