Block Statements

Formal Definition

The block statements provide a means of grouping two or more statements in the block.

Simplified Syntax

begin : name

  statement;

  ...

end

fork : name

  statement;

  ...

join

Description

The Verilog HDL contains two types of blocks:

· Sequential (begin-end blocks).

· Parallel (fork-join blocks).

These blocks can be used if more than one statement should be executed.

All statements within sequential blocks (Example 1) are executed in the order in which they are given. If a timing control statement appears within a block, then the next statement will be executed after that delay.

All statements within parallel blocks (Example 2) are executed at the same time. This means that the execution of the next statement will not be delayed even if the previous statement contains a timing control statement.

Examples

Example 1

begin
  a = 1;
  #10 a = 0;
  #5 a = 4;
end

During the simulation, this block will be executed in 15 time units. At time 0, the 'a' variable will be 1, at time 10 the 'a' variable will be 0, and at time 15 (#10 + #5) the 'a' variable will be 4.

Example 2

fork
  a = 1;
  #10 a = 0;
  #5 a = 4;
join

During the simulation this block will be executed in 10 time units. At time 0 the 'a' variable will be 1, at time 5 the 'a' variable will be 4, and at time 10 the 'a' variable will be 0.

Example 3

fork
  a = 1;
  @(b);
  a = 0;
join

During the simulation when this block is executed 'a' becomes 1 and when a change occurs on 'b', then 'a' changes to 0. Flow of the procedural block is suspended when the @(b) statement is reached awaiting a change in value of 'b' before procedure block activities resume.

Important Notes

· In parallel blocks all statements are executed at the same time, so there must not be any mutual dependent assignments.