In last page we saw how to generate radom values. This random values if total random does not do much good in verification. To control the value range that a variable takes we constrain it. This is what we do in Specman, VERA and SystemVerilog. SystemC also provids ways to constrain the random values generated.

A constraint is derived from the scv_constraint_base class; the data to be randomized is specified as scv_smart_ptr class variable(s). The basic components of an expression can be created from scv_smart_ptr objects through operator()(), which can then be composed into more complicated expressions by using the following operators:

• Arithmetic operators +, -, *
• Relational operators ==, !=, >, >=, <, <=
• Logical operators !, &&, ||

In general, operator() is used to create expressions that can be analyzed or evaluated at the later point. Both the scv_extensions classes and the scv_smart_ptr templates from the data introspection facility implement this operator.

Basic Constrains are constructed with operators show above. SCV provides macros like SCV_CONSTRAINT_CTOR and SCV_CONSTRAINT for constraining a data object. There are two types of constaints.

• Hard Constraints : This is are hard rules, which should be always true when a object is randomized. Hard Constraints can not be bypassed.
• Soft Constraints : This are soft rules, this can be bypassed with Hard Constraints

We can have have multiple contraints and then select the contraint to use. This can be done with using use_constraint() method.

There are other basic constraints types which allow to specify range. You can use keep_only/keep_out only if the element you are constraining is NOT involved in SCV_CONSTRAINTS in any way.

• keep_only : This constraint is used for specifying range in which the random value should be generated.
• keep_out : This constraint is used for specifying range in which the random value should not be generated.

  1 #include <scv.h>
  2 
  3 struct packet_t {
  4   sc_uint<32> src_addr;
  5   sc_uint<32> dest_addr;
  6   sc_uint<16> length;
  7 };
  8 // nbcode "packet" end
  9 
 10 //define an ostream for a packet object
 11 ostream& operator<<(ostream& os, const packet_t& p) {
 12   os << "   src_addr : " << p.src_addr  << "\n"
 13      << "   dest_addr: " << p.dest_addr << "\n"
 14      << "   length   : " << p.length    << endl;
 15   return os;
 16 }
 17 
 18 // Create packet extension
 19 template<>
 20 class scv_extensions<packet_t> : public scv_extensions_base<packet_t> {
 21 public:
 22   scv_extensions<sc_uint<32> > src_addr;
 23   scv_extensions<sc_uint<32> > dest_addr;
 24   scv_extensions<sc_uint<16> > length;
 25 
 26   SCV_EXTENSIONS_CTOR(packet_t) {
 27     //must be in order
 28     SCV_FIELD(src_addr);
 29     SCV_FIELD(dest_addr);
 30     SCV_FIELD(length);
 31   }
 32 };
 33 
 34 //Create a basic default constraint for the packet generator
 35 struct packet_base_constraint : public scv_constraint_base {
 36   //create a packet object
 37   scv_smart_ptr<packet_t> packet;
 38   //put the base constraints on the packet variables
 39   SCV_CONSTRAINT_CTOR(packet_base_constraint) {
 40     // Soft Constraint
 41     SCV_SOFT_CONSTRAINT ( packet->length() < 1500 ); // Max Frame Size 
 42     SCV_SOFT_CONSTRAINT ( packet->length() > 64 );   // Mix Frame Size 
 43     // Hard Constraint
 44     SCV_CONSTRAINT ( packet->src_addr()    ! = packet->dest_addr());
 45     // Hard limit on min frame size
 46     SCV_CONSTRAINT ( packet->length() > 20 );  
 47   }
 48 };
 49 
 50 // Create a actual contraint for the testcase
 51 struct packet_basic_constraint : public packet_base_constraint {
 52   //add config variable
 53   scv_smart_ptr<sc_uint<32> > dest_min;
 54   scv_smart_ptr<sc_uint<32> > dest_max;
 55 
 56   SCV_CONSTRAINT_CTOR(packet_basic_constraint) {
 57     //use the base constraint
 58     SCV_BASE_CONSTRAINT(packet_base_constraint);
 59     //add extra constraints
 60     SCV_CONSTRAINT ((packet->dest_addr() > dest_min()) &&  
 61                    (packet->dest_addr() < dest_max()) );
 62     SCV_CONSTRAINT (
 63       ((packet->src_addr() > (packet->dest_addr() + 0x100000) ) &&   
 64        (packet->src_addr() < (packet->dest_addr() + 0x200000) ) ) ||   
 65       ((packet->src_addr() < (packet->dest_addr() - 0x10000) )) &&   
 66        (packet->src_addr() > (packet->dest_addr() - 0xfffff) ) );   
 67     SCV_CONSTRAINT ( packet->length() == 64 );  
 68   }
 69 };
 70 
 71 int sc_main(int argc, char** argv) {
 72   // Set the Seed to 1
 73   scv_random::set_global_seed(1);
 74   //instatiate test specific constraints
 75   packet_basic_constraint pkt("Constrained Packet");
 76   // Disable randomization 
 77   pkt.dest_min->disable_randomization();
 78   pkt.dest_max->disable_randomization();
 79   // Set the values manually
 80   *pkt.dest_min = 0x100000;
 81   *pkt.dest_max = 0x800000;
 82   for(int i=0; i<5; ++i) {
 83     pkt.next();
 84     cout << pkt.packet->get_name() << *(pkt.packet) << endl;
 85   }
 86   cout << endl;
 87   return (0);
 88 }

    src_addr : 6975369
    dest_addr: 5279859
    length   : 64
 
    src_addr : 4482536
    dest_addr: 3148491
    length   : 64
 
    src_addr : 8796910
    dest_addr: 7006346
    length   : 64
 
    src_addr : 7331724
    dest_addr: 7924165
    length   : 64
 
    src_addr : 5611803
    dest_addr: 3737188
    length   : 64

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Deepak Kumar Tala - All rights reserved

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