Advantage of using Specman Elite stimulus variation feature

The Specman elite e-language stimulus Variation concept mainly helps in,

· Reusability
· Maintainability
· Extendibility

An Approach to Creating SUBTYPES using e-language:
Consider a Network Packet example having address, length, data as its fields and using the defined fileds need to calculate parity . The defined Network Packet may be either GOOD or BAD.
Whenever the packet is of type GOOD assign the parity calculated to parity field and if the packet is of type BAD then do not assign the parity to the parity field.
In order to achieve the required condition, we need to create a subtype using when inheritance.

Following are the steps to be taken while creating subtypes.
· Define a new enumerated type (i.e. Good and Bad)
· Define a field on enumerated type in the struct to represent various subtypes.
· Write a when block and add struct members in the when block that correspond to properties of the specific subtypes.

filename : packet.e
<’
---- Declaration of enumerated types
type packet_kind_t: [Good, Bad];
struct packet_s {
pkt_Kind: packet_kind_t;
Address : uint (bits: 2);
Length : uint (bits: 6);
Data : list of byte;
Keep data. size () = = length;
Parity : byte;
---- User Method for calculating parity Value
Parity_calc () : byte is {
----- User Actions Needs to be entered for calculating parity;
};
--- Subtype creation
When Good ‘pkt_kind packet_s {
Keep parity = = parity_calc ();
};
When Bad ‘pkt_kind packet_s {
Keep parity! = Parity_calc ();
};
};
‘> ---- End of struct

filename : packet_top.e

<'
import packet.e;
extend sys {
packet : packet_s ;
};
'>

filename : packet_test.e
<'
import packet_top.e;
// For Writing Test cases using constraint use the extend feature supported by e-language
extend packet_s {
// Apply the Constraints Required
keep soft pkt_kind == GOOD;
keep soft length == 10;
keep soft Address in [0x1000..0x2000];
};
'>

Specman elite example for creating subtype packet structure

The below example demonstrates how to generate WR only packet using specman elite e-language without applying the constraints to the enumerated fields declared.

< '
type packet_wr_rd_t : [ WR, RD];
struct packet_s {
packet_wr_rd : packet_wr_rd_t ; // Declaration of the field
Address : byte ; // Common field for both RD / WR
keep soft Address == 0x10;// Apply the Subtype to control the generation
when WR' packet_s {
wr_data : byte ; // default value, can be overwritten using testcase
keep soft wr_data == 0x0 ;
}; // end of WR subtype creation.
}; // end of struct definition
'>

filename : packet_top.e
<'
extend sys {
// The below assignment make sure that only WR related Packet is generated, which is indirect
// way of applying the constraints
pkt : WR packet_s;
};
'>

specman elite e-language supports subtype stimulus creation

Specman elite e-language supports the formation of subtype stimulus creation.
In order to achieve we need to use the enumerated define types.
e.g If we want to control the generation of fields w.r.t subtypes, it is better to use enumerated types as a controlling parameters.
Let's have requirements in mind for the Read / Write scenario. We all Know that Address field is common for both Read / Write Transcation, whereas we need to generated write data when we are performing the Write Transcation.
Here is how we control the generation using the specman e-language subtype declaration.
The following example demonstrates , if the enumerate type declared is WR in nature it generates Write Address and Write Data else it only performs Read Transcation.
< '
type packet_wr_rd_t : [ WR, RD];
struct packet_s {
packet_wr_rd : packet_wr_rd_t ; // Declaration of the field
keep soft packet_wr_rd == WR;
Address : byte ; // Common field for both RD / WR
keep soft Address == 0x10;
// Apply the Subtype to control the generation
when WR' packet_s {
wr_data : byte ;
// default value, can be overwritten using testcase
keep soft wr_data == 0x0 ;
}; // end of WR subtype creation.
}; // end of struct definition

'>

specman e-language constraint usage example

Adding a small example for using the constraint feature with the help of specman elite e-language.

file name : packet_def.e
<'
packet_kind_t : [ small, medium, large]

'>
file name : packet.e
<’
struct packet_s {
Addr: uint (bits: 32);
Kind: packet_kind_t;
my_method () is {
// define the actions required e.g parity_calculation
};

// call the above mentioned method in run phase

run() is also {

my_method();

};

}; // end of struct def

'>

file name : packet_top.e

<'

import packet_def.e;

import packet.e;

extend sys {

pkt : packet_s ;

};

'>

// Writing the Top Test case for applying constraints

file name : packet_test.e

<'

import packet_top.e;

extend packet_s {

keep soft Kind = = Small ;

keep soft addr in [10..40];
};
‘>

specman elite possible ways of generating fields

Specman Elite e-language supports three possible ways of generating the fields.

