Assign all zeroes to unsigned

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Unsigned is a base 10 nuber without a sign. So assigning 0 should work.

unsigned_sig <= 0;

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You can't do that because 0 is an integer. Also note that unsigned is binary, not base 10.

You need to convert to an unsigned from an integer.

Op <= to_unsigned(0, op'length);

This should also work.

Op <= (others => '0');

The ops problem sounds like a syntax error. Why not post the actual code?