{"id":17656,"date":"2022-10-17T11:20:51","date_gmt":"2022-10-17T15:20:51","guid":{"rendered":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/?p=17656"},"modified":"2026-03-27T08:50:11","modified_gmt":"2026-03-27T12:50:11","slug":"dig-a-pool-of-specialized-systemverilog-classes","status":"publish","type":"post","link":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/2022\/10\/17\/dig-a-pool-of-specialized-systemverilog-classes\/","title":{"rendered":"Dig a Pool of Specialized SystemVerilog Classes"},"content":{"rendered":"\n
Introduction<\/h2>\n\n\n\n
SystemVerilog classes are a great way to encapsulate both variables and the routines that operates on them. What if you want to reuse the methods but change the type of properties? Use a parameter and specialize it!<\/p>\n\n\n\n
Definition<\/h2>\n\n\n\n
In Object-Oriented Programming (OOP), encapsulate variables and routines in a class. A class variable is called a property and a class routine is a method. By grouping them together, the result is more reusable than if they were kept separate.<\/p>\n\n\n\n
The First Assignment<\/h2>\n\n\n\n
Your manager asks you to write a class to represent a stack. When you tell her that SystemVerilog already has a queue that can be used as a LIFO (Last In, First Out), she responds, \u201cYes, but on the last project, someone kept switching between push_front(), push_back(), and pop_front(), pop_back(), causing testbench bugs. Give me a stack class with just push() and pop().\u201d Phew!<\/p>\n\n\n\n
Your first attempt might look like the following, with no error checking.<\/p>\n\n\n\n
class Stack;\n bit[31:0] items[100];\n int idx = -1;\n string name;\n\n function new(string name);\n this.name = name;\n endfunction\n\n function push(input bit[31:0] val);\n items[++idx] = val;\n endfunction\n\n function bit[31:0] pop();\n return items[idx--];\n endfunction\nendclass<\/code><\/pre>\n\n\n\n
Here is a short test.<\/p>\n\n\n\n
Stack s; initial begin s = new(\"s32\"); \/\/ Construct a Stack object s.push('1); \/\/ Push all 1\u2019s $display(\"pop()=%x\", s.pop()); \/\/ Expect FFFF_FFFF end<\/code><\/pre>\n\n\n\n
That was good, now specialize it<\/h2>\n\n\n\n
Your manager liked that solution but asked why you only used 32-bit numbers. After all, the next project has a 64-bit data path. Can you make a compatible stack class? How about 128-bits?<\/p>\n\n\n\n
Rather than make separate classes for each width, create a parameterized class. This one has a default width of 4 bits so it will obvious if you forget to specify the correct width.<\/p>\n\n\n\n
class Stack #(parameter int WIDTH=4);\n bit[WIDTH-1:0] items[100];\n int idx = -1;\n\n function push(input bit[WIDTH-1:0] val); \u2026\n function bit[WIDTH:0] pop(); \u2026<\/code><\/pre>\n\n\n\n
Declare two handles for this class, first with the default width, and another for 64-bit values.<\/p>\n\n\n\n
Stack s4; \/\/ Handle to a 4-bit stack object Stack #(64) s64; \/\/ Handle to a 64-bit stack object<\/code><\/pre>\n\n\n\n
Try these two stack types with the test code from the first section.<\/p>\n\n\n\n
When you give a value for a parameter when declaring a class handle (or use the default value), it is called “specialization”. Under the hood, it is as if the compiler creates a new type (but you can’t use it directly). You can imagine it creates the following for a specialization of WIDTH=64.<\/p>\n\n\n\n
class _Stack__64<\/strong>; \/\/ Equivalent class for specialization: Stack #(64) s64\n bit[64<\/strong>-1:0] items[100];\n int idx = -1;\n\n function push(input bit[64<\/strong>-1:0] val); \u2026\n function bit[64<\/strong>-1:0] pop(); \u2026\nendclass<\/em><\/code><\/pre>\n\n\n\n
A group of stacks<\/h2>\n\n\n\n
A verification engineer saw this parameterized class and said that he wanted to make a group of these stacks as part of the reference model. However, when he tried the following code, it wouldn\u2019t compile.<\/p>\n\n\n\n
Stack q[$]; \/\/ A queue of stack handles\ninitial begin\n q.push_back(s4);\n q.push_back(s64); \/\/ Bad assign to Stack#(4) from Stack#(64)\nend<\/code><\/pre>\n\n\n\n
The problem is that the two specializations, Stack#(4) and Stack#(64), are different types, and you can’t assign between them, not even with the $cast() system task. You can’t group dissimilar types.<\/p>\n\n\n\n
Twisting by the Pool<\/h2>\n\n\n\n
You can group multiple objects together if they have the same type. Wrap this group in a class with methods to make it easier to manipulate them. UVM calls this a \u201cpool\u201d.<\/p>\n\n\n\nA pool of specialized classes<\/figcaption><\/figure>\n\n\n\n
(The formal name for this group is an \u201caggregate\u201d which means each object can exist on its own. The opposite is \u201ccomposition\u201d, such as an Animal class that is composed from Head<\/a> and Body classes.)<\/p>\n\n\n\n
![\"A](\"https:\/\/blogs.sw.siemens.com\/wp-content\/uploads\/sites\/54\/2022\/10\/pool_of_stacks.png\")