base_specifier " [ bit_value ] "
extended_digit { [ underline ] extended_digit }
B means binary, O means octal and X means hexadecimal.
Bit_8: bit_vector(7 downto 0) := B"1111_1111" -- equivalent to "11111111" Bit_8 : bit_vector(7 downto 0) := X"FF" -- equivalent to B"1111_1111" Bit_9 : bit_vector(8 downto 0) := O"777" -- equivalent to B"111_111_111" Bit_12: bit_vector(11 downto 0) := X"777" -- equivalent to B"0111_0111_0111"