An union is used to stored multiple different kind/size of data in the same STORAGE location.

1.typedef union{
2.bit [31:0] a;
3.int b;
4.} data_u;

Now here XYZ union can contain either bit [31:0] data or an int data. It can be written with a bit [31:0] data and read-back with a int data. There is no TYPE-checking done.

In the case where we want to enforce that the read-back data-type is same as the written data-type we can use TAGGED union which is declared using the QUALIFIER tagged. Whenever an union is defined as tagged, it stores the tag information along with the value (in expense of few extra bits). The tag and values can only be updated together using a statically type-checked tagged union expression. The data member value can be read with a type that is consistent with current tag value, making it impossible to write one type and read another type of value in tagged union. (the details of which can be found in section 3.10 and 7.15 of SV LRM 3.1a).

01.typedef union tagged{
02.bit [31:0] a;
03.int b;
04.} data_tagged_u;
05. 
06.// Tagged union expression
07.data_tagged_u data1 = tagged a 32'h0;
08.data_tagged_u data2 = tagged b 5;
09. 
10.// Reading back the value
11.int xyz = data2.b;

An union is used to stored multiple different kind/size of data in the same storage location.

1.typedef union{
2.bit [31:0] a;
3.int b;
4.} data_u;

Now here XYZ union can contain either bit [31:0] data or an int data. It can be written with a bit [31:0] data and read-back with a int data. There is no type-checking done.

In the case where we want to enforce that the read-back data-type is same as the written data-type we can use tagged union which is declared using the qualifier tagged. Whenever an union is defined as tagged, it stores the tag information along with the value (in expense of few extra bits). The tag and values can only be updated together using a statically type-checked tagged union expression. The data member value can be read with a type that is consistent with current tag value, making it impossible to write one type and read another type of value in tagged union. (the details of which can be found in section 3.10 and 7.15 of SV LRM 3.1a).

01.typedef union tagged{
02.bit [31:0] a;
03.int b;
04.} data_tagged_u;
05. 
06.// Tagged union expression
07.data_tagged_u data1 = tagged a 32'h0;
08.data_tagged_u data2 = tagged b 5;
09. 
10.// Reading back the value
11.int xyz = data2.b;