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This coding example declares an ADD function within a package. The ADD function is a single-bit Adder and is called four times to create a 4-bit Adder. The following example uses a function:
Filename: function_package_1.vhd
-- Declaration of a function in a package
--
-- function_package_1.vhd
--
package PKG is
function ADD(A, B, CIN : BIT) return BIT_VECTOR;
end PKG;
package body PKG is
function ADD(A, B, CIN : BIT) return BIT_VECTOR is
variable S, COUT : BIT;
variable RESULT : BIT_VECTOR(1 downto 0);
begin
S := A xor B xor CIN;
COUT := (A and B) or (A and CIN) or (B and CIN);
RESULT := COUT & S;
return RESULT;
end ADD;
end PKG;
use work.PKG.all;
entity function_package_1 is
port(
A, B : in BIT_VECTOR(3 downto 0);
CIN : in BIT;
S : out BIT_VECTOR(3 downto 0);
COUT : out BIT
);
end function_package_1;
architecture ARCHI of function_package_1 is
signal S0, S1, S2, S3 : BIT_VECTOR(1 downto 0);
begin
S0 <= ADD(A(0), B(0), CIN);
S1 <= ADD(A(1), B(1), S0(1));
S2 <= ADD(A(2), B(2), S1(1));
S3 <= ADD(A(3), B(3), S2(1));
S <= S3(0) & S2(0) & S1(0) & S0(0);
COUT <= S3(1);
end ARCHI;