When reading and writing associative arrays the

When reading and writing associative arrays, the simulator must search for the ele-
ment in memory. The LRM does not specify how this is done, but popular ways are
hash tables and trees. These require more computation than other arrays, and there-
fore associative arrays are the slowest.
2.8.4 Sorting
Since SystemVerilog can sort any single-dimension array (fixed-size, dynamic, and
associative arrays plus queues), you should pick based on how often the values are
added to the array. If the values are received all at once, choose a fixed-size or
dynamic array so that you only have to allocate the array once. If the data slowly drib-
bles in, choose a queue, as adding new elements to the head or tail is very efficient.
If you have unique and noncontiguous values, such as
Õ{1, 10, 11, 50}
, you can
store them in an associative array by using them as an index. Using the routines
first
,
next
, and
prev
, you can search an associative array for a value and find suc-
cessive values. Lists are doubly linked, and so you can find values both larger and
smaller than the current value. Both of these support removing a value. However, the
associative array is much faster in accessing any given element, given an index.
For example, you can use an associative array of bits to hold expected 32-bit values.
When the value is created, write to that location. When you need to see if a given

Chapter 2:Data Types 48
value has been written, use the
exists
function. When done with an element, use
delete
to remove it from the associative array.
2.8.5 Choosing the Best Data Structure
Here are some suggestions on choosing a data structure.
z
Network packets . Properties: fixed size, accessed sequentially. Use a fixed-
size or dynamic array for fixed- or variable-size packets.
z
Scoreboard of expected values . Properties: size not known until run-time,
accessed by value, and a constantly changing size. In general, use a queue, as
you are continually adding and deleting elements during simulation. If you
can give every transaction a fixed id, such as 1, 2, 3, …, you could use this as
an index into the queue. If your transaction is filled with random values, you
can just push them into a queue and search for unique values. If the score-
board may have hundreds of elements, and you are often inserting and delet-
ing them from the middle, an associative array may be faster.
z
Sorted structures . Use a queue if the data comes out in a predictable order,
or an associative array if the order is unspecified. If the scoreboard never
needs to be searched, just store the expected values in a mailbox, as shown
in Section 7.6.
z
Modeling very large memories, greater than a million entries . If you do not
need every location, use an associative array as a sparse memory. If you do
need every location, try a different approach where you do not need so much
live data. Still stuck? Be sure to use 2-state values packed into 32-bits.