Emulating a hardware signed multiplier

[Torbjorn Granlund]

Sorry, my previous advice about signed long long was not very clever.

  The hardware multiplier takes two signed 64-bit numbers and produces a
  signed 128-bit number. I'm new to GMP and I'm looking for a way to
  make it interpret numbers the same way the hardware multiplier does.
  
That might be hard.  GMP stores negative numbers not in two's complement
form, but in sign+magnitude form.  But if you generate numbers in the
range -2^63 ... 2^63-1 then GMP and your hardware should agree on the
result since that always fits a 128-bit two's complement number.

Exactly how to program things is something I cannot help with.

-- 
Torbjörn