Xilinx Synthesis Technology User Guide

XST User Guide
If...Else Statement
If... else statements use true/false conditions to execute statements. If
the expression evaluates to true, the first statement is executed. If the
expression evaluates to false (or x or z), the else statement is executed.
A block of multiple statements may be executed using begin and end
keywords. If...else statements may be nested. The following example
shows how a MUX can be described using an If...else statement.
Example 7-2 MUX Description Using If... Else Statement
module mux4 (sel, a, b, c, d, outmux);
input [1:0] sel;
input [1:0] a, b, c, d;
output [1:0] outmux;
reg [1:0] outmux;
always @(sel or a or b or c or d)
begin
if (sel[1])
if (sel[0])
outmux = d;
else
outmux = c;
else
if (sel[0])
outmux = b;
else
outmux = a;
end
endmodule
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.
Note Do not use unsized integers in case statements. Always size
integers to a specific number of bits, or results can be unpredictable.
7-12 Xilinx Development System