error in simulation of floating point adder

[mostafa.khairy]

Hi all,
i'm trying to design a floating point adder using advantage pro and i simulating it using modelsim
this my code

LIBRARY ieee;
USE ieee.std_logic_1164.all;
USE ieee.std_logic_arith.all;


ENTITY fp_adder IS
-- Declarations
port(a,b: in  std_logic_vector(31 downto 0);
	   c: out std_logic_vector(31 downto 0)
	);
END fp_adder ;

-- hds interface_end
ARCHITECTURE adder OF fp_adder IS
--declaration of sign 
signal sa,sb,sc: std_logic;
--declaration of exponent
signal ea,eb,ec: unsigned(7 downto 0);
--declaration of mantissa
signal ma,mb,mc: unsigned(22 downto 0);
BEGIN
--asignement of sign signals
sa <= a(31);
sb <= b(31);
--assignement of exponent signals
ea <= unsigned(a(30 downto 23));
eb <= unsigned(b(30 downto 23));
--assignement of mantissa signals
ma <= unsigned(a(22 downto 0));
mb <= unsigned(b(22 downto 0));
------------------------------------------------------------------------------------------------------
process(ea,eb,ec,ma,mb,mc,sa,sb,sc)
begin
	if(ea > eb)then loop
	eb <= eb+1;
	mb <= '0'& mb(7 downto 1);
	exit when ea=eb;
	end loop;
	elsif(eb > ea) then loop
	ea <= ea+1;
	ma <= '0'& ma(7 downto 1);
	exit when ea=eb;
	end loop;
	else
	mc<= ma+mb;
	ec<= ea;
	sc<=sa xor sb;
	end if;
end process;
	c(22 downto  0) <= std_logic_vector(mc);
	c(30 downto 23) <= std_logic_vector(ec);
	c(31)		    <= sc;
END adder;

and i found a problem which i couldn't recognize it
there is no error in compiling the code
but at simulation the o/p is UUUUUUUUUUUUUUUUUUUU

and there is some warnings at modelsim command window
this is the error massage

# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 1  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 1  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 1  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 1  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 1  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 1  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 2  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 2  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 2  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 2  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 2  Instance: /fp_adder
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 2  Instance: /fp_adder

can anyone kindly help me to solve the error