Xilinx Synthesis Technology User Guide

XST User Guide
Case Statement
Case statements perform a comparison to an expression to evaluate
one of a number of parallel branches. The case statement evaluates
the branches in the order they are written; the first branch that
evaluates to true is executed. If none of the branches match, the
default branch is executed. Example 6-13 shows the use of a Case
statement.
Example 6-13 MUX Description Using the Case Statement
library IEEE;
use IEEE.std_logic_1164.all;
entity mux4 is
port (a, b, c, d: in std_logic_vector (7 downto 0);
sel: in std_logic_vector (1 downto 0);
outmux: out std_logic_vector (7 downto 0));
end mux4;
architecture behavior of mux4 is
begin
process (a, b, c, d, sel)
begin
case sel is
when "00" => outmux outmux outmux
outmux