You are developing a primitive microprocessor with only 4 instructions: The firs

Question

You are developing a primitive microprocessor with only 4 instructions: The first instruction adds two numbers presented at the two inputs. The second instruction multiplies two numbers presented at the two inputs. The third instruction adds the number presented at the first input with the result from the previous instruction. The fourth instruction multiplies the number presented at the first input with the result from the previous instruction. Assume that other mechanisms are present in the other parts of the system that take care of selecting inputs from memory and placing results in memory. See figure. Design a circuit that takes the machine code for the instructions as input and the 'sel' inputs of the two selectors as outputs, with 'sel1' being the left selector controller and 'sel0' being the right selector. In other words, the key to implementing this microprocessor's instruction set is merely making sure the 'sel' lines of the selectors are working correctly. Assume that a '0' input to a 'sel' selects the top input of the selector to connect to the output, and a 'l' input to a 'sel' selects the bottom input. Write out the VHDL for the entity and architecture of your design.

Explanation / Answer

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

entity mux is

    Port ( SEL : in STD_LOGIC_VECTOR (1 downto 0);     -- select input

           IN   : in STD_LOGIC_VECTOR (3 downto 0);     -- inputs

           X   : out STD_LOGIC);                        -- output

end mux;

architecture Behavioral of multiplexer4_1 is
begin

process(i0,i1,i2,i3,sel)
begin
case sel is
  when "00" => bitout <= i0;
  when "01" => bitout <= i1;
  when "10" => bitout <= i2;
  when others => bitout <= i3;
end case;

end process;

end Behavioral;

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