SystemVerilog Associative Arrays

In previous post we learn in detail about SystemVerilog Dynamic arrays which is useful for dealing with contiguous collections of variables whose number changes dynamically. Now consider if the size of array is unknown then how much size will you allocate to array?

An Associative array is one to use when the size of the collection is unknown or the data space is sparse. So the associative arrays are mainly used to model the sparse memories. In the associative arrays the storage is allocated only when we use it not initially like in dynamic arrays. Associative arrays can be assigned only to another Associative array of a compatible type and with the same index type.

Another main difference between Associative array and normal arrays is in that in assoc arrays the arrays index can be any scalar value. 

Properties of Associative arrays:
  • Dynamically allocated, non-contiguous elements
  • Accessed with integer, or string index, single dimension
  • Great for sparse arrays with wide ranging index
  • Array functions: exists, first, last, next, prev
Declaration Syntax:

data_type array_name [ index_type ];

Where,
data_type : data type of the array element. This can be any type that is allowed for Fixed Arrays
array_name : name of the array being declared.
index_type : data type to be used as index

Example : Associative array declaration

int array_name[*];//Wildcard index. can be indexed by any integral datatype.
int array_name [string];// String index
int array_name [some_Class];// Class index
int array_name [integer];// Integer index
typedef bit signed [4:1] Nibble;
int array_name [Nibble]; // Signed packed array

Accessing the Associative arrays
SystemVerilog provides various in-built methods to access, analyze and manipulate the associative arrays.
  • num() or size() returns the number of entries in the associative arrays.
  • delete() removes the entry from specified index.
  • exist() checks weather an element exists at specified index of the given associative array.
  • first() assigns to the given index variable the value of the smallest/first index in the associative array. Returns 0 if array is empty; else returns 1.
  • last() assigns to the given index variable the value of the largest/last index in the associative array. Returns 0 if array is empty; else returns 1.
  • next() finds the entry whose index is greater than the given index. If next entry exists then the index variable is assigned to the index of next entry and returns 1. Otherwise the index is unchanged and the function returns 0.
  • previous() finds the entry whose index is smaller than the given index. If previous entry exists then the index variable is assigned to the index of previous entry and returns 1. Otherwise the index is unchanged and the function returns 0.

Example : Associative Arrays in-built methods

// SystemVerilog Associative arrays
module associative_array ();
 
 integer associative_array [integer];
 
 integer i;
 
 initial begin
   // Add element array
   associative_array[100] = 101;
   $display ("value stored in 100 is %d", associative_array[100]);
   associative_array[1]   = 100;
   $display ("value stored in 1   is %d", associative_array[1]);
   associative_array[50]   = 99;
   $display ("value stored in 50  is %d", associative_array[50]);
   associative_array[250] = 22;
   $display ("value stored in 250 is %d", associative_array[250]);
   // Print the size of array
   $display ("size of array is %d", associative_array.num());
   // Check if index 2 exists
   $display ("index 2 exists   %d", associative_array.exists(2));
   // Check if index 100 exists
   $display ("index 100 exists %d", associative_array.exists(100));
   // Value stored in first index
   if (associative_array.first(i)) begin
     $display ("value at first index %d value %d", i, associative_array[i]);
   end
   // Value stored in last index
   if (associative_array.last(i)) begin
     $display ("value at last index  %d value %d", i,  associative_array[i]);
   end
   // Delete the first index
   associative_array.delete(100);
   $display ("Deleted index 100");
   // Value stored in first index
   if (associative_array.first(i)) begin
     $display ("value at first index %d value %d", i, associative_array[i]);
   end
    #1  $finish;
 end
 
 endmodule

Simulation Result:
value stored in 100 is 101
value stored in 1 is 100
value stored in 50 is 99
value stored in 250 is 22
size of array is 4
index 2 exists 0
index 100 exists 1
value at first index 1 value 100
value at last index 250 value 22
Deleted index 100
value at first index 1 value 100

Try simulation yourself here

Now as you know that we can use any data type as index of associative arrays, below are the things to keep in mind while using different index datatypes.

1. Integer or int index : While using integer in associative arrays, following rules need to be kept in mind.

  • A 4-state index containing X or Z is invalid.
  • Indices smaller than integer are sign extended to 32 bits.
  • The ordering is signed numerical.
  • Indices can be any integral expression.
  • Indices are signed.
  • Example: int array_name [ integer ];

2. String index : While using string in associative arrays, following rules need to be kept in mind.

  • An empty string "" index is valid.
  • The ordering is lexicographical (lesser to greater).
  • Indices can be strings or string literals of any length.
  • Example: int array_name [ string ];

3. Class index : While using class in associative arrays, following rules need to be kept in mind.
  • A null index is valid.
  • The ordering is deterministic but arbitrary.
  • Indices can be objects of that particular type or derived from that type.
  • Example: int array_name [ some_Class ];

4. Wild Character index : While using wild characters in associative arrays, following rules need to be kept in mind.
  • A 4-state Index containing X or Z is invalid.
  • Indices are unsigned.
  • Indexing expressions are self-determined; signed indices are not sign extended.
  • The ordering is numerical (smallest to largest).
  • A string literal index is auto-cast to a bit-vector of equivalent size.
  • The array can be indexed by any integral data type.
  • Example: int array_name [*];

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