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SystemVerilog Tutorial PART V: by Abhiram Rao

Arrays and Queues in SystemVerilog

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5.1 Introduction

An array is a collection of variables, all of the same type, and accessed using the same name plus one or more indices.

In C, arrays are indexed from 0 by integers, or converted to pointers. Although the whole array can be initialized, each element must be read or written separately in procedural statements.

In Verilog-2001, arrays are indexed from left-bound to right-bound. If they are vectors, they can be assigned as a single unit, but not if they are arrays. Verilog-2001 allows multiple dimensions.

In Verilog-2001, all data types can be declared as arrays. The reg, wire and all other net types can also have a vector width declared. A dimension declared before the object name is referred to as the �vector width� dimension.

The dimensions declared after the object name are referred to as the �array� dimensions.

 reg [7:0] r1 [1:256];//[7:0] is the vector width,[1:256] is the array size

 SystemVerilog uses the term �packed array� to refer to the dimensions declared before the object name (what Verilog-2001 refers to as the vector width). The term �unpacked array� is used to refer to the dimensions declared after the object name.

 SystemVerilog enhances packed arrays by allowing multiple dimensions. SystemVerilog adds the ability to procedurally change the size of one of the dimensions of an unpacked array. Fixed-size unpacked arrays can be multi-dimensional and have fixed storage allocated for all the elements of the array. Each dimension of an unpacked array can be declared as having a fixed or un-fixed size. A dynamic array allocates storage for elements at runtime along with option of changing the size of one of its dimensions. An associative array allocates storage for elements individually as they are written. Associative arrays can be indexed using arbitrary data types. A queue type of array grows or shrinks to accommodate the number elements written to the array at runtime.

 5.2 Packed and unpacked arrays

 A packed array is a mechanism for subdividing a vector into subfields which can be conveniently accessed as array elements. Consequently, a packed array is guaranteed to be represented as a contiguous set of bits. An unpacked array may or may not be so represented. A packed array differs from an unpacked array in that, when a packed array appears as a primary, it is treated as a single vector.

If a packed array is declared as signed, then the array viewed as a single vector shall be signed. The individual elements of the array are unsigned unless they are of a named type declared as signed. A part-select of a packed array shall be unsigned.

Packed arrays allow arbitrary length integer types, so a 48 bit integer can be made up of 48 bits. These integers can then be used for 48 bit arithmetic. The maximum size of a packed array can be limited, but shall be at least 65536 (216) bits.

Packed arrays can only be made of the single bit types (bit, logic, reg, wire, and the other net types) and recursively other packed arrays and packed structures.

Integer types with predefined widths cannot have packed array dimensions declared. These types are: byte, shortint, int, longint, and integer. An integer type with a predefined width can be treated as a single dimension packed array. The packed dimensions of these integer types shall be numbered down to 0, such that the right-most index is 0. 

 Unpacked arrays can be made of any type. SystemVerilog enhances fixed-size unpacked arrays in that in addition to all other variable types, unpacked arrays can also be made of object handles (see Section 11.4) and events (see Section 13.5).

SystemVerilog accepts a single number, as an alternative to a range, to specify the size of an unpacked array, like C. That is, [size] becomes the same as [0:size-1].

 example:

int Array[8][32]; is the same as: int Array[0:7][0:31]; 

The following operations can be performed on all arrays, packed or unpacked. The examples provided with these rules assume that A and B are arrays of the same shape and type.

•   Reading and writing the array, e.g., A = B

•   Reading and writing a slice of the array, e.g., A[i:j] = B[i:j]

•   Reading and writing a variable slice of the array, e.g., A[x+:c] = B[y+:c]

•   Reading and writing an element of the array, e.g., A[i] = B[i]

•   Equality operations on the array or slice of the array, e.g. A==B, A[i:j] != B[i:j]

The following operations can be performed on packed arrays, but not on unpacked arrays. The examples provided with these rules assume that A is an array.

•   Assignment from an integer, e.g., A = 8�b11111111;

•   Treatment as an integer in an expression, e.g., (A + 3)

If an unpacked array is declared as signed, then this applies to the individual elements of the array, since the whole array cannot be viewed as a single vector.

When assigning to an unpacked array, the source and target must be arrays with the same number of unpacked dimensions, and the length of each dimension must be the same. Assignment to an unpacked array is done by assigning each element of the source unpacked array to the corresponding element of the target unpacked

array. Note that an element of an unpacked array can be a packed array.

For the purposes of assignment, a packed array is treated as a vector. Any vector expression can be assigned to any packed array. The packed array bounds of the target packed array do not affect the assignment. A packed array cannot be directly assigned to an unpacked array without an explicit cast.

5.3 Multiple dimensions 

Like Verilog memories, the dimensions following the type set the packed size. The dimensions following the

instance set the unpacked size.

 Note that the dimensions declared following the type and before the name ([3:0][7:0] in the preceding declaration) vary more rapidly than the dimensions following the name ([1:10] in the preceding declaration).

When used, the first dimensions ([3:0]) follow the second dimensions ([1:10]).

 5.4 Indexing and slicing of arrays

An expression can select part of a packed array, or any integer type, which is assumed to be numbered down to 0.

SystemVerilog uses the term �part select� to refer to a selection of one or more contiguous bits of a single dimension packed array. This is consistent with the usage of the term �part select� in Verilog.

SystemVerilog uses the term �slice� to refer to a selection of one or more contiguous elements of an array. Verilog only permits a single element of an array to be selected, and does not have a term for this selection.

A single element of a packed or unpacked array can be selected using an indexed name.

 5.5 Array querying functions

SystemVerilog provides new system functions to return information about an array. These are: $left, $right, $low, $high, $increment, $size, and $dimensions.

 5.6 Dynamic arrays

A dynamic array is one dimension of an unpacked array whose size can be set or changed at runtime. The space for a dynamic array doesn�t exist until the array is explicitly created at runtime.

The syntax to declare a dynamic array is:

data_type array_name [];

 where data_type is the data type of the array elements. Dynamic arrays support the same types as fixed-size arrays.

For example:

bit [3:0] nibble[]; // Dynamic array of 4-bit vectors

integer mem[]; // Dynamic array of integers

 The new[] operator is used to set or change the size of the array.

 The size() built-in method returns the current size of the array.

 The delete() built-in method clears all the elements yielding an empty array (zero size).

 5.6.1 new[]

The built-in function new allocates the storage and initializes the newly allocated array elements either to their default initial value or to the values provided by the optional argument.

For example:

 The new operator follows the SystemVerilog precedence rules. Since both the square brackets [] and the parenthesis () have the same precedence, the arguments to this operator are evaluated left to right:

[ expression ] first, and ( expression ) second.

In the above example [200] indicates the number of elements in the array. Must be a non-negative integral expression and (addr) is Optional. An array with which to initialize the new array. If it is not specified, the elements of the newly allocated array are initialized to their default value. This array identifier must be a dynamic array of the same data type as the array on the left-hand side, but it need not have the same size. If the size of this array is less than the size of the new array, the extra elements shall be initialized to their default value. If the size of this array is greater than the size of the new array, the additional elements shall be ignored.

 5.6.2 size ()

The prototype for the size () method is:

function int size();

The size() method returns the current size of a dynamic array, or zero if the array has not been created.

Note: The size method is equivalent to $length(addr, 1 ).

 5.6.3 delete()

The prototype for the delete() method is:

function void delete();

The delete() method empties the array, resulting in a zero-sized array.

 4.9 Associative arrays

Dynamic arrays are useful for dealing with contiguous collections of variables whose number changes dynamically.

When the size of the collection is unknown or the data space is sparse, an associative array is a better option. Associative arrays do not have any storage allocated until it is used, and the index expression is not restricted to integral expressions, but can be of any type.

An associative array implements a lookup table of the elements of its declared type. The data type to be used as an index serves as the lookup key, and imposes an ordering.

The syntax to declare an associative array is:

data_type array_id [index_type];

where:

� data_type is the data type of the array elements. Can be any type allowed for fixed-size arrays.

� array_id is the name of the array being declared.

� index_type is the data-type to be used as an index, or *. If * is specified, then the array is indexed by any integral expression of arbitrary size. An index type restricts the indexing expressions to a particular type.

 5.10 Associative array methods

In addition to the indexing operators, several built-in methods are provided that allow users to analyze and manipulate associative arrays, as well as iterate over its indices or keys.

 5.10.1 num()

The syntax for the num() method is:

function int num();

The num() method returns the number of entries in the associative array. If the array is empty, it returns 0.

5.10.2 delete()

The syntax for the delete() method is:

function void delete( [input index] );

Where index is an optional index of the appropriate type for the array in question.

If the index is specified, then the delete() method removes the entry at the specified index. If the entry to be deleted does not exist, the method issues no warning.

If the index is not specified, then the delete() method removes all the elements in the array.

 5.10.3 exists()

The syntax for the exists() method is:

function int exists( input index );

Where index is an index of the appropriate type for the array in question.

The exists() function checks if an element exists at the specified index within the given array. It returns 1 if the element exists, otherwise it returns 0.

 5.10.4 first()

The syntax for the first() method is:

function int first( ref index );

Where index is an index of the appropriate type for the array in question.

The first() method assigns to the given index variable the value of the first (smallest) index in the associative array. It returns 0 if the array is empty, and 1 otherwise.

 5.10.5 last()

The syntax for the last() method is:

function int last( ref index );

Where index is an index of the appropriate type for the array in question.

The last() method assigns to the given index variable the value of the last (largest) index in the associative array. It returns 0 if the array is empty, and 1 otherwise.

 5.10.6 next()

The syntax for the next() method is:

function int next( ref index );

Where index is an index of the appropriate type for the array in question.

The next() method finds the entry whose index is greater than the given index. If there is a next entry, the index variable is assigned the index of the next entry, and the function returns 1. Otherwise, index is unchanged, and the function returns 0.

 5.10.7 prev()

The syntax for the prev() method is:

function int prev( ref index );

Where index is an index of the appropriate type for the array in question.

The prev() function finds the entry whose index is smaller than the given index. If there is a previous entry, the index variable is assigned the index of the previous entry, and the function returns 1. Otherwise, the index is unchanged, and the function returns 0.

If the argument passed to any of the four associative array traversal methods first, last, next, and prev is smaller than the size of the corresponding index, then the function returns �1 and shall copy only as much data as can fit into the argument. For example:

 5.11 Queues

SystemVerilog adds dynamic queues to Verilog

�      A dynamic array � can grow and shrink in size during simulation

�      Can represent FIFO, LIFO or other types of queues

A queue is declared like an array, but using $ for the range

�      Optionally, a maximum size for the queue can be specified

     A queue can only hold one data type, which is specified when the queue is   declared

5.11.1 Queue methods

  SystemVerilog queues have several built-in methods

•  insert(value) �  The method inserts the given item at the specified index position.

•  delete(value) �  The method deletes the item at the specified index position.

•  push_front(<value>) � adds a new location at the beginning of the queue with the value.

•  push_back(<value>) � adds a new location at the end of the queue with the value.

•  variable = pop_front() � removes the first element of the queue and returns its value.

•  variable = pop_back() � removes the last element of the queue and returns its value.

•  insert(<index>,<value>) � changes the value of a queue location without changing the queue size

•  variable = <queue_name>[<index>] � retrieves the value of a queue location without changing the queue size

•  variable = size() � returns the current number of elements in the queue.

           It is a run-time error to write to a full queue or to read from an empty queue

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