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

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.

bit [7:0] c1; // packed array

real u [7:0]; // unpacked array

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 the 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 of elements written to the array at runtime.

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 bits integer can be made up of 48 bits. These integers can then be used for 48 bits 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.

byte c2; // same as bit [7:0] c2;

integer i1; // same as logic signed [31:0] i1;

Unpacked arrays can be made of any type. System Verilog enhances fixed-size unpacked arrays in that in addition to all other variable types, unpacked arrays can also be made of object handles and events.

System Verilog 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]. For 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.

Multiple dimensions

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

bit [3:0] [7:0] john [1:10]; // 10 entries of 4 bytes (packed into 32 bits)

can be used as follows:

john[9] = john[8] + 1; // 4 byte add
john[7][3:2] = john[6][1:0]; // 2 byte copy

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