digital design - AND all elements of an n-bit array in VHDL

Question

lets say I have an n-bit array. I want to AND all elements in the array. Similar to wiring each element to an n-bit AND gate.

How do I achieve this in VHDL?

Note: I am trying to use re-usable VHDL code so I want to avoid hard coding something like

    result <= array(0) and array(1) and array(2)....and array(n); 

Thanks Oshara

Answer 1

Solution 1: With unary operator

VHDL-2008 defines unary operators, like these:

outp <= and "11011";
outp <= xor "11011";
outp <= and inp; --this would be your case

However, they might not be supported yet by your compiler.

Solution 2: With pure combinational (and traditional) code

Because in concurrent code you cannot assign a value to a signal more than once, your can create a temp signal with an "extra" dimension. In your case, the output is one-bit, so the temp signal should be a 1D array, as shown below.

-------------------------------------------
entity unary_AND IS
    generic (N: positive := 8); --array size
    port (
        inp: in bit_vector(N-1 downto 0);
        outp: out bit);
end entity;
-------------------------------------------
architecture unary_AND of unary_AND is
    signal temp: bit_vector(N-1 downto 0);
begin
    temp(0) <= inp(0);
    gen: for i in 1 to N-1 generate
        temp(i) <= temp(i-1) and inp(i);
    end generate; 
    outp <= temp(N-1); 
end architecture;
-------------------------------------------

The inferred circuit is shown in the figure below.

Solution 3: With sequential code

This is simpler than solution 2, though you are now using sequential code to solve a purely combinational problem (but the hardware will be the same). You can either write a code similar to that in solution 2, but with a process and loop (the latter, in place of generate) or using a function. Because in sequential code you are allowed to assign a value to a signal more than once, the temp signal of solution 2 is not needed here.