Just as it's helpful to define intermediate variables in a software program, it is often useful to define intermediate signals to represent part of a hardware computation.

Declaring a signal is just like declaring an input or output port, except that it happens at the top of the architecture, and begins with the keyword signal.

architecture synth of adderlogic is
  signal onebit : std_logic;
  signal mybus : std_logic_vector(2 downto 0);
  -- regular architecture

Implement a 3-bit adder without using the addition operator, by writing boolean assignments for each bit. A and B are 3-bit inputs; the result should be a 4-bit output (so there should be no overflow.)

Use intermediate signals for the carry bits.

library IEEE; use IEEE.std_logic_1164.all; entity adderlogic is port( a : in std_logic_vector(2 downto 0); b : in std_logic_vector(2 downto 0); sum : out std_logic_vector(3 downto 0) ); end adderlogic; architecture synth of adderlogic is begin sum <= "0000"; end;

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Compiler/test output: