Extending struct And subtypes
 Expending the existing structs are done to add struct members to a previously defined struct or struct subtype.
 Members added to the base struct type in extensions apply to all other extensions of the same struct.

 Thus, for example, if you extend a method in a base struct with is only, it overrides that method in every one of the like children.

The extension of a struct can be empty, containing no members.
Example

  1 <'

  2 struct structs_units4 { 

  3   addr : byte; 

  4   data : byte;

  5   rd_wr: bool; 

  6 }; 

  7 

  8 // Empty Extension

  9 extend structs_units4 {

 10 

 11 };

 12 

 13 // New elements added

 14 extend structs_units4 {

 15   drive_delay : uint;

 16 };

 17 

 18 // Add new method 

 19 extend structs_units4 {

 20   say_hi() is {

 21     out ("Hello Dude");

 22   };

 23 };

 24 '>

A struct subtype is an instance of the struct in which one of its fields has a particular value. A struct subtype can optionally be specified with extend, so that the extension only applies to that subtype.

  1 <'

  2 struct structs_units5 { 

  3   addr : byte; 

  4   rd_wr: bool; 

  5 

  6   when TRUE'rd_wr structs_units5 {

  7     data : byte;

  8   };

  9 }; 

 10 

 11 // Short Way to extend subtype 

 12 extend TRUE'rd_wr structs_units5 {

 13   write_delay : uint;

 14 };

 15 

 16 // Second natual way to extend subtype 

 17 extend structs_units5 {

 18   when TRUE'rd_wr structs_units5 {

 19     no_writes : byte;

 20     keep no_writes   < 10;

 21     keep write_delay < 10;

 22   };

 23 };

 24 

 25 extend sys {

 26   obj : structs_units5;

 27    // This method shows a sub type can accessed

 28   print_obj () is {

 29     if (obj.rd_wr == TRUE) {

 30       out ("Access Type           is : Write");

 31       outf("Access Address        is : %x\n",

 32          obj.addr);

 33       outf("Access Data           is : %x\n",

 34          obj.as_a(TRUE'rd_wr structs_units5).data);

 35       outf("Access write_delay    is : %x\n",obj.

 36          as_a(TRUE'rd_wr structs_units5).write_delay);

 37       outf("Access no_writes      is : %x\n",obj.

 38          as_a(TRUE'rd_wr structs_units5).no_writes);

 39     } else {

 40       out ("Access Type           is : Read");

 41       outf("Access Address        is : %x\n",obj.addr);

 42     };

 43   }; 

 44   // Just generate the obj and print it

 45   run() is also {

 46     for {var i : int = 0; i < 2; i = i + 1} do {

 47       gen obj;

 48       print_obj(); 

 49     };

 50   };

 51 };

 52 

 53 '>
 Access Type           is : Write

Access Address        is : 74

Access Data           is : 2b

Access write_delay    is : 7

Access no_writes      is : 9

Access Type           is : Read

Access Address        is : e6

Defining Fields
Defines a field to hold data of a specific type. You can specify whether it is a physical field or a virtual field, and whether the field is to be automatically generated. For scalar data types, you can also specify the size of the field in bits or bytes.

Note : You can reference a field before it is declared as long as the declaration of the field is in the same file. In the case of cyclic import, the field may be declared in one of the current set of imported files.

Syntax :[!][%] field-name[: type] [[min-val .. max-val]][((bits | bytes):num)]

Physical Fields

A field defined as a physical field (with the % option) is packed when the struct is packed. Fields that represent data that is to be sent to the HDL device in the simulator or that are to be used for memories in the simulator or in Specman Elite, need to be physical fields. Nonphysical fields are called virtual fields and are not packed automatically when the struct is packed, although they can be packed individually.
If no range is specified, the width of the field is determined by the field's type. For a physical field, if the field's type does not have a known width, you must use the (bits | bytes : num) syntax to specify the width.

Ungenerated Fields
A field defined as ungenerated (with the ! option) is not generated automatically. This is useful for fields that are to be explicitly assigned during the test, or whose values involve computations that cannot be expressed in constraints.
Ungenerated fields get default initial values (0 for scalars, NULL for structs, empty list for lists). An ungenerated field whose value is a range (such as [0..100]) gets the first value in the range. If the field is a struct, it will not be allocated and none of the fields in it will be generated.

Assigning Values to Fields
Unless you define a field as ungenerated, Specman Elite will generate a value for it when the struct is generated, subject to any constraints that exist for the field. However, even for generated fields, you can always assign values in user-defined methods or predefined methods such as init(), pre_generate(), or post_generate(). The ability to assign a value to a field is not affected by either the ! option or generation constraints.

Example Of Fields

  1 <'

  2 struct structs_units6 { 

  3   %addr : byte; 

  4   %data : byte;

  5   %rd_wr: bool; 

  6   rd_wt : uint [0..100];

  7   wr_wt : uint [0..100];

  8    ! drive_delay : uint [0..10];

  9 }; 

 10 

 11 extend sys {

 12   obj : structs_units6;

 13   run() is also {

 14     for {var i : int = 0; i < 2 ; i = i + 1} do {

 15       gen obj;

 16       print obj;

 17     };

 18   };

 19 };

 20 '>
   obj = structs_units6-@0: structs_units6

               ----------------------------------------------         @structs_units6

0              %addr:                          116

1              %data:                          2

2              %rd_wr:                         FALSE

3              rd_wt:                          77

4              wr_wt:                          78

5              !drive_delay:                   0

  obj = structs_units6-@1: structs_units6

               ----------------------------------------------         @structs_units6

0              %addr:                          230

1              %data:                          240

2              %rd_wr:                         TRUE

3              rd_wt:                          58

4              wr_wt:                          26

5          !drive_delay:                   0
drive_delay is always 0, as it not generated.

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