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Constraint blocks are class members just like variables, functions and tasks and have unique names within a class. These blocks of expressions are typically used to limit the values of random variables within certain values as specified within the constraint block.

Syntax

 
  constraint  [name_of_constraint] {  [expression 1];
                                      [expression N]; }
 

Expressions listed within the curly braces specify the conditions a solver has to take into account when assigning random values for the variables. It is not necessary to have a single constraint for every variable nor is it required to restrict a constraint block to have conditions pertaining only to a single variable. However you cannot have conflicting constraints spread out in multiple blocks, unless they are turned off using constraint_mode() method which we'll see later.

 
  constraint  valid_addr {    addr [1:0] == 2'b0;
                              addr <= 32'hfaceface;
                              addr >= 32'hf0000000; }
 
  constraint  fast_burst {    burst >= 3;
                              len   >= 64;
                              size  >= 128; }
 
  // Error - valid_addr already declared
  constraint valid_addr { ... }
 
  // Runtime error - solver fails due to conflict on addr
  constraint valid { addr >= 32'hfaceffff; }
 
  // Valid because solver can find an address that satisfies all conditions
  // eg :-  f200_0000 is below f400_0000 and face_face; and above f000_0000
  constraint valid2 { addr <= 32'hf4000000; }
 

There are some restraints on these blocks because of their declarative nature. Note that constraint blocks are enclosed within curly braces { } instead of the begin end keywords of procedural statements.

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