· Random generation

· Directed-Random generation

· Directed generation

Random generation:

The Random generation is not supported by constraints.

Ex: address: int (bits: 32);

Directed-Random generation:

The Directed-Random generation supports the generation by constraining to a range of values. Here, even though the generation is random it is been restricted within the Range of 1000 to 2000 address locations.

Ex: keep address in [0x1000..0x2000];

Directed generation:

The Directed generation supports the generation by constraining to a specific value.Here, the generation of address value is specified to the location of 1245.

Ex: keep address = = 0x1245;


Soft Constraints

For constraints that might need to be overridden, we use soft constraints. Basically, Soft constraints are obeyed if not contradicted by hard constraints.
The last loaded soft constraint prevails if there is a contradiction with other soft constraints.

Syntax :

keep soft Boolean-expression;

Ex: keep soft length = = 64;

· Soft constraints are used to define the default range of values of fields:

Ex: keep soft packet_length in [60..100];

· The test writer has the option to ignore the soft constraint by using predefined method reset_soft().

Ex: keep packet_length.reset_soft ();

· Soft constraints are used to set initial settings for tests

Ex: keep soft errors = = FALSE;

Controlling order of generation using specman e-language

Specman elite e-language generates the fields based on the Order of declaration.
Consider an example, which illustrates the order of generation.

Ex: <’

type packet_kind_t: [Small, Medium, Large];

struct packet_s {

Kind: packet_kind_t;

Address: uint (bits: 32);

keep Kind = = Small => Address < 40; };

‘>

The above example explains that the packet_kind_t (kind) of enumerated type is generated first followed by address.The constraints definition says if the kind is Small in nature then generate address below 40.

Suppose, unknowingly if the address is declared first followed by Kind then the above constraint statement doesn’t hold good.

In order to achieve the same result, add an explicit generation order constraint supported by Specman Elite e-language.

The below example shows the constraints for such Operation.

Syntax:

keep soft gen (control- field) before (control –field);

Ex: <’

type packet_kind_t: [Small, Medium, Large];

struct packet_s {

Address: uint (bits: 32);

Kind: packet_kind_t;

keep Kind = = Small => Addr < 40;

keep soft gen (Kind) before (Address); };

‘>

specman e-language supports constraint random generation

specman e-language supports the concept of constrained random generation.

. Constraints are applied on struct members e.g Fields and methods.

· Constraints are Boolean equations

· Constraints are declarative statements

Specman e-language categories the constrained mechansim as follows :

1) Random Generation without any constraints parameters
2) Directed Random constrained Generation [ e.g from .. to ]
3) Directed constrained Generation

For constraining a Field use the following syntax :
keep Boolean-expression

Ex: struct packet_s {

Length : uint(bits:6);

Address : uint(bits:2);

// application of Directed Constraints Parameter for Length field

keep Length = = 10;

// application of Directed Random Constraints Parameter for Address field

keep Address in [0..2];
};




For constraining elements in a list (array):

keep for each (item) in list_name {

Boolean-expression; };


Ex: Struct packet_driver_s {

Packets : list of packet_s;

// Constraints list for the individual fields

keep for each (pkt) in packets {

pkt.len < 10; }; };



The syntax for implication constraints are given below:

keep Boolean-expr1 => Boolean-expr2;

Ex:

keep size = = SHORT => length < 10;

keep size = = LONG => length > 20;

Or, suppose if we want to constraint using lists:

Ex:

keep for each (pkt) in packet {

index = = 0 => pkt.addr = = 1 and pkt.kind = = GOOD;

index = = 1 => pkt.addr = = 2 and pkt.kind = = BAD; };


Weighted Constraints:

Weighted constraints are used on a specific application. It allows selection weight for value or range of values. In order to achieve a weighted constraint we need to apply a soft control on the distribution of generated values.

syntax: keep soft gen-item = = select {weight: value ;};

Ex: <’

keep soft length = = select {

20:14;

25:10;

10:20; };

‘>