The conditional statement (or if-else statement) is used to make a decision about whether a statement is
executed. Formally, the syntax is below
if ( expression ) statement_or_null
{ else if ( expression ) statement_or_null }
[ else statement_or_null ]
If the expression evaluates to true (that is, has a nonzero known value), the first statement shall be executed. If it evaluates to false (that is, has a zero value or the value is x or z), the first statement shall not execute. If there is an else statement and expression is false, the else statement shall be executed.
This sequence of if statements (known as an if-else-if construct) is the most general way of writing a
multiway decision. The expressions shall be evaluated in order. If any expression is true, the statement
associated with it shall be executed, and this shall terminate the whole chain. Each statement is either a
single statement or a block of statements.
The last else part of the if-else-if construct handles the none-of-the-above or default case where none of the other conditions were satisfied. Sometimes there is no explicit action for the default. In that case, the trailing else statement can be omitted, or it can be used for error checking to catch an impossible condition.
For example:
The following module fragment uses the if-else statement to test the variable index to decide whether one of three modify_segn regs has to be added to the memory address and which increment is to be added to the index reg. The first ten lines declare the regs and parameters.
// declare regs and parameters
reg [31:0] instruction, segment_area[255:0];
reg [7:0] index;
reg [5:0] modify_seg1,
modify_seg2,
modify_seg3;
parameter
segment1 = 0, inc_seg1 = 1,
segment2 = 20, inc_seg2 = 2,
segment3 = 64, inc_seg3 = 4,
data = 128;
// test the index variable
if (index < segment2) begin
instruction = segment_area [index + modify_seg1];
index = index + inc_seg1;
end
else if (index < segment3) begin
instruction = segment_area [index + modify_seg2];
index = index + inc_seg2;
end
else if (index < data) begin
instruction = segment_area [index + modify_seg3];
index = index + inc_seg3;
end
else
instruction = segment_area [index];