Signal assignments - tomas-fryza/vhdl-labs GitHub Wiki

Outside process

The following two assignments can only be written outside a process. Assignment With-Select is so called selected signal assignment:

with addr_i select
    y_o <= a_i when "000",  -- If addr_i = "000" then y_o = a_i
           b_i when "001",
           c_i when "010",
           d_i when others; -- All other combinations

Assignment When-Else is called conditional signal assignment:

y_o <= a_i when (addr_i = "000" and en_i = '1') else
       b_i when (addr_i = "001" and en_i = '1') else
       c_i when (addr_i = "010" and en_i = '1') else
       d_i;                 -- All other combinations

Inside process

The following two assignments can only be written inside a process. Assignment If-Then-Else:

p_label : process (a)
begin
    if (a = '0') then
        q <= '0';
    else
        q <= '1';
    end if;
end process p_label;

Assignment Case-When:

p_label : process (a)
begin
    case a is
        when '0' =>
            q <= '0';
        when others =>
            q <= '1';
    end case;
end process p_label